Compressed Air Manual

Compressed Air Manual

Co

mp

ressed A

ir Man

ual 1998

The

face

of

com

mitm

ent

What sets Atlas Copco apart as a company is ourconviction that we can only excel in what we do if weprovide the best possible know-how and technology toreally help our customers produce, grow and succeed.

There is a unique wayof achieving that - we simplycall it the Atlas Copco way. It builds on interaction, onlong-term relationships and involvement in the customers’process, needs and objectives. It means having theflexibility to adapt to the diverse demands of the peoplewe cater for.

It’s the commitment to our customers’ business thatdrives our effort towards increasing their productivitythrough better solutions. It starts with fully supportingexisting products and continuously doing things better, butit goes much further, creating advances in technologythrough innovation. Not for the sake of technology, but forthe sake of our customers’ bottom line and peace-of-mind.

That is how Atlas Copco will strive to remain the firstchoice, to succeed in attracting new business and to main-tain our position as the industry leader.

9780

038

0-10

Kaft 27.04.2000 11:46 Pagina 1


6th Edition

Inhoud 27.04.2000 11:54 Pagina 1

This Manual is published by:Atlas Copco Compressor ABUddvägen 7S-131 82 NackaSweden

Reproduction of the contents of this publication, fully orin part, is forbidden in accordance with copyright lawswithout prior written permission from Atlas CopcoCompressor AB. This applies to any form of reproduc-tion through printing, duplication, photocopying,recording, etc.

During the production of this material we have grate-fully received pictures and contributions from ourcustomers and suppliers, of which we would especiallylike to name: ABB, Siemens, Vattenfall, AGA andHamrin Adsorptions & Filterteknik.

Atlas Copco Compressor ABProject manager: Paul LindebergProduction: In Red Design ABTechnical Production: Blueprint AB

ISBN: 91-630-7342-0

Inhoud 27.04.2000 11:54 Pagina 2

Introduction

The Compressed Air Manual is a resource for everyone who wishes to know more about

compressed air. This edition, the sixth, is in many respects extended, updated and improved

compared to previous editions, of which the last was issued in 1976. Naturally a great deal

has happened during these twenty or more years, nevertheless the fundamentals remain

and make up the core of this Manual, which has been desired and requested by many.

The Manual addresses the essentials of theoretical and practical issues faced by every-

one working with compressed air on a day-to-day basis, from the fundamental theoretical

relations to more practical advice and tips. The main addition to this edition is an increased

concentration on environmental aspects, air quality issues, energy savings and compressed

air economy. Furthermore, we conclude with calculation examples as well as diverse, helpful

table information and a complete keyword index. The Manual’s contents have been produ-

ced by our leading compressed air technicians and I hope that the different sections act both

as a textbook for newcomers and a reference book for more experienced users.

It is my belief that the Manual will be useful and perhaps even an enjoyment to many

within the industry. Many questions can surely be answered with its help, while others re-

quire further investigation. In the case of the latter, I believe the reader can also receive help

through the support and structure for continued discussion provided by the Manual. With

this in mind, each reader is always more than welcome to contact us for answers to unresol-

ved questions.

Stockholm, September 1998

Atlas Copco Compressor AB

Robert Robertson, MD

Inhoud 27.04.2000 11:54 Pagina 3

2

1.1 Physics, General 101.1.1 The structure of matter 101.1.2 The molecule and the

different states of matter 101.2 Physical units 11

1.2.1 Pressure 111.2.2 Temperature 121.2.3 Thermal capacity 131.2.4 Work 131.2.5 Power 131.2.6 Volume rate of flow 13

1.3 Thermodynamics 141.3.1 Main principles 141.3.2 Gas laws 141.3.3 Heat transfer 151.3.4 Changes in state 16

1.3.4.1 Isochoric process 161.3.4.2 Isobaric process 171.3.4.3 Isothermic process 171.3.4.4 Isentropic process 181.3.4.5 Polytropic process 18

1.3.5 Gas flow throughnozzles 18

1.3.6 Flow through pipes 191.3.7 Throttling 19

1.4 Air 201.4.1 Air in general 201.4.2 Moist air 20

1.5 Types of compressors 211.5.1 Two basic principles 211.5.2 Displacement

compressors 211.5.3 The compressor diagram for

displacement compressors 231.5.4 Dynamic compressors 241.5.5 Compression in

several stages 241.5.6 Comparison between

displacement andcentrifugal compressors 25

Chapter 1Theory

1.6 Electricity 251.6.1 Basic terms and

definitions 251.6.2 Ohm’s law for

alternating current 261.6.3 Three-phase system 271.6.4 Power 271.6.5 The electric motor 28

1.6.5.1 Speed 281.6.5.2 Efficiency 281.6.5.3 Insulation class 281.6.5.4 Protection classes 281.6.5.5 Cooling methods 291.6.5.6 Installation method 291.6.5.7 Star (Y) and delta

(∆) connections 291.6.5.8 Torque 30

Chapter 2Compressors and

auxiliary equipment

2.1 Displacementcompressors 342.1.1 Displacement compressors

in general 342.1.2 Piston compressors 342.1.3 Oil-free piston

compressors 372.1.4 Diaphragm compressors 372.1.5 Screw compressors 37

2.1.5.1 Oil-free screwcompressor 39

2.1.5.2 Liquid injectedscrew compressor 39

2.1.6 Tooth compressor 412.1.7 Scroll compressor 412.1.8 Vane compressor 432.1.9 Liquid-ring compressor 432.1.10 Blower 44

2.2 Dynamic compressors 442.2.1 Dynamic compressors

in general 44

Inhoud 27.04.2000 11:54 Pagina 4

3

2.2.2 Centrifugal compressor 452.2.3 Axial compressor 46

2.3 Other compressors 472.3.1 Vacuum pump 472.3.2 Booster compressor 472.3.3 Pressure intensifier 48

2.4 Treatment ofcompressed air 482.4.1 Drying compressed air 48

2.4.1.1 Aftercooler 502.4.1.2 Refrigerant dryer 502.4.1.3 Over-compression 512.4.1.4 Absorption drying 512.4.1.5 Adsorption drying 52

2.4.2 Filters 542.5 Control and

regulation systems 562.5.1 Regulation in general 562.5.2 Regulation principles for

displacement compressors 572.5.2.1 Pressure relief 572.5.2.2 Bypass 572.5.2.3 Throttling the intake 572.5.2.4 Pressure relief with

throttled intake 582.5.2.5 Start/stop 582.5.2.6 Speed regulation 582.5.2.7 Variable discharge

port 592.5.2.8 Balancing of valves 592.5.2.9 Clearance volume 592.5.2.10 Full load-unload

-stop 592.5.3 Regulation principles

for dynamic compressors 602.5.3.1 Throttling the intake 602.5.3.2 Intake vane control 602.5.3.3 Outlet vane control

(diffuser) 602.5.3.4 Pressure relief 602.5.3.5 Load-unload-stop 602.5.3.6 Speed regulation 60

2.5.4 Control and monitoring 62

2.5.4.1 General 622.5.4.2 Load-unload-stop 622.5.4.3. Speed control 63

2.5.5 Control and monitoring 632.5.5.1 Temperature

measurement 632.5.5.2 Pressure

measurement 642.5.5.3 Monitoring 64

2.5.6 Comprehensivecontrol system 652.5.6.1 Start sequence

selector 652.5.7 Central control 662.5.8 Remote monitoring 67

Chapter 3Dimensioning and installation

3.1 Dimensioning compressorinstallations 703.1.1 General 70

3.1.1.1 Calculating theworking pressure 70

3.1.1.2 Calculating the airrequirement 71

3.1.1.3 Measuring the airrequirement 72

3.1.2 Centralisation ordecentralisation 723.1.2.1 General 733.1.2.2 Centralised

compressorinstallations 73

3.1.2.3 Decentralisedcompressorinstallations 74

3.1.3 Dimensioningat high altitude 743.1.3.1 General 743.1.3.2 The ambient conditions

effect on a compressor 753.1.3.3 Power source 76

Inhoud 27.04.2000 11:54 Pagina 5

3.1.3.3.1 Electric3.1.3.3.1 motors 763.1.3.3.2 Combustion3.1.3.3.2 engines 76

3.2 Air treatment 773.2.1 General 773.2.2 Water vapour in

the compressed air 773.2.3 Oil in the compressed air 783.2.4 Microorganisms in the

compressed air 793.2.5 Filters 803.2.6 Aftercooler 813.2.7 Water separator 813.2.8 Oil as droplets 81

3.3 Cooling systems 823.3.1 Water cooled compressors 82

3.3.1.1 General 823.3.1.2 Open system,

withoutcirculating water 82

3.3.1.3 Open system,circulating water 82

3.3.1.4 Closed system 833.4 Energy recovery 85

3.4.1 General 853.4.2 Calculation of the recovery

potential 873.4.3 Recovery methods 87

3.4.3.1 General 873.4.3.2 Air cooled systems 883.4.3.3 Water

cooled system 893.5 The compressor room 90

3.5.1 General 903.5.2 Placement and design 913.5.3 Foundation 913.5.4 Intake air 923.5.5 Compressor room

ventilation 933.6 The compressed air

networks structure 963.6.1 General 96

3.6.1.1 Air receiver 963.6.2 Design of the compressed

air network 983.6.3 Dimensioning the

compressed air network 993.6.4 Flow measurement 101

3.7 Portable compressors 1013.7.1 General 1013.7.2 Noise and gaseous

emissions 1013.7.3 Transport and auxiliary

equipment 1023.8 Electrical installation 102

3.8.1 General 1023.8.2 Motors 1023.8.3 Starting methods 1033.8.4 Control voltage 1033.8.5 Short-circuit protection 1043.8.6 Cables 1043.8.7 Phase compensation 104

3.9 Sound 1053.9.1 General 1053.9.2 Sound pressure 1053.9.3 Absorption 1063.9.4 Room constant 1063.9.5 Reverberation 1063.9.6 Relation between sound

power and soundpressure 106

3.9.7 Sound measurements 1073.9.8 Interaction of several

sources 1083.9.9 Sound reduction 1083.9.10Noise with compressor

installations 1093.10 Standards, laws

and provisions 1093.10.1General 1093.10.2Standards 109

4

Inhoud 27.04.2000 11:54 Pagina 6

5

Chapter 4Economy

4.1 Economy 1124.1.1 Costs for compressed

air production 1124.1.1.1 General 1124.1.1.2 Apportioning costs 113

4.2 Opportunities for saving 1134.2.1 Power requirement 1134.2.2 Working pressure 1134.2.3 Air consumption 1154.2.4 Regulation method 1164.2.5 Air quality 1174.2.6 Energy recovery 1174.2.7 Maintenance 118

4.2.7.1 Maintenanceplanning 119

4.2.7.2 Auxiliaryequipment 120

4.3 Other economic factors 1204.3.1 General 1204.3.2 LCC 120

Chapter 5Calculation example

5.1 Example of dimensioningcompressed airinstallations 124

5.2 Input data 1255.2.1 Requirement 1255.2.2 Ambient conditions

(dimensioning) 1255.2.3 Miscellaneous 125

5.3 Component selection 1265.3.1 Dimensioning the

compressor 1275.3.2 Assumption for the

continued calculation 1275.3.3 Dimensioning of the

receiver volume 1285.3.4 Dimensioning of the dryer 129

5.3.5 Assumptions for thecontinued calculation 129

5.3.6 Control calculations 1305.4 Other dimensioning 131

5.4.1 Condensation quantitycalculation 131

5.4.2 Ventilation requirementin the compressor area 132

5.5 High altitude 1335.6 Intermittent output 1345.7 Water borne energy

recovery 1355.7.1 Assumption 1365.7.2 Calculation of the cooling

water flow in the energyrecovery circuit 136

5.7.3 Energy balance across therecovery heat exchanger 137

5.7.4 Compilation of the answer 1375.8 Pressure drop in

the piping 138

Chapter 6Appendices

6.1 The SI-system 1406.2 Drawing symbols 1436.3 Diagrams and tables 1456.4 Compilation of current

standards and norms 1506.4.1 Safety related regulations

and standards 1506.4.1.1 Machine safety 1506.4.1.2 Pressure safety 1506.4.1.3 Environment 1506.4.1.4 Electrical safety 150

6.4.2 Technical relatedstandards and norms 1516.4.2.1 Standardization 1516.4.2.2 Specifications 1516.4.2.3 Measurements 151

Index 152

Inhoud 27.04.2000 11:54 Pagina 7

6

Inhoud 27.04.2000 11:54 Pagina 8

7

Chapter 1Theory

Chapter 2Compressors and auxiliaryequipment

Chapter 3Dimensioningand installation

Chapter 4Economy

Chapter 5Calculation examples

Chapter 6Appendices

Inhoud 27.04.2000 11:54 Pagina 9

8

Inhoud 27.04.2000 11:54 Pagina 10

Chapter 1 Theory

Kap 1 dok engelsk 27.04.2000 12:15 Pagina 1

10

number of protons in the nucleus is equalto the atom’s atomic number.

The total number of protons and thenumber of neutrons are approximatelyequal to the atom’s total mass.

This information is a part of the datathat can be read off from the periodicsystem. The electron shell contains thesame number of electrons as there are pro-tons in the nucleus. This means the atom iselectrically neutral.

The Danish physicist, Niels Bohr, produceda theory as early as 1913 that proved to cor-respond with reality where he, amongothers, demonstrated that atoms can onlyoccur in a so-called, stationary state with adetermined energy. If the atom transformsfrom one energy state to another a radia-tion quantum is emitted, a photon.

It is these different transitions thatmakes themselves known in the form oflight with different wavelengths. In aspectrograph they appear as lines in theatom’s spectrum of lines.

1.1.2 The molecule and the diffe-rent states of matter

Atoms held together by chemical bondingare called molecules. These are so smallthat, for example, 1 mm3 of air at atmosp-heric pressure contains approx. 2.55 x 1016

molecules.All matter can in principle exist in four

different states: solid state, liquid state,gaseous state and plasma state. In the solidstate the molecules are tightly packed in alattice, with strong bonding. At all temp-eratures above absolute zero a certaindegree of molecular movement occurs, inthe solid state as a vibration around abalanced position, the faster the greater thetemperature becomes. When a substance in

1.1 Physics General

1.1.1 The structure of matter

Matter primarily consists of protons, neu-trons and electrons. There are also a num-ber of other building blocks however theseare not stable.

All of these particles are characterisedby four properties: their electrical charge,their rest mass, their mechanical momen-tum and their magnetic momentum. The

+

_

+

+

__

_

_

_

++

_

+

+

neutron

electron

proton

1:1

The electron shell gives elements their chemicalproperties. Here are a few simple examples.

Hydrogen (top) has one electron in an electronshell. Helium (middle) has two electrons in an

electron shell. Lithium (bottom) has a third electron in a second shell.


ken, and it transforms into a gaseous stateduring expansion in all directions andmixes with the other gases in the room.When gas molecules are cooled, they loosespeed and bond to each other again, andcondensation starts. However, if the gasmolecules are heated further, they are bro-ken down into individual particles andform a plasma of electrons and atomic nuc-lei.

1.2 Physical units

1.2.1 Pressure

The force on a square centimetre area of anair column, which runs from sea level tothe edge of the atmosphere, is about 10.13N. Therefore the absolute atmosphericpressure at sea level is approx. 10.13 x 104 Nper square metre, which is also called 1 Pa(Pascal), the SI unit for pressure. A basicdimension analysis shows that 1 bar = 1 x105 Pa. The higher above sea level you arethe lower the atmospheric pressure andvisa versa.

a solid state is heated so much that themovement of the molecules cannot be pre-vented by the rigid pattern (lattice), theybecome loose the substance melts andtransforms to a fluid. If the liquid is heatedmore, the bonding of the molecules is bro-

11

1:2

A salt crystal has a cubic structure. Commontable salt (NaCl) is a typical example. The

lines show the bonding between the sodium(red) and the chlorine (white).

super heating

evaporation at atmospheric pressure

(water + steam)

(water)

(ice)

ice melts

(steam)

0 1000 2000 3000 kJ/kgHeat added

Temperature°C

200

100

0-20

1:3

By leading off or applying thermal energy the state of a substance changes. This is how the relation forcommon water appears.


1.2.2 Temperature

The temperature of a gas is more difficultto clearly define than the pressure. Temp-erature is an indication of the kinetic ener-gy in the molecules. They move more rap-idly the higher the temperature and themovement stops at absolute zero. TheKelvin scale is based on this, but otherwiseis graduated the same as the Celsius scale.The relation is:

T = t + 273.2T = absolute temperature (K)t = temperature (°C)

1.2.3 Thermal capacity

Thermal capacity refers to the quantity ofheat required to increase the temperatureof 1 kg of a substance by 1 K. Accordingly,the dimension of the thermal capacity will12

water boils

water freezes

absolute zero

400373350

300273250

200

150

100

50

0-273-250

-200

-150

-100

-50

0

50

100

KCo

1:5

This is how the relation between Celsius andKelvin scales appear. For Celsius, 0° is set atwater’s freezing point; for Kelvin, 0° is set at

absolute zero.

actual pressure

effectivte pressure(gauge pressure)bar (g) = bar (e)

vacuum bar (u)

absolutepressurebar (a)

localatmosphericpressure(barometicpressure)bar (a)

absolutepressurebar (a)

zero pressure (perfect vaccum)

variable level

normal atmosphericpressure (a)

Most pressure gauges register the difference between the pressure in a vessel and the local atmosphericpressure. Therefore to find the absolute pressure the value of the local atmospheric pressure must be added.

1:4


1.2.4 Work

Mechanical work can be defined as the pro-duct of a force and the distance over whichthe force affects a body.

Exactly as for heat, work is an energythat is transferred from one body to an-other. The difference is that it is a questionof force instead of temperature.

An example is the compression of a gas in acylinder by a moving piston. Compressiontakes place through a force moving thepiston. At the same time energy is transfer-red from the piston to the enclosed gas.This energy transfer is work in a thermody-namic meaning. The sum of the appliedand transmitted energy is always constant.Work can give different results, for examp-le, changes to the potential energy, kineticenergy or the thermal energy.

The mechanical work associated withchanges in the volume of a gas or gas mix-ture is one of the most important processeswithin thermodynamics. The SI unit forwork is Joule. 1 J = 1 Nm = 1 Ws.

1.2.5 Power

Power is work per time unit. SI unit forpower is Watt. 1 W = 1 J/s.

For example, the power or energyflow to a drive shaft on a compressor isnumerically similar with the heat emittedfrom the system plus the heat applied tothe compressed gas.

1.2.6 Volume rate of flow

The SI unit for volume rate of flow is m3/s.However, the unit litre/second (l/s) is fre-quently used, when speaking about thevolume rate of flow, for example, given bya compressor. This volume rate of flow iscalled the compressor’s capacity and is ei-ther stated as normal litre/second (Nl/s) or

be J/kg x K. Consequently, the molar ther-mal capacity is dimensioned J/mol x K.The designations commonly used are:

cp = thermal capacity at a constant pressure

cv = thermal capacity at a constant volume

Cp = molar thermal capacity at a constant pressure

Cv = molar thermal capacity at a constant volume

The thermal capacity at a constant pressureis always greater than thermal capacity at aconstant volume. The thermal capacity fora substance is however not constant, butrises in general with the temperature.

For practical usage a mean value canfrequently be used. For liquids and solidsubstances it is cp ≈ cv ≈ c. The power con-sumed to heat a mass flow from t1 to t2 willthen be:

Q ≈ m x c x (t2 - t1)

Q = heat power (W)m = mass flow (kg/s)c = specific thermal capacitivity

(J/kg x K)t = temperature (K)

The explanation as to why cp is greater thancv is the expansion work that the gas at aconstant pressure must perform. The rela-tion between cp and cv called kappa κ, is afunction of the number of atoms in themolecule.

13

cp Cp=

cv Cv=κ


as free output air rate (l/s). With the unitnormal litre/second (Nl/s) the air flow rateis recalculated to “the normal state”, i.e.1.013 bar and 0°. The unit is primarily usedwhen you wish to specify a mass flow.

With free output air rate the compres-sor’s output air rate is recalculated to itsstandard intake condition (intake pressureand intake temperature). Accordingly, youstate how many litres of air would fill if itonce again were allowed to expand to theambient condition. The relation betweenthe two volume rates of flow is (note thatthe formula below does not take the humi-dity into consideration):

Qi = volume rate of flow as free output air flow rate (l/s)

Qn= volume rate of flow as normal litres / second (Nl/s)

Ti = intake temperature (°C)pi = intake pressure (bar)

1.3Thermodynamics

1.3.1 Main principles

Thermodynamics’ first main principle is alaw of nature that can not be proved, but isaccepted without reservation. It says thatenergy can neither be created nordestroyed and from that it follows that thetotal energy in a closed system is constant.Thermodynamics’ second main principlesays that heat can never of “its own effort”be transferred from one source to a hottersource. This means that energy can only beavailable for work if it can be convertedfrom a higher to a lower temperature level.

Therefore in, for example, a heat engine theconversion of a quantity of heat to mecha-nical work can only take place if a part ofthis quantity of heat is simultaneously ledoff without being converted to work.

1.3.2 Gas laws

Boyle’s law says that if the temperature isconstant, so the product of pressure andvolume are constant. The relation reads:

p1 x V1 = p2 x V2

p = absolute pressure (Pa)V = volume (m3)

This means that if the volume is halvedduring compression, then the pressure isdoubled.

Charles’s law says that the volume of agas changes in direct proportion to thechange in temperature. The relation reads:

V = volume (m3)T = absolute temperature (K)∆V = volume difference∆T = temperature difference

The general law of state for gases is a combi-nation of Boyle’s and Charles’s laws. Thisstates how pressure, volume and tempera-ture affect each other. When one of these var-iables is changed, this affects at least one ofthe other two variables. This can be written:

= R = gas constant

p = absolute pressure (Pa)v = specific volume (m3/kg)T = absolute temperature (K)R = R/M = individual gas constant

(J/kg x k)14

Qi =Qn x (273 + Ti ) x 1,013

273 x pi

>__V1

T1

V2

T2= ∆V

V1

T1= x ∆T

p x v

T


Generally the relation below applies:

q = k x A x ∆T x t

q = the quantity of heat (J)k = total heat transfer coefficient

(W/m2 x K)A = area (m2)∆T = temperature differencet = time (s)

Heat transfer frequently occurs betweentwo bodies, separated by a wall. The totalheat transfer coefficient is dependent on theheat transfer coefficient on respective sidesof the wall and the coefficient of thermalconductivity for the wall. For such a clean,flat wall the relation below applies:

1/k = 1/α1 + d/λ + 1/α2

α = heat transfer coefficient on respective sides of the wall(W/m2 x K)

d = thickness of the wall (m)λ = coefficient of thermal conduc-

tivity for the wall (W/m x K)k = total heat transfer coefficient

(W/m2 x K)

The transferred quantity of heat, forexample, in a heat exchanger, is at eachpoint a function of the prevailing heat dif-ference and the total heat transfer coeffici-ent. Applicable to the entire heat transfer surface is:

Q = k x A x ϑm

Q = transferred quantity of heat (W)k = total heat transfer coefficient

(W/m2 x K)A = heat transferring surface (m2)ϑm = logarithmic mean temperature

difference (K)

The constant R is called the individual gasconstant and only concerns the propertiesof the gas. If the mass m of the gas takes upthe volume V, the relation can be written:

p x V = m x R x T

p = absolute pressure (Pa)V = volume (m3)m = mole mass (kmol)R = universal gas constant

= 8314 (J/kmol x K)T = absolute temperature (K)

1.3.3 Heat transfer

Each heat difference within a body, orbetween different bodies, always leads tothe transfer of heat, so that a temperaturebalance is obtained. This heat transfer cantake place in three different ways: throughconductivity, convection or radiation. Inreality heat transfer takes place in parallel,in all three ways.

Conductivity takes place between solidbodies or between thin layers of a liquid orgas. Molecules in movement emit theirkinetic energy to the adjacent molecules.

Convection can take place as free con-vection, with the natural movement thatoccurs in a medium or as forced convectionwith movement caused by, for example, afan or a pump. Forced convection gives sig-nificantly more intense heat transfer.

All bodies with a temperature above0°K emit heat radiation. When heat rays hita body, some of the energy is absorbed andtransforms to heat. Those rays that are notabsorbed pass through the body or arereflected. Only an absolute black body cantheoretically absorb all radiated energy.

In practice heat transfer is the sum ofthe heat transfer that takes place throughconductivity, convection and radiation. 15


The logarithmic mean temperature dif-ference is defined as the relation betweenthe temperature differences at the heat ex-changer’s two connection sides accordingto the expression:

ϑm = logarithmic mean temperature difference (K)

ϑ = the temperature differences (K)according to figure 1:6.

1.3.4 Changes in state

You can follow the changes in state for agas from one point to another in a p/V dia-gram. It should really need three axes forthe variables p, V and T. With a change instate you move along a curve on the sur-face in space that is then formed. However,you usually consider the projection of thecurve in one of the three planes, usually thep/V plane. Primarily a distinction is madebetween five different changes in state:

Isochoric process (constant volume),

isobaric process (constant pressure), isothermic process (constant temperature)isentropic process (without heat exchangewith the surroundings) and polytropic process (where the heat exchange with thesurroundings is stated through a simplemathematical function).

1.3.4.1 Isochoric process

16

Counter flow Parallel flow2

Surface

t

1

ϑ2

t 2

t1

ϑ1

1

2

Surface

ϑ2

t1

2t

ϑ1

t

1:6

This is how the temperature process appears in counter flow respective parallel flow heat exchangers.

1

2

q12

P

V

P T2 2

P T1 1

V = V1 2

= applied energy

P

1:7

Isochoric change of state means that the pres-sure increases, while the volume is constant.

ϑm =ϑ1 – ϑ2

ϑ1

ϑ2ln


1.3.4.3 Isothermic process

If a gas in a cylinder is compressed isother-mally, a quantity of heat that is equal to theapplied work must be gradually led off.This is practically impossible, as such aslow process can not be realised.

The relation for the quantity of heat led off is:

q = m x R x T x 1n

q = p1 x V1 x 1n

q = quantity of heat (J)m = mass (kg)R = individual gas constant

(J/kg x K)T = absolute temperature (K)V = volume (m3)p = absolute pressure (Pa)

Heating a gas in an enclosed container is anexample of the isochoric process. The rela-tion for the applied quantity of heat is:

q = m x cv x (T2 – T1)

q = quantity of heat (J)m = mass (kg)cv = the heat capacity at constany

volume (J/kg x K)T = absolute temperature (K)

1.3.4.2 Isobaric process

Heating of a gas in a cylinder with a con-stant loaded piston is an example of theisobaric process. The relation for the appli-ed quantity of heat is:

q = m x cp x (T2 - T1)

q = quantity of heat (J)m = mass (kg)cp = the heat capacity at constant

pressure (J/kg x K)T = absolute temperature (K) 17

V

21

P

P

q12

V T1 1 V T2 2

= applied energy

1:8

Isobaric change of state means that the vol-ume increases, while the pressure is constant.

P

P

V

12q = quality of heat led off

P2

P1V1

2

1

V2

1:9

Isothermic change of state means that thetemperature of a gas mixture is constant,

when the pressure and volume are changed.

p2

p1

V2

V1


1.3.4.4 Isentropic process

An example of an isentropic process is if agas is compressed in a fully insulated cylin-der without heat exchange with the sur-roundings. Or if a gas is expanded througha nozzle so quickly that no heat exchangewith the surroundings has time to takeplace. The relation for such a process is:

p = absolute pressure (Pa)V = volume (m3)T = absolute temperature (K)

1.3.4.5 Polytropic process

The isothermic process involves full heatexchange with the surroundings and theisotropic process involves no heat exchangeat all. In reality all processes are somethingbetween these extremes and this generalprocess is called polytropic.

The relation for such a process is:

p x Vn = konstant

p = absolute pressure (Pa)V = volume (m3)n = 0 means isobaric processn = 1 means isothermic processn = κ means isentropic processn = ∞ means isochoric process

1.3.5 Gas flow through a nozzle

The gas flow through a nozzle depends onthe pressure ratio on respective sides of thenozzle. If the pressure after the nozzle islowered the flow increases, however, onlyuntil its pressure before the nozzle isapproximately double so high. A furtherreduction of the pressure after the openingdoes not bring about an increase in flow.

This is the critical pressure ratio and itis dependent on the gas’s isentropic expo-nent (κ). The critical pressure ratio occurswhen the flow velocity is equal to the sonicvelocity in the nozzle’s narrowest section.

The flow becomes supercritical if thepressure after the nozzle is reduced further,under the critical value. The relation for theflow through the nozzle is:

G =

G = mass flow (kg/s)α = nozzle coefficientψ = flow coefficientA = minimum flow area (m2)R = individual gas constant (J/kg K)T1 = absolute temperature before the

nozzle (K)p1 = absolute pressure before the

nozzle (bar)18

x ψ x p1 x 105 x A xR x T1

2

P

P

V

isentropic

1

P2

P1

VV

2

2 1

1:10

When the entropy in a gas that has been com-pressed or expanded is constant, no heat

exchange with the surroundings takes place.This change in state follows Poisson’s law.

V1

V2

T2

T1

κ κκ-1p2

p1

p2

p1

cp

cv

= =

κ =

>__


19

1.3.6 Flow through pipes

Reynold’s number is a dimensionless ratiobetween inertia and friction in a flowingmedium. It is defined as:

Re= D x w x η/ρ x = D x w/v

D = a characteristic measurement (for example the pipe diameter) (m)

w = mean flow velocity (m/s)ρ = the density of the flowing medium

(kg/m3)η = the flowing medium’s dynamic

viscosity (Pa x s)v = η/ρ = the flowing medium’s

kinematic viscosity (m2/s).

In principal there are two types of flow in apipe. With Re<2000 the viscous forcesdominate in the media and the flow be-comeslaminar. This means thatdifferent lay-ers of the medium move in relation to eachother in good order. The velocity distribu-tion across the laminar layers is usuallyparabolic shaped. With Re≥4000 the inertiaforces dominate the flowing medium andthe flow becomes turbulent, with particlesthat move randomly in the flow’s cross sec-tion. The velocity distribution across a layerwith turbulent flow becomes diffuse.

In the critical area, between Re≤2000 andRe≥4000, the flow conditions are undeter-mined, either laminar or turbulent or a mix-ture of the both. The conditions are govern-ed by factors such as the surface smooth-ness of the pipe or other disturbances.

To start a flow in a pipe requires a specificpressure difference or pressure drop, to overcome the friction in the pipe and couplings. The size of the pressure drop

depends on the diameter of the pipe, itslength and form as well as the surfacesmoothness and Reynold’s number.

1.3.7 Throttling

When an ideal gas flows through a restric-tor, with a constant pressure before andafter the restrictor, the temperature remainsconstant. However, there occurs a pressuredrop across the restrictor, through the innerenergy transforming into kinematic energy,which is why the temperature falls.However, for real gases this temperaturechange becomes lasting, even if the gas’senergy content is constant. This is calledthe Joule Thomson effect. The temperaturechange is equal to the pressure changeacross the throttling multiplied by the JouleThomson coefficient.

If the flowing medium has a sufficient-ly low temperature (≤+329°C for air) a tem-perature drop occurs across the restrictor,but if the flow medium is hotter, a tempera-ture increase occurs. This condition is usedin several technical applications, forexample, in refrigeration technology andthe separation of gases.

W2

1:11

When an ideal gas flows through a small open-ing between two large containers, the energybecomes constant and no heat exchange takes

place. However, a pressure drop occurs with thepassage through the restrictor.


air’s humidity can vary within broad limits.Extremities are completely dry air and airsaturated with moisture. The maximumwater vapour pressure that air can holdincreases with rising temperatures. A maxi-mum water vapour pressure correspondsto each temperature.

Air usually does not contain so much watervapour that maximum pressure is reached.Relative vapour pressure (also known asrelative humidity) is a state between theactual partial vapour pressure and the satu-rated pressure at the same temperature.

The dew point is the temperature when airis saturated with water vapour. Thereafterwith a fall in temperature the condensationof water takes place. Atmospheric dewpoint is the temperature at which watervapour starts to condense at atmosphericpressure. Pressure dew point is the equiva-lent temperature with increased pressure.The following relation applies:

(p - ϕ x ps) x 105 x V = Ra x ma x Tϕ x ps x 105 x V = Rv x mv x T

p = total absolute pressure (bar)ps = saturation pressure at the actual

temperature (bar)ϕ = relative vapour pressureV = total volume of the moist air (m3)Ra = gas constant for dry air

= 287.1 J/Kg x KRv = gas constant for water vapour

= 461.3 J/Kg x Kma= mass of the dry air (kg)mv= mass of the water vapour (kg)T = absolute temperature of the

moist air (K)

20

1.4Air

1.4.1 Air in general

Air is a colourless, odourless and tastelessgas mixture. It consists of many gases, butprimarily oxygen and nitrogen. Air can beconsidered a perfect gas mixture in mostcalculation contexts. The composition isrelatively constant, from seal level and upto an altitude of 25 kilometres.

Air is always more or less contaminatedwith solid particles, for example, dust, sand,soot and salt crystals. The degree of conta-mination is higher in populated areas, lessin the countryside and at higher altitudes.

Air is not a chemical substance, but amechanically mixed substance. This is whyit can be separated into its constituent ele-ments, for example, by cooling.

1.4.2 Moist air

Air can be considered as a mixture of dryair and water vapour. Air that containswater vapour is called moist air, but the

Others 1%

Nitrogen78%

Oxygen 21%

1:12

Air is a gas mixture that primarily consistsof oxygen and nitrogen. Only approx. 1% is

made up of other gases.


1.5Types of compressors

1.5.1 Two basic principles

There are two basic principles for the com-pression of air (or gas), the displacementprincipal and dynamic compression. Among displacement compressors are, forexample, piston compressors and differenttypes of rotary compressors. They are themost common compressors in mostcountries.

On a piston compressor for example,the air is drawn into a compression cham-ber, which is closed from the inlet.Thereafter the volume of the chamber dec-reases and the air is compressed. When thepressure has reached the same level as thepressure in the outlet manifold, a valve isopened and the air is discharged at a con-stant pressure, under continued reductionof the compression chamber’s volume.

In dynamic compression air is drawn into arapidly rotating compression impeller andaccelerates to a high speed. The gas is thendischarged through a diffuser, where thekinetic energy is transformed to static pres-sure. There are dynamic compressors withaxial or radial flow. All are suitable forlarge volume rates of flow.

1.5.2 Displacement compressors

A bicycle pump is the simplest form of adisplacement compressor, where air isdrawn into a cylinder and is compressedby a moving piston. The piston compressorhas the same operation principle, with apiston whose forward and backwardmovement is accomplished by a connectingrod and a rotating crankshaft. If only one 21

side of the piston is used for compressionthis is called single acting. If both the pis-ton’s top and undersides are used the com-pressor is called double acting. The diffe-rence between the pressure on the inlet sideand the pressure on the outlet side is ameasurement of the compressor’s work.

The pressure ratio is the relation betweenabsolute pressure on the inlet and outletsides. Accordingly, a machine that draws inair at atmospheric pressure and compressesit to 7 bar overpressure works with a pres-sure ratio of (7 + 1)/1 = 8.

1:13

Single stage, single acting piston compressor.


22

Ejector Radial Axial

Rotary

Dynamic Displacement

Compressors

Piston compressors

Single acting Double acting Labyrinth sealed Diaphragm

Single rotor Double rotor

Screw Tooth BlowerVane Liquid ring Scroll

The chart shows the most common types of compressor, divided according to their working principles.They can also be divided according to other principles, for example, air or liquid cooled, stationary or

portable, etc.

1:14


a real compressor diagram for a pistoncompressor. The stroke volume is the cylin-der volume that the piston travels duringthe suction stage. The clearance volume isthe area that must remain at the piston’sturning point for mechanical reasons,together with the area required for the val-ves, etc.

The difference between the strokevolume and the suction volume is due tothe expansion of the air remaining in theclearance volume before suction can start.The difference between the theoretical p/Vdiagram and the real diagram is due to thepractical design of a compressor, e.g. apiston compressor. The valves are neverfully sealed and there is always a degree ofleakage between the piston and the cylin-der wall. In addition, the valves can notopen and close without a delay, whichresults in a pressure drop when the gasflows through the channels. Due to reasonsof design the gas is also heated when itflows into the cylinder.

Compression work with isothermiccompression becomes:W = p1 x V1 x 1n(p2/p1)Compression work with isentropiccompression becomes:

W =

W = compression work (J)p1 = initial pressure (Pa)V1 = initial volume (m3)p2 = final pressure (Pa)κ = isentropic exponent in most

cases x κ ≈ 1,3 – 1,4 applies.

These relations show that more work isrequired for isentropic compression thanwith isothermic compression. In reality therequisite work lies between the limits (κ ≈ 1,3 – 1,4).

1.5.3The compressor diagram fordisplacement compressors

Figure 1:15 illustrates a theoretical com-pressor diagram and figure 1:16 illustrates 23

Pressure

Volume

1:16

This is how the actual p/V diagram can appearfor a piston compressor. The pressure drop on

the inlet side and the overpressure on thedischarge side are minimised primarily by

giving the compressor sufficient valve area.

Suction volume

Stroke volume

VolumeClearance

volume

Pressure

3 2

14

Compression

Discharge

Pressurereduction

Suction

1:15

This is how a piston compressor works intheory with self-acting valves. The p/V dia-gram shows the theoretical process, withoutlosses with complete filling and emptying of

the cylinder.

κκ-1 (p2V2 - p1V1)x


1.5.4 Dynamic compressors

A dynamic compressor is a flow machinewhere the pressure increase takes place atthe same time as the gas flows. The flowinggas accelerates to a high velocity by meansof the rotating blades, after which the velo-city of the gas is transformed to pressurewhen it is forced to decelerate underexpansion. Depending on the main direc-tion of the flow they are called radial oraxial compressors.

In comparison with displacementcompressors, dynamic compressors have acharacteristic where a small change in theworking pressure results in a large changein the capacity. See figure 1:19.

Each speed has an upper and lowercapacity limit. The upper limit means thatthe gas’s flow velocity reaches sonic veloci-ty. The lower limit means that the counterpressure is greater than the compressor’spressure build-up, which means returnflow in the compressor. This in turn resultsin pulsation, noise and the risk for mech-anical damage.

1.5.5 Compression in several stages

Theoretically a gas can be compressedisentropically or isothermally. This can take

place as a part of a reversible process. If thecompressed gas could be used immediate-ly, at its final temperature after compres-sion, the isentropic process would have cer-tain advantages. In reality the gas can rare-ly be used directly without being cooledbefore use. Therefore the isothermal pro-cess is preferred, as this requires less work.

In practice attempts are made to reali-se this process by cooling the gas duringcompression. How much you can gain bythis is shown, for example, with an effect-ive working pressure of 7 bar that theoreti-cally requires 37% higher output forisentropic compression compared withisothermal compression.

A practical method to reduce the heat-ing of the gas is to divide the compressioninto several stages. The gas is cooled aftereach stage, to then be compressed further.This also increases the efficiency, as thepressure ratio in the first stage is reduced.The power requirement is at its lowest ifeach stage has the same pressure ratio.

The more stages the compression isdivided into the closer the entire process

24

Isentropic compression

Reduced work requirement through 2-stage compression

Stage 1

Stage2

P

V

Isothermal compression

1:18

Diffusor

Intake

1:17

The shaded area represents the work savedby dividing the compression into two stages.

Radial compressor


25

1.6 Electricity

1.6.1 Basic terminology and definitions

The alternating current used for example topower lighting and motor operations regu-larly changes strength and direction in asinusoidal variation. The current strengthgrows from zero to a maximum value, thenfalls to zero, changes direction, grows to amaximum value in the opposite directionto then become zero again. The current hasthen completed a period. The period T isthe time in seconds under which the cur-rent has gone through all its values. Thefrequency states the number of completecycles per second.

f = 1/Tf = frequency (Hz)T = time for a period (s)

When speaking about current or voltage itis usually the root mean square value thatis meant. With a sinusoidal current the rela-tion for the current’s respective voltage’sroot mean square value is:

gets to be isothermal compression. How-ever there is an economic limit for howmany stages a real installation can bedesigned with.

1.5.6Comparison between displace-

ment and centrifugal compressors

The capacity curve for a centrifugal com-pressor differs significantly from an equiv-alent curve for a displacement compressor.The centrifugal compressor is a machinewith a variable capacity and constant pres-sure. On the other hand a displacementcompressor is a machine with a constantcapacity and a variable pressure.

Examples of other differences is that adisplacement compressor gives a higherpressure ratio even at a low speed,unlike themore significantly higher speed centrifugalcompressors. The centrifugal compressorsare well suited to large air flow rates.

Centrifugalcompressor

Displacementcompressor

Flow

Pressure

1:19

This is how the load curves for centrifugalrespective displacement compressors appear

when the load is changed at a constant speed.

peak value

2root mean square value =

Time = 1period = 1/50 sec

Voltage

Root Mean Square Value

Root Mean Square Value

Peak-value

325 V

230

0

230

325 V

Peak-value

1:20

This is how a period of a sinusoidal voltageappears (50 Hz).


Voltage under 50V is called extra low volt-age. Voltage under 1000V is called lowvoltage. Voltage over 1000V is called highvoltage. Standard voltages at 50 Hz are230/400V and 400/690V.

1.6.2 Ohm’s law for alternating current

An alternating current that passes a coilgives rise to a magnetic flow. This flowchanges strength and direction in the sameway as the current. When the flow changesan emf (electromotive force) is generated inthe coil, according to the laws of induction.This emf is counter directed to the connect-ed pole voltage. The phenomenon is calledself-induction.

Self-induction in an alternating currentunit partly gives rise to phase displacementbetween the current and the voltage, andpartly to an inductive voltage drop. Theunit’s resistance to the alternating currentbecomes apparently greater than that calcu-lated or that measured with direct current.

Phase displacement between the cur-rent and voltage is represented by the angleϕ. Inductive resistance (reactance) is repre-sented by X. Resistance is represented by R.Apparent resistance in a unit or conductoris represented by Z.

Applicable for impedance is:

Z = impedance (Ω)R = resistance (Ω)X = reactance (Ω)

Ohm’s law for alternating current reads:

U = I x Z

U = voltage (V)I = current (A)Z = impedance (Ω)

26

Z

R

X

ϕ

1:21

Relation between Reactance (X) – Resistance(R) – Impedance (Z) – Phase displacement (ϕ).

Phase 1

Phase 2

Phase 3

0 Neutral wire

Uh

Uh

Uh

Uf Uf

Uf

Uh Uh

Uh

3-phase ∆3-phase Y

1-phase

Uf

Uf

Uf

1:22

This is how the different connection options for a three-phase system appears. The voltage between the twophase conductors is called the main voltage (Uh). The voltage between one phase conductor and the

neutral wire are called phase voltage (Uf). The phase voltage = Main voltage/ 3 .

Z = R2 + X2


The following relation applies:

Single phase: P = U x I x cosϕQ = U x I x sinϕS = U x Icosϕ = P/S

Three phase: P = 3 x Uh x I x cosϕQ = 3 x Uh x I x sinϕS = 3 x Uh x Icosϕ = P/S

U = voltage (V)Uh= main voltage, (V)Uf = phase voltageI = current (A)Ih = main current (A)If = phase current (A)P = active power (W)Q = reactive power (VAr)S = apparent power (VA)ϕ = phase anglecosϕ = power factor

1.6.3 Three-phase system

Three-phase alternating current is produ-ced in a generator with three separate wind-ings. All values on the sinusoidal voltageare displaced 120° in relation to each other.

Different units can be connected to athree-phase unit. A single phase unit can beconnected between the phase and zero.Three-phase units can be connected in twoways, star (Y) or delta (∆) connection. Withthe star connection a phase voltage liesbetween the outlets. With a delta connec-tion a main voltage lies between the outlets.

1.6.4 Power

Active power, P, is the useful power thatcan be used for work. Reactive power, Q, isthe “useless” power and can not be usedfor work. Apparent power, S, is the powerthat must be consumed from the mainssupply to gain access to active power. Therelation between active, reactive and appar-ent power is usually illustrated by a powertriangle.

27

voltage U U U

Time240°120°

1 2 3

1:24

The displacement between the generator’s windings gives a sinusoidal voltage curve on the system. Themaximum value is displaced at the same interval as the generator’s windings.

S

P

Q

ϕ

1:23

This is how the relation between apparentpower (S), reactive power (Q) and active

power (P) The angle ϕ between S and P givesthe power factor cosϕ.


1.6.5 The electric motor

The most common electric motor is a threephase, short circuit induction motor. Thistype of motor can be found within allindustries. Silent and reliable, it is a part ofmost systems, for example, compressors.The electric motor consists of two mainparts, the stationary stator and the rotatingrotor. The stator produces a rotating mag-netic field and the rotor converts this ener-gy to movement, i.e. mechanical energy.

The stator is connected to the mainssupply’s three phases. The current in thestator windings give rise to a rotating mag-netic force field, which induces currents inthe rotor and gives rise to a magnetic fieldthere too. The interaction between the sta-tor’s and the rotor’s magnetic fields createsturning torque, which makes the rotor shaftrotate.

1.6.5.1 Rotation speed

If the motor shaft should rotate at the samespeed as the magnetic field, the inducedcurrent in the rotor would at the same timebe zero. However, due to losses in, forexample the bearings, this is impossibleand the speed is always approx. 1-5%lower than the magnetic field’s synchro-nous speed (slip). Applicable for this syn-chronous speed is:

n = 2 x f x 60/p

n = synchronous speed (r/min)f = main supply’s frequency (Hz)p = number of poles

1.6.5.2 Efficiency

Energy conversion in a motor does not takeplace without losses. These are due to,among others, resistive losses, ventilationlosses, magnetisation losses and frictionlosses. Applicable for efficiency is:

η = efficiencyP2 = stated power, shaft power (W)P1 = applied power (W)

It is always the stated power, P2, stated onthe motor’s rating plate.

1.6.5.3 Insulation class

The insulation material in the motor’swindings is divided into insulation classesin accordance with IEC 85 (InternationalElectrotechnical Commission). A letter cor-responding to the temperature, which isthe upper limit for the isolation’s calculatedapplication area, designates each class. Ifthe upper limit is exceeded by 10°C the ser-vice life of the insulation is shortened byabout half.

1.6.5.4 Protection classes

Protection classes state, according to IEC34-5, how the motor is protected againstcontact and water. These are stated withthe letters IP and two digits. The first statesthe protection against contact and penetra-tion by a solid object. The other digit statesthe protection against water. For exampleIP23 represents: (2) protect against solidobjects greater than 12 mm, (3) protect a-gainst direct sprays of water up to 60° fromthe vertical. IP 54: (5) protection againstdust, (4) protection against water sprayedfrom all directions. IP 55: (5) protectionagainst dust, (5) protection against low-pressure jets of water from all directions.28

Insulation class

Ambient temp. °C

Temp. increase °C

Thermal marginal °C

Max. final temp. °C

40

80

10

130

40

105

10

155

40

125

15

180

B=130°C F=155°C H=180°C

η =P2

P1


“ends” of motor winding’s phases are join-ed together to form a zero point, whichlooks like a star (Y).

A phase voltage (phase voltage =main voltage/ 3; for example 400V =690/ 3 ) will lie across the windings. Thecurrent Ih in towards the zero point be-comes a phase current and accordingly aphase current will flow If = Ih through thewindings.

With the delta (∆) connection you join thebeginning and ends between the differentphases, which then form a delta (∆). Therewill then lie a main voltage across thewindings. The current Ih into the motor isthe main current and this will be dividedbetween the windings and give a phasecurrent through these, Ih/ 3 = If. The samemotor can be connected as 690V star con-nection or 400V delta connection. In bothcases the voltage across the windings willbe 400V. The current to the motor will belower with a 690V star connection thanwith a 400V delta connection. The relationbetween the current levels is 3.

1.6.5.5 Cooling methods

Cooling methods state, according to IEC34-6, how the motor shall be cooled. This isdesignated with the letters IC and twodigits. For example IC 01 represents: Freecirculation, own ventilation and IC 41:Jacket cooling, own ventilation.

1.6.5.6 Installation method

The installation method states, according toIEC 34-7, how the motor should be instal-led. This is designated by the letters IM andfour digits. For example IM 1001 repre-sents: two bearings, shaft with free journal-led end, stator body with feet. IM 3001: twobearings, shaft with free journalled end,stator body without feet, large flange withplain securing holes.

1.6.5.7 Star (Y) and delta (∆) connections

A three-phase electric motor can beconnected in two ways, star (Y) or delta (∆).The winding phases in a three-phase motorare marked U, V and W (U1-U2; V1-V2;W1-W2). With the star (Y) connection the

29

Uf

If

400V

400V

400V

Motor windings

L1

L2

L3

V2V1

W1

W2

U1

U2

690V

690V

690V

W2 U2 V2

U1 V1 W1

L1 L2 L3

Star connection

Motor terminal

Uf

Uf

1:25

This is how the motor wind-ings are connected with a star

connection. The top figureshows how the connection

strips are placed on the starconnected unit. The example

shows the connection to a690V supply.


On the motor plate it can, for example,state 690/400 V. This means the star con-nection is intended for the higher voltageand the delta connection for the lower. The

current, which can also be stated on the plate, shows the lower value for the starconnected motor and the higher for thedelta connected motor.

30

Uh

If

400V

Motor winding

V V

400V

400V

400V

400V

W U V

U V W

L L LDelta connection

Motor terminal

400V

If

1 2

Ih

Uh

Uh

2 2 2

21 3

L1

L3

1 1 1W

2

W1

U1

U2

L2

1:26

This is how the motor windings are connected with a delta connection. The top figure shows how the con-nection strips are placed on the delta connected unit. The example shows the connection to a 400V supply.

D

1:27

The mains supply is connected to a three-phase motor’s terminals marked U, V and W. The phase sequenceis L1, L2 and L3. This means the motor will rotate clockwise seen from “D” the drive end. To make the

motor rotate anticlockwise two of the three conductors connected to the starter or to the motor are switched.Check the operation of the cooling fan when rotating anticlockwise.


1.6.5.8 Torque

An electric motor’s turning torque is anexpression for the rotor’s turning capacity.Each motor has a maximum torque. A loadabove this torque means that the motordoes not have the power to rotate. With anormal load the motor works significantlyunder its maximum torque, however, thestart phase involves an extra load. The cha-racteristics of the motor are usuallypresented in a torque curve. 31

Torque

Mst Mmin Mmax

Mn

rpm

1:28

The torque curve for a short circuited, induction motor. When the motor starts the torque is high, duringacceleration the torque first drops a little, to then rise to its max. value before dropping. M = torque, Mst = start torque, Mmax = max torque (“cutting torque”), Mmin = min. torque (“saddle torque”),

Mn = rated torque.

Torque

Torque curve (∆)

Loaded compressor

Unloaded compressor

Switch Star Y-∆

Torque curve (Y)

Time

1:29

A star/delta started induction motor’s torque cure inserted in a torque curve for a screw compressor. Thecompressor is unloaded (idling) during star operations. When the speed has reached approx. 90-95% ofthe rated speed the motor is switched to the delta mode, the torque rises, the compressor is loaded and

finds its working point.


32


Chapter 2 Compressors andauxiliary equipment


2.1 Displacement compressors

2.1.1 Displacement compressorsin general

A displacement compressor is character-ised by enclosing a volume of gas or air andthen increasing the pressure by reducingthe area of the enclosed volume.

2.1.2 Piston compressors

The piston compressor is the oldest andmost common of all compressors. It is avail-able as single or double acting, oil lubricat-ed or oil–free with a different number ofcylinders in different configurations. Withthe exception of really small compressorswith vertical cylinders, the V configurationis the most common for small compressors.

On double acting, large compressorsthe L-type with vertical low pressure cylin-der and horizontal high pressure cylinder,offers immense benefits and is why this isthe most common design.

34

2:1

Piston compressor


35

Oil lubricated compressors normally workwith splash lubrication or pressure lubrica-tion. Most compressors have self-acting

valves. A self-acting valve opens and clo-ses through pressure differences on respec-tive sides of the valve disk.

Vertical

Single acting

V-type W-type Stepped piston (Two stage)

L-type

V-type

W-type

Horizontal stepped pistonsBoxer

Inline

Double acting (cross head type)

2.2

Examples of cylinder placement on piston compressors.


36

2:3

A piston compressor with a valve systemconsisting of two stainless steel valve discs.

When the piston moves downwards anddraws in air into the cylinder the largest discis sufficiently flexible to fold downwards to

allow the air to pass.

When the piston moves upwards, the largedisc folds upwards and seals against theseat. The small disc’s flexi-function then

allows the compressed air to be forcedthrough the hole in the valve seat.

1

23

4

5

6

2:4

Labyrinth sealed, double acting oil-free piston compressor with crosshead.

1. Crosshead2. Guide bearing3. Oil scraper4. Oil thrower ring5. Stuffing box6. Valve disc


37

2.1.3 Oil-free piston compressors

Oil-free piston compressors have pistonrings of PTFE or carbon, alternatively thepiston and cylinder wall can be toothed ason labyrinth compressors. Larger machinesare equipped with a crosshead and sealson the gudgeon pins, ventilated intermedi-ate piece to prevent oil from being transfer-red from the crankcase and into the com-pression chamber. Smaller compressorsoften have a crankcase with sealed for lifebearings.

2.1.4 Diaphragm compressors

Diaphragm compressors form anothergroup. The diaphragm is actuated mecha-nically or hydraulically. The mechanicaldiaphragm compressors are used with asmall flow and low pressure or as vacuumpumps. The hydraulic diaphragm com-pressors are used for high pressure.

2.1.5 Screw compressors

The principle for a rotating displacement compressor with piston in a screw form

was developed during the 1930s, when arotating compressor with a high capacityand stable flow in varying conditions wasrequired.

The screw element’s main parts arethe male and female rotors, which movetowards each other while the volumebetween them and the housing decreases.Each screw element has a fixed, integratedpressure ratio that is dependent on itslength, the pitch of the screw and the formof the discharge port. To attain the bestefficiency the pressure ratio must be adap-ted to the required working pressure.

The screw compressor is not equip-ped with valves and has no mechanicalforces that cause unbalance. This means itcan work at a high shaft speed and com-bine a large flow rate with small exteriordimensions. An axial acting force, depen-dent on the pressure difference betweenthe inlet and outlet, must be taken up bythe bearings. The screw, which originallywas symmetrical, has now been developedin different asymetrical helical profiles.

Diaphragm

Connecting rod

Flywheel

Shaft

Cam

Counterbalance weight

Clutch

2:5

Mechanical diaphragm compressor, where the movement of the diaphragm is transferred from a conventio-nal crankshaft with connecting rod, which is connected to the diaphragm.


38

1 2

3 4

2:6

This is how air is compressed in a screw compressor. In figure 1 air fills the space between the rotors, butfor each turn the space decreases more and more.

2:7

An oil lubricated screw compressor element.


39

2.1.5.1 Oil-free screw compressorsThe first screw compressors had a symetricprofile and did not use liquid in the com-pression chamber, so-called oil-free or dryscrew compressors. At the end of the 1960sa high speed, oil-free screw compressorwas introduced with an asymetric screwprofile. The new rotor profile resulted insignificantly improved efficiency, due toreduced internal leakage.

An external gear is used in dry screw com-pressors to synchronise the counter rota-ting rotors. As the rotors neither come intocontact with each other nor with the com-pressor housing, no particular lubricationis required in the compression chamber.Consequently the compressed air is com-

pletely oil-free. The rotors and housing aremanufactured with great precision to mini-mise leakage from the pressure side to theinlet. The integrated pressure ratio is limit-ed by the temperature difference betweenthe intake and the discharge. This is whyoil-free screw compressors are frequentlybuilt with several stages.

2.1.5.2 Liquid injected screw compressors

A liquid injected screw compressor is cooled and lubricated by liquid that is injected to the compression chamber andoften to the compressor bearings. Its function is to cool and lubricate the com-pressor element and to reduce the returnleakage to the intake.

2:8

A stage in an oil-free screw compressor. Male and female rotors are journalled in the rotor housing, which here is water-cooled. The front rotor, with four lobes, is the male, this is connected to the gearbox.The distant rotor, with six lobes, is the female, this is held in place by the synchronising gear to the left.


Today oil is the most common liquid dueto its good lubricating properties, however,other liquids are also used, for example,water. Liquid injected screw compressorelements can be manufactured for high

pressure ratio, which why one compres-sion stage is usually sufficient for pressureup to 13 bar. The element’s low returnleakage also means that relatively smallscrew compressors are efficient.

40

Suction filterSuction valve

Non-return valve

Minimum pressure valve

Aftercooler

Compressor element

Oil separator

2:9

Suction filterSuction valve Intercooler

Water separator

Non-return valve

Aftercooler

Water separator

Compressor element

Compressor element

2:10

Oil injected screw compressor.

Oilfree screw compressor.


41

2.1.6 Tooth compressor

The compression element in a tooth com-pressor consists of two rotors that rotatetowards each other in a compressionchamber.

The compression process consists ofintake, compression and outlet. During theintake phase air is drawn into the compres-sion chamber until the rotors block theinlet. During the compression phase thedrawn in air is in the compression cham-ber, which gets smaller and smaller as therotors move.

The outlet is blocked during com-pression by one of the rotors, while the in-let is open to draw in new air into the op-posite section of the compression chamber.

Discharge takes place when one of therotors opens the channel and the compres-sed air is forced out of the compressionchamber. Intake and outlet take place ra-dially through the compression chamber,which allows the use of simpler bearingdesign and improve filling properties.

Both rotors are synchronised via agear wheel. The maximum pressure ratioobtainable with an oil-free tooth compres-sor is 4.5. Consequently several stages arerequired for higher pressures.

2.1.7 Scroll compressor

A scroll compressor is a type of oil-freerotating displacement compressor, i.e. itcompresses a specific amount of air in anever decreasing volume. The compressorelement consists of a fixed spiral in an ele-ment housing and a motor poweredeccentric, moveable spiral. The spirals aremounted with 180° phase displacement toform air pockets with a varying volume.

This provides the elements with ra-dial stability. Leakage is minimised as thepressure difference in the air pockets is lessthat the pressure difference between theinlet and the outlet.

1. Intake 2. Compression starts

3.Compression completed 4. Outlet

Inlet ports

Female rotor

Male rotor

Outlet ports

2:11

2:12

Rotors in a tooth compressor


The moving spiral is driven by a short stroke crankshaft and runs eccentricallyaround the centre of the fixed spiral. Theintake is situated at the top of the elementhousing.

When the moving spiral runs antic-lockwise air is drawn in, and is captured inone of the air pockets and compressed

variably in towards the centre where theoutlet and a non-return valve are situated.The compression cycle is in progress for2.5 turns, which virtually gives constantand pulsating free air flow. The process isrelatively silent and vibration free, as theelement has hardly any torque variationcompared to, e.g. a piston compressor.

2:13

42

A scroll compressor in cross section.


43

2.1.8 Vane compressor

The operating principle for a vane com-pressor is the same as for many compres-sed air motors. The vanes are usuallymanufactured of special cast alloys andmost compressors are oil lubricated.

A rotor with radially movable blades iseccentrically mounted in a stator housing.When it rotates the vanes are pressedagainst the stator walls by centrifugalforce. Air is drawn in when the distancebetween the rotor and stator is increasing.The air is captured in different compressorpockets, which decrease in volume withrotation. The air is discharged when thevanes pass the outlet port.

2.1.9 Liquid-ring compressor

The liquid ring compressor is a displacementcompressor with built-in pressure ratio. Therotor has fixed blades and is eccentricallymounted in a housing, which is partly filledwith a liquid. The blade wheel transports the

liquid around in the compressor housingand a ring of liquid is formed around thecompressor housing wall by means of centri-fugal force. The liquid ring lies eccentricallyto the rotor as the compressor housing hasan oval form. The volume between the bladewheel varies cyclically. The compressor isusually designed with two symmetrical,opposite compression chambers to avoidradial thrust on the bearings.

Cooling in a liquid ring compressor isdirect, due to the contact between the li-quid and the air, and means the tempera-ture increase on the compressed air is verylittle. However, losses through viscous fric-tion between the housing and the bladesare high. The air becomes saturated withcompressor liquid, which normally iswater. Other liquids can also be used, forexample, to absorb a specific constituentpart of the gas to be compressed or to pro-tect the compressor against corrosion whenaggressive gases are compressed.

Outlet

IntakeCompression

2:15

2:14

Compression

Intake

Outlet

2:16

Operating principle for a scroll compressor


2.1.10 Blowers

A blower is not a displacement compressoras it works without internal compression.When the compression chamber comesinto contact with the outlet, compressed airfloods in from the pressure side. It is firsthere that compression takes place, whenthe volume of the compression chamberdecreases with continued rotation.Accordingly, compression takes placeagainst full counter-pressure, which resultsin low efficiency and a high noise level.

Two identical, normally symmetrical,counter-rotating rotors work in a housingwith flat ends and a cylindrical casing. Therotors are synchronised by means of a gearwheel. Blowers are usually air cooled andoil-free. The low efficiency limits the blow-ers to low pressure applications and com-pression in a single stage, even if two andthree stage versions are available. Blowersare frequently used as vacuum pumps andfor pneumatic conveyance.

2.2 Dynamic

compressors

2.2.1 Dynamic compressors ingeneral

Dynamic compressors are available in axialand radial designs. The latter are frequent-ly called turbo or radial turbo and the for-mer are called centrifugal compressors. Adynamic compressor works with a con-stant pressure, unlike for example a displa-cement compressor, which works with aconstant flow. The performance of a dyna-mic compressor is affected by external con-ditions, for example, a small change in theinlet pressure results in a large change inthe capacity.

44

B

A

B

A

BA

A

B

2:17

Operating principle of a blower


45

2.2.2 Centrifugal compressors

The centrifugal compressor is character-ised by the radial discharge flow. Air isdrawn into the centre of a rotating impellerwith radial blades and is thrown outtowards the periphery of the impeller bycentrifugal forces. Before the air is led tothe centre of the next impeller, it passes adiffuser and a volute where the kineticenergy is converted to pressure.

The pressure ratio across each stage isdetermined by the compressor’s final pres-sure. This also gives a suitable velocity inc-rease for the air after each impeller. The airtemperature at the inlet of each stage has adecisive significance for the compressor’s

power requirement, which is why coolingbetween stages is needed. Centrifugal com-pressors with up to six stages and pressureup to 25 bar are not uncommon. Theimpeller can have either an open or closeddesign. Open is the most common with airapplications. The impeller is normallymade of special stainless steel alloy or alu-minium. The speed is very high comparedwith other types of compressor, 15,000-100,000 r/min are common.

This means that journalling on thecompressor shaft takes place using plainbearings instead of rolling bearings.Rolling bearings are used on single stagecompressors with a low pressure ratio.

2:18

Three stage centrifugal compressor


Often multi-stage compressors have twoimpellers mounted on each end of thesame shaft to counteract the axial loadscaused by the pressure differences. Thelowest volume flow rate through a centri-fugal compressor is primarily determinedby the flow through the last stage. A prac-tical limit value of 160 l/s in the outletfrom a horizontal split machine is often arule-of-thumb.

Each centrifugal compressor must besealed in a suitable manner to reduce leak-age along the shaft where it passes through the compressor housing. Manytypes of seal are used and the most advan-ced can be found on compressors with ahigh speed intended for high pressures.The four most common types are labyrinthseals, ring seals, (usually graphic seals thatwork dry, but even sealing liquids areused), mechanical seals and hydrostaticseals.

2.2.3 Axial compressors

A axial compressor has axial flow, the airor gas passes along the compressor shaftthrough rows of rotating and stationaryimpellers. In this way the velocity of the airis gradually increased at the same time asthe stationary blades convert the kineticenergy to pressure.

The lowest volume flow rate throughsuch a compressor is about 15 m3/s. Abalancing drum is usually built into thecompressor to counterbalance axial thrust.

Axial compressors are generallysmaller than equivalent centrifugal com-pressors and work ordinarily with about a25% higher speed. They are used for con-stant high volume rate of flow at a relative-ly moderate pressure. With the exceptionof gas turbine applications the pressureratio is seldom higher than 6. The normalflow is approx. 65 m3/s and effective pres-sure up to approx. 14 bar(e).

46 Axial compressor

2:19


47

2.3 Other

compressors

2.3.1 Vacuum pumps

A vacuum means a lower pressure thanatmospheric pressure. A vacuum pump isa compressor that works in this pressurerange. A typical characteristic of a vacuumpump is that they work with a very highpressure ratio, however despite this, multi-stage machines are common. Multi-stagecompressed air compressors can also beused for vacuums within the pressurerange 1 bar(a) and 0.1 bar(a).

2.3.2 Booster compressors

A booster compressor is a compressor thatworks with air that has been compressedand compresses it to a higher pressure. It isused to compensate the pressure drop inlong pipelines or in applications where ahigher pressure is required for a sub-pro-cess. Compression may be single or multi-staged and the compressor can be of a

dynamic or displacement type, but pistoncompressors are the most common. Thepower requirement for a booster compres-sor increases with a rising pressure ratio,while the mass flow drops. The curve forpower requirement as a function of theintake pressure has the same general formas the curve for a vacuum pump.

2:20

The working range for some types of vacuum pumps

2:21

Adiabatic power requirement for a booster com-pressor with an absolute final pressure of 8 bar.


2.3.3 Pressure intensifiers

Pressure intensifiers increase the pressurein a medium, for example, for laboratorytests of valve, pipes and hoses. A pressureof 7 bar can be amplified in a single stageto 200 bar or up to 1700 bar in multi-stagedequipment. The pressure intensifier is onlyavailable for very small flows.

When the high pressure chamber is filled,the low pressure piston is lifted. When thepropellant flows in, the piston is presseddownwards and forces the medium outunder high pressure. The intensifier canwork in a cycling process, up until a presetpressure level. All inert gases can be com-pressed in this way. Air can also be com-pressed in a pressure intensifier, but mustbe completely oil-free to avoid selfignition.

2.4 Treatment of

compressed air

2.4.1 Drying compressed air

All atmospheric air contains water vapour,more at high temperatures and less atlower temperatures. When the air is com-pressed the water concentration increases.For example, a compressor with a workingpressure of 7 bar and a capacity of 200 l/sthat draws in air at 20˚C with a relativehumidity of 80% will give off 80 litres ofwater in the compressed air line during aneight hour working day.

The term pressure dew point (PDP) isused to describe the water content in thecompressed air. It is the temperature atwhich water vapour transforms into waterat the current working pressure. Low PDPvalues indicate small amounts of watervapour in the compressed air.

It is important to remember thatatmospheric dew point can not be com-pared with PDP when comparing differentdryers. For example, a PDP of +2˚C at 7 baris equivalent to –23˚C at atmospheric pres-sure. To use a filter to remove moisture(lower the dew point) does not work. Thereason is because further cooling means48

2:22

1. Propellant2. Low pressure piston3. High pressure piston4. Seal5. Compression chamber6. Intake7. Outlet

A cross section of a single stage pressureintensifier.


49

continued precipitation of condensationwater. You can select the main type of dry-ing equipment based on the pressure dewpoint. Seen from a cost point of view, thelower the dew point required the higherthe acquisition and operating costs for air

drying. In principle, there are four me-thods to remove the moisture from com-pressed air: Cooling, over-compressionabsorption and adsorption. There is equip-ment available, based on these methods fordifferent types of compressed air systems.

2:23

A compressor that delivers 200 litres of air per second, also supplies approx. 240 litres of water per day ifworking with air at 20˚C. To avoid problems and disturbances due to water precipitation in the pipes and

connected equipment the compressed air must be dried. This takes place in an aftercooler and drying equipment as set out in the figure.

2:24

This is how the relation between dew point and compressed air looks.


2.4.1.1 Aftercooler

An aftercooler is a heat exchanger, whichcools the hot compressed air to precipitatethe water that otherwise would condensatein the pipe system. It is water or air cooled,generally equipped with a water separatorwith automatic drainage and should beplaced next to the compressor.

80–90% of the precipitated condensa-tion water is collected in the aftercooler'swater separator. A common value for thetemperature of the compressed air after theaftercooler is approx. 10˚C above the cool-ant temperature, but can vary dependingon the type of cooler. An aftercooler isused in virtually all stationary installa-tions. In most cases an aftercooler is builtinto modern compressors.

2.4.1.2 Refrigerant dryer

Refrigerant drying means that the com-pressed air is cooled, whereby a largeamount of the water condenses and can beseparated. After cooling and condensingthe compressed air is reheated to aroundroom temperature so that condensationdoes not form on the outside of the pipesystem. Cooling of the compressed airtakes place via a closed coolant system. By

cooling the incoming compressed air withthe cooled air in the heat exchanger theenergy consumption of the refrigerantdryer is reduced. Refrigerant dryers areused with dew points between +2˚C to+10˚C and are limited downwards by thefreezing point of the condensed water.

50

Different aftercoolers and waterseparators. The water separatorcan, for example, work with cyc-

lone separating or separationthrough changes indirection and

speed.

2:26

Examples of how different parameters change withcompression, aftercooling and refrigerant drying.

2:25


51

2.4.1.3 Over-compression

Over-compression is perhaps the easiestmethod to dry compressed air.

Air is first compressed to a higherpressure than the intended working pres-sure, which means the concentration ofwater vapour increases. Thereafter the airis cooled, whereby the water is separated.Finally the air is allowed to expand to theworking pressure, whereby a lower PDP isattained. However, this method is onlysuitable for very small air flow rates, dueto the high energy consumption.

2.4.1.4 Absorption drying

Absorption drying is a chemical process,where water vapour is bound to theabsorption material. The absorption mate-rial can either be a solid or liquid. Sodiumchloride and sulphuric acid are frequentlyused, which means the possibility of corro-sion must be taken into consideration.

This method is unusual and has ahigh consumption of absorption material.The dew point is only lowered to a limiteddegree.

2:27

This is how refrigerant cooling works in principle.


2.4.1.5 Adsorption drying

There are two types of adsorption dryer,cold regenerative and hot regenerative.Cold regenerative dryers are best suited tosmaller air flow rates. The regenerationprocess takes place with the help of com-pressed air and requires approx. 15–20% ofthe dryer's nominal capacity at 7 bar(e)working pressure, PDP 20˚C. Lower PDPrequires a greater leakage air flow. Hotregenerative adsorption drying regenera-tes the desiccant by means of electrical orcompressor heat, which is more economi-cal than cold regeneration. Very low dewpoints (-30˚C or lower) can be obtained.

Guaranteed separation and drainageof the condensation water shall always bearranged before adsorption drying. If the

compressed air has been produced usingoil lubricated compressors, an oil separat-ing filter should also be fitted before thedrying equipment. In most cases a particlefilter is required after adsorption drying.

There are adsorption dryers for oil-free screw compressors that use the heatfrom the compressor to regenerate thedesiccant. These types of dryers are gene-rally fitted with a rotating drum withdesiccant of which one sector (a quarter) isregenerated by means of a partial flow ofhot compressed air (130–200˚C) from thecompressor stage. Regenerated air is thencooled, the condensationdrainedand theairreturned via the ejector to the main airflow. The rest of the drying drum's surface(three-quarters) is used to dry the com-52

2:28

Cold regenerative adsorption drying


53

pressed air from the compressor's aftercool-er. The system gives no compressed airlosses. The power requirement for such adryer is limited to that required for power-ing the drum. For example, a dryer with acapacity of 1000 l/s only requires 120 W.In addition, no compressed air is lost andneither oil nor particle filters are required.

2:30

2:29

In the diagram the left tower dries the compressed air while the right tower regenerates. After cooling andpressure equalisation the towers are automatically switched.

Oil-free screw compressor with a MD type adsorption dryer.


2.4.2 Filters

Particles in an air stream that pass a filtercan be removed in several different ways.If the particles are larger than the openingin the filter material they are separatedmechanically.

This usually applies for particles greater than 1 µm. The filter’s efficiency inthis regard increases with a tighter filter material, consisting of finer fibres. Particlesbetween 0.1 µm and 1 µm can be separatedby the air stream going around the filtermaterial's fibres, while the particlesthrough their inertia continue straight on.

These then hit the filter material's fibresand adhere to the surface. The efficiency ofthe filter in this regard increases with anincreased flow velocity and a tighter filtermaterial consisting of finer fibres.

Very small particles (<0.1 µm) moverandomly in the air stream influenced bycollisions with air molecules. They "hover"in the air flow changing direction thewhole time, which is why they easily col-lide with the filter material's fibres andadhere there. The efficiency of the filter inthis regard increases with a reduction inthe stream velocity and a tighter filtermaterial consisting of finer fibres.

54

2:32

This is how filter material with mechanicalseparation works in theory. Particles that are

> 1µm are separated.

2:33

Particles between 0.1–1 µm move randomlyin the air stream and are separated when they

collide with the fibres in the filter material.

2:31

MD dryer


55

The separating capacity of a filter is aresult of the different sub-capacities as setout above. In reality, each filter is a com-promise, as no filter is efficient across theentire particle scale, even the effect of thestream velocity on the separating capacityfor different particle sizes is not a decisivefactor. For this reason particles between 0.2µm and 0.4 µm are the most difficult toseparate.

The separating efficiency for a filter is spe-cified for a specific particle size. A separa-tion efficiency of 90–95% is frequently stated, which means that 5–10% of allparticles in the air go straight through thefilter. Furthermore, a filter with a stated95% separation efficiency for the particlesize 10 µm can let through particles thatare 30–100 µm in size. Oil and water inaerosol form behave as other particles andcan also be separated using a filter.

Drops that form on the filter mater-ial's fibres sink to the bottom of the filterdue to gravitational forces. The filter canonly separate oil in aerosol form. If oil invapour form is to be separated the filtermust contain a suitable adsorption mate-rial, usually active carbon.

2:35

A filter to remove oil, water and dust particles. Thefilter element has a small diameter and consists of

spun glass fibre

2:34

Particles (<0.1 µm) that collide with fibresin the filter are separated by adhering to the

surface.

This is how a particle filter can look in reality. Alarge filter housing and large area mean a low air

velocity, less pressure drop and a longer service life.


All filtering results in a pressure drop, thatis to say, an energy loss in the compressedair system. Finer filters with a tighter struc-ture cause a greater pressure drop andbecome blocked more quickly, whichdemands more frequent filter replacementresulting in higher costs.

Accordingly, filters must be dimen-sioned so that they not only handle thenominal flow, but also have a greater capa-city threshold so they can manage a pres-sure drop due to a degree of blockage.

2.5 Control and

regulation systems

2.5.1 Regulation, general

Frequently you require a constant pressurein the compressed air system, which makes

demands on the ability to be able to controlthe compressed air flow from the compres-sor centre. There are a number of methodsfor this depending on, e.g. the type of com-pressor, permitted pressure variations, con-sumptionvariations and acceptable losses.

Energy consumption represents ap-prox. 80% of the total cost for compressedair, which means that you should carefullyconsider the choice of regulation system.Primarily this is because the differences inperformance broadly overshadow the dif-ferences between compressor types andmanufacturers. It is ideal when the com-pressor's full capacity can be exactly adap-ted to an equal consumption, for example,through carefully choosing the gearbox'stransmission ratio, something that is frequently used in process applications. Anumber of consumers are self-regulating,i.e. increased pressure gives an increased

56

2:36


57

flow rate, which is why they form stablesystems. Examples can be pneumatic conveyors, ice prevention, chilling, etc.However, normally the flow rate must becontrolled, which often takes place usingequipment integrated in the compressor.There are two main groups of such regula-tion systems:

1. Continuous capacity regulation involvesthe continuous control of the drive motoror valve according to variations in pres-sure. The result is normally small pressurevariations (0.1 to 0.5 bar), depending onthe regulation system's amplification andits speed.

2. Load/unload regulation is the mostcommon regulation system and involvesthe acceptance of variations in pressurebetween two values. This takes place bycompletely stopping the flow at the higherpressure (off-loading) and resume the flowrate (loading) when the pressure has drop-ped to the lowest value. Pressure varia-tions depend on the permitted number ofload/unload cycles per time unit, but nor-mally lie within the range 0.3 to 1 bar.

2.5.2 Regulation principles for displacement compressors

2.5.2.1 Pressure relief

The original method to regulate a compres-sor is a pressure relief valve, which relea-ses excess pressure into the atmosphere.The valve in its simplest design can bespring loaded, where the spring tensiondetermines the final pressure.

Frequently a servo-valve is usedinstead, which is controlled by a regulator.The pressure can then be easily controlledand the valve can also act as an off-loading

valve when starting a compressor underpressure. Pressure relief makes large ener-gy demands, as the compressor must workcontinuously against full counter pressure.

A variant, which is used on smallercompressors, is to unload the compressorby fully opening the valve so that the com-pressor works against atmospheric pres-sure. Power consumption is significantlymore favourable using this method.

2.5.2.2 Bypass

Bypass regulation has in principle thesame function as pressure relief. Thedifference is that the pressure relieved airis cooled and returned to the compressor'sintake. The method is often used on pro-cess compressors where the gas is unsuit-able or too valuable to release into theatmosphere.

2.5.2.3 Throttling the intake

Throttling is an easy method to reduce theflow. By increasing the pressure ratioacross the compressor, depending on theinduced underpressure in the intake, the

2:37

2:38


method is however limited to a small regu-lation range. Liquid injected compressors,which have a large permitted pressureratio, can however be regulated down to10% of the maximum capacity. This method makes relatively high energy demands, due to the high pressure ratio.

2.5.2.4 Pressure relief with throttled intake

The most common regulation method cur-rently used that unites a maximum regula-tion range (0-100%) with low energy con-sumption, only 15–20% of full load powerwith an off-loaded compressor (zero flow).The intake valve is closed, but with a smallopening remaining, at the same time as ablow off valve opens and relieves the out-going air from the compressor.

The compressor element then works with avacuum in the intake and low counter pres-sure. It is important the pressure relief iscarried out quickly and that the relievedvolume is small to avoid unnecessary los-ses during the transition from loaded to unloaded. The system demands a systemvolume (air receiver), the size of which isdetermined by the acceptable differencebetween loading and off-loading pressureand by the permitted number of unloadingcycles per hour.

2.5.2.5 Start/stop

Compressors less than 5–10 kW are oftencontrolled by completely stopping theelectric motor when the pressure reachesan upper limit value and restarting it whenthe pressure passes the lower limit value.The method demands a large system vol-ume or large pressure difference betweenthe start and stop pressure, to minimise theload on the electric motor. This is an effec-tive regulation method under the condi-tion that the number of starts per time unitis kept low.

2.5.2.6 Speed regulation

A combustion engine, turbine or frequencycontrolled electric motor controls the com-pressor's speed and thereby the flow. It is anefficient method to attain an equal outgoingpressure and a low energy consumption.

The regulation range varies with the typeof compressor, but is greatest for liquidinjected compressors. Frequently speedregulation and pressure relief are com-bined, with or without a throttled intake,at low degrees of loading.58

P

2:40

P

2:41

2:39

2:42


59

2.5.2.7 Variable discharge port

The capacity of screw compressors can beregulated by moving the position of thedischarge port in the housing, in the screw’s lengthways direction, towards theintake. However, the method demandshigh power consumption with sub-loadsand is relative unusual.

2.5.2.8 Suction valve unloading

Piston compressors can be effectively relie-ved by mechanically forcing the intake val-ves to the open position. Air is then pump-ed out and in under the position of thepiston, with minimal energy losses as aresult, often lower than 10% of the loadedshaft power. On double acting compres-sors there is generally multi-stage off-loa-ding, where one cylinder at a time is balan-

ced to better adapt the capacity to thedemand. An odd method used on processcompressors is to allow the valve to beopen during a part of the piston stroke andthereby receive a continuous flow control.

2.5.2.9 Clearance volume

By varying the clearance volume on apiston compressor the degree of filling de-creases and thereby the capacity. The clear-ance volume is varied by means of anexternally connected volumes.

2.5.2.10 Load–unload–stop

The most common regulation method usedfor compressors greater than 5 kW thatcombines a large regulation range withlow losses. In practice a combination of thestart/stop and different off-loadingsystems. See further under 2.5.4.2.

2:43

Off-loading device for a piston compressor


60

2.5.3 Regulation principles fordynamic compressors

2.5.3.1 Throttling the intake

The intake can be throttled on a dynamiccompressor to continuously reduce thecapacity of the compressor. The minimumflow is determined when the pressure ratioreaches the pump limit and the machinebecomes unstable (surge).

The regulation range is determinedby the design of machine, for example, thenumber of stages and the impeller design,but also to a large degree by external fac-tors such as counter pressure, suction tem-perature, and the coolant temperature. Theminimum flow often varies between 60%and 85% of the maximum flow.

2.5.3.2 Inlet guide vanes

Vanes arranged as radial blades in the in-take cause the in-drawn gas to rotate, atthe same time as the flow is throttled. Themethod acts as throttling, but with a grea-ter regulation range and with improvedenergy utilisation. Regulation down to50–60% of the design flow is a typicalvalue. There is also the possibility of in-creasing the capacity and pressure of thecompressor to a certain degree, by turning

the vanes in the opposite direction, how-ever, this does impair performance a little.

2.5.3.3 Outlet guide vanes (diffuser)

To further improve the regulation rangeyou can also control the flow in the com-pressor stage's diffuser. Regulation downto 30% with maintained pressure is com-mon. Normally usage is limited to singlestage compressors, due to the complexityand increased costs.

2.5.3.4 Pressure relief

The original method of regulating a dyna-mic compressor was also to use a pressurerelief valve, which releases excess com-pressed air into the atmosphere. The method works in principle as pressure relief on a displacement compressor.


While throttling the compressor's inlet islimited by the pump limit, this can be sol-ved in one of two ways:1. Modulating. The excess flow is releasedinto the atmosphere (or intake), however,with unchanged energy consumption.2. Auto Dual. The regulation system virtu-ally fully closes the intake valve at thesame time as the compressor's outlet isopened to the atmosphere (compare thedisplacement compressor). However, theoff-loading power is relatively high, over20% of the full load power, depending onthe design of the impeller, etc.

2.5.3.6 Speed regulation

Speed regulation is commonly used oncompressors where the flow shall be regu-lated and the pressure is permitted to vary.With constant pressure regulation, speedregulation gives no benefits when compa-red with other regulation systems.

2:44


61

2:45

The relation between the shaftpower and flow for oil lubricatedcompressors with different regu-lation systems and combinations

of regulation systems.

The relation between the shaftpower and flow for oil-free screw

compressors with different regula-tion systems and combinations of

regulation systems.

The relation between the shaftpower and flow for dynamic com-pressors with different regulation

systems and combinations ofregulation systems.


2.5.4 Control and monitoring

2.5.4.1 General

Regulation principles for different com-pressors are taken up in chapters 2.5.2 and2.5.3. To control compressors according tothese principles requires a regulationsystem that can either be intended for anindividual compressor or an entire com-pressor installation.

Regulation systems are becomingmore advanced and development goesquickly. Relay systems have been replacedby programmable equipment (PLC), whichin turn are being replaced by productadapted systems based on microcompu-ters. The designs are often an attempt tooptimise operations and cost.

This section deals with a few of thecontrol and monitoring systems for themost common types of compressor.


The most common regulation principlesfor displacement compressors are "produceair"/"don't produce air" (loaded/unload-ed), see 2.5.2.4 and 2.5.2.5.

When air is required a signal is sentto a solenoid valve, which in turn guidesthe compressor's intake damper to thefully open position. The damper is eitherfully opened (loaded) or fully closed (un-loaded), there is no intermediate position.

The traditional control, now commonon smaller compressors, has a pressureswitch placed in the compressed air systemthat has two settable values, one for theminimum pressure (= loaded) and one formaximum pressure (unloaded). The com-pressor will then work within the limits ofthe set values, for example, 0.5 bar. If theair requirement is small or nothing thecompressor runs off-loaded (idling). The

length of the idling period is limited by atimer (set e.g. to 20 minutes). When thetime elapses, the compressor stops anddoes not start again until the pressure hasdropped to the minimum value. This is thetraditional tried and trusted control method. The disadvantage is that it givesslow regulation.

A further development of this traditionalsystem is to replace the pressure switchwith an analogue pressure transducer anda fast electronic regulation system. Theanalogue transducer can, together with theregulation system, sense how quickly thepressure in the system changes. Thesystem starts the motor and controls the opening and closing of the damper at theright time. This gives quick and good regulation within ± 0.2 bar.

If no air is used the pressure will remainconstant and the compressor will run off-loaded (idling). The length of the idlingperiod is controlled by how many startsand stops the electric motor can withstandwithout becoming too hot and by the over-all operating economy. The latter is possib-le as the system can analyse trends in airconsumption and thereby decide whetherto stop the motor or continue to idle.62

2:46

2:47

Pressure band, min–max, within which the com-pressor works. "Min" = load, "Max" = off-load.

An advanced regulation system can send signals tothe motor, starter and regulator at "the right time".


63

2.5.4.3.Speed control

Compressors with a power source whosespeed is controlled electronically provide agreat opportunity to keep the compressedair constant within a very tight pressurerange.

A frequency converter, which regula-tes the speed on a conventional inductionmotor, is an example of such a solution.The compressor's capacity can be adaptedexactly to the air requirement by contin-uously and accurately measuring the system pressure and then allow the pres-sure signals to control the motor'sfrequency converter and thereby themotor's speed. The pressure within the system can be kept within ± 0.1 bar.

2.5.5 Control and monitoring

All compressors are equipped with someform of monitoring equipment to protectthe compressor and prevent productiondowntime. The transducer is used to sensethe current condition of the installation.Information from the transducers is pro-cessed by the monitoring system, whichgives a signal to, e.g. an actuator.

A transducer for measuring the pres-sure or temperature often consists of a sen-sor and a measurement converter. The sen-sor senses the quantity to be measured. The measurement converter converts the

sensor's output signal to an appropriateelectrical signal that can be processed bythe control system.

2.5.5.1 Temperature measurement

A resistance thermometer is normally usedto measure the temperature. This has ametal resistor as a transducer, whoseresistance increases with the temperature.The change in resistance is measured andconverted to a signal of 4–20 mA. Pt 100 isthe most common resistance thermometer.The nominal resistance at 0°C is 100Ω.

The thermistor is a semiconductor, whoseresistance changes with the temperature. Itcan be used as a temperature controller, forexample, on an electric motor. PTC, Posi-tive Temperature Coefficient, is the mostcommon type. The PTC has an insignifi-cant change in resistance with increased

2:49

An example of a 3 wire connection using a resistan-ce thermometer of 100Ω. The resistance thermome-

ter and connectors are connected on a bridge.

2:48

A system with a speed controlled compressor


temperature up to a reference point, wherethe resistance increases with a jump. ThePTC is connected to a controller, whichsenses the "resistance jump" and gives asignal, for example, stop the motor.

2.5.5.2 Pressure measurement

A pressure sensing body, e.g. a diaphragmis used to measure the pressure. Themechanical signal from the diaphragm isthen converted to an electrical signal, 4–20mA or 0–5 V.

The conversion from a mechanical to anelectrical signal can take place in differentmeasurement systems. In a capacitivesystem, pressure is transferred to a diaph-ragm. The position of the measurementdiaphragm is sensed by a capacitor plateand is converted in a measurement conver-ter to a direct voltage or direct current,proportional to the pressure.

The resistive measurement system consistsof a strain gauge, connected in a bridgeconnection and attached to the diaphragm.When the diaphragm is exposed to

pressure a low voltage (mV) is received.This is then amplified to a suitable level.The piezo electric system is based on speci-fic crystals (e.g. quartz) generating electri-cal charges on the surface of the crystal.The charges are proportional to the force(pressure) on the surface.

2.5.5.3 Monitoring

Monitoring equipment is adapted accord-ing to the type of compressor, whichentails a large range when concerning thescope of equipment. A small piston com-pressor is only equipped with a conventio-nal overload cut-out for the motor, while alarge screw compressor can have a numberof cut-outs/transducers for overloading,temperature and pressure, etc.

On smaller, more basic machines thecontrol equipment switches off the com-pressor and the machine is blocked for res-tarting when a cut-out gives an alarmvalue. A warning lamp, can in some cases,indicate the cause of the alarm.

Compressor operations can be fol-lowed on a control panel on more advan-ced compressors, for example, by directlyreading off the pressure, temperature andstatus, etc. If a transducer value approachesan alarm limit the monitoring equipmentgives off a warning. Measures can then betaken before the compressor is switchedoff. If the compressor is stilled stopped byan alarm, the restart of the compressor isblocked until the fault has been rectified oris reset by hand.64

2:50

Example of a capacitive system for pressuremeasurement

2:51

Bridge connection with strain gauge.


65

Trouble shooting is significantly facilitatedon compressors equipped with a memorywhere data on, e.g. temperatures, pressureand operating status are logged. The capa-city of the memory covers, for example, thelast 24 hours. By using this it's possible toproduce trends over the last day and then,using logical trouble shooting, quicklytrack down the reason for the downtime.

2.5.6 Comprehensive control system

Compressors that are a part of a system ofseveral machines should have co-ordinatedcompressor operations. There are manyfactors that speak for a comprehensivecontrol system. The division of operatingtimes between machines reduces the riskof unexpected stoppages. Servicing com-pressors is also easier to plan. Standbymachines can be connected if somethingshould occur during operations.

2.5.6.1 Starting sequence selector

The simplest and most common form of

master control system is the well tried andtested start sequence selector. This has thetask of equally dividing the operatingtimes and starts between the connectedcompressors. The start sequence can beswitched manually or automatically accord-ing to a time schedule. This basic selectorutilises an on/off pressure transducer,with one transducer per compressor,which is a simple and practical solution.

The disadvantage is that there are rel-atively large steps between the differentcompressor's loading and off-loading levels, which in turn gives relatively broadpressure bands (the span between maxi-mum and minimum levels) for the installa-tion. Therefore this type of selector shouldnot be used to control more than 2–3 com-pressors.

A more advanced type of start se-quence selector has the same type of se-quence control, but with only one, centrallypositioned, analogue pressure transducer.This manages to keep the installation'stotal pressure band within a few tenths of

R

1

2

Compressor Outlet

Automatically LoadedMenu Show More Unload

7.7 barIntercooler 2.1 bar

F1 F2 F3

2:52

A user-friendly monitoring panel shows all the operating parameters for the compressor, for example,pressure and temperatures, with data logically grouped for direct read-off.


a bar and can control 2–7 machines. A startsequence selector of this type, whichselects the machines in fixed sequences,does not take the capacity of the compres-sors into consideration. It is thereforeappropriate that the connected compres-sors are of approximately the same size.

2.5.7 Central control

Central control in association with com-pressors usually means relatively intelli-gent control systems. The basic demand isto be able to maintain a predeterminedpressure within tight limits and that theinstallation's operation shall be as econo-mic as possible. To achieve this, the systemmust be capable of predicting what willhappen in the system and at the same timesense the load on the compressor.

The system senses how quickly the pres-sure changes, upwards or downwards (i.e.the time derived pressure). Using these

values the system can perform calculationsthat make it possible to predict the air requirement and, for example, off load/load or start/stop the machines. In acorrectly dimensioned installation thepressure will be within ± 0.2 bar.

It is extremely important for the operatingeconomy that the central control systemselects a compressor or compressor combi-nations, if compressors of different capa-city are included in the system.

The compressors shall run virtuallycontinuously loaded and thereby minimiseidling to give the best economy.

An another advantage of a comprehensivecontrol system is that it is generally possib-le to connect older machines to thesesystems and thereby, in a relatively easymanner, modernise the entire compressorinstallation. Operations become more eco-nomic and availability increases.66

2:53

The difference in the pressure band for five compressors controlled by conventional pressure switches(left-hand field) and the same machines controlled by a regulation system with an analogue transducer

(right-hand field).


67

2.5.8 Remote monitoring

In a number of compressor installationsthere may be a need of monitoring andcontrolling compressor operations from aremote location. On smaller installations itis fairly easy to connect an alarm, operat-ing indicator, etc. from the compressor.Normally it is also possible for remotestarting and stopping.

On larger installations, whereimmense value is at stake, central monitor-ing can be motivated. It should consist ofequipment that gives a continuous over-view of the system, but where it is alsopossible to access individual machines tocontrol details such as the intercooler pres-sure, oil temperature/ etc.

The monitoring system should alsohave a memory, so that it is possible toproduce a log of what has happened

during the past 24 hours. The log makes up the basis for trend curves, where it ispossible to read off whether any valueshave a tendency of deviating from thedefault. The curves can form the basis forcontinued operations or a planned stop.

The system frequently presents compres-sor installation status reports in the form ofdifferent levels, from a total overview todetail status for individual machines.

2:54

A centrally controlled compressor installation


68

2:55

An overview display with remote monitoring. The upper section shows the installation status. Threemachines in operation, one stopped. In the lower section details for compressor 4 are shown, among

others, flow chart for the compressed air, cooling water and oil as well as prevailing compressor data.


Chapter 3 Dimensioning compressor installations

Hoofdstuk3 27.04.2000 12:28 Pagina 1

70

3.1 Dimensioning compressor installations

3.1.1 General

A number of decisions must be madewhen dimensioning a compressed airinstallation for it to suit the user’s needs,give the best operating economy and beprepared for future expansion.

The foundation is the applications orprocess that will use the compressed air.Therefore, you must start by mapping outthese to gain the correct basis for continu-ed dimensioning.

The areas to be looked at are calculation orassessment of the air requirement and thereserve capacity and the space for futureexpansion. The working pressure is a criti-cal factor, as this significantly affects theenergy consumption. Sometimes it can beeconomical to use different compressorsfor different pressure ranges.

The quality of the compressed air isnot just a question of the water content,but has also become increasingly directedtowards environmental issues. Odour and

the microorganism content are importantfactors that can affect the product quality,rejections, the working environment andthe outdoorenvironment.The issue of whe-ther the compressor installation should becentralised or decentralised affects the space requirement and perhaps futureexpansion plans. From economic and environmental standpoints it is becomingmore important to investigate the possibi-lities of recovering energy at an earlystage, this often gives very quick return onthe investment.

It is important to analyse these typesof issues with regard to current as well asfuture requirements. It is then possible,and only then, to design an installationoffering sufficiently flexibility.

3.1.1.1 Calculating the working pressure

The compressed air equipment in aninstallation determines the requisite work-ing pressure. The right working pressuredoes not just depend on the compressorbut also on the design of the compressedair system with piping, valves, compres-sed air dryers, filters, etc.

Different types of equipment candemand a different pressure in the samesystem. Normally the highest pressuredetermines the requisite installation pres-

3:1

The air requirement for connected equipment is obtained from, e.g. tool catalogues and descriptions ofproduction equipment. By analysing and assessing the utilisation factor you can easily attain upper and

lower limits for the overall air requirement.


71

for all tools, machines and processes to beconnected, bearing in mind the utilisationfactor that experience tells us will be perti-nent. Additions for leakage, wear andfuture changes in the air requirement mustalso be considered.

A simple method to estimate the presentand future air requirement is to compilethe air requirement for connected equip-ment and the utilisation factor.

This type of calculation requires a listof machines, supplemented with respec-tive machine’s air consumption and expect-ed utilisation factor. If you do not havedata for the air consumption or utilisationfactor, standard values can be used. Theutilisation factor for tools can be difficultto estimate, therefore calculation valuesshould be compared with measured con-sumption in similar applications.

For example, when large power con-sumers such as grinders and sand-blastingmachines are used, it is frequently for longperiods (3–10 min) under continuous op-eration, despite the low overall utilisationfactor. This does not really correspond tointermittent operation, which is why it isnecessary to estimated how many machi-nes will be used simultaneously to judgethe total maximum air consumption.

The compressor capacity is essenti-ally determined by the total nominal com-pressed air requirement. The compressor’sfree output flow rate should cover this rateof air consumption. The calculated reservecapacity is primarily determined by thecost of lost production with a possiblecompressed air failure.

The number of compressors and the mutu-al size is determined principally by therequired degree of flexibility, control sys-

sure and other equipment is fitted withreducing valves at the point of consump-tion. In more extreme cases the methodcan be uneconomical and a separate com-pressor for special needs can be a solution.

Also bear in mind that the pressuredrop increases quickly with an increasingflow. If a change in consumption can beexpected, it is makes economic sense toadapt the installation to these conditions.

Filters, special dust filters, have a lowinitial pressure drop, but in time becomeblocked and are replaced at the recom-mended pressure drop, which here will bea factor in the calculation. The compres-sor’s flow regulation also brings aboutpressure variations and shall be includedin the assessment. It may be appropriate tosystematise the calculations according tothe following example:

Description Pressure drop bar(e)

End user 6

Final filter 0.1–0.5

Pipe system 0.2

Dust filter 0.1–0.5

Dryer 0.1

Compressor’s regulation range 0.5

Compressor’s max

working pressure 7.0–7.8

Primarily it is the end user together withthe pressure drop between the compressorand the consumer that determines thepressure that the compressor needs to pro-duce. By, as in this example, adding thepressure drop in the system the workingpressure can be determined.

3.1.1.2 Calculating the air requirement

The nominal compressed air requirementis determined by the air consumers. This iscalculated as a sum of the air consumption


72

tem and energy efficiency. In an installa-tion where, due to reasons of cost, only onecompressor shall answer for the compres-sed air supply, the system can be prepared

for the quick connection of a portable com-pressor in connection with servicing. Anolder compressor, used as a reserve source,can be used for inexpensive reserve power.

3:3

During an operating analysis the compressed air production is continuously measured, for example,during a week. This is the results in diagram form for a compressor with the capacity 260 m3/min where

the different curves represent the days of the week.

3:2

The principle for an operating analysis.


73

as well as the operating method for theinstallation. For example, the workingpressure can often be reduced at certaintimes and the control system can be modi-fied in order to improve compressor usagewith changes in production. Anotherimportant factor is to check whether thereis any leakage.

For the production of small quanti-ties of air during the night and weekends,you must consider whether it is worth-while installing a smaller compressor tocover this requirement.

3.1.2 Centralisation or decentralisation

3.1.2.1 General

There are several factors that affect thechoice between one large or several smal-ler compressors to meet the same compres-sed air requirement. For example, the costof a production stoppage, the availabilityof electricity, loading variations, costs forthe compressed air system and the avail-able floor space.

3.1.2.2 Centralised compressor installations

A centralised compressor installation is inmost cases the solution of choice, as it is

3.1.1.3 Measuring the air requirement

An operating analysis provides key factorsabout the compressed air requirement andforms the basis for assessing how muchcompressed air it is best to produce. Mostindustrial companies develop continuous-ly, which means that the compressed airrequirement also changes. It is thereforeimportant that the compressed air supplyis based on the current prevailing condi-tions, at the same time as an appropriatemargin for expansion is built into theinstallation.

Operating analysis involves the measure-ment of operating data, possibly supple-mented with the inspection of an existingcompressed air installation during a suit-able period of time. The analysis shouldcomprise at least one week of operationsand the measurement period should beselected with care in order to serve as atypical case to ensure that a relevant pic-ture is obtained. The stored data also pro-vides an opportunity to simulate differentmeasures and changes in compressor op-erations and to analyse the significance forthe installation’s overall economy.

Factors such as the loading times andoff-loading times also have a bearing onthe total assessment of compressor opera-tions. These provide the basis to assess theloading factor and the compressed airrequirement, spread over a day or work-ing week. Accordingly, the loading factorcan not just be read off on the compres-sor’s running hour meter.

An operating analysis also gives a basis forthe potential energy recovery. Frequentlymore than 90% of the supplied energy canbe recovered. Furthermore, the analysis canprovide answers relating to dimensioning

3:4

Even small leakage can result in large costsand downtime.


short peaks is to solve the problem bypositioning a buffer (air receivers) at stra-tegic places.

A unit or building normally suppliedfrom a compressed air central and which isthe sole consumer of compressed air atspecific periods can be sectioned off andsupplied with its own compressor. Theadvantage of this is you avoid “feeding”any leakage in the remaining part of thesystem and that the localised compressorcan be adapted to the smaller requirement.

3.1.3 Dimensioning at high altitude

3.1.3.1 General

The ambient pressure and temperaturediminish at heights above sea level. Thisaffects the pressure ratio, for compressorsas well as the connected equipment, whichin practice means an influence on thepower and air consumption. At the sametime the changes also affect the availablerated power from electric motors and com-bustion engines.

You should also be aware of how theambient conditions influence the end user.Is it a specific mass flow rate, e.g. in a pro-cess or is it volume flow rate you require?Is it the pressure ratio, absolute pressureor over pressure that was used for dimen-sioning? Is the compressed air temperaturesignificant?

All these create different conditionsfor dimensioning a compressed air instal-lation installed at a high altitude and canbe fairly complex to calculate. If you feelunsure you should always contact themanufacturer of the equipment.

74

less expensive than several, locally situa-ted compressors. Compressor plant can beefficiently interconnected, which result inlower energy consumption. A centralinstallation also involves lower monitoringand maintenance costs as well as betterconditions for recovering energy. Theoverall, requisite floor area for the com-pressor installation is less. Filters, coolersand other auxiliary equipment and the airintake can be optimally dimensioned andinstalled. Noise insulation will also be easi-er to fit.

A system comprising several, diffe-rent sized compressors in a central installa-tion can be sequence controlled to improveefficiency. One large compressor has dif-ficulty in meeting large variations in thecompressed air requirement, without effi-ciency dropping.

For example, systems with one largecompressor are often supplemented with asmaller compressor, for use during periodssuch as a night shift or at weekends.Another factor worth considering is theeffect the start of a large electric motor hason the mains supply.

3.1.2.3 Decentralised compressor installations

A system with several decentralised com-pressors involves a smaller, simpler com-pressed air system. The disadvantages ofdecentralised compressors is the difficultyin inter-regulating the compressed air sup-ply, the expense and that maintenancework is more demanding as well as it ishard to maintain a reserve capacity. De-centralised compressors can be utilised tomaintain the pressure in a system with alarge pressure drop if the intermediateprocesses temporarily draw too much air.Otherwise an alternative with extremely


75

3.1.3.2 The effect on a compressor

To choose the right compressor where theambient conditions differ from those sta-ted on the data sheet, you should take thefollowing factors into consideration:

• Height above sea level

or ambient ressure

• Ambient temperature

• Humidity

• Coolant temperature

• Type of compressor

• Power source

These factors primarily affect the follow-ing:

• Max. working pressure

• Capacity

• Power consumption

• Cooling requirement

The most important factor is the intakepressure variations at altitude. For examp-le, this means a compressor, with a pressu-re ratio of 8.0 at sea level, will have a pres-sure ratio of 11.1 at an altitude of 3000metres (under the condition that the work-ing pressure is constant). This affects theefficiency and thereby the power require-ment. To what degree is dependent on thetype of compressor and the design as setout in figure 3:6.

The ambient temperature, humidityand coolant temperature interact and

3:6

A rule of the thumb for the altitude’s effect on the compressor at 7 bar(e) working pressure and constantambient temperature. Bear in mind that each compressor has a top pressure ratio that can not be exceeded.

The table shows the standardised pressure andtemperature variations at different heights. Thepressure is dependent on the weather and variesapprox. ± 5%, while the local season dependent

temperature variations can be considerable.

3:5


76

affect the compressor’s performance to dif-ferent degrees on single or multi-stagecompressors, dynamic compressors anddisplacement compressors.

3.1.3.3 Power source

3.1.3.3.1 Electric motors

Cooling becomes impaired on electricmotors by the thinner air at high altitude.It should be possible for standard motorsto work up to 1000 m and with an ambienttemperature of 40°C without the rateddata deteriorating. With greater heightstable 3:7 can be used as a guideline forstandard motors. Notice that for sometypes of compressor the motor perform-ance is impaired more than thecompressor’s requisite shaft power at highaltitude.

3.1.3.3.2 Combustion engines

A reduction in the ambient pressure, atemperature increase or a reduction inhumidity reduce the oxygen content in theintake air and thereby the extractablepower from the engine. The degree ofshaft power degradation depends on thetype of engine and its breathing method asset out in figure 3:8. The humidity plays alesser part (<1%/1000 m) when the tempe-rature falls below 30°C.

Notice that the engine power fallsmore rapidly than the compressor’s requi-site shaft power, which means that foreach compressor/engine combinationthere is a maximum working height.Generally, you should let respective sup-pliers calculate and state the specific datathat applies to the compressor, engine andair consumption equipment in question.

3:7

The table shows the permitted load in % of the electric motor’s rated power.

3:8

The table shows how combustion engines are affected by altitude and temperature.


77

compressed air quality specified by theconsumer.

When the compressed air’s part inthe process is clearly defined, the answerto which system is the most profitable andefficient will be found. It is a question,among others, of establishing whether thecompressed air will come into direct con-tact with the product or whether, e.g. oilmist can be accepted in the working envi-ronment. A systematic method is necessa-ry to select the right equipment.

3.2.2 Water vapour in the compressed air

Air in the atmosphere always containsmoisture in the form of water vapour.Some follows with the compressed air andcan cause problems. Examples of whichare: High maintenance costs, shortenedservice life and impaired tool performance,high rate of rejection with spray paintingand plastic injection, increased leakage,disturbances in the control system andinstruments, shorter service life for the

3.2 Air treatment

3.2.1 General

It is a matter of vital importance to theuser that the compressed air is of the rightquality. If air that contains contaminationcomes into contact with the final product,rejection costs can quickly become unac-ceptably high and the cheapest solutioncan quickly become the most expensive. Itis important that you select the compres-sed air quality in line with the company’squality policy and even attempt to judgefuture requirements.

The compressed air can containunwanted substances, for example, waterin drop or vapour form, oil in drop oraerosol form as well as dust. Dependingon the compressed air’s application area,these substances can impair the produc-tion result and even increase costs. Thepurpose of air treatment is to produce the

3:9

The main components in a compressed air system when set up and installed. The equipment for air treatment determines the quality of the compressed air,

which has a great effect on the economy of the installation.


78

pipe system due to corrosion and moreexpensive installation. The water can beseparated by using accessories, e.g. after-coolers, condensation sepators, refrigerantdryers and adsorption dryers.

A compressor that works with 7bar(e) overpressure, compresses air to 7/8the volume. This also reduces the air’s abi-lity to hold water vapour by 7/8. Thequantity of water that is released is consi-derable. For example, a 100 kW compres-sor that draws in air at 20°C and with 60%relative humidity will give off approx. 85litres of water during an 8 hour shift.Consequently, the amount of water to beseparated depends on the compressed air’s

application area. This in turn is decideswhich combination of coolers and dryersare suitable.

3.2.3 Oil in the compressed air

The quantity of oil in the compressed air isdependent on several factors, amongothers, the type of machine, design, age,condition, etc. There are two main types ofcompressor design in this respect, thoseworking with lubricant in the compressionchamber and those working without lubri-cant. In lubricated compressors the oiltakes part in the compression process andalso follows with the compressed air fullyor partly. However, on modern, lubricated

3:10

3:11

A compressor that works with 7 bar(e) overpressure, compresses air to 1/8 of the volume.

ISO has quality classified compressed air with regard to the degree of contamination.


79

control over any bacteria growth after thefilter.

The picture becomes even more com-plicated as gases and aerosol can be con-centrated into drops (through concentra-tion or electric charging) even after pas-sing several filters. Microorganisms germi-nate through the filter walls and thereforeexist in the same concentrations on theinlet and outlet sides of the filter.

During investigations it has been estab-lished that microorganisms thrive in com-pressed air systems with undried air andthereby high humidity (100%). Contamina-tion smaller than 1 µm and thereby micro-organisms, can pass unimpeded throughthe compressor’s intake filter.

Oil and other contamination act asnutrients for growth. The most decisivefactor is to dry the air to a humidity of<40%, which is achieved by using anadsorption dryer and at room temperaturealso with a refrigerant dryer.

piston and screw compressors the oilquantity is only small. For example, in anoil injected, screw compressor the oil con-tent in the air is less than 3 mg/m3 at 20°C.The oil content can be reduced by meansof multi-stage filters. If such a solution ischosen it is important to consider the qua-lity limitations, risks and energy costs thisbrings about.

3.2.4 Microorganisms in the compressed air

More than 80% of the particles that conta-minate the compressed air are less than 2µm and thereby easily pass through thecompressor’s intake filter. Thereafter theparticles are spread in the pipe system andmix, among others, with the water and oilresidue and pipe deposits. This can resultin the growth of microorganisms. A filterdirectly after the compressor can eliminatethese risks. Nevertheless, to have pure orsterile compressed air you must have full

3:12

A view of an installation that can produce compressed air indifferent quality classes in accordance with ISO 8573-1.


3.2.5 Filters

Modern fibre filters are very efficient atremoving oil. However, it is difficult toexactly control the quantity of oil remain-ing in the air after filtration as the tempe-rature, among others, has an importanteffect on the separation process. Efficiencyis also affected by the oil concentration inthe compressed air and the amount of freewater.

To achieve the best results the air shouldbe as dry as possible. Oil, carbon and ste-rile filters all give bad results if there isfree water in the air (the filter specificationsdo not apply in such conditions). Fibre filters can only remove oil in the form ofdroplets or as aerosols. Oil vapour must beremoved using a filter with active carbon.A correctly installed fibre filter, togetherwith a suitable prefilter, can reduce thequantity of oil in the compressed air toapproximately 0.01 mg/m3 at 21°C. A filterwith active carbon can reduce the quantityof oil to 0.003 mg/m3 at 21°C.

Carbon filters should have the correctquality of carbon and dimensioned to givethe lowest possible pressure drop. To havethe best effect the filters should also be plac-ed as close to the application in question aspossible. In addition, they must be check-ed carefully and replaced relatively fre-quently. Filters with active carbon onlyremove air contamination in the form ofvapour, for example, oil. Sterile filters shallwithstand sterilisation on site in the pipesystem with the help of, e.g., steam or beremoved for treatment. A filter’s capacityto separate oil from compressed air variesat different operating temperatures.

Data stated in the filter specificationalways applies at a specific air tempera-

ture, normally 21°C. This correspondsapproximately with the temperature afteran air cooled compressor working in anambient temperature of 10°C. However,climate and seasonal changes give tempe-rature variations, which in turn affect thefilter’s separation capacity.

An oil free compressor eliminates theneed of an oil filter. This means the com-pressor can work at a low pressure, whichreduces energy consumption. It has beenshown in many cases that oil-free com-pressors are the best solution, both econo-mically and for quality.80

3:13

Temperature variations in compressed airresult in quality variations with the

use of filters. Note that standard and high effect filters do

not have an effect on oil in vapour form.


81

can be achieved. The remainder followswith the compressed air as water mist intothe air receiver.

3.2.8 Oil as droplets

Oil in the form of droplets is separatedpartly in, e.g. an aftercooler, condensationseparator or a condensation tap and fol-lows with the condensation water. Thisoil/water emulsion is classed from anenvironmental point of view as waste oiland must not be lead off in the sewagesystem or directly into nature.

New and more stringent laws arecontinuously being introduced withregard to the handling of environmentallyhazardous waste. The drainage of conden-sation, as well as the collection and drain-age, is involved and expensive.

An easy and cost effective solution to theproblem is to install an oil/water separa-tor, for example, with a diaphragm filter,which gives clean drainage water andleads the oil off into a special receiver.

3.2.6 Aftercooler

The compressed air from the compressor ishot after compression, often 70–200°C. Anaftercooler is used to lower the tempera-ture, which also reduces the water contentand now is frequently included as stan-dard equipment in a compressor installa-tion. The aftercooler should always beplaced directly after the compressor. It isthe heat exchanger that cools the hot air, tothen precipitate the main part of the con-densation water as quickly as possible thatwould otherwise follow out into thesystem. The aftercooler can either be wateror air cooled and is generally fitted with awater separator with automatic drainage.

3.2.7 Water separator

Most compressor installations are fittedwith an aftercooler as well as a water sepa-rator, in order to separate as much conden-sation water as possible from the compres-sed air. With the right choice and sizing ofthe water separator an efficiency of 80-90%

3:14

This is how a diaphragm filter for oil separation works. The diaphragm lets through small molecules(clean water), while larger molecules (oils) are kept in the system and can be collected in a container.


3.3 Cooling system

3.3.1 Water cooled compressors

3.3.1.1 General

A water cooled installation makes littledemand on the ventilation of the compres-sor room, as the larger part of the heat pro-duced is led off by the cooling water. Thecooling water from a water cooled com-pressor contains, in the form of heat,approx. 90% of the energy taken up by theelectric motor.

A compressor’s cooling water systemcan be designed based on one of threemain principles. As an open system with-out circulating water, as an open systemwith circulating water, and as a closed cir-culating system.

3.3.1.2 Open system, without circulating water

An open system without circulating watermeans that the water comes from the

municipal water mains, a lake, a stream, ora well and is used to cool the compressorand is then discharged as waste water. Thesystem should be controlled by a thermo-stat, to maintain the desired temperatureas well as to govern water consumption.The cooling water pressure should belower than the pressure the componentsparts are designed for.

Generally an open system is easy and inex-pensive to install, but expensive to run,especially if the cooling water is takenfrom the municipal water mains. Waterfrom a lake or stream is normally free, butmust be filtered and purified to be usedwithout the risk of blocking the coolingsystem. Furthermore, water rich in limecan result in boiler scale forming in thecoolers, bringing about impaired cooling.The same applies to salt water, whichhowever is possible to use if the system isdesigned and dimensioned accordingly.

3.3.1.3 Open system, circulating water

An open system with circulating watermeans that cooling water from the com-

82

3:15

An open cooling system with circulating cooling water.


83

thermal dissipation capacity of the coolingtower. The water must be regularly ana-lysed and treated with chemicals to prevent algae from growing in the water.During the winter, when the compressor isnot operating, the cooling tower must eit-her be emptied or the water heated to pre-vent freezing.

3.3.1.4 Closed system

In a compressor cooled with a closedsystem the same water circulates betweenthe compressor and some form of cooler.This cooler is in turn cooled either bymeans of another water circuit or the sur-rounding air. Generally when the water iscooled against another water circuit a flatheat exchanger is used.

pressor is recooled in a cooling tower.Water is cooled in the cooling tower byallowing it to sprinkle down into a cham-ber at the same time as surrounding air isblown through, whereby a part of thewater vaporises and the remaining wateris cooled to 2˚C under the ambient tempe-rature (can vary depending on the tempe-rature and relative humidity). Opensystems with circulating water are prima-rily used when the availability of water islimited. The disadvantage here is that thewater becomes contaminated by the sur-rounding air. The system must be contin-uously diluted using fresh water due toevaporation.

Dissolvable salts are deposited onthe hot metal surfaces, this reduces the

3:17

A basic air heater (air cooledheat exchanger) can be found inclosed cooling systems for cool-ing liquids such as water/glycol,oil, etc. In aggressive environ-

ments or with aggressive liquids,materials such as stainless steel

or titan are used.

3:16

This is how a flatheat exchanger canappear. The advan-tage of a flat heat

exchanger is that itis easy to clean,

which makes it pos-sible to indirectly

cool the compressorusing lake or stream

water.


When the water is cooled against the sur-rounding air a cooling battery is used con-sisting of pipes and cooling flanges. Thesurrounding air is forces to circulatearound the battery by means of one ormore fans. The air is normally filtered firstto prevent blockage. This method is suit-able if the availability of water is limited.The cooling capacity of open or closed cir-cuits is about the same, i.e. the compressorwater is cooled by 5°C above the coolanttemperature.

If the water is cooled by thesurrounding air, the addition of an anti-freeze, e.g. glycol is required. The closedcooling water system is filled with pure,softened water.

When glycol is added the compressorsystem’s water flow must be recalculated,as the type and concentration of glycolaffects the thermal capacity and the visco-sity. The addition of chemical agents alsoaffects the cooling water’s capacity tocrawl through the connection points.

It is also important that the entire system iswell cleaned before being filled for the firsttime. A correctly implemented closedwater system requires very little supervi-sion and has low maintenance costs. Forinstallations where the available coolingwater is aggressive, it is appropriate to usea cooler designed using a corrosion retar-dant material such as incoloy.

84

3:19

3:18

The water must be protected from freezing atlow temperatures. It is important to

remember that the size of the cooler mayneed to be increased as, for example, a

water/glycol mixture has a lower thermalcapacity than pure water.

This is how a closed cooling system can be made up. The heat exchanger can be water or air cooled.


85

pressors. Energy costs can, in largerindustries, amount to 80% of the total costfor the production of compressed air. Asmuch as 94% of the compressor’s suppliedenergy can, for example, be recovered as90° hot water from large, oil-free screwcompressors. This means that each savingmeasure quickly gives noticeable divi-dends.

Presuppose that a compressorcentral in a large industry consumes 500kW during 8,000 operating hours perannum. This corresponds to no less thanapproximately 4 million kWh/year. Thepossibilities to recover this waste heat viahot air or hot water are good.

The return on the investment forenergy recovery is usually as short as 1–3years. In addition, energy recovered bymeans of a closed cooling system is advan-tageous to the compressor’s operating con-ditions, reliability and service life due to

3.4Energy recovery

3.4.1 General

When air is compressed heat is formed.The heat energy is concentrated in the dec-reasing volume and the excess is led offbefore the air goes out into the pipesystem. For each compressed air installa-tion you must assure yourself that there issufficient and reliable cooling capacity forthe installation. This can take place eitherby means of the outdoor air or a watersystem such as municipal water, streamwater or process water in an open or clo-sed system.On many installations producing compres-sed air there are significant and frequentlyunutilised energy saving possibilities inthe form of energy recovery from the com-

3:20

As heat is the natural by-product of compression, energy can be recovered in the form of hot water fromthe compressor’s cooling system.


an equal temperature level and high cool-ing water quality to name but a few. TheNordic countries are somewhat of a fore-

runner and energy recovery has for longtime been praxis when it comes to com-pressors.

86

3:22

Each compressor installation represents large possibilities for energy recovery. A whole 95% of the compres-sor’s supplied energy can, for example, be recovered from large, oil-free screw compressors.

3:21

The diagram illustrates some of the typical application areas for energy recovery from the compressor’scooling water in different temperature ranges. In the highest temperature levels the degree of recovery is

the greatest.


87

When it is a question of a large thermalflow it can be of interest to look at the pos-sibility of selling the recovered heat ener-gy. A purchaser can be the energy supplierand you can seek agreement forms forinvestment, suborder and delivery. Thereis also the possibility of co-ordinatingenergy recovery from several processes.

3.4.3 Recovery methods

3.4.3.1 General

Energy recovery from compressed airinstallations does not always give heatwhen it is required and perhaps not in suf-ficient quantities. The quantity of recov-

Most compressors from the major suppli-ers are today adapted to be supplementedwith standard equipment for recovery.

3.4.2 Calculation of the recovery potential

Virtually all energy supplied to a compres-sor installation is converted to heat. Themore energy you can recover and use inother processes, the higher the system’sefficiency. The quantity that can be recover-ed can easily be calculated by the relation:

Recovered energy kWh/year:

W = [ (K1 x Q1) + (K2 x Q2)] x TR

Saving/year: W x ep/ηSaved oil m3/year: W/68000 x η

W =Recovered energy Wh/year)TR =Time per year when there is a

need of recovered power hours/year)

K1 =Part of TR with loaded compressor

K2 =Part of TR with off-loaded compressor

Q1 =Available power in coolant with load compressor (kW)

Q2 =Available power in coolant withoff-loadedcompressor (kW)

ep =Energy priceη =Normal heat source´s efficiency

In many cases the degree of recovery canexceed 90%, if the energy you gainthrough cooling the compressor installa-tion can be taken care of efficiently. Thefunction of the cooling system, distance tothe point of consumption, and the degreeand continuity of the demand are all deci-sive factors.

3:23

Example of recovery potentialfrom compressors.


ered energy will vary if the compressorhas a variable load. In order for recoveryto be possible a corresponding energyrequirement is needed, which is normallymet through an ordinary system supply.Recovered energy is best utilised as addi-tional energy to the ordinary system, sothat the available energy is always utilisedwhen the compressor is running.

3.4.3.2 Air cooled systems

Options for air cooled compressors, whichgive off a large hot air flow rate at a rela-tively low temperature, are direct buildingheating or heat exchanging to a preheatingbattery. The heated cooling air is then dis-tributed using a fan.

When the buildings do not require additio-nal heat, the hot air is led off into theatmosphere either automatically using

88

3:24

Energy recovery from an air cooled compressor.

3:25

An example of water-borne energy recovery for an oil-free screw compressor.


89

rature up to 90° can supplement a hotwater flow. If hot water is used for wash-ing, cleaning or showering a normal hotwater boiler is still required. The energyrecovered from the compressed air systemprovides an addition that reduces the loadon the boiler, saves fuel and possibly canresult in the use of a smaller boiler.

Prerequisites for energy recovery fromcompressed air compressors differs partlydepending on the type of compressor. Oil-free compressors even in the standarddesign are easy to modify for energy reco-very. This type of compressor in a hotwater design reaches the water tempera-

thermostat control or manually by control-ling the air damper. A limiting factor isthat the distance between the compressorsand the building to be heated should beshort, preferably it should be a question ofan adjoining building. Furthermore, thepossibility of recovery is limited to thecolder parts of the year. Air-borne energyrecovery is more common on small andmedium sized compressors. Recoveryresults in small losses and requires littleinvestment.

3.4.3.3 Water cooled system

On a water cooled compressor, the coolingwater from the compressor with a tempe-

3:26

Example of water-borne energy recovery for oil lubricated screw compressor. The residue cooler withregulation system is integrated in the compressor.


tures (90°C) required for efficient energyrecovery. On oil lubricated compressorsthe oil, which takes part in compression, isthe factor that limits the possibilities toreach higher cooling water temperatures.

In centrifugal compressors the tem-perature levels are lower an thereby, so isthe degree of recovery. In addition the per-formance of the compressor is negativelyaffected by the increased water tempera-tures.

Water-borne energy recovery is bestsuited to compressors with an motorpower over 10 kW. Water-borne recoveryof energy brings about a more complexinstallation than air-borne energy reco-very. The basic equipment consists ofpumps, heat exchanger and regulation val-ves.

Heat can also be distributed to remotebuildings using relatively small pipedimensions (40-80 mm) without significantheat losses using water-borne energy reco-very. The high initial temperature means

that energy can be used to increase thetemperature of the return water from a hotwater boiler. Thereby the normal heatingsource can be periodically switched offand replaced by the compressor’s wasteheat. Waste heat from compressors in theprocess industry can also be used to in-crease the temperature of the process. It isfully possible even with the use of air cooled, oil lubricated screw compressors toarrange water-borne energy recovery. Thisrequires a heat exchanger in the oil circuit,but the system gives lower temperaturelevels than with an oil-free compressor.

3.5 The compressor room

3.5.1 General

Not so long a go the acquisition of a com-pressor meant that you needed to buy theelectric motor, starter equipment, aftercool-er, intake filters, etc. You then needed togo through capacity and quality demands

90

3:27

Installation in a compressor room is easier today than before. The compressor plant is now a turnkey solu-tion ready to install and connect to requisite auxiliary equipment.


91

requirements for future expansion andaccessibility for service. However, installa-tion in, e.g. a workshop or warehouse canfacilitate the installations for energy reco-very. If there are no facilities to install thecompressor indoors it can also be placedoutdoors under a roof. You must howeverbear in mind the risk of freezing in con-densation pockets and discharges, rain andsnow protection on the air intake, suctioninlet and ventilation, demands of a solidand flat foundation, e.g. asphalt, concreteslab or a flattened bed of shingle, the riskof dust, inflammable or aggressive sub-stances and unauthorised access protec-tion.

3.5.2 Placement and design

The compressed air central should be pla-ced to facilitate routing of the distributionsystem in large installations with longpiping. It can be advantageous for serviceand maintenance to place the compressedair central close to auxiliary equipmentsuch as pumps and fans; even a locationclose to the boiler room can be beneficial.

The building should have access to liftingequipment dimensioned to handle theheaviest components in the compressorinstallation, (usually the electric motor)and/or the possibility of using a fork lifttruck. It should also have floor space forthe installation of an extra compressorwith future expansion.

In addition, the clearance height must besufficient to allow the lift of an electricmotor or the like if the need arises. Thecompressed air central should have a floordrain or other facilities to handle conden-sation from the compressor, aftercooler, airreceiver, dryers, etc. The floor drain shall

with each supplier of the different compo-nents This was necessary to ensure thatthey would work well together with thecompressor. Nowadays, the compressorand accessories are purchased in as a turn-key solution. A compressor package consi-sts of a box frame, on which the compres-sor and accessories are mounted. All inter-nal connections between the different partsare already made. The complete compres-sor package is enclosed in a sound redu-cing hood to reduce noise levels.

This has resulted in a significant simplifi-cation of the installation and you can becompletely assured from the outset thatthe system will work. Irrespective of this,it is important to remember that the instal-lation method and technology still have asignificant influence on the compressorsystem’s performance and reliability.

The main rule for an installation isfirst and foremost to arrange a separatecompressor central. Experience says thatcentralisation is preferable, irrespective ofthe type of industry. This gives, amongothers, improved operating economy, abetter designed compressed air system,service and user friendliness, protectionagainst authorised access, good noise con-trol and simpler possibilities for controlledventilation.

Secondly, a demarcated area in abuilding used for other purposes can beused for the compressor installation. Therisk for other problems should be observedwith such an installation, for example,disturbances from noise, the compressor’sventilation requirements, physical risksand/or the risk of overheating, drainagefor condensation, hazardous surroundingse.g. dust or inflammable substances,aggressive substances in the air, space


be implemented in accordance with muni-cipal directives.

3.5.3 Foundation

Normally only a flat floor of sufficientbearing capacity is required to set-up thecompressor plant. In most cases vibrationdampening is integrated in the plant. It isusual with new installations to cast a plin-th for each compressor package to allowthe floor to be cleaned.

Large piston and centrifugal compressorscan require a concrete slab foundation,which is anchored to the bedrock or on asolid soil base. The effects of externallyproduced vibration has been reduced to aminimum on advanced, complete com-pressor plants. In systems with centrifugalcompressors it may be necessary to vibra-tion dampen the compressor room’s foun-dation.

3.5.4 Intake air

The compressor’s intake air must be cleanand free from solid and gaseous contami-

nation. Particles of dirt that cause wearand corrosive gases can be particularlydamaging.

The compressor’s air intake normallytakes place via the sound reducing hood,but can also be placed where the air is asclean as possible. Gas contamination suchas vehicle exhaust fumes, can be fatal ifmixed in air to be breathed. For example,hospital applications usually make specialdemands on the placement of the air in-take. A prefilter (cyclone, panel or rotaryband filter) should be used on installationswhere the surrounding air has a high dustconcentration. In such cases the pressuredrop caused by the prefilter must be ob-served, so that it does not exceed the maxi-mum limits prescribed by the manufac-turer.

It is also beneficial for the intake air to becold. It can therefore be appropriate toroute this via a separate pipe from the out-side of the building to the compressor.

It is important that corrosion resi-stant pipes, fitted with mesh over the inlet

92

3:28

It is important that the compressor installation has a design that is service friendly and flexible to accom-modate future expansion. The minimum area at service points in front of the machine’s electrical cabinets

should be 1200 mm.


93

to 10% of the energy consumed by theelectric motor. The heat must be removedto maintain the temperature in the com-pressor room at an acceptable level. Thecompressor manufacturer should providedetailed information regarding the requi-site ventilation, but it can also becalculated according to the following:

Pv = requisite quantity of ventilationair (m3/s)

Qv = heat flow (kW)∆T = permitted temperature rise (°C)

A better way to deal with the problem is torecover the energy and use it in the enter-prise.

Ventilation air should be taken from out-doors, preferably without long ducting.The inlet for the ventilation air should beplaced on a north facing wall if possible, or

and designed so that there is no risk ofdrawing in snow or rain into the compres-sor, are used for this purpose. It is alsoimportant to use pipes of a sufficient largedimension to gain as low a pressure dropas possible.

The design of the inlet pipes onpiston compressors is particularly critical.Pipe resonance caused by the compressor’scyclic pulsating frequency, can damage thecompressor, cause vibration and affect thesurroundings through low frequencynoise.

3.5.5 Compressor room ventilation

Heat in the compressor room is generatedfrom all compressors. This heat is led offthrough ventilating the compressor room.The quantity of ventilation air is deter-mined by the size of the compressor andwhether it is air or water cooled.

The ventilation air with air cooled com-pressors contains close to 100% of the ener-gy consumed by the electric motor in theform of heat. The ventilation air withwater cooled compressors contains down

3:29

3:30

This is how a basic ventilation solution can bedesigned. The disadvantage is that ventilationis constant irrespective of the outer temperatu-

re. In addition difficulties can occur if twocompressors are installed. The fans will be overspecified if only one of the compressors is used.The problem can be solved by fitting the fans

with speed controlled motors, which start via amulti-stage thermostat.

This is how a system with several thermostatcontrolled fans, which together can handle thetotal ventilation requirement, can be designed.The thermostats on the individual fans are setfor different ranges, which means the quantityof ventilation air can vary depending on the

outer temperature and/or the number of com-pressors in use (as the thermostats will switchon the fans one after another depending on the

temperature in the compressor room).Alternatively, the fans can be started via a

multi-stage thermostat.

1,25 x ∆ TPv =

QV


in another place in the shade so that theintake air is as cool as possible during thesummer. A grille should be fitted to theoutside of the intake and an air streamoperated damper on the inside to preventforeign objects from entering and colddraughts.

Furthermore, the intake should beplaced as low as possible, yet avoiding therisk of being covered with snow duringthe winter. Even the possible risk of dustand explosive or corrosive substancesentering the compressor room must betaken into consideration.

The ventilation fan/fans should beplaced high up on one of the compressorroom’s end walls, while the air intake isplaced on the opposite wall. The air velo-city at the intake should not exceed 4 m/s.

Thermostat controlled fans are themost appropriate. These must be dimen-sioned to handle the pressure drop in theducting, outer wall grille, air stream opera-ted damper, etc. The quantity of ventila-tion air must be sufficient to limit the tem-perature increase in the room to 7–10°C.The possibility in using water cooled com-pressors should be considered, if there is aproblem in arranging sufficient ventilationin the room.

94

3.31

Examples of different ventilation solutions


95

3:32

A small compressor installation in practice. The pipe for draining condensation must not open out underthe water surface in the floor drain. An oil separator must be installed, if there is a risk that the waste

water contains oil.

Example of a hospital installation with closed supply on the suction side and 100% reserve system (doubleindependent system).

3:33


3.6 The compressed airnetwork’s structure

3.6.1 General

Three demands are placed on a distribu-tion system to provide reliable operationsand good economy: a low pressure dropbetween the compressor and point of con-sumption, a minimum of leakage, and thebest possible condensation separation inthe system if a compressed air dryer is notinstalled.

This primarily applies to the main pipes.The cost of installing larger pipe dimen-sions as well as fittings than that initiallydemanded is low compared with the costof rebuilding the system at a later date.The air line network’s routing, design anddimensioning are important for the effi-ciency of the installation, reliability and cost. Sometimes a large pressure drop inthe pipeline is compensated by increasingthe working pressure of the compressorfrom, e.g. 7 bar(e) to 8 bar(e). This givesinferior compressed air economy. Whenthe compressed air consumption falls, thepressure drop also falls and the pressure atthe point of consumption rises above thepermitted level.

Fixed compressed air installationsshould be dimensioned so that the pres-sure drop in the pipes does not exceed 0.1bar between the compressor and the furt-hest point of consumption. Added to thisis the pressure drop in hoses, hose coup-lings and other fittings. It is particularlyimportant how these components aredimensioned, as the greatest pressure dropfrequently occurs at such connections.96

The longest permitted length in the pipenetwork for a specific pressure drop can becalculated from the following empiricalrelation:

l = overall pipe length (m)∆p = largest permitted pressure drop

in the network (bar)p = absolute inlet pressure (bar)Qc = flow, FAD (l/s)d = internal pipe dimension (mm)

In general the best solution is to design apipe system as a ring line around the areawhere air consumption will take place.Branch pipes are then taken off of the mainpipeline to the points of consumption. Thisgives an even compressed air supply,despite heavy intermittent usage as the airis led to the actual point of consumptionfrom two directions.

This system should be used for all installa-tions, even if some points of consumptionare at a great distance from the compressorinstallation. A separate main pipe is thenrouted to these areas.

3.6.1.1 Air receiver

One or more air receivers are included ineach compressor installation. The size isadapted, e.g. according to the compressorcapacity, regulation system and the consu-mer’s air requirement. The air receiverforms a storage area for the compressedair, balances pulsation from the compres-sor and cools the air and collects condensa-tion. Accordingly, the air receiver must befitted with a drainage device.

450 x Qc1,85

∆ p x d5 · pl =


97

V = air receiver’s volume (m3)Q = capacity of the largest

compressor (m3/min)∆p = desired pressure difference (bar)

When air is required in large quantitiesduring short periods it is not economic todimension the compressor or pipenetwork according to this. A separate airreceiver is then placed close to the consu-mer and is dimensioned according to themaximum air output.

In more extreme cases, a smaller highpressure compressor is used together witha large receiver to meet short-term, largeair requirements between long intervals.The compressor will then be dimensionedfor the mean consumption. The followingrelation applies for such a receiver:

V = air receiver’s volume (l)Q = air flow during the emptying

phase (l/s)t = length of the emptying phase (s)P1 = normal working pressure in the

network (bar)P2 = minimum pressure for the con

sumer’s function (bar)L = filling phase’s air requirement

(1/work cycle)

The formula does not take into considera-tion the fact that the compressor can supp-ly air during the emptying phase. A com-mon application is the start of large shipengines, where the receiver’s filling pres-sure is 30 bar.

The following relation applies whendimensioning the receiver’s volume. Notethat the relation only applies for compres-sors with offloading/loading regulation.

V =

V = air receiver’s volume (l)Qc= Compressor’s capacity

(l/s) FADp1 = Compressor’s intake pressure

(bar(a))T1 = Compressor’s maximum intake

temperature (K)T0 = Compressed air temperature in

receiver (K)(pu-pL) = set pressure difference bet

ween the loaded and off-loaded

fmax = maximum frequency = 1 cycle/30 seconds (applies to Atlas Copco compressors)

Simplified formula that applies with anambient relation 1 bar(a) and approx. 20˚Cand 30 s cycle time.

0,25 x Qc x p1 x T0

fmax x (pU - pL) x T1

V =Q

8 x ∆p

Q x tp1 - p2

V = Lp1 - p2

3:34

The air receiver is always dimensionedaccording to the capacity of the largest compressor when a system comprises of

several compressors.

=


3.6.2 Design of the compressed air network

In smaller installations the same pipe canserve as the riser and distribution pipe.The starting point when designing anddimensioning a compressed air network isan equipment list with all the compressedair consumers, and a drawing indicatingtheir placement. The consumers are group-ed in logical units and are supplied via thesame distribution pipe. The distributionpipe is fed in turn by risers from the compressor central. A larger compressed 98

air network can be divided into four mainparts: Risers, distribution pipes, servicepipes and compressed air fittings. Therisers transport the compressed air fromthe compressor central to the consumptionarea.

Distribution pipes divide the air across thedistribution area. Service pipes feed the airfrom the distribution pipes to the workpla-ces. The compressed air fittings are theconnections between the service pipe andthe compressed air consumer.

3:35


99

that for a straight pipe can be calculatedwith the relation:

∆p = 450

∆p = pressure drop (bar)qv = air flow, free air (l/s)d = internal pipe diameter (mm)l = length of the pipe bar(a)p = absolute initial pressure

3.6.3 Dimensioning the compres-sed air network

The pressure obtained immediately afterthe compressor can generally never be uti-lised fully, accordingly, you must calculatethat the distribution of compressed airclaims some losses, primarily friction los-ses in the pipes. In addition, throttling andchanges in the direction of flow occur invalves and pipe bends. Losses, which areconverted to heat, result in a pressure drop

3:36

x d5 x pqv

1,85 x l

Some fittings and their influence on losses in pipes of a different diameter. The losses are recalculated to acorresponding increase in the length of the pipe network (m).


When calculating different parts of thecompressed air network the followingvalues can be used for the permitted pres-sure drop:

Pressure drop across

service pipes 0.03 bar

Pressure drop across

distribution pipes 0.05 bar

Pressure drop across risers 0.02 bar

Total pressure drop across

the fixed pipe installation 0.10 bar

The requisite pipe lengths for the differentparts of the network (risers, distributionand service pipes) are estimated. A scale

100

drawing of the probable network plan is asuitable basis. The length of the pipe iscorrected through the addition of equiva-lent pipe lengths for valves, pipe bends,unions, etc as set out in figure 3:36. Whencalculating the pipe diameter a nomogram,as set out in figure 3:37, can be used to findthe most appropriate pipe diameter as analternative to the formula (page 99). Theflow, pressure, permitted pressure dropand the pipe length must be known tomake a calculation. Standard pipe of theclosest, greater diameter is then selectedfor the installation.

The equivalent pipe lengths for all

3:37


101

3.7 Portable

compressors

3.7.1 General

Today, virtually all portable compressorsconsist of a diesel engine powered, oil in-jected, screw compressor. Oil-free com-pressors only occur, for example, with ser-vice work in the process industry.

3.7.2 Noise and gaseous emissions

Modern designs of diesel powered com-pressors have a very low noise levelaccording to applicable EU standards (ISO84/536/EC) and can therefore be used inpopulated areas and close to hospitals, etc.

During the passed years fuel econo-my has been improved dramaticallythrough efficient screw elements and moreeffective diesel engines. This is especiallyvaluable, e.g. for well drilling, where thecompressor works intensively under along period. At the current time, there areengines with exhaust gas emissions thatcomply with the stringent demands set outin EURO-1. Contractors carrying out workin large towns must today (1998) usemachinery that complies with this stan-dard.

the parts of the installation are calculatedby using a list of fittings and pipe compo-nents as well as the flow resistance expres-sed in pipe length. These “extra” pipelengths are added to the starting pipe leng-th. The network’s selected dimensions arethen recalculated to ensure that the pressu-re drop will not be too great. The individu-al sections (service pipe, distribution pipeand risers) should be calculated separatelyon large installations.

3.6.4 Flow measurement

Strategically placed flow meters permitinternal debiting and economic allocationof compressed air utilisation within thecompany. Compressed air is a productionmedia that should be a part productioncosts for individual departments withinthe company. With such a viewpoint itbecomes interesting for all concerned totry to reduce consumption within the dif-ferent departments.

The modern flow meters available onthe market can give everything fromnumerical values for manual reading, tomeasurement data directly to a computeror debiting module.

The flow meters are generally moun-ted close to shutoff valves. Ring measure-ment makes particular demands as themeter needs to be able to measure bothforwards and backwards.

Portable compressors are chiefly available in three different pressure ranges.

3:38

Pressure range Pressure (bar) Application area

Low 7 Contract work

Medium 10 - 12 Blasting, ground work

High 20 Water and energy welldrilling, geotechnicalinvestigations

>

<


3.7.3 Pressure range

Modern portable compressors have a goodoverall economy through high operatingreliability, good service characteristics,compact dimensions and a low totalweight. They have a chassis normallydesigned for a transport speed of 30km/hor 80km/h. As for stationary compressorsthere is auxiliary equipment such as after-coolers, different filter packs (dust filters,carbon, etc.), after heaters and lubricatingoil systems available. They can also beequipped with cold start equipment and agenerator 230V/400V. There are portable,diesel powered generators built in a simi-lar way to portable compressors for grea-ter power requirements. The power classesstart from 10 kVA and upwards.

3.8 Electrical installation

3.8.1 General

To dimension and install a compressorinstallation requires knowledge on howcomponent parts affect each other andwhich regulations and provisions apply.

Here follows an overview of theparameters that should be especially consi-

102

dered to obtain a compressor installationthat functions satisfactorily with regard tothe electrical installation.

3.8.2 Motors

Short-circuited, three phase inductionmotors are used for compressor opera-tions. Low voltage motors are generallyused up to 450 kW and there above highvoltage is the best option.

The motor’s protection class is regu-lated by standards. The dust and waterspray resistant design (IP54) is preferred toopen motors (IP23), which require regulardismantling and cleaning. In other cases,dust deposits in the machine will eventu-ally cause overheating, resulting in a redu-ced service life.

The motor, usually fan cooled, is intendedto work at a maximum ambient temperatu-re of 40˚C. At high temperatures the outputmust be reduced. The motor is normallyflange mounted and directly connected tothe compressor. The speed is adapted tothe compressor, however, in practice 2 poleor 4 pole motors with a speed of 3000 rpmrespective 1500 rpm are solely used.

The rated output of the motor is also deter-mined by the compressor and should be as

3:39

Example of a portable compressor


103

The star/delta–start is used to limit thestarting current. The starter consists ofthree contactors, overload protection and atimer. The motor is started with the starconnection and after a set time (when thespeed has reached 90% of the rated speed)the timer switches the contactors so thatthe motor is delta connected, which is theoperating mode. See 1.6.5.7.

The star/delta–start reduces the startingcurrent to approximately 1/3 comparedwith a direct start, however, the startingtorque falls at the same time to a quarter.The relatively low starting torque meansthe motor’s load should be low during thestarting phase, so that the motor virtuallyreaches its rated speed before switching tothe delta connection. If the speed is toolow, a current/torque peak as great aswith direct start, will occur duringswitching to the delta connection.

Gradual start, which can be an alter-native start method to star/delta–start is astarter built up of semiconductors (thy-ristors) instead of mechanical contactors.The thyristors are controlled according to atime ramp, so that an equal rising currentfeeds the motor. The start is gradual andthe starting current is limited to approx.three times the rated current.

The starters for direct start and star/delta-start are in most cases integrated in thecompressor. It can be motivated with largecompressor plant to place the units sepa-rately in the switchgear, due to spacerequirements, heat developmentand accessfor service.

A starter for gradual start is usuallyset-up separately next to the compressor.High voltage fed compressors always havetheir start equipment in the switchgear.

close to the compressor’s requirement aspossible.

A too large motor is more expensiveto buy, requires an unnecessarily highstarting current, requires larger fuses, haslow output factor and somewhat inferiorefficiency. A too small motor becomesoverloaded and consequently a risk forbreakdown.

The starting method should also beincluded as a parameter when selecting amotor. The motor is only started with aquarter of its rated torque with star/-delta–start, which is why a comparisonbetween the motor’s and the compressor’storque curves can be justified to ensurethat the compressor starts correctly. See 3.8.3.

3.8.3 Starting methods

The most common starting methods aredirect start, star/delta–start and gradualstart. Direct start is easy and only requiresa contactor and overload protection. Thedisadvantage is the high starting current,6–10 times the motor’s rated current andsometimes the high starting torque, whichcan, for example, damage shafts andcouplings.

3:40

Connection current/time at different starting methods


3.8.4 Control voltage

Normally no separate control voltage isconnected to the compressor, as most com-pressors are fitted with an integrated con-trol transformer. The transformer’s prima-ry side is connected to the compressor’spower supply. This arrangement givesmore reliable operations. In the event ofdisturbances in the power supply the com-pressor will be stopped immediately andblocked for restart.

This function, with one internally fedcontrol voltage, should be copied in thosecases where the starter is placed awayfrom the compressor.

3.8.5 Short-circuit protection

Short-circuit protection, which is placed onone of the cables’ starting points, can bemade up of fuses or a circuit-breaker.Irrespective of which of the solutions youselect it will give, if correctly matched,good protection.

Both methods have advantages and disad-vantages. Fuses are well-known and workbetter than a circuit-breaker with largeshort-circuit currents, but they do notmake a fully isolating break and have along tripping time with small fault cur-rents. A circuit-breaker breaks fully isola-ting and rapidly even with small fault cur-rents, but demands more work during theplanning stage compared to fuses.Dimensioning of the short-circuit protec-tion is based on the expected load, but alsoon the limitations of the starter unit.

With regard to the starter’s short-circuitprotection see the standard according toIEC (International Electrotechnical Com-mission) 947-4-1 Type 1 & Type 2. How ashort-circuit will affect the starter is deter-104

mined by which of the options, Type 1 orType 2, is selected.

Type 1: “damage to contactors and overload relays

can occur. The replacement of components may be

necessary”.

Type 2: “Damage does occur to the overload relays.

Light welding of the contactors is permitted. It shall

be possible using basic measures to reset the starter

in the operating mode.”

3.8.6 Cables

Cables shall, according to the provisions“be dimensioned so that during normaloperations they do not accept hazardoustemperatures and that they shall not bedamaged thermally or mechanically with ashort-circuit”. The dimensioning andselection of cables is based on the load,permitted voltage drop, routing method(on a rack, on a wall, etc.) and the ambienttemperature. Fuses can be used, forexample, to protect the cables and canmake up both a short-circuit protectionand an overload protection. For motoroperations a short-circuit protection isused (for example, fuses) and a separateoverload protection (usually the motorprotection included in the starter).

The overload protection protects themotor and motor cables by tripping andbreaking the starter, when the load currentexceeds the pre-set value. The short-circuitprotection protects the starter, overloadprotection and the cables. How cables aredimensioned taking the load into conside-ration is set out in IEC 364 5 523 (SS4241424).

There is a further parameter to bearin mind when dimensioning cables andthe short-circuit protection, namely the“tripping condition”. This conditionmeans the installation shall be designed so,


105

phase compensation can be that the powersupplier may charge for drawing reactivepower over a predetermined level and thatheavily loaded transformers and cablesneed to be off-loaded.

3.9 Sound

3.9.1 General

Noise is an energy form that propagates ina room as longitudinal waves through theair, which is an elastic medium. The wavemovement causes changes in pressure,which can be registered by a pressure sen-sitive instrument, for example a micro-phone. The microphone is therefore one ofthe main parts in all equipment for soundmeasurement.

To measure the sound power in the SI unitWatt is difficult, due to the range coveredby the sounds that surround us. Withinacoustics they speak of levels instead, andmeasure sound in relation to a referencepoint. Measurement becomes manageableby apply a logarithm to the relation. Theformula is:

LW = 10 x log10 x W/W0

LW = sound power level (dB)W = actual sound power (W)W0 = reference power,

usually 10–12 (W)

a short-circuit anywhere in the installationshall result in quick and safe breaking.Whether the condition is met is deter-mined by, among others, the short-circuitprotection, the length and cross-section ofthe cable.

3.8.7 Phase compensation

The electric motor does not only consumeactive power, which can be converted tomechanical work, but also reactive power,which is needed for the motor’s magneti-sation. The reactive power loads the cablesand transformer. The relation between theactive and reactive power is determinedby the power factor, cosϕ. This usually isbetween 0.7 and 0.9 where the lower valuerefers to small motors.

The power factor can be raised to virtually1 by generating the reactive power directlyby the machine using a capacitor. Thisreduces the need of drawing the reactivepower from the mains. The motive for

3:42

Reactive power Qc is supplied to increasethe motor’s power factor (cosϕ) to close to 1.

3:41

A basic system for the connection of an electric motor to the mains supply.

ϕ


3.9.2 Sound pressure

The sound pressure level is a measurementof the sound’s intensity. The relation is:

Lp = 20 x log10 x p/p0

Lp = sound pressure level (dB)p = actual sound pressure (bar)po = reference sound pressure,

usually 0.0002 x 10-6 (bar)

The sound pressure level always refers to aspecific distance to the power source, e.g. amachine. For a stationary compressor thedistance is 1 metre and for a portable com-pressor the distance is 7 metres (accordingto CAGI Pneurop).

Information about the sound pres-sure level must always be supplementedwith a room constant for the room wherethe measurement was made. Otherwisethe room is assumed to be limitless, i.e. anopen field. In a limitless room there are nowalls that can reflect the sound waves,which would affect the measurement.

3.9.3 Absorption

When sound waves come into contact witha surface, some of the waves are reflectedand some absorbed into the material ofwhich it consists. The sound pressure at acertain moment therefore always consistspartly of a sound that the sound sourcegenerates, partly of sound that is reflectedfrom surrounding surfaces (after one ormore reflections).

How effectively a surface can absorbsound depends on the material it is madeup of and is usually stated as an absorp-tion factor (between 0 and 1).

3.9.4 Room constant

A room constant is calculated for a roomwith several surfaces, walls and other sur-

106

K=A x α1 - α

α =A1 x α1+A2 x α2+

A1+A2+total absorption

total area=

faces, which depends on the different sur-faces’ absorption characteristics. The rela-tion is:

K = room constantα = average absorption factor

for the room (m2)A = total room area (m2)

A1, A2, etc. are the parts of the room surface that have absorption factors α1, α2, etc.

3.9.5 Reverberation

The reverberation time is defined as thetime it takes for the average sound pres-sure to decrease by 60 dB once the soundsource has become silent. The average orequivalent absorption factor for the roomis calculated as:

V= volume of the room (m3)T = reverberation time (s)

The room constant is obtained if thisexpression in put in relation to:

A = total room area (m2)

3.9.6 Relation between soundpower and sound pressure

If sound is sent out from a point soundsource in a room without reflecting surfa-ces, the sound is distributed equally in all

α = 0,163 x VT

K = A x α1 - α


107

Lp = Lw + 10 log

If this relation was drawn as a serie of cur-ves it would show that in the proximity ofthe power source the sound pressure leveldrops by 6 dB for each doubling of thedistance. However, at greater distancesfrom the power source the sound powerlevel is dominated by the reflected soundand thereby there is no decrease at all withincreased distance.

The machines, which transmit soundthrough their bodies or frames, do notbehave as point sources if the listener is ata distance greater than 2–3 times the mach-ine’s greatest dimension from its centre.

3.9.7 Sound measurements

The human ear distinguishes sound at dif-ferent frequencies with different clarity.Low or very high frequency sounds mustbe stronger than sounds around 1000–2000Hz to be perceived as equally strong.

directions and the measured intensity willtherefore be the same at all points at thesame distance from the sound source.Accordingly, the intensity is constant at allpoints on a spherical surface around thesound source.

From this you can derive that thesound level falls by 6dB for each doublingof the distance to the sound source.However, this does not apply if the roomhas hard, reflective walls. You must thentake the sound reflected by the walls intoconsideration. If you then introduce adirection factor the relation becomes:

Lp = LW + 10log

Lp = sound pressure level (dB)Lw= sound power level (dB)Q = direction factor (m2)r = distance to the sound source

For Q the empirical values apply (for otherpositions of the sound source the value ofQ must be estimated):

If the sound source is placed in a roomwhere its border surfaces do not absorb allthe sound, the sound pressure level willincrease due to the reverberation effect.This addition is inversed proportionally tothe room constant:

Q4πr2

Q4πr2

+ 4K

3:43

This is how the frequency curves appear forthe different filters used to weight sound

levels when measuring sound.

Q = 2

Q = 8

Q = 4

if the sound source is suspended in the middle of a large room.

if the sound source is placed on a hard, reflective floor, close to the center of a wall or ceiling.

if the sound source is placed close to the transition wall-floor or wall-ceiling.

if the sound source is placed in a corner close to the intersectionof three surfaces.

Q = 1


same levels shall be added the increase is10 dB.

Background sound is a special case. It istreated as a separate sound source and thevalue is deducted from the total of theother sound sources in order to give thesespecial treatment.

3.9.9 Sound reduction

There are five different ways to reducesound. Sound insulation, sound absorp-tion, vibration insulation, vibration dam-pening and dampening of the sound sour-ce. Sound insulation involves an acousticbarrier being placed between the soundsource and the receiver.

This means that only a part of thesound can be insulated, depending on the

Different filters that adjust the measuredlevels at low and high frequencies are usedto emulate the human ear’s ability to hearsounds. When measuring noise an A-filteris usually used and the sound is measuredas dB(A).

3.9.8 Interaction of several sound sources

When there is more than one sound sourcein a room the sound pressure increases.However, as the sound pressure level is

defined logarithmically you can not addthe sound pressure levels algebraically.When more than two sound sources areactive, you start by adding two and there-after the next is added to the sum of thefirst and so on. As a mnemonic rule, whentwo sound sources with the same levelsshall be added the result is an increase by3 dB. When ten sound sources with the108

3:44

The nomogram defines how many dB shall beadded to the most powerful sound source whenthe power of two sound sources shall be added.

3:45

The nomogram defines how many dB shallbe deducted from the total sound level at dif-ferent background sound levels in order to

estimate the total noise level.


109

the presence of other equipment (and itspossible noise level) in the room are all sig-nificant.

Furthermore, the positioning of thecompressor in the room also affects thenoise level, precisely as the set-up and con-nection of pipes and the like. Sound radia-ting from compressed air pipes are fre-quently a more problematic noise sourcethan the noise from the compressor and itspower source. It can be a question of vibra-tion transferred mechanically to the pipe,often in combination with vibration trans-ferred through the compressed air. It istherefore important to fit vibration insula-tors and even enclose part of or the entirepipe system with a combination of soundreducing material and a sealed barrier.

3.10 Standards,

laws and provisions

3.10.1 General

In the compressed air sector, as in manyother sectors, there are regulations thatapply. It can be demands that are definedin laws and provisions as well as optionalregulations, as in national and interna-tional standards. Sometimes the regula-tions in standards are also binding, forexample, when they come into forcethrough legislation. If a standard is quotedin an agreement it can thereby also bemade binding.

Binding regulations can apply, forexample, to safety for people and propertywhile optional standards are used to facili-tate activities such as work with specifica-tions, selection of quality, measurements,manufacturing drawings etc.

area of the barrier and the insulation cha-racteristics. A heavier barrier is more effec-tive than a lighter barrier.

Sound absorption involves the soundsource being surrounded by light, porousabsorbents attached to a barrier. Thickerabsorbents are more effective than thinnerabsorbents and typical densities areapprox. 30 kg/m3 for polyurethane foamrespective approx. 150 kg/m3 for mineralwool. Vibration insulation is used to pre-vent the transfer of vibrations from onepart of a structure to another. A commonproblem is the transfer of vibrations from abuilt-in machine to the surrounding bar-rier or down to the floor. Steel springs,cork, plastic, and rubber and examples ofmaterial used for vibration insulation. Thechoice of material and dimensioning isdetermined by the frequency of thevibration and demands of stability on themachine set-up.

Vibration dampening involves astructure being fitted with a dampeningexternal surface of an elastic material witha high hysteresis factor. When the damp-ening surface is sufficiently thick, a wall,for example, is effectively prevented fromvibrating and thus starting to emit sound.Dampening of a sound source gives smallresults, yet a good exchange in relation tothe cost. In this way a reduction in themachine’s total sound level by approx. 5dB can be achieved, while integration canmean a reduction by approx. 15–25 dB.

3.9.10 Noise with compressor installations

A compressor’s noise level is measured ona machine in free field. When it is installedin a room the noise level is affected by theproperties of the room. The size of theroom, material in the walls and ceiling as


3.10.2 Standards

Standards are quoted in many cases bylegislators as a way of creating the desiredlevel of safety. You can be considered ascomplying with the legislation’s differentdemands if you follow the detailed directi-ves given by the standards with regard todesign, equipment and testing. Standardsare useful to the manufacturer and theconsumer. They increase interchangeabili-ty between components from differentmanufacturers and the possibility of com-parison under the same conditions.

Standards are produced and issuednationally as well as on European andinternational levels. International stan-dards, ISO or EN usually come into forceas national standards (SIS in Sweden).

It is the standardization body ISO (Interna-tional Organization for Standardization)with its base in Geneva respective CEN(Commission Européenne pour laStandardization), that organise the interna-tional work.

SIS (Standardiseringskommissionen iSverige) is the Swedish party in this workand in addition manages national standar-disation in Sweden with the help of anumber of specialised standardisationbodies. You can purchase all standards,national as well as international from SIS.

Besides official standards there arealso documents produced by trade bodiessuch as PNEUROP an association ofEuropean manufacturer’s of compressedair equipment. An example of such a docu-ment is the measurement standards for,e.g. compressor capacity, oil content in thecompressed air, etc. which are issuedwhile awaiting a standard to be drawn up.

110


Chapter 4 Economy


4.1 Economy

4.1.1 Costs for compressed airproduction

4.1.1.1 General

Electrical energy is the dominant energytype with virtually all industrial compres-sed air production. In many compressedair installations there are often significantand unutilised energy-saving possibilitiesthrough, e.g. energy recovery, pressurelowering, leakage reduction and by opti-mising operations through the choice ofthe control and regulation system.

It is profitable to look to the future as far aspossible and try to assess the affects ofnew situations and demands that mightapply to the installation when planning anew investment. Typical examples areenvironmental demands, energy savingdemands, increased quality requirementsfrom production and future productioninvestments.

Optimised compressor operations

are becoming more important, especiallyfor larger, compressed air dependentindustries. Production changes over timein a developing industry and thereby theconditions for compressor operations. It istherefore important that the compressedair supply is based both on the actualrequirement and on plans for the future.Experience shows that an extensive andunbiased analysis of the operating situa-tion results, on nearly every occasion, inimproved overall economy.

Energy costs are clearly the dominatingfactor for the installation’s overall econo-my. It is therefore important to concentrateon finding solutions that comply withdemands of performance and quality aswell as demands on efficient energy utili-sation. The added cost involved withacquiring compressors and other equip-ment that comply with both of thesedemands will been seen in time as a goodinvestment.

As energy consumption often repre-sents approx. 80% of the overall cost youshould exercise care when selecting theregulation system. The difference in regu-lation systems overshadows the difference

112

4:1

When analysing differentexpenses for the production ofcompressed air you will find abreakdown similar to the onein the diagram. However, theapportionment between diffe-

rent types of costs change withthe number of operating

hours/year, the equipmentincluded in the calculation, thetypes of machine and the selec-

ted cooling system, etc.


culation’s interest rate. The size of energycosts are linked to the operating time peryear, degree of utilisation and the energy price, etc. Some acquisition costs, forexample, equipment for energy recoverygive a direct pay off in the form of reducedoperating and maintenance costs.

4.2 Opportunities

for saving

4.2.1 Power requirement

When performing calculations it is impor-tant to bear in mind the overall powerrequirement. All energy consumers thatbelong to machines, should be observed,for example, intake filters, fans andpumps.

With comparisons between differentinvestment options particular importancemust be placed on the use of comparablevalues. Therefore you must be assured thatthe values are stated in accordance with

between types of compressor. The idealsituation is when the compressor’s fullcapacity is adapted exactly to balancedconsumption, something frequently ap-plied in process applications. Most typesof compressors are supplied with theirown control and regulation system, but theaddition of equipment for co-control withother compressors in the installation canfurther improve the operating economy.

Speed regulation is becoming apopular regulation method, due to thepower requirement being virtually propor-tional to the speed, drawn capacity. Tothink carefully and allow the requirementgovern the selection of regulation equip-ment gives good results.

If a small amount of compressed air isrequired during the night and weekends, itcan be profitable to install a small com-pressor adapted to this requirement. If, forsome reason, you need another workingpressure, the requirement should be analy-sed to discover whether the entire produc-tion can take place from a compressorcentre or whether the network should bedivided up for different pressure levels.Sectioning of the compressed air networkcan also come into question, in order toshutdown certain sections during the nightand at weekends, to reduce air consump-tion or when you wish to apportion costsinternally based on flow measurements.

4.1.1.2 Apportioning costs

Investment costs are a fixed cost made upof the purchasing price, building costs,installation and insurance. The cost of theinvestment as a part of the overall cost isconnected partly with the selection of thequality of the compressed air and partlywith the amortisation period and the cal- 113

4:2

This is how costs can be divided between 3 com-pressors and auxiliary equipment. The large dif-ferences can be due to how the included machi-

nes are valued, that the capital value for an oldermachine may be higher than for a newly purcha-sed machine, that the selected level of safety can

affect the accounted maintenance costs, etc.


internationally agreed regulations, for ex-ample, as set out in ISO 1217. Ed3, supple-ment C-1996.

4.2.2 Working pressure

The working pressure directly affects thepower requirement. High pressure meanshigher energy consumption. To increasethe working pressure to compensate forpressure drop always results in impairedoperating economy.

Despite this it is a common method,for example, with pressure drop caused bya too small pipe system or blocked filters.As an example , an increase in the workingpressure by 1 bar, brings about an increasein the power requirement by approx. 6% .In an installation with several filters, espe-cially if they have been operational for along period of time without being repla-ced, the pressure drop can be significantlyhigher and therefore very costly.

114

4:3

The figure shows a simple, but extremelyuseful model that can be used to give a truepicture of a compressor’s electrical energy

requirement.

4:4

The relation between pressure and electrical energy consumption. For a compressor of 15 m3/min a reduction of the working pressure from 7 to 6 bar means that the power requirement decreases from

91 to 85 kW. With a running time of 4000 hours/year this represents 24,000 kWh/year.


In many installations it is not possible toimplement large pressure reductions, butusing modern regulation equipment it isfrequently fully realistic to lower the pres-sure by 0.5 bar. This means a saving ofperhaps a few per cent, which may seemto be low, but if you consider that the totalefficiency of the installation is increased toan equivalent degree it becomes more evi-dent what the pressure reduction means.

4.2.3 Air consumption

By analysing routines and the use of com-pressed air, you can find solutions thatgive a more equal load on the compressed air system. The need of increased air pro-duction can thereby be kept low, whichreduces operating costs.

Unprofitable consumption, whichusually depends on leakage, worn equip-ment, processes that have not been adap-

115

4:5

This is how the pressure drop across different components in the network affects the requisite working pressure.

4:6

The diagram shows how consumption can be spread during a week and 24 hours per day. Consumption islow during the night shift, high during the day shift, it drops during breaks, but is constant during the

weekends (leakage or production).


ted or the incorrect use of compressed airis best rectified by increasing general awa-reness. Dividing the compressed airsystem into sections, that can be separatedusing valves, can be a method of reducingconsumption during the night and at week-ends. In most installations there is someleakage, which is a pure loss that must the-refore be minimised. Frequently leakageclaims 10-15% of the produced compres-sed air sometimes even more. Leakage isproportional to the working pressure,which is why one method of reducing

leakage is to repair leaking equipment andlower the working pressure, e.g. at night.

A lowering of the pressure by only0.3 bar reduces leakage by 4%. If the leak-age in an installation of 100 m3/min is 12%and the pressure is reduced by 0.3 bar, thisrepresents a saving of approx. 3 kWh/hour, which is equivalent to the electricityconsumption in a normal electrically hea-ted home. Even the air consumption formachines and equipment increases with anincreased working pressure.

4.2.4 Regulation method

Using a modern, master control system thecompressorcentral can be run optimally fordifferent operating situations at the sametime as safety and availability increase.

Selecting the right regulation methodallows the supplied quantity of energy tobe reduced through a lower system pres-sure and the degree of utilisation is optimi-sed for each machine in the installation. Atthe same time availability increases, whichreduces the risk of unplanned downtime.Besides, central control allows program-ming for automatic pressure reduction inthe entire system, e.g. during operations atnight and weekends.

116

4:7

The table shows the relation between leakageand power consumption for some different(small) holes at a system pressure of 7 bar.

4:9

A cost comparison between different drying methods.


As compressed air consumption is seldomconstant, thecompressor installation shouldhave a flexible design, for example,through the use of compressors with diffe-rent capacities and speed controlled motors. Compressors with a prudent design can be run with speed control andscrew compressors are especially suitedfor this, as their capacity and power requirement are virtually proportional tothe speed.

4.2.5 Air quality

High quality compressed air reduces theneed of maintenance, increases operatingreliability of the pneumatic system, controlsystem and instrumentation at the sametime as wear to the air-powered machinesis reduced.

If the compressed air system is adapted fordry compressed air from the outset theinstallation will be less expensive andsimpler, as the pipe system does not needto be fitted with a water separator and so-called, swan-necks. When the air is dry itis not necessary to discharge air to the

atmosphere to remove condensation. Con-densation drainage on the pipe system isalso not required, which means lowercosts for installation and maintenance. Thebest economy can be gained by installing acentralcompressed air dryer.Decentralisingair treatment, with several smaller unitsplaced in the system, is more expensiveand makes the system harder to maintain.

It is normally calculated that the reducedinstallation and maintenance costs for asystem with dry compressed air cover thecost of the drying equipment. Profitabilityis very good, even when it is a question ofsupplementing existing installations withdrying equipment.

Oil-free compressors do not need anoil separator or cleaning equipment forcondensation. At the same time filters arenot needed and therefore the cost for filterreplacement is avoided. Consequently,there is no need to compensate for thepressure drop in the filter so compressor’sworking pressure can be lowered, whichimproves the installation’s economy yetfurther.

117

4:10

An oil-free compressor gives constant compressed air quality to a fixed energy cost.


4.2.6 Energy recovery

If you use electricity, gas or oil for anyform of heating within the production faci-lities or in the process, there is good reasonto investigate the possibilities of fully orpartly replacing the energy with surplusenergy from the compressor installation.The decisive factors are the alternativevalue in SEK/kWh for the surplus energy,degree of utilisation and the amount ofadditional investment necessary. A wellplanned investment in energy recoveryoften gives a repayment time of only 1–3years. Over 90% of the power supplied tothe compressor can be recovered in theform of highly valuable heat. The tempera-ture level of the recovered energy deter-mines the possible application areas andthereby the value.

The highest degree of efficiency is gene-rally obtained from water cooled installa-tions, when the compressor installation’soutgoing cooling water can be connecteddirectly to a continuous heating require-ment, for example, the heating boiler’sreturn circuit. Surplus energy can then beeffectively utilised all year round.Different compressor designs give diffe-rent prerequisites. In situations requiring alarge heat flow, long transporting distan-ces to the point of utilisation, a generallylower requirement or a requirement thatvaries during the year, it can be of interestto look at the possibilities of selling therecovered energy.

4.2.7 Maintenance

As all other equipment, even a compressor

118

4:11

At the same time as the compressor produces compressed air, it also converts the supplied energy to heat,which is transferred to the coolant, air or water. Only a small part follows with the compressed air and isemitted as radiation from the machine and pipes. An air cooled compressor involves the simplest recoverysystem, while a water cooled compressor involves more efficient and more flexible recovery possibilities.

Recovered energy kWh/year:W = [(K1 x Q1) + (K2 x Q2)] x TR

Saving/year: W x ep/η

Saved oil m3/year: W/6800 x η

W = Recovered energy (kWh/year)TR = Time per year when there is a need

of power recovery (hours/year)K1 = PartofTRwith loaded compressorK2 = Part of TR with off-loaded

compressorQ1 = Available power in coolant with

loaded compressor (kW)Q2 = Available power in coolant with

off-loaded compressor (kW)ep = Energy priceη = Normal heat source’s efficiency.


installation requires some form of mainte-nance. However, maintenance is low inrelation to other costs, but can be reducedfurther through planning measures. Thechoice of the level of maintenance is deter-mined by the installation’s reliability andperformance.

Maintenance makes up the smallestpart of the installation’s total cost. This isconnected to a high degree on how theinstallation has been planned in generaland the choice of compressor and the auxi-liary equipment.

You can reduce costs by combiningmonitoring with other functions whenusing equipment for fully automatic ope-rations and monitoring of the compressorcentral. The total budget for maintenanceis affected by:

• Type of compressors

• Auxiliary equipment

(dryers, filters, control and

regulation equipment)

• Operating situation

• Installation conditions

• Media quality

• Maintenance planning

• Choice of safety level

• Energy recovery/cooling system

• Degree of utilisation

The annual cost usually lies between5–10% of the machine’s investment value.

4.2.7.1 Maintenance planning

Well planned maintenance means that youcan anticipate costs and extend the servicelife of the machine and auxiliary equip-ment. At the same time costs for repair ofsmall faults decrease and downtime be-comes shorter.

By utilising advanced electronics to agreater degree machines are equippedwith instruments for diagnostic examina-tion. This means that component parts canbe utilised optimally and replacementtakes place only when there is a need. The

119

4:12

The highest possible degree of utilisation reduces the cost of service and maintenance, counted inSEK/operating hour. It is realistic to plan for 100% utilisation and at least 98% availability.


need of reconditioning can be discoveredat an early stage before becoming immen-se, thus avoiding subsequent damage andunnecessary downtime.

By utilising the supplier’s service staff andoriginal spare parts, you can expect themachine to have a high technical standardand you have the possibility of adoptingmodifications based on the latest findingsduring the machine’s service life. Theassessment of the maintenance require-ment is made by specially trained techni-cians, who also answer for the training ofin-house maintenance staff. You shouldalways have you own skilled staff to takecare of daily inspection, as ears and eyescan hear and see things that monitoringequipment cannot.

4.2.7.2 Auxiliary equipment

It is easy to expand an installation withnumerous pieces of auxiliary equipmentto, for example, increase the air quality ormonitor the system. However, even auxil-iary equipment needs service and bringsabout costs for maintenance, e.g. in theform of filter replacement, drying agentreplacement, adaptation to other equip-ment and the training of staff.

In addition there are secondarymaintenance costs, for example, the distri-bution network and production machines,which are affected by the quality of thecompressed air and deposit costs for oiland filter cartridges. All of these costsmust be evaluated in the calculation thatforms the basis of an investment.

4.3 Other economic

factors

4.3.1 General

You can describe and analyse a product, amaterial or service in a systematic way (yetsimplified), with the help of a life cycleanalysis, LCA. The LCA analyses all thestages in the product’s live cycle. Thismeans everything, from the selection ofthe raw material to the final waste deposit-ing are included in the study.

The analysis is often used as a com-parison between different options, forexample, products with an equivalent fun-ction. The result is often used to provideguidance in issues concerning processes orproduct design. LCA can also be used bycompanies in communication with sub-contractors, customers or the authorities todescribe their product’s characteristics.

The results from an LCA primarily act toform the basis for making decisions in thework minimise a product’s effect on theenvironment. LCA does not give the an-swer to every question, which is why otheraspects such as quality and the availabletechnology must be examined to providecomprehensive background material.

4.3.2 LCC

LCC calculations (LCC = Life Cycle Cost)are used more and more as a tool to eva-luate the different investment options.Included in the LCC calculation are theproduct’s combined costs during a specificperiod, thus the capital cost, operating costand the service cost.

The LCC calculation is often imple-120


mented based on a planned installation ora working installation and with this as abasis, defines the requirement level for thenew installation. However, it is right topoint out that a LCC calculation is oftenonly a qualified guess with regard to fu-ture costs and is limited as it is based ontoday’s knowledge of an installation’s con-dition and energy price changes.

Neither does it bear in mind “soft”values that can be just as important, forexample, production safety and subse-quent costs.

To make an LCC calculation requiresknowledge and preferably experience of

compressed air installations. For the bestresult it should be made in consultationbetween the purchaser and the salesman.Central issues are, e.g. how different inve-stment options affect factors such as pro-duction quality, production safety, theneed of subsequent investment, mainte-nance of production machines and the dis-tribution network, environment, the quali-ty of the final product, risk assessment fordowntime and rejections. An expressionthat must not be forgotten in this context isLCP, Life Cycle Profit, i.e. the earnings thatcan be made through, e.g. energy recoveryand reduced rejections.

121

4:13


When assessing service and maintenancecosts you must also consider the equip-ment’s expected condition after the calcu-lation period has passed, that is whether itshould be seen as consumed or be restoredto its original condition.

Furthermore, the calculation model mustbe adapted to the type of compressor. Theexample below can serve as a model forthe economic evaluation of a compressorinstallation, with or without energy re-covery.

122

4:14

4:15

This is how costs are divided without energy recovery.

This is how costs are divided with energy recovery.


Chapter 5 Calculations


124

5.1 Example of dimensioning compressed air installations

Below follows some normal calculations for dimensioning a compressed air installation. The

intention is to show how some of the formulas and data from previous chapters are used.

The example is based on a desired compressed air requirement and result in dimensioned

data, based on components that can be chosen for the compressed air installation. After the

example are a few additions that show how special cases can be handled.


125

5.2 Input data

The compressed air requirements and the ambient conditions must be established before

dimensioning is started. In addition to this requirement, a decision as to whether the com-

pressor shall be oil lubricated or oil-free and whether the equipment shall be water cooled or

air cooled must be made.

5.2.1 Requirement

Assume that the need consists of three compressed air consumers.

They have the following data:

Consumer Flow Pressure Dew point

1 12 Nm3/min 6 bar(e) +5°C

2 67 l/s (FAD) 7 bar(a) +5°C

3 95 l/s (FAD) 4 bar(e) +5°C

5.2.2 Ambient conditions (dimensioning)

Dimensioning ambient temperature: 20°C

Maximum ambient temperature: 30°C

Ambient pressure: 1 bar(a)

Humidity: 60%

5.2.3 Miscellaneous

Air cooled equipment

Compressed air quality from an oil lubricated compressor is regarded as sufficient.


5.3 Component selection

It is a good idea to recalculate all input data from the requirement table under 5.2.1 so that it

is uniform with regard to type before dimensioning of the different components is started.

Flow: In general the unit l/s is used to define the compressor capacity, which is why consu-

mer 1, given in Nm3/min, must be recalculated.

12 Nm3/min = 12 x 1000/60 = 200 Nl/s.

Insertion of the current input data in the formula gives:

Pressure: The unit generally used to define pressure for the compressed air components is

overpressure in bar, i.e. bar(e).

Consumer 2 is stated in absolute pressure, as 7 bar(a). The ambient pressure shall be detrac-

ted from this 7 bar to give the overpressure. As the ambient temperature in this case is 1 bar

the pressure for consumer two can be written as 7-1 bar(e) = 6 bar(e).

With the recalculations set out above the table for uniform requirement:

Consumer Flow Pressure Dew point

1 225 l/s (FAD) 6 bar(e) +5°C

2 67 l/s (FAD) 6 bar(e) +5°C

3 95 l/s (FAD) 4 bar(e) +5°C

126


127

5.3.1 Dimensioning the compressor

The total consumption is the sum of the three consumers 225 + 67 + 95 = 387 l/s. A safety

marginal of approx. 10-20% should be added to this, which gives a dimensioned flow rate of

387 x 1.15 ≈ 445 l/s (with 15% safety marginal).

The maximum required pressure for consumers is 6 bar(e). The dimensioned consumer is the

one, including the pressure drop, that requires the highest pressure.

A reducing valve should be fitted to the consumer with the requirement of 4 bar(e).

Assume at the moment that the pressure drop in the dryer, filter and pipe together does not

exceed 1.5 bar. Therefore a compressor with a maximum working pressure of 7.5 bar(e) is

suitable.

5.3.2 Assumption for the continued calculation

A compressor with the following data is selected:

Maximum pressure = 7.5 bar(e)

State flow at 7 bar(e) = 450 l/s

Total supplied power at 7 bar(e) = 175 kW

Supplied shaft power at 7 bar(e) = 162 kW

The compressed air temperature out of the compressor = ambient temperature +10°C.

Furthermore, the selected compressor has loading/unloading regulation with a maximum

cycle frequency of 30 seconds. Using loading/unloading regulation the selected compressor

has a pressure that varies between 7.0 and 7.5 bar(e).


5.3.3 Dimensioning of the air receiver volume

Qc = Compressor’s capacity (l/s) = 450 l/s

P1 = Compressor’s intake pressure (bar(a)) = 1 bar(a)

T1 = Compressor’s maximum intake temperature (K) = 273 + 30 = 303 K

fmax = maximum cycle frequency = 1 cycle/30 seconds

(pU - pL) = set pressure difference between loaded and unloaded compressor (bar) = 0.5 bar.

T0 = Compressed air temperature out of the selected compressor is 10°C higher that the

ambient temperature which is why the maximum temperature in the air receiver will be (K)

= 273 + 40 = 313 K.

Compressor with loading/unloading regulation gives the right formula for the air receiver

volume:

This is the minimum recommended air receiver volume. The next standard size up is selected.

128


129

5.3.4 Dimensioning of the dryer

As the required dew point in this example is +6°C, a refrigerant dryer is the most suitable

choice of dryer. When selecting the size of the refrigerant dryer a number of factors must be

taken into consideration by correcting the refrigerant dryer’s capacity using correction fac-

tors. These correction factors are unique for each refrigerant dryer model. Below the correc-

tion factors applicable for Atlas Copco’s refrigerant dryers are used and are stated on the

Atlas Copco’s data sheet. The four correction factors are:

1. Refrigerant dryer’s intake temperature and pressure dew point.

As the compressed air temperature out of the selected compressor is 10°C higher than

the ambient temperature, the refrigerant dryer’s intake temperature will be maximum

30 + 10 = 40°C. In addition, the desired pressure dew point is +5°C.

The correction factor 0.95 is obtained from Atlas Copco’s data sheet.

2. Working pressure

The actual working pressure in the compressor central is approx. 7 bar, which repre-

sents a correction factor of 1.0.

3. Ambient temperature

With a maximum ambient temperature of 30°C a correction value of 0.95 is obtained.

Accordingly the refrigerant dryer should be able to handle the compressor’s fully capacity

multiplied by the correction factors above.

450 : 0.95 x 1.0 : 0.95 = 406 l/s.

5.3.5 Assumptions for the continued calculation

An air cooled refrigerant dryer with the followed data is selected:

Capacity at 7 bar(e) = 450 l/s

Total power consumption = 5.1 kW

Emitted heat flow to surroundings = 14.1 kW

Pressure drop across the dryer = 0.09 bar


5.3.6 Control calculations

When all the components for the compressor installation have been chosen, it should be

checked that the pressure drop is not too great. This is done by adding together all the pres-

sure drops for the components and pipes.

It may be appropriate to draw a schematic diagram of the compressed air installation as

shown in fig. 5:1.

The pressure drop for the components is obtained from the component suppliers, while the

pressure drop in the pipe system should not exceed 0.1 bar.

The total pressure drop can now be calculated:

Component Pressure drop (bar)

Oil filter (pressure drop when filter is new) 0.14

Refrigerant dryer 0.09

Dust filter (pressure drop when filter is new) 0.2

Pipe system in compressor central 0.05

Pipe system from compressor central to consumption points 0.1

Total pressure drop: 0.58

The maximum pressure of 7.5 bar(e) and on-load pressure of 7.0 bar(e) for the selected com-

pressor gives a lowest pressure at the consumers of 7.0 - 0.58 = 6.42 bar(e). You should add to

this, the pressure drop increase across the filter that occurs over time. This pressure drop inc-

rease can be obtained from the filter supplier.130

5:1


131

5.4 Other dimensioning

5.4.1 Condensation quantity calculation

As an oil lubricated compressor has been chosen, the water condensation separated in the

compressor and refrigerant dryer will contain oil. The oil must be separated before the water

is released into the sewer, which can be done in an oil separator. Information on how much

water is condensed is needed in order to dimension the oil separator.

The total flow of water in the air taken in is obtained from the relation:

f1 = relative humidity x the amount of water (g/litre) the air can carry at the maximum

ambient temperature 30°C x air flow = 0.6 x 0.030078 x 445 ≈ 8.0 g/s.

The amount of air remaining in the compressed air after drying is subtracted from this quan-

tity (saturated condition at +6°C).

The total condensation flow from the installation f3 then becomes

f1 - f2 = 8.0 - 0.4 = 7.6 g/s ≈ 27.4 kg/hour

With help of the calculated condensation flow the right oil separator can be chosen.


5.4.2 Ventilation requirement in the compressor room

The principle that the supplied power to the room air shall be removed with the ventilation

air is used to determine the ventilation requirement in the compressor room.

For this calculation the relationship for the power at a specific temperature change for a spe-

cific mass of a specific material is used.

Q = m x cp x ∆T

Q = the total heat flow (kW)

m = mass flow (kg/s)

cp = specific heat capacity (kJ/kg, K)

∆T = temperature difference (K)

The formula can be written as:

where:

∆T = the ventilation air’s temperature increase; presuppose that an increase of the air tempe-

rature with 10K can be accepted ∆T = 10 K.

cp = specific heat capacity for the air = 1.006 kJ/kg x K (at 1 bar and 20°C)

Q = the total heat flow (in kW) = (94% of the supplied shaft power to the compressor + the

difference between the supplied total power to the compressor and the supplied shaft

power to the compressor + the stated heat flow from the refrigerant dryer)

= 0.94 x 162 + (175 - 162) + 14.1 ≈ 180 kW

which gives the ventilation air

which at an air density of 1.2 kg/m3 is equivalent to 17.9/1.2 ≈ 15m3/s.

132

v


133

5.5 (Addition 1) At high altitude

Question: Presuppose the same compressed air requirement as described in the previous

example at height of 2500 metres above sea level with a maximum ambient temperature of

35°C. How large compressor capacity (expressed as free air quantity) is required?

Answer: Air is thinner at altitude, which must be considered when dimensioning the com-

pressed air equipment that has a compressed air requirement specified for a normal state

(e.g. Nm3/min). In those cases that the consumer’s flow is stated in free air quantity (FAD)

no recalculation is necessary.

As consumer 1 in the example above is given in the unit Nm3/min the requisite flow for this

consumer must be recalculated. The state at which the compressor’s performance is normally

stated is 1 bar and 20°C, which is why the state at 2500 metres above sea level must be recal-

culated into this state.

By using the table the ambient pressure 0.74 bar at 2500 metres above sea level is obtained. If

the flow is recalculated to Nl/s (12 Nm3/min = 12000/60 Nl/s = 200 Nl/s) and is set in the

formula the following is obtained:

The total compressor capacity demanded is then 309 + 67 + 95 = 471 l/s (FAD).


5.6 (Addition 2) Intermittent output

Question: Presuppose that in the calculation example above there is an extra requirement

from consumer 1 of a further 200 l/s for 40 seconds on the hour. It is accepted during this

phase that the pressure in the system drops to 5.5 bar(e). How large should the receiver

volume be to meet this extra requirement?

Answer: It is possible, during a short period, to take out more compressed air than what the

compressor can manage by storing the compressed air in an air receiver. However, this

requires that the compressor has a specific over capacity. The following relation applies for

this:

where

Q = air flow during the emptying phase = 200 l/s

t = length of the emptying phase = 40 seconds

P1 - P2 = permitted pressure drop during the emptying phase = normal pressure in the

system - minimum accepted pressure during the emptying phase = 6.46 - 5.5 = 0.96 bar

Inserted in the formula to give the requisite air receiver volume:

In addition, it is required that the compressor has a specific over capacity, so that it can fill

the air receiver after the emptying phase. The selected compressor has an over capacity of 5

l/s = 18000 litres/hour. As the air receiver is emptied once per hour the compressor’s over

capacity is more than sufficient.

134


135

5.7 (Addition 3) Water borne energy recovery

Question: A water borne energy recovery circuit is to be built for the compressor in the

example. Presuppose that the water to be heated is in a warm water return (boiler return)

with an ingoing return temperature of 55°C. Calculate the flow required for the energy

recovery circuit and the power that can be recovered. Also calculate the flow and outgoing

temperature for the boiler return.

Answer: Start be drawing the energy recovery circuit and name the different power, flow

and temperatures. Now follow the calculation below.

QE = power transferred from the compressor to the energy recovery circuit (kW)

QP = power transferred from the energy recovery circuit to the boiler return (kW)

mE = water flow in the energy recovery circuit (l/s)

mP = water flow in the boiler return (l/s)

tE1 = water temperature before the compressor (°C)

tE2 = water temperature after the compressor (°C)

tP1 = ingoing temperature on the boiler return (°C)

tP2 = outgoing temperature on the boiler return (°C)

5:2


5.7.1 Assumption

The following assumption has been made:

A suitable water temperature out of the compressor for energy recovery can be obtained

from the compressor supplier and is assumed to be in this example tE2= 80°C.

Assumption for the water circuit through the energy recovery heat exchanger:

tE1 = tP1 +5°C

tP2 = tE2 -5°C

In addition it is assumed that the pipe and heat exchanger have no heat exchange with the

surroundings.

5.7.2 Calculation of the cooling water flow in the energy recovery circuit

Q = m x cp x ∆T

∆T = temperature increase across the compressor = tE2 - tE1 = 80°C - 60°C = 20°C

cp = specific heat capacity for water = 4.18kJ/kg x K

Q = QE = the power that can be taken care of = 70% of the supplied shaft power = 0.70 x 162

= 132 kW.

This is the power possible to recover for the selected compressor.

m = mass flow in the energy recovery circuit = mE.


136


137

5.7.3 Energy balance across the recovery heat exchanger

For the recovery heat exchanger applies:

QE = mE x cp x (tE2-tE1)

Qp = mp x cp x (tP2-tP1)

However, as it has been presupposed that no heat exchange shall take place with the sur-

roundings, the power transferred to the energy recovery circuit from the compressor will be

equal to the power transferred in the recovery heat exchanger, i.e. QP = QE = 113 kW.


5.7.4 Compilation of the answer

It can be established from the calculation that the power which can be recovered is 113 kW.

This requires a water flow in the energy recovery circuit of 1.35 l/s. An appropriate flow for

the boiler return is also 1.35 l/s with an increase of the boiler temperature by 20°C.


5.8 (Addition 4) Pressure drop in the piping

Question: A 23 metre pipe with an inner diameter of 80 mm shall lead a flow of 140 l/s. The

pipe is routed with 8 elbows that all have a bend diameter equal to the inner diameter. How

great will the pressure drop across the pipe be if the absolute initial pressure is 8 bar(a)?

Answer: First the equivalent pipe length for the 8 elbows must be determined. The equiva-

lent pipe length of 1.3 metres per elbow can be read off from the table 3:36. The total pipe

length is then 8 x 1.3 + 23 = 33.4 meters. The following formula is used to calculate the pres-

sure drop:

Insertion gives:

Accordingly, the total pressure drop across the pipe will be 0.0054 bar

138


Chapter 6 Quantities, units and symbols

Kap 6 dok eng 27.04.2000 13:26 Pagina 1

140

Since 1964 SI has been the Swedish stan-dard within the area of quantities andunits where fundamental information canbe found in SIS 01 61 18 (General princi-ples and writing rules) and SIS 01 61 26(Prefixes for multiple units) and SIS 01 6132 (SI-units, derived and additional units).

Units are divided into four differentclasses:

Base unitsSupplementary unitsDerived unitsAdditional units

If a prefix (micro, milli, kilo, mega, etc.) isplaced before a unit, the formed unit isthen called a multiple unit.

Base units, supplementary units andderived units are called SI units and mul-tiple units formed by SI units are calledunits in SI. Note that additional units andnot units in SI.

6.1 The SI-system

Base units are one of the established,independent units in which all other unitscan be expressed.

There are 7 base units in SI:

for length metre mfor mass kilogram kgfor time second sfor electrical current ampere Afor temperature kelvin Kfor luminous intensity candela cdfor the amount of substance mole mol

Supplementary units are units of a basicnature, but are not classified as base unitsor derived units.

Two supplementary units are included inSI:

for plane angles radian radfor solid angles steradian sr

Derived units and formed as a power orproducts of powers of one or more baseunits and/or supplementary units accord-ing to physical laws for the relation betwe-en different units.

15 derived units have been given their own names:

Expressed Expressed in Quantity Designation Symbol in other base and

SI units supplementary units

frequency hertz Hz - s-1

force newton N - m x kgxs-2

pressure, mechanical stress pascal Pa N/m2 m-1 x kg x s-2

energy, work joule J N x m m2 x kg x s-2

power watt W J/s m-2 x kg x s-3

electrical quantity, charge coulomb C A x s s x Aelectrical voltage volt V W/A m2 x kg x s-3 x A-1

capacitance farad F C/V m-2 x kg-1 x s4 x A-2

resistance ohm W V/A m2 x kg x s-3 x A-2

conductivity siemens S A/V m-2 x kg-1 x s3 x A2

magnetic flux density tesla T Wb/m2 kg x s-2 x A-1

magnet flux weber Wb V x s m2 x kg x s-2 x A-1

inductance henry H Wb/A m2 x kg x s-2 x A-2

luminous flux lumen lm cd x sr cd x srlight lux lx lm/m2 cd x sr x m2


There are also a further four additionalunits primarily for use within astronomyand physics. All of these additional unitsare approved by Comité International desPoids et Mesures (CIPM) 1969 and areused together with SI units.

Additional units. There are a number ofunits outside of SI, which for different rea-sons cannot be eliminated despite that cor-responding units in principle can beexpressed in SI units. A number of theseunits have been selected to be used withthe units in SI and are called additionalunits.

141

The following additional units for technical use occur:

Additional unitQuantity Designation Symbol Remarks

plane angle degree ...° 1° = rad

plane angle minute ... 1 =

plane angle second ...” 1” =

volume litre l 1 l = 1 dm3

time minute min 1 min = 60 sectime hour h 1 h = 60 min = 3.600 sectime day d 1 d = 24 hmass tonne (metric) t 1 t = 1.000 kgpressure bar bar 1 bar = 105 Pa = 105 N/m2

180

60

60

π

1°

1,

, ,


142

Fourteen such prefixes are stated in inter-national recommendations (standards) asset out in the table below.

Multiple units. A multiple unit is formedby an SI unit or an additional unit by theunit being preceded by a prefix that invol-ves the multiplication by the power of ten.

PrefixPower Designation Symbol Example

1012 tera T 1 terajoule 1 TJ109 giga G 1 gigawatt 1 GW106 mega M 1 megavolt 1 MV

103 kilo k 1 kilometre 1 km102 hectoh h 1 hectogram 1 hg101 deca da 1 decalumen 1 dalm

10–1 deci d 1 decimetre 1 dm10–2 centi c 1 centimetre 1 cm10–3 milli m 1 milligram 1 mg

10–6 micro µ 1 micrometre 1 µm10–9 nano n 1 nanohenry 1 nH10–12 pico p 1 picofarad 1 pF

10–15 femto f 1 femtometre 1 fm10–18 atto a 1 attosecond 1 as

Pressure

Pa=N/m2 bar kp/cm2 Torr m vp mm vp

1 10–5 1.02 x 10–5 7.5 x 10–3 1.02 x 10–4 0.102105 1 1.02 750 10.2 1.02 x 104

9.81 x 104 0.981 1 735 10 104

133,3 1.33 x 10–3 1.36 x 10–3 1 1.36 x 10–2 13,69.81 x 103 9.81 x 10–2 0.1 73.5 1 103

9.81 9.81 x 10–5 10–4 7.35 x 10–2 10–3 1

Energy

J kJ kWh kpm kcal

1 10–3 2.78 x 10–7 0.102 2.39 x 10–4

1000 1 2.78 x 10–4 102 0.2393.6 x 106 3.6 x 103 1 3.67 x 105 860

9.81 9.81 x 10–3 2.72 x 10–6 1 2.39 x 10–3

4.19 x 103 4.19 1.16 x 10–3 427 1

Power

W kpm/s kcal/s kcal/h hk

1 0.102 0.239 x 10–3 0.860 1.36 x 10–3

9.81 1 2.34 x 10–3 8.43 1.33 x 10–2

4.19 x 103 427 1 3.6 x 103 5.69

1.163 0.119 0.278 x 10–3 1 1.58 x 10–3

735 75 0.176 632 1


143

Air filter

Silencer

Diffusor

Compensator

Damper valve

Screw compressor

Non-return valve

Stop valve

Safety valve

Manual valve

Pressure closing valve

Pressure opening valve

Oil separator

Ejector

Water cooled cooler

Water separator

Condensation receiver

Direction of flow

Air cooled cooler

Fan

Expansion vessel

Minimum pressure valve

Thermostat valve

Thermostat valve

6.2Drawing symbols


M

144

Electric motor

Electricaldistribution box

Motor coupling

Blind flange

Sensor for pressure, temperature, etc.



Fluid filter

Fluid pump

Oil tank withmanual drainage

Restrictor

Bypass

Pressure regulator

Oil stop valve

Air

Oil

Water

Drainage

Ventilation ducting

Mechanicalconnection

Signal air

Electrical signal

Border line

Electrical energy


145

Heat capacity for some materials.

Flow coefficients as a function of thepressure relation for different κ-values.

Some physical constants for dry air at sealevel (=15°C and 1013 bar).

6.3Diagramsand tables


146

Water content in air at different relative vapour pressures (ϕ). The maximum water content is doubled with each temperature rise of 11 K.

The diagram show the temperature relationship T2/T1 for different gases with different κ-values during isentropic compression.


147Saturation pressure (Ps) and density (ρw)

for saturated water vapour.

Composition of clean, dry air at sea level.This composition is relatively constant up to

a height of 25 km.


148Some examples of air consumption for some common power tools and machines based on experience.

These values from the basis for calculating the requisite compressor capacity.


149The water content in the air at different dew points.


150

6.4 Compilation of current

standards and norms

Here follows a compilation of current(1997) standards and norms within thecompressed air field. The compilationrefers to Swedish regulations, but in mostcases there are equivalent national regula-tions in other countries. The listed stan-dards are all, with some exceptions,European or international. Pneurop docu-ments are usually issued with a parallelCAGI issue for the American market.

It is always important to check with theissuing body that the latest issue is beingused, unless the requirement/demandrefers to a dated issue.

6.4.1 Safety related regulations and standards

6.4.1.1 Machine safety

EU directive 89/392/EEG, Machinery direc-tive. In Sweden this has been made law as AFS93:10 (modified as AFS 94:48). NationalSwedish Board of Occupational Safety andHealth regulations for machinery

EN 1012-1 Compressors and vacuum pumps –Safety demands – Del 1: Compressors

EN 1012-2 Compressors and vacuum pumps –Safety demands – Del 2: Vacuum pumps

6.4.1.2 Pressure safety

Directive 87/404/EEG, Simple pressure ves-sels. In Sweden this has been made law as AFS

93:41 I (modified as AFS 94:53) NationalSwedish Board of Occupational Safety andHealth regulations for simple pressure vessels

Directive 76/767/EEG Covers common legisla-tion for pressure vessels and methods ofinspecting. Directive 97/23/EG for pressureequipment (applies from 1999-11-29)

AFS 86:9 (modified as AFS 94:39) ) NationalSwedish Board of Occupational Safety andHealth regulations for pressure vessels andother pressure equipment

EN 764 Pressure equipment - Terminologyand symbols - Pressure, temperature.

EN 286-1 Simple unfired pressure vesselsdesigned to contain air or nitrogen - Part 1:Design, manufacture and testing

EN 286-2 Simple unfired pressure vesselsdesignated to contain air or nitrogen - Part 2:Pressure vessels for air braking and auxiliarysystems for motor vehicles and their trailers

EN 286-3 Simple unfired pressure vesselsdesigned to contain air or nitrogen - Part 3:Steel pressure vessels designed for air brakingequipment and auxiliary pneumatic equipmentfor railway rolling stock

EN 286-4 Simple unfired pressure vesselsdesigned to contain air or nitrogen - Part 4:Aluminum alloy pressure vessels designed forair braking equipment and auxiliary pneuma-tic equipment for railway rolling stock

6.4.1.3 Environment

Pneurop PN8NTCI, Noise test code for com-pressors. ISO 84/536/EC. Sound level demandsfor machinery. A special standard for soundmeasurements is being drawn up within ISO.


151

6.4.2.2 Specifications

SS-ISO 1217 Compressed air technology –displacement compressors – delivery tests

ISO 5389 Turbo-compressors - Performancetest code ISO 7183-1 Compressed air dryers -Part 1: Specifications and testing

ISO 7183-1 Compressed air dryers - Part 2:Performance ratings

ISO 8010 Compressors for the process industry- Screw and related types - Specifications anddata sheets for their design and construction

ISO 8011 Compressors for the process industry- Turbo types - Specifications and data sheetsfor their design and construction

ISO 8012 Compressors for the process industry- Reciprocating types - Specifications and datasheets for their design and construction

SS-ISO 8573-1 Compressed air for general use- Part 1: Contaminants and quality classes. Forenforcement of the current national Swedishregulations there are:

Pressure vessel norms 1987Piping norms 1978 Air container norms 1991 Issued by the Pressure vessel standardizationEG directive 73/23/EEG Low voltage directive

6.4.2.3 Measurements

ISO 8573-2 Compressed air for general usePart 2: Test methods for oil aerosol content

(Draft) ISO 8573-3 Compressed air - Part 3:Measurement of humidity

(Draft) ISO 8573-4 Compressed air - Part 4:Measurement of solid particles

For the emission of exhaust fumes from com-bustion engines a decision is expected accord-ing to "EU stage 1'

6.4.1.4 Electrical safety

ELSAK-FS 1994:9 National Swedish Board ofElectrical Safety’s regulations for electricalmaterial.

ELSAK-FS 1994:7 National Swedish Board ofElectrical Safety’s regulations for heavy cur-rent installations (equivalent to IEC 364)

EU directive 89/336/EEG Electromagneticcompatibility

ELSAK-FS 1995:5 National Swedish Board ofElectrical Safety’s regulations for electromag-netic compatibility

EN 60204-1 Machinery, Electrical safetyinstructions

EN 60439-1 Low-voltage switchgear and con-trol gear assemblies

6.4.2 Technical related standards and norms

6.4.2.1 Standardization

SS 1796 Compressed air technology -Terminology

ISO 3857-1 Compressors, pneumatic tools andmachines - Vocabulary - Part 1: General

ISO 3857-2 Compressors, pneumatic tools andmachines - Vocabulary - Part 2: Compressors

ISO 5390 Compressors - Classification

ISO 5941 Compressors, tools and machinesPreferred pressures


152

Index

absolute pressure 1.2.1absolute zero 1.2.2absorption 2.4.1absorption dryers 2.4.1.4active power 1.6.4adsorption 2.4.1adsorption dryers 2.4.1.5after cooler 2.4.1.1air 1.4air composition 1.4.1air consumption 1.1.2, 4.2.3air intake 3.5.4air receiver 3.6.1.1air requirement 3.1.1.2air-borne energy recovery 3.4.3.2alternating current 1.6.1apparent power 1.6.4assembly method 1.6.5.6atmospheric pressure 1.2.1atomic number 1.1.1axial compressors 2.2.3

booster compressors 2.3.2Boyle’s law 1.3.2by pass regulation 2.5.2.2

cables 3.8.6capacitive measurement system 2.5.5.2carbon filter 2.4.2, 3.2.5Celsius scale 1.2.2central control 2.5.7centralised installation 3.1.2.2centrifugal compressors 2.2.2changes in state 1.3.4Charles’ law 1.3.2circuit-breaker 3.8.5clearance volume 1.5.3closed cooling system 3.3.1.4comprehensive control system 2.5.6compressed air central, 3.5.1, 3.5.2, 3.5.3compressed air distribution 3.6.1compressed air quality 3.2.2compression in several stages 1.5.5compressor calculation 4.3.2compressor central 3.1.2, 3.5compressor package 3.5.1compressor’s sound level 3.7.2, 3.9.10contactor 3.8.3continuous capacity regulation 2.5.1control transformer 3.8.4convection 1.3.3cooling methods 3.3cost division 4.1.1.1, 4.3.2critical flow 1.3.5

critical pressure ratio 1.3.5

decentralised installations 3.1.2.3decibel 3.9.1degree of recovery 3.4.2, 4.2.6degree of utilisation 3.1.1.1delta connection 1.6.5.7dew point 1.4.2diaphragm compressors 2.1.4diaphragm filter 3.2.8dimensioning 3.1, 5.1direct start 3.8.3displacement compressors 1.5.2double acting compressors 1.5.2, 2.1.2dynamic compressors 2.2.1

electrical current 1.6.1electrical voltage 1.6.1electromotive force, emf 1.6.2electron scale 1.1.1electrons 1.1.1energy recovery 3.4equivalent pipe length 3.6.3exhaust emissions 3.7.2extra low voltage 1.6.1

filter 2.4.2, 3.2.5free output air rate 1.2.6frequency 1.6.1frequency converter 2.5.4.3frequency curves 3.9.7fuses 3.7.5

gas constant 1.3.2gases’ general state 1.3.2generators 3.7.3glycol mixture 3.3.1.4gradual start 3.8.3

heat transfer 1.3.3heat transfer number 1.3.3high pressure compressor 3.6.1.1high voltage 1.6.1

idling time 2.5.4.2impedance 1.6.2individual gas constant 1.3.2installation’s overall economy 4.1.1.1insulation class 1.6.5.3intake air 3.5.4intake pressure variation 3.1.3.2isentropic process 1.3.4.4isobaric process 1.3.4.2isochoric process 1.3.4.1

E

D

C

B

A

F

G

H

I


153

phase displacement 1.6.2phase voltage 1.6.3pipe resonance 3.5.4pipes 1.3.3piston compressors 1.5.1-1.5.2, 2.1.2plasma 1.1.2polytropic process 1.3.4.5portable compressors 3.7portable generators 3.7.3power 1.2.5prefilter 3.5.4pressure 1.2.1pressure amplifier 2.3.3pressure band 2.5.4.2pressure dew point 2.4.1pressure drop 1.3.6, 4.2.2pressure measurement 1.2.1, 2.5.5.2pressure ratio 1.5.2pressure relief 2.5.2.1, 2.5.3.4pressure relief with throttled intake 2.5.2.4pressure switch 2.5.4.2protection classes 1.6.5.4protons 1.1.1quality class in accordance with ISO 3.2.2quantity of ventilation air 3.5.5

radial compressors 2.2.1, 2.2.2radiation 1.1.1reactance 1.6.2reactive power 1.6.4recovery potential 3.4.2, 4.2.6refrigerant dryer 2.4.1.2regulation 2.5.1regulation system 2.5.1resistance 1.6.2resistance thermometer 2.5.5.1resistive measurement system 2.5.5.2reverberation 3.9.5Reynolds’ number 1.3.6ring piping 3.6.2room constant 3.9.4

saving possibilities 4.2screw compressors 2.1.5scroll compressors 2.1.7separation efficiency 2.4.2sequence control 2.5.6.1short circuit induction motor 1.6.5short-circuit protection 3.8.5single acting compressor 1.5.2, 2.1.2sound absorption 3.9.9sound dampening 3.9.9sound measurement 3.9.7sound power level 3.9.1sound pressure level 3.9.2speed regulation 2.5.4.3star connection 1.6.3star/delta start 3.8.3

isothermic process 1.3.4.3

Joule-Thomson effect 1.3.7

Kelvin scale 1.2.2

labyrinth seal compressors 2.1.2laminar flow 1.3.6leakage 3.1.1.3, 4.2.3life cycle analysis, LCA 4.3.2life cycle cost, LCC 4.3.2liquid injected screw compressors 2.1.5.2liquid ring compressors 2.1.9loading 2.5.1logarithmic mean temperature difference 1.3.3low voltage 1.6.1

main voltage 1.6.3maintenance costs 4.2.7maintenance planning 4.2.7.1MD dryer 2.4.1.5metal resistor 2.5.5.1microorganisms 3.2.4modulation 2.5.3.4moist air 1.4.2molecular movement 1.1.2molecules 1.1.1monitoring 2.5.5.3, 2.5.8multi-stage off-loading 2.5.2.8

neutrons 1.1.1new investment 4.1.1.1normal litre 1.2.6norms and standards 3.9,6.4nozzle 1.3.5

off loading/loading 2.5.1Ohm’s law 1.6.2oil filter 3.2.5oil free compressors 2.1.3, 2.1.5.1oil free screw compressors 2.1.5.1oil/water emulsion 3.2.8open cooling system 3.3.1.2, 3.3.1.3operating analysis 3.1.1.3operating costs 4.1.1.1-4.1.1.2optimised compressor operations 4.1.1.1over compression 2.4.1.3overall cost 4.3.2overload protection 3.8.6

part flow measurement 3.6.4particle filter 2.4.2phase compensation 3.8.7

K

J

M

L

N

O

PQ

R

S


154

start sequence selector 2.5.6.1start/stop regulation 2.5.2.5starter 3.8.3, 3.8.5sterile filters 3.2.5stroke volume 1.5.3synchronous speed 1.6.5.1

temperature 1.2.2temperature measurement 2.5.5.1thermal capacity 1.2.3thermal conductivity number 1.3.3thermistor 2.5.5.1thermodynamics 1.3three phase mains 1.6.3throttling 1.3.7throttling the intake 2.5.2.3, 2.5.3.1tooth compressor 2.1.6turbo compressors 2.2.1turbulent flow 1.3.6

vacuum pumps 2.3.1valve off-loading 2.5.2.8vane compressors 2.1.8vane compressors 2.1.8vane regulation 2.5.3.2, 2.5.3.3variable discharge port 2.5.2.7ventilation fan 3.5.5water content in compressed air 2.4.1, 3.2.2water cooled compressor 3.3.1water separator 2.4.1.1water vapour 3.2.2water-borne energy recovery 3.4.3.3work 1.2.4working pressure 3.1.1.1

VW

T


155


156

Notes


157


158


159


160


161



Co

mp

ressed A

ir Man

ual 1998

The

face

of

com

mitm

ent

What sets Atlas Copco apart as a company is ourconviction that we can only excel in what we do if weprovide the best possible know-how and technology toreally help our customers produce, grow and succeed.

There is a unique wayof achieving that - we simplycall it the Atlas Copco way. It builds on interaction, onlong-term relationships and involvement in the customers’process, needs and objectives. It means having theflexibility to adapt to the diverse demands of the peoplewe cater for.

It’s the commitment to our customers’ business thatdrives our effort towards increasing their productivitythrough better solutions. It starts with fully supportingexisting products and continuously doing things better, butit goes much further, creating advances in technologythrough innovation. Not for the sake of technology, but forthe sake of our customers’ bottom line and peace-of-mind.

That is how Atlas Copco will strive to remain the firstchoice, to succeed in attracting new business and to main-tain our position as the industry leader.

9780

038

0-10

Kaft 27.04.2000 11:46 Pagina 1

Documents

Compressed Air Manual