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COMPRESSED AIRFUNDAMENTALS
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PART 1
Fundamentals of Compressed Air Technology
Compressed air is used so often in technologicalsituations that any listing of applications mustremain incomplete. No industrial or handicraftoperation can forgo compressed air; no hospital,hotel, power plant or ship functions without it. It isneeded in mining, in laboratories, in airports andin harbours. Compressed air is just as important inproducing foodstuffs as in a cement plant and is asnecessary in the production of glass, paper andtextiles as in the lumber and pharmaceutical indu-stries.Compressed air tools are used to tension, spray,polish and grind, as well as to stamp, blow, clean,drill and transport. Using compressed air, all typesof machines and devices in countless applicationsare pneumatically started, driven and controlled.In addition, many chemical, technical and physicalprocesses and procedures are regulated andmonitored using compressed air.
Not using compressed air as an energy source isunthinkable in our highly technological world.
But what is compressed air?
Compressed air is compressed atmospheric air.Atmospheric air is the air that we breathe. It is amixture of different gases:
78% nitrogen,
21% oxygen and
1% other gases.
The state of a gas is described by three units:
the pressure p
the temperature T
the specific volume Vspec
Air behaves like an ideal gas over broad ranges inpressure and temperature.Therefore, a linear cor-relation (ideal gas law) exists between the threeunits p, T and Vspec. This is described by the
General gas equation:
Naturally, atmospheric air, as with all gases, ismade up of molecules. If the air molecules are hin-dered in their heat motion, for example, by sealingit into a container, they collide with the containerwall, resulting in a pressure p. The force exertedby the pressure p on a flat surface A is then:
2
constantTspec
Vp=
×
F = p x A
3
What is pressure?
We are constantly surrounded by atmosphericpressure, as a glance at a barometer easily shows.The various possible pressure ranges are diffe-rentiated as follows:
- Atmospheric air pressure = Pamb- Overpressure = P0- Underpressure = -P0- Absolute pressure = Pabs
(see Figure 1)
Figure 1: Representation of the pressure ranges
Units:
l The recommended unit of pressure, which wasbindingly introduced in 1978 with the interna-tional unit system (SI system), is the Pascal
(Pa):
l An additional unit of pressure is the bar:
1 bar = 105 Pa = 0.1 MPa
In compressed air technology, the operatingpressure is usually given as the overpressu-re in bar. Previously used units such as atm (1 atm = 0.981 bar overpressure ) are nolonger permitted.
l The SI unit for temperature is degrees Kelvin(K). The relationship with degrees Celsius(degrees C), which is also permitted, is:
T(K) = t(degrees C) + 273.15
l The volume V is used in compressed air tech-nology to specify, for example, receiver sizes.To determine the delivered quantity of amachine that is generating or consuming com-pressed air, the air volume flow Veff (equals theair volume V per time unit) is used. If com-pressed air flows at a speed v through a tubewith the cross-section A, volume flow Veff
Veff = A x v
l The air volumeflow represents the compres-sed air throughflow of a machine. The usualunits for the air volume flow are:
- l/min
- m3/min
- m3/h
In practical applications with piston compres-sors, the unit l/min is used for specifying the airvolume flow; with screw compressors, the unitm3/min is used.
P ab
s
P am
b
-Pe
P e
100% vacuum
Underpressure
Barometric air pressure
Overpressure
m²N
Pa =
4
Air volume flows can only be compared ifthey are at the same pressure and thesame temperature.
In today's compressed air technology, the airvolume flow is nearly exclusively given for theair delivery amount of air compressors. Inaddition to the measurement of other perfor-mance data, the determination of the volumeflow is specified in the German DIN 1945 andin the ISO 1217 regulations.
Standardised and oft-used reference units forpressure and temperature of the air are:
p0 = 1.013 bar/t0 = 20 degrees Corp0 = 1.013bar/t0 = 0 degrees C
l The volume flow is often given in norm cubicmetres per hour (m3N/h). The norm cubicmetre of air determined according to DINequals an air volume of 1 m3 at p = 1.013 barand t = 0 degrees C.
During the comparison of volume flows as ameasurement of the air delivery of compres-sors, the location of the measurement also hasa significant influence on the result. It dependson whether the measurement is made on thesuction side or on the pressure side of thecompressor or, for example, at the exit of acomplete compressor unit. Volume flows canonly be compared if they are measured at thesame pressure, temperature and location.
l Another unit is of fundamental interest for com-paring compressors:the specific power consumption PspecThis shows in kW (kilowatt) how much perfor-mance is required to generate a volume flow of1 m3/min.
For example, if a compressor has a volumeflow of 6.95 m3/min and consumes 42.9 kW ofpower, the result is a specific power con-sumption of
The specific power consumption is probablythe most important parameter for comparingdifferent compressors in terms of their con-structive quality. It provides information aboutthe compressed air one obtains for the inputenergy. However, it is important that the com-parisons are always made at the same ope-rating pressure.
For reasons of comparison, one must also payattention - at which final pressure the values were mea-sured,- whether the power consumption was meas-ured on the compressor shaft or on the drivemotor input side.Finally, the effectiveness of the drive motor andany existing belt drive or gear drive must alsobe taken into account.
(m³/min)flow Volume(kW)n consumptioPower
Pspec. =
n) kW/(m³/mi6.18m³/min 6.95
kW42.9Pspec. ==
Generation of Compressed Air
What are compressors?Compressors are machines for compressing gasand steam. In these machines, a compressingstage provides the compression itself.
Compressor models
The figure below provides an overview of thevarious compressor models.
Figure 2: Overview of the most important compressor models
Rotation and piston compressors form part of thedisplacement compressors. In this case, the air tobe compressed placed in a space and compres-sed by decreasing the space.Turbo compressors,on the other hand, are dynamic compressors. Theair to be compressed is subjected to energy andaccelerated to a high speed. Pressure is increasedby delaying the accelerated air.
Figure 3: Areas of use of the most important compressormodels
Figure 3 provides an overview of the areas of useof the most important compressor models.
Compressed air technology uses mainly pistonand screw compressors. In this chapter, there-fore,we will limit the discussion to these two models.
5
Compressor
TurboPiston
Lifting PistonRotation
Screw
Lamella
Fluid ring
Roots
Reciprocating
Crosshead
Radial
Axial
Free Piston
Membrane
10
10
100
100
1000
1000
10000 100000
Suction volume flow in m³/h
Max
imum
pre
ssur
e in
bar
Piston Compressor
Screw Compressor
Turbo Compressor
Piston compressors
In the case of piston compressors, pistons aremoved linearly to and fro in cylinders. The pistonsare generally driven by a crank pinion with acrankshaft and connecting rods. Up to five connec-ting rods can be arrayed on one crank of thecrankshaft.The inflowing and outflowing air is con-trolled by autonomously opening and closing val-ves.
There are piston compressors with one or severalcylinders and with a fixed V- or W-shaped cylinderarray. An additional differentiation criterion is thenumber of compression stages.
Using the example of a twin-cylinder compressorwith a V-shaped cylinder array (see Figure 4), thedifference between single and double stage com-pression, as well as the actual compression proce-dure, will be explained in detail:
Figure 4: Double stage compression in a piston compressor
1: Suction filter2: Inlet valve3: Outlet valve4: First compression stage5: Intermediary cooler6: Second compressor stage7: Crankshaft
Single stage model: The cylinders are equal insize. Both suction in air, compress it and transportit in a common pressure line.
Double stage model: In the first stage, the air iscompressed to an intermediate pressure. Afterintermediate cooling, it is brought to the final pres-sure in a second cylinder. The relationship of thecylinder diameters to each other constructivelydetermines the value of the intermediate pressu-re.The piston displacement of the second stage issignificantly less than that of the first stage becau-se the pre-compressed air at the entrance of thesecond stage has a significantly lower volume.Autonomous compact valves control the inflowand outflow. The relationship of the stage pressureis determined in such a way that approximatelythe same amount of work is carried out in both sta-ges. A V-shaped array of the cylinders and anequal weight of the pistons for the first and secondstages, aided by the counterweight on the cranks-haft, permit a good mass balance.
Double stage piston compressors offer a lowerdrive performance per m³ of generated compres-sed air compared to single-stage machines. Theintermediate cooling after the first stage results ina volume decrease of the compressed air andthus approaches isothermal compression. In prac-tical terms, this savings in work compared tosingle stage compression at the same motor per-formance means that the volume flow at 10 bar isca. 20% higher. An additional advantage is thelower temperature in the cy1inder space; for thisreason, this model is extremely stable for largerunits up to 15 bar, even if the unit is permanentlyoperating.
Piston compressors are usually driven by electricor combustion motors. The force between thedrive unit and the compressor is transferred direc-tly, using a clutch or, if a more flexible adaptationof the speed is required, using V-belt(s).
6
Function
Compression occurs according to the followingprocedure (see figure 5):
Figure 5: Air compression stages
When the piston moves downward from the topdead centre, the pressure in the compressionspace decreases to below the suction-side pres-sure (point 4). The input valve opens and the airflows from the suction side into the compressionspace.
The piston now moves upward and the pressure inthe compression space increases. As soon as it isgreater than the suction-side pressure, the inputvalve closes (point 1).
The pressure now continues to rise until it is hig-her than the outlet-side pressure (point 2). Theoutlet valve opens and the compressed air isreleased until the top dead centre is attained.
At the start of the concluding downward motion,the pressure in the cylinder space decreases veryquickly and the outlet valve closes again (point 3).
Temperature increase
An increase in temperature is connected to theincrease in pressure; this can be represented bythe following equation:
T2 = T1 x (P2/P1)(K-1)/K, where K = 1.38 to 1.4
For oil-lubricated air compressors, the maximumpossible pressure increase in a compressionstage is limited by the maximum permitted finalcompression temperature. The upper limits are,for example, depending on operating conditionsaccording to the German Accident PreventionRegulations (UVV,VBG 16), between 160 degreesC and 220 degrees C. The result of these tempe-rature limits is the following rough approximationof the required number of compression stages forthe desired final pressure (see Table 1):
Table 1: Number of compression stages depending on thefinal operating pressure
The air heated during compression is cooledagain in air coolers incorporated in the compres-sor and which follow after the compression stages.Due to physical factors, the entire driving energyrequired for operating a compressor is convertedinto heat, which must be removed. In the case ofpiston compressors, this functions using air orwater cooling. Due to their simpler construction,air-cooled piston compressors are more commondesign.
7
Final compression pressureNumber of
compression stages
to 10 bar
6 - 40 bar
20 - 250 bar
120 - 350 bar
1
2
3
4
5200 - 450 bar
3
41
2
Pressure
Operatingpressure
Release
Top
dead
cen
tre
Return expansion
Com
pression
Suctionpressure
Suction
Piston movement
Bot
tom
dea
d ce
ntre
Screw compressors
Screw compressors form part of rotation com-pressors. These are compressors whose com-pression spaces are decreased by a rotary move-ment. Single and double stage rotation compres-sors have become popular on the market. Asignificant advantage of most compressors of thissystem is the total mass balance which allows avibration-free installation.
The air end of a screw compressor of the con-struction mentioned above contains two rotorsarrayed in parallel. One of these has a convexscrew profile while the other has a concave screwprofile. The two profiles are in gear. In an oppositerotary direction, the air is compressed betweenthe profiles due to the different tooth numbers ofthe rotors according to the displacement princi-ple.
The procedure can basically be divided into fourphases (see figure 6):
Figure 6: Compression phases for screw compressors
1st phase:The air enters the compression housing throughthe inflow opening. The thread gaps of the rotorsare then filled with air in a manner similar to thesuction stroke of the piston compressor.
2nd and 3rd phases:When the rotors have turned past the inlet ope-ning, they form a closed compression space bet-ween the thread gaps and the housing. Thisdecreases in size due to the counter-rotatingmovement of the rotors; the enclosed air is com-pressed.
Compression continues until the compressionspace, which is steadily decreasing in size, rea-ches the outside edge of the outlet opening.
4th phase:The compressed air flows out.
8
1st phase 2nd phase
3rd phase 4th phase
Oil-injected screw compressors
In the case of oil-injected screw compressors,generally only one rotor is driven: the male rotor.Since the pair of rotors is interlocked, the femalerotor turns automatically with every movement ofthe male rotor. The oil that is continuously injectedinto the air end prevents the metallic contact bet-ween the rotors. In addition to lubricating the com-pressor air end, the oil has two other importantjobs: it seals the gap between the rotors as well asbetween the rotors and the housing, and it remo-ves the heat produced during compression
The oil is injected during phase 2 into the com-pression spaces - ca. 1 litre per minute per kilo-watt of drive power. It flows through the air endtogether with the air. That what leaves the air endis therefore a compressed air-oil mixture. Due tothe very high oil content, the CE Machinery Direc-tive does not permit the compression temperatu-re to exceed 120 degrees C.
The compressed air-oil mixture first flows into thecompressed air/oil tank. There, the air and the oilseparate mechanically. The oil, which has absor-bed the main portion of the heat energy produ-ced, is then cooled in an oil cooler and can thenbe re-injected.
Any remaining oil particles are then removed fromthe compressed air in a downstream oil separatorbefore the air leaves the compressor.
Oil-injected screw compressors work with finalcompression overpressures of 4 to 15 bar.Volumeflows of 0.5 m3/min to 70 m3/min are attained withmotor performances of 4 kW to 400 kW.The noiselevel with noise insulation is between some 63 and80 dB(A).
Due to their low-vibration running, screw com-pressors can be set up directly on a flat hall floorwithout a special foundation; due to their goodnoise insulation, they can also be set up in workingrooms. When setting them up, heed the corre-sponding Accident Prevention Regulations.
Heat recycling
Screw compressors are often used at a high baseload at continuous operation (100% full load ope-ration). Since ca. 80% of the installed motor per-formance is transferred to the oil in the case of oil-injected screw compressors (at an oil temperatu-re of 85 degrees C), this energy can be used toheat household water and heating water (up to 70degrees C).
Comparison
When a comparison of screw compressors ismade, a significant amount of attention must bepaid to the specific performance. Also pay attenti-on which performances (main driver or ventilatormotor) are included! In addition, make sure thatthe volume flow was measured at the outlet side ofthe system and not on the compressor block. Per-formance trials of compressors must be carriedout according to the German DIN 1945 T1 or ISO1217, where the required measurement conditi-ons are determined.
9
Oil-free compressors
There are many areas of application, especially inthe chemical, pharmaceutical and foodstuff indus-tries, in which oil-free compressed air is required.For this reason, various oil-free compressors weredevelopped: oil-free piston compressors, dry-compressing screw compressors, rotary toothcompressors and many others. An alternative incertain areas is paraffin oil-lubricated compres-sors, because paraffin oil, as opposed to mineraloil, is not toxic.
Dry-compressingscrew compressors
In the case of dry-compressing screw compres-sors, a synchronous drive that drives both rotorsprevents metallic contact between the two rotors.However, this drive significantly increases theprice of the air end and the lack of cooling by theoil permits compression in only one stage to 3.5bar.A intermediate cooler and a second stage arerequired for further compression to 10 bar.Dry-compressing compressors have a significant-ly lower effectiveness than the oil-injected ones.
Water-injected screw compressors
This is not the case for water-injected screw com-pressors. They are currently state of the art andcombine the advantages of oil-lubricated and oil-free compressors: absolutely oil-free compressionsingle-stage to 13 bar at an optimum effective-ness.
The main characteristic of the new compressorgeneration is the replacement of compressor oilby the most natural, most environmentally friendlyand, at the same time, least expensive fluid: water.Water is distinguished by its high specific heatcapacity and heat conduction. Using specificallydosed injections into the compression space, thetemperature increase during the compressionprocedure can be limited to ca. 12 degrees,regardless of the final pressure. Return cooling ofthe produced compressed air is thus no longerrequired. The circulating water can be cooled tonear the ambient temperature. Then the moisture
that is contained in the suction air condenses. Inthe case of oil-injected compressors, condensatewould lead to damage - in the case of water-injec-ted compressors, it even has a positive effect: itrenews the water in the plant circulation (duringcontinu-ous operation under normal ambient con-ditions) by itself within a few hours. This conti-nuous re-generation practically eliminates thecollection of dirt within the compression plant.
Water-injected screw compressors come close toideal "isothermal" compression. Compared to theusual oil-injected machines, this means a drasticenergy savings - up to 20%! In addition, the tem-perature load of the components is minimised.Therefore, the water-injected system guaranteesan especially high operational safety, particularlyin critical application areas. In addition, the remo-val of used oil, oil-containing condensate, oil filtersand oil removal cartridges are no longer neces-sary; the corresponding removal costs are also eli-minated.
The technical realisation of water-injected screwcompressors occurred through the use of paten-ted polymer-ceramic materials and newly develo-ped, highly precise manufacturing procedures. Anew injection system that has also been patentedatomises and optimally distributes the water. Thisguarantees the practically complete transfer of theproduced compression heat with a low waterthroughput.
10
Application Areas forCompressors inCompressed Air Technology
Low pressure range (4-15 bar)
In the largest application area for compressed air,in the low pressure range of 4-15 bar, single anddouble stage oil-lubricated piston compressorsand single stage oil-injected screw compressorsare chiefly used. All of the applications describedin the introduction are covered by these compres-sors.
Mid-pressure range (16-40 bar)
At mid-range operating pressures of 16 to 40 bar,generally 2- or 3-stage piston compressors areused; in the case of very large compressor ratings,double stage screw compressors are used. Thesecompressors are mainly used to start dieselmotors with higher ratings such as those in shipdrives and stationary diesel power plants. In addi-tion, they are used in industry for, for example,sealing tests and in the processing of plastics (i.e.PET blowing).
High pressure range (up to 400 bar)
In this pressure range, only multi-stage pistoncompressors or the related membrane compres-sors are used (ignoring turbocompressors forvery high ratings). There are many different appli-cation areas for high-pressure compressors. Thehigh pressure is required in most applicationareas to store large amounts of air in as small aspossible receivers. An example is the generationand storage of breathing air in bottles with 200and 300 bar pressure, such as those used bydivers and fire-fighters.
In addition, high pressure compressors are usedin hydropower plants, other power plants, rollingmills, in the oil and gas industry, for pressure andsealing tests, in the airline and shipping indus-triesand for marine application.
Figure 7: Four-stage water-cooled marine high pressure com-pressor of Sauer design
Figure 7 shows a four-stage water-cooled pistoncompressor for the Navy with a air delivery of160m3/h at a final pressure of 350 bar and a speedof 1800 rpm. Due to the star-shaped ar-rangementof its four cylinders and the balance of its crankmechanism, this compressor is practically vibrati-on-free in operation.
11
CompressorControl Units - Principles
The job of the compressor control unit is to adaptthe generation of compressed air to the consump-tion of compressed air. There are three types ofcontrol units for compressors: start/stop control,fullload/unload control and the delayed idling control.
Start/Stop control
The compressor works between two pressurevalues set by pressure switches. The runningcompressor increases the working pressure untilthe cut-out pressure is attained. Then the pressureswitch switches off the compressor. Due to theconsumption of compressed air in the network,the network pressure decreases again. When itreaches the restart (cut-in) pressure, the pressureswitch restarts the compressor to produce com-pressed air (see Figure 8).
12
P
0 t
Dropout portions
Cut-out pressure
Cut-in pressure
Figure 8: Start/Stop control
Full load / Unload control
In this controller type, the compressor is switchedto idling when the cut-out pressure is attained. Itcontinues to run at idling without generating com-pressed air. The idling lasts until the restart pres-sure is reached, where the compressor enters anew production phase. The idling operation hastwo important functions: it limits the number ofmotor switchings, and the residual heat after com-pression is removed more efficiently(see Figure 9).
Figure 9:Full load / unload control
Figure 10: Delayed idling control
Delayed dropout controller
This type of control combines the advantages ofthe other two. If the cut-in pressure is not reached,the compressor switches off after a specified time.When the cut-in pressure is attained, the com-pressor starts up from the standstill (see Figure 10).
13
P
0 t
Idling portions
Idling portions Stop portions
Cut-out pressure
Cut-in pressure
P
0 t
Cut-out pressure
Cut-in pressure
Control Units - Practice
The following pages provide a representation ofthe range of functions of modern high-performan-ce compressor control systems.
The introduction of microprocessors in the last fewyears has increased the performance of the con-trollers enormously. This is not merely due to thenumber of possible functions that can now be putinto practice; rather, it also indicates a significantlymore intelligent method of operation using theinput energy, in addition to a strong increase in thecost effectiveness of compressor systems.
14
Controllers for multiple systems
If several compressors within a compressorsystem work on the same compressed air net-work, it is sensible and required that the compres-sors have the operating modes "basic load" and"peak load". In order to evenly load several com-pressors of the same size within a station, thesecompressors are alternatingly switched into thebasic load mode and the peak load mode using abasic load switching system. Such a switchingsystem works automatically (time-dependent) orcan be operated manually.
Figure 11: Operating panel of a microprocessor controller formultiple systems
15
@MULTI CONTROLENTER
CODE
RESET
16
ALUP control unitMulti
control
DISPLAYS
Air delivery amount X
Load operation of all compressors X
Time and date X
Total operating hours X
Load hours X
Automatic functions
Switching on/off the compressors by reaching the cut in/cut off pressure
X
Restart after power failure (programable) X
Lead / Lag control system for several compressors X
Group controller for up to 10 compressors X
MONITORING
Motor switching frequency X
Network pressure too high X
Cable defects X
DOCUMENTATION
Load and operating hours X
LINK
External malfunction alarm X
Basic values in case of interruptions X
Annual switching calendar X
Protection from incorrect entries X
Password authorisation X
- Network pressure- Operating state
X
Description of symbols: X = standard; O = option
Microprocessor controllers
The greater the number of controller and monito-ring functions that are to be put into practice, theearlier the use of an electronic microprocessorcontroller for the compressor makes sense (see Figure 11a).Especially in the case of screw compressors, thesecontrollers, which fulfil all the required controllerand monitoring functions of a compressor system,are offered in series.
Figure 11a: Operating panel of a microprocessor controller
On the following pages, you will find an overviewof the range of functions of modern powerful com-pressor controllers.
17
18
ALUP control units AirControl
Lead/Lagcontrol
Suitable for ALUP compressors of series
DISPLAYS
O O- Network pressure- Final compression temperature- Operating state
X XMaximum compression temperature
X XOperating mode
X XCut-in and cut-off pressure
X XTime and date
X
X
Total operating hours
X XLoad hours
X
X
Remaining lifetime for- air filter- oil- oil filter- oil separation cartridge
XXDirection of motor rotation
X XSelection of the most cost effective operating mode
X XSwitching off the system in case the limit values are exceeded
Restarting after power failure (programmable)
X XMultiple units� controlMaximum number of grouped compressors
AUTOMATIC FUNCTIONS
O OLead/lag control system for several compressors
O OOil heater
O OSwitching on the heater if oil temperature is too low
CT - SCK - SCG
Description of symbols: X = standard; O = option
19
ALUP control units AirControl
Lead/Lagcontrol
Suitable for ALUP compressors of series
MONITORING
Monitoring
O O
- Suction filter- Oil- Oil filter- Oil separation cardridge
X X- Oil change intervals
X X
Temperatures- min.- max- compression end temperature
X XMotor switching frequency
X XNetwork pressure too high
X XCable defects
X XProtection from incorrect entries
X XPassword authorisation
X XMalfunctions
X XLoad and operating hours
Basic values in case of interruptions
X XMalfunction memory
X XAnnual switching calendar
DOCUMENTATION
X XTwo additional selectable outputs
X
X
XExternal malfunction alarm
Remote display using PC
LINK
CT - SCK - SCG
Description of symbols: X = standard; O = option
Sound Insulation
Sound results from mechanical vibrations that aretransferred to the air. The vibrations can be cau-sed by very many sources: from a vibrating area(for example, the covering of a machine) as wellas from flowing gases or liquids. Sound is therefo-re pressure distorting the air, a wave that is super-imposed on the atmospheric pressure. For humandetection of sound, it is required that the frequen-cy of the vibrations lies in the range between 16 Hz and 20 kHz. The detection of most soundconsists of an overlapping of several sound sour-ces with different frequencies.
Sound power level
Energy is required to generate every sound wave,and a portion of this energy carries each soundwave with it.The remainder is emitted into the air asfriction heat. The power range is extremely large:quiet whispering has a value of 0.00000001 wattswhile starting a jet aeroplane has a value of 100,000watts. To simplify the use of these values, they arelogarithmically represented as a "sound powerlevel" with the unit "decibels" (dB).(see Table 2)
Table 2: Relationship between sound power and soandpower level
20
Sound sourceSound power
(W)Sound power
level (dB)
Jet Aeroplane 100.000 170
10.000 160
1.000 150
100 140
Disco 10 130
1 120
0,1 110
0,001 90
Screwcompressor
without soundinsulation
0,01 100
Screwcompressorwith soundinsulation
0,0001 80
Normalconversation
0,00001 70
Sound pressure level
Although the sound power level defines the ener-gy of a sound source, it provides no informationregarding how it is detected by human hearing.This is defined by the sound pressure level, whichis the term for the logarithmic sound pressure thatis based on the human threshold of hearing at afrequency of 1000 Hz. The sound pressure level isrepresented in decibles (dB). Since it is depen-dent on the distance from the sound source, thedistance between the sound source and the mea-surement point must always be specified.
During a measurement of the sound pressurelevel according to DIN 45635, the measurementpoints are located on the surface of a "quadraticsurface".This is a theoretical space with a height of1.5 m and 1 m distance from the main surfaces ofthe compressor (see Figure 12).
Figure 12: Arrangement of the measurement points on thequadratic surface.
Sound insulation
During the operation of compressors, sound pres-sure level values of above 85 dB(A) can occur. Ac-cording to, i.e. the German Worker's ProtectionRegulations, sound protection must be used star-ting at 85 dB(A). Therefore, it is often not onlyadvantageous but also necessary to equip com-pressors with sound insulation.
Sound insulated compressors can be set up nearthe workplace, preventing the costs for longpiping systems and for separate compressorrooms. In addition, the pressure loss in the pipeli-nes is kept to a minimum.
Certain requirements are made on the soundinsulation materials; they must be both non-com-bustible and impervious against dust and oil. Forthis reason, mainly mineral wool and fluorocarb-on-free self-extinguishing foams that are integra-ted into the covering plates are used.
21
Point 3
Point 4
Point 1
Point 2
Cooling and Room Aeration
When you design a compressor station, take intoconsideration that the compressors convert theentire consumed power into heat. This requiresthat the room where the compressor is set up hasadequate ventilation and aeration. This can beachieved with air inflow and exhaust openingsand can be supported using ventilators. In somecases, the installation of air inflow or exhaust chan-nels is required. Precise information regardingventilation and aeration is given in the GermanVDMA unit sheet 4363.
While heat is removed directly at the point of ori-gin by air or water cooling in the case of pistoncompressors, the heat from oil-injected screwcompressors is first transported out of the com-pression block to the air or water cooler, wherethe heat exchange and removal then take place.Figures 14 and 15 show how the heat is distribu-ted in different compressor types.
22
72% Oilcooler
40% Oilcooler
10% motor loss
5% compressed air residual heat
3% radiation
10% compressed air post cooling
10% motor loss
5% compressed air residual heat
5% radiation
40% compressed airintermediate cooling
100%electrical
energy
100%electrical
energy
Figure 14: Heat flow in oil-injected screw compressors
Figure 15: Heat flow in double stage piston compressors
Utilisation of Waste Heat
During operation of a compressor, the input ener-gy is converted completely into heat and must beremoved by cooling. Heat can be covered andused from the cooling media air, water and oil either directly or using heat exchangers; it canthen be used. This makes it possible to reduceenergy costs.
As can be seen in Figure 14, heat recycling fromthe 85 degree C hot oil is particularly attractive forscrew compressors. This is very simple using oil-water heat exchangers.
The three possibilities for utilising heat are repre-sented in the following figures:
Hot air for heating
The heated cooling air is used for heating roomsusing a airduct system. Using temperature-con-trolled flaps, a controlled, adjustable room tempe-rature is attained.The length of the airducts is limi-ted to some 4 to 8 metres. Additional ventilatorsare required for longer channels. It is recommen-ded to contact the compressor manufacturer forgetting detailed data.
In winter, the heat of the exhaust air is partially orcompletely used for heating; in summer, it isblown outside via an exhaust airduct.(see Figure 16)
Figure 16: Heated air for room heating
1: Screw compressor2: Temperature controller3: Air distributor with flap controller4: Air duct
23
Heated airsummer operation
Heated airwinteroperation
4
3
2
1
Hot water for heating
Simple bundled tube heat exchangers are used toprepare hot water.
The heating water is fed through a bundled tubein a closed sleeve. The hot compressor oil flowsbetween the tubes and the sleeve, releasing heatenergy into the heating water.
The arrangement is not complicated (see Figure 17) and the extra investment is ratherlow. Due to the savings in heating costs, the systemcan be amortised in less than one year.
Figure 17: Hot water for heating
1: Screw compressor2: Heat exchanger3: Circulating pump for heat recycling4: Expansion vessel for heat recycling5: Additional heating boiler6: Circulating pump for heating circulation7: Heater thermostat8: Heater
Heat for household water
The heat recycling procedure is the same as thatfor warming heating water (see the arrangementin Figure 18). Using safety heat exchangers orintermediate circuits prevents oil from enteringthe household water, even if there are defects.Thisis attained using a double tube in which two tubesare coupled. The water that is to be heated flowsthrough the inner tube. A blocking medium is pla-ced in the space between the two tubes, whosepressure is monitored. In case of a breakthrough,the monitor triggers an alarm.
Figure 18: Heat for potable water
1: Screw compressor2: Safety heat exchanger3: Circulating pump4: Hot water tank5: Hot water consumer6: Water inlet7: Additional heater (electric)
24
6
3
5
4
2
1
7
8
4
7
6 2
3
1
5
What is the Use ofCompressed AirPurification?
In order for you to understand the material, hereare a few basic remarks:In order to generate a cubic metre of compressedair with an overpressure of 10 bar, a compressormust suction 11 cubic metres of ambient air.Together with this air, it also suctions, just like avacuum cleaner, impurities contained in the air:dust, steam, oil vapour and chemicals, to namejust a few, not to mention the natural humidity.
Despite high-quality suction filters, all of thesecomponents of the suction air are found in com-pressed air.The materials, which were distributedthroughout the 11 cubic metres before compres-sion, are now concentrated in a single cubic metreof compressed air.
Let us observe the subjects "Impurities" and"Humidity" separately and first examine the sub-ject "Humidity". One can illustrate the subject asfollows:
Picture the ambient air as adamp sponge. In a
relaxed state, it cancontain a certain
amount ofwater withoutdripping.
If one presses this spongetogether, a portion of the waterruns out of it - but only a part.
Even if one wrings the sponge out, notall of the water will be
removed.
Therefore, parts of the humidity are removed ascondensate due to the compression. In water,impurities are dissolved, and the whole thingforms an aggressive mixture that can attack thecompressor and the pipes. Corrosion may result.And a disgusting mixture of condensate, rust par-ticles and starter traces from the compressed airpipes is transported to the connected machineswith the compressed air. It should be obvious thatsuch machines wear significantly faster thanothers that are operated with clean compressedair; this has been proven by several on-site expe-riments.
Let us now analyse this process physically:
Humidity is nothing more than moisture in the(basically) dry air.The pressure of the humid air isa sum of PA (air pressure) and PM (moisture pres-sure). Dry air can take up moisture only until itreaches the dewpoint PD. If the moisture pressureincreases beyond the dewpoint (PM > PD), theextra moisture condenses as fog.The capability ofdry air to take up moisture varies depending onits temperature, but is independent of the pressu-re. This results in the
relative humidity of air
For example, PD at 3 degrees C is 0.007576 bar,but at 20 degrees C, it is 0.02337 bar. At a relativehumidity of 70% and 20 degrees C, PM is 0.01636bar. (See the following table, where the moisturepressure is already converted into mass proporti-ons g/m3.)
Since the capability of air to take up moistureis independent of the pressure,these relationships do not change if the air is com-pressed.
25
D
Mrel P
P=ϕ
Examples
8 m3 of moist air with a pressure of 1 barabs equals1 m3 of moist air with a pressure of 8 bar, but thedewpoint pressure does not change. The moistu-re pressure Pd equals 0.02337 bar before com-pression and 0.18696 bar after compression(without a temperature change). Since the dew-point pressure Ps remains constant at 0.02337 bar,7/8 of the contained moisture condense.
Starting conditions:
- 8 m3 of air- 20 degrees C- 1 bar (abs)- ϕrel = 70%- PD = 0.0234 bar = 17.3 g/m3
- PM = 0.0164 bar = 12.11 g/m3
Conditions after compression:
- 1 m3 of air- 20 degrees C- 8 bar (abs)- ϕrel = 100%- PD = 0.0234 bar = 17.3 g/m3
- PM = 8 x 0.0164 bar = 0.1312 bar- PM is thus greater than PDResult: moisture condensesAmount: 8 x 12.11 g/m3 - 17.3 g/m3 = 79.58 g/m3
Since you have worked with this before, you willcorrectly object that moisture does not condensein the compressor.
This is correct. During compression, the air tem-perature and thus the dewpoint pressure PDincrease significantly.Therefore, moisture general-ly can not condense in the compressor.
Here is an example for this:
Starting conditions:- 8 m3 of air- 20 degrees C- 1 bar (abs)- ϕrel = 70%- Moisture content = 17.3 x 0.7 = 12.11 g/m3
Conditions after compression:- 1 m3 of air- 80 degrees C- ϕrel = 96.88/293.3 = 33%- Maximum moisture content of the air
at 80 degrees C = 293.3 g/m3
- Actual moisture content of the compressed air = 8 x 12.11 = 96.88 g/m3, which is less than 293.3 g/m3;therefore, moisture does not condense.
Therefore, the fact that moisture does not conden-se in the compressor is solely due to the tempera-ture rise during compression. Compressor manu-facturers make use of this fact and design theirmachines for an operating temperature of around80 degrees C to prevent the formation of waterpools. Compressor units, working under tropicalconditions and high relative humidity, should workat even higher compression temperatures inorder to prevent water condensate formation.Depending on a number of factors, including thecompressor type, the final compression tempera-ture and the final operation pressure, as well as themodel of the intermediate and post-coolers, thedewpoint temperature is reached in preciselythese coolers, and condensate begins to form.
26
27
DDEEWWPPOOIINNTT
°°CC
MMOOIISS--TTUURREE gg//mm³³
DDEEWWPPOOIINNTT
°°CC
MMOOIISS--TTUURREE gg//mm³³
DDEEWWPPOOIINNTT
°°CC
MMOOIISS--TTUURREE gg//mm³³
DDEEWWPPOOIINNTT
°°CC
MMOOIISS--TTUURREE gg//mm³³
DDEEWWPPOOIINNTT
°°CC
MMOOIISS--TTUURREE gg//mm³³
DDEEWWPPOOIINNTT
°°CC
MMOOIISS--TTUURREE gg//mm³³
DDEEWWPPOOIINNTT
°°CC
MMOOIISS--TTUURREE gg//mm³³
DDEEWWPPOOIINNTT
°°CC
MMOOIISS--TTUURREE gg//mm³³
100,0 588,208 79,0 279,278 58,0 118,199 37,0 43,508 16,0 13,531 -4,0 3,513 -25,0 0,550 -46,0 0,060
99,0 569,071 78,0 268,806 57,0 113,130 36,0 41,322 15,0 12,739 -5,0 3,238 -26,0 0,510 -47,0 0,054
98,0 550,375 77,0 258,827 56,0 108,200 35,0 39,286 14,0 11,987 -6,0 2,984 -27,0 0,460 -48,0 0,048
97,0 532,125 76,0 248,840 55,0 103,453 34,0 37,229 13,0 11,276 -7,0 2,751 -28,0 0,410 -49,0 0,043
96,0 514,401 75,0 239,351 54,0 98,883 33,0 35,317 12,0 10,600 -8,0 2,537 -29,0 0,370 -50,0 0,038
95,0 497,209 74,0 230,142 53,0 94,483 32,0 33,490 11,0 9,961 -9,0 2,339 -30,0 0,330 -51,0 0,034
94,0 480,394 73,0 221,212 52,0 90,247 31,0 31,744 10,0 9,356 -10,0 2,156 -31,0 0,301 -52,0 0,030
93,0 464,119 72,0 212,648 51,0 86,173 30,0 30,078 9,0 8,784 -11,0 1,960 -32,0 0,271 -53,0 0,027
92,0 448,308 71,0 204,286 50,0 82,257 29,0 28,488 8,0 8,243 -12,0 1,800 -33,0 0,244 -54,0 0,024
91,0 432,885 70,0 196,213 49,0 78,491 28,0 26,970 7,0 7,732 -13,0 1,650 -34,0 0,220 -55,0 0,021
90,0 417,935 69,0 188,429 48,0 74,871 27,0 25,524 6,0 7,246 -14,0 1,510 -35,0 0,198 -56,0 0,019
89,0 403,380 68,0 180,855 47,0 71,395 26,0 24,143 5,0 6,790 -15,0 1,380 -36,0 0,178 -57,0 0,017
88,0 389,225 67,0 173,575 46,0 68,056 25,0 22,830 4,0 6,359 -16,0 1,270 -37,0 0,160 -58,0 0,015
87,0 375,471 66,0 166,507 45,0 64,848 24,0 21,578 3,0 5,953 -17,0 1,150 -38,0 0,144 -59,0 0,013
86,0 362,124 65,0 159,654 44,0 61,772 23,0 20,386 2,0 5,570 -18,0 1,050 -39,0 0,130 -60,0 0,011
85,0 340,186 64,0 153,103 43,0 58,820 22,0 19,252 1,0 5,209 -19,0 0,960 -40,0 0,117 -65,0 0,0064
84,0 336,660 63,0 146,771 42,0 55,989 21,0 18,191 00,,00 44,,886688 -20,0 0,880 -41,0 0,104 -70,0 0,0033
83,0 324,469 62,0 140,659 41,0 53,274 20,0 17,148 -21,0 0,800 -42,0 0,093 -75,0 0,0013
82,0 311,616 61,0 134,684 40,0 50,672 19,0 16,172 -1,0 4,487 -22,0 0,730 -43,0 0,083 -80,0 0,0006
81,0 301,186 60,0 129,020 39,0 48,181 18,0 15,246 -2,0 4,135 -23,0 0,660 -44,0 0,075 -85,0 0,0003
80,0 290,017 59,0 123,495 38,0 45,593 17,0 14,367 -3,0 3,889 -24,0 0,600 -45,0 0,067 -90,0 0,0001
Moisture content of the air at various temperatures
Example: At a dewpoint of 0 degrees C, one m3 of air contains 4.868 g of moisture.
Cyclone Separator
When the compressed air leaves the compressor,it contains moisture in the form of tiny water drop-lets and steam.As the first stage in compressed airpreparation, the droplets are to be mechanicallyremoved using a cyclone separator at the com-pressed air outlet of the compressor. This func-tions as follows:
Compressed air and the included water dropletsenter the cyclone. Due to the guide nozzle device(1), they are affected by a strong twisting motionso that they rotate in the cyclone space (2) at highspeed around the cylinder axis. Due to the strongoutwards-directed centrifugal forces, the waterdroplets are catapulted to the separator wall andthen flow into the collection space (3). The collec-tion space is separated from the cyclone space bya curved shield (4) so that the air flow cannotbring any condensate with it. The compressed airleaves the cyclone separator through a pipe andthe pure gas outlet (5). The condensate is remo-ved through an opening (6) in the floor of thecollection space. It makes sense that there shouldbe installed an electronically controlled condensa-te drain because the condensate level must notrise to the curved shield or beyond. Large cyclo-ne separators have an additional inspection ope-ning on the container floor (7); small ones mustgenerally be disassembled for cleaning.
28
5
1
2
4
3
7
6
29
Drying of Compressed Air
After the compressed air leaves the cyclone separator, it theoretically contains only residual moisture inthe form of vapour because this is not mechanically separated and goes through the cyclone sepa-ratortogether with the compressed air. For further drying, various procedures can be used, depend-ing onthe use of the compressed air.The following overview presents these procedures. Since we do not wishthis presentation to be a purely academic list, we have marked the relevant procedures by large arro-ws and a black background. All other procedures require special applications (especially absorption)or are simply to uneconomic (over-compression; heat-regenerated adsorber with heating of the desic-cant).
Dryingmethods
Heatingof a desiccant
(internal heat rege-neration)
Regeneration
Solid dryingmaterial
Fluid dryingmaterial
Viscous dryingmaterial
Solid dryingmaterial
Adsorption(physical binding to
a desiccant)
Sorption(removal of liquid)
Condensation
Over-compressionand depression
Adsorption(chemical reaction
of a desiccant)Cooling
Cold regenerationwith dried compres-
sed air
Heatingof the regeneration
air(external heat gene-
ration)
Utilisation of thecompression heat
(oil-free compressor,full load and partial
flow principle)
Regarding the individual procedure
Refrigeration drying
Figure 19: Electronically controlled compressed air refrigera-tion dryer for volume flows up to 8000 m3/h.
Refrigeration drying is a procedure in which com-pressed air is cooled by a refrigerant in a heat ex-changer. The moisture contained in the compres-sed air condenses and is removed. The larger thedifference between the compressed air inlet andoutlet temperatures, the larger the amount of con-densed moisture. The lower the cooling tempera-ture of the compressed air, the lower the amountof remaining moisture.
Drying takes place in two phases:
First phase: In an air/air heat exchanger, the warm,inflowing compressed air cools in the oppositelydirected current of the already cold outflowing air.No additional energy must be consumed for this.Here, ca. 60% of the contained moisture alreadycondenses.
Second phase: The compressed air flows throughan refrigerant heat exchanger and cools down tothe set pressure dewpoint. The remaining moistu-re cools to the pressure dewpoint, condenses andis automatically removed.
Figure 20: Functional schematic of a compressed air refrige-ration dryer
Figure 20a: Funktional schematic of a �TRISAB� heat exchan-ger.
The refrigerant circulation is driven by a com-pressor. This compressor compresses thegaseous cooling material and presses it into thecondensator. There, it is liquefied and injectedthrough an expansion valve into the refrigerantcircuit, where it returns to gaseous form. Thisrequires heat energy which is removed from thecompressed air. The compressed air cools until itreaches the set pressure dewpoint pressure(see Figure 20).
30
Kompressoren
Test
Power
Oekodry DT
Together with the condensate,a large portion of the oilthat the compressed air contains in the case of oil-lubricated or oil-injected compressors is removed. Itmixes with the water.This mixture must not be dispo-sed of directly in the sewer system; rather, it must firstbe separated in a suitable separator into water and oil.
Refrigeration dryers are generally fully equippedand wired; they must only be connected to thepower supply. They are available in various sizes,differing in the volume flow, the permitted ambienttemperature and the pressure dewpoint tempera-ture.
The performance range of freeze dryers is fromca. 15 to ca. 5400m3/h, where a dewpoint pressu-re of +2 degrees C or higher is attained at anambient temperature of up to +50 degrees C.Therequired energy increases with the cooling per-formance that must be carried out; for example, to14,5 kW at 8000m3/h.
For 90% of all applications, refrigeration drying isthe most economical way because both, therequired energy and the operating costs, are sig-nificantly less than for other procedures.
Sorption
Sorption means that fluid is removed from com-pressed air either chemically or physically. Sinceabsorption drying plays a subordinate role inpractice, we will concentrate solely on adsorptionin the following sections.
The principle of adsorption:The moisture that the compressed air contains isbound by adhesion force to the surface of a dry-ing desiccant in granulated form (= adsorbate). Inthis process, a pressure dewpoint of up to -70degrees C is attained. As opposed to refrigerantdrying, the compressed air is not cooled.The pro-cess of adsorption itself requires no energy; ener-gy is required only for the regeneration of theadsorbate, i.e. the removal of the deposited moi-sture. Since the regeneration procedure requires acertain amount of time, an adsorption dryeralways consists of two vessels; one is in operationwhile the other is being regenerated. Until thisstage, all dryers work according to the same prin-ciple; however, they differ in the type of regenera-tion. Basically, two procedures are available forthis: cold and warm regeneration.
Cold regeneration:
In cold regeneration, a part flow is branched offthe dried main flow of compressed air; this is usedas cleaning and regeneration air.This part air flowis first depressed. Thereby, it is strongly under-saturated with moisture. If it is then transportedover the bed of desiccant to be regenerated, itabsorbs the contained moisture and transports itto the outside. The regeneration air cannot bereturned to the compressed air flow again and itleaves the dryer as loss air. If you lay out a com-pressed air plant, you must therefore calculate theregeneration air as an additional consumer!
31
Figure: Functional principle of adsorption drying1: Adsorption Vessel2: Desorption Vessel3: Main valve4: Upper valve block5: Diaphragm6: Outlet valve7: Regeneration air exit with silencer
Since the balance between the remaining load ofthe desiccant with moisture and the partial pres-sure of the cleaning gas flow is attained relativelyquickly, cold regeneration requires automatic swit-ching cycles of ca. 3-10 minutes.
Warm regeneration:
In warm regeneration, hot regeneration air is usedfor desorption. This is generated by an externalheat source.Ambient air is sucked in, heated and transportedthrough the bed of desiccant that is to be regene-rated. In this procedure, temperatures between150 and 300 degrees C are required, dependingon the type of desiccant used. Regeneration isfinished when the temperature at the exit of theregeneration air has reached ca. 100 degrees C.Then the bed of desiccant is cooled to the utilisa-tion temperature with a cold cleaning air flow. Asopposed to a cold-regenerated adsorption dryer,you do not need to calculate additional compres-sed air consumption for a warm-regeneratedadsorption dryer. In a few cases only a small amo-unt of compressed air for switching of the controlvalves is required (ca. 1%).
The automatic switching cycles lie at ca.4-8 hours,depending on the operating conditions.
Life time of the desiccant:The life time duration of the desiccant is ca. 2000to 4000 regeneration cycles.The following criteriacan reduce the long-term ability of the desiccantto absorb water and thus the utilisation duration:
- ageing and reduced affinity- reduction of the granulate surface due to
constant abrasion - soiling due to oil particles in the compressed
air
32
Filtration
Several factors influence the generation of techni-cally pure compressed air:
• suction air that contains greater or lesser amounts of solid particles and/or chemicals,depending on the local air pollution;
• the occurrence of condensate and the formati-on of rust;
• oil-lubricated or oil-cooled compression spa-ces in compressors;
• inadequate maintenance.
In order to attain malfunction-free operation, dirt,water and oil must therefore be removed from thecompressed air.The removal of water was descri-bed in detail in the chapter "Drying methods".
After drying, the air contains very fine oil dropletsand dirt only in very small quantities. Therefore, itmakes sense to use filters at this point. Withoutpre-cooling and the preliminary removal of con-densate and dirt, the filter elements would beco-me soiled quickly. Due to the rapidly increasingpressure loss that would result, the filter elementswould have to be continuously replaced - a costfactor that should not be underestimated.
Figure 23: Compressed air filter
On the other hand,a small pressure loss cannot beavoided. This loss can be measured with a diffe-rential pressure manometer and provides infor-mation about the degree of soiling of the filters. Forthis reason, manometers are attached to the filterhead in the case of high-performance filters (seeFigure 23). Using the manometer allows
the determination of the correct time to change afilter. Generally, the time has been reached whenthe differential pressure is ca. 0.6 bar. A even hig-her economical operation, especially on biggersize (and more expansive) units, can be achievedwith a microprocessor-equipped filter unit: theactual pressure loss through the filter element ispermanentely monitored and its energy costs(through higher compression for equalizing thepressure drop) are permanently compared withthe costs for a new filter element. As soon as theenergy costs will become more expensive than anew replacement filter element, a signal for filterelement change will be given.
The filters and separators used in compressed airtechnology can be categorised according to diffe-rent viewpoints:• the purpose (suction filter, pre-filter, sterile fil-
ter, oil steam adsorption filter, etc.);• the working procedure (mass force separator,
electrical separator, surface filter, membranefilter, depth filter);
• the degree of fineness (coarse filter, fine filter,microfilter);
• the filter material (fabric filter, paper filter, fibrefilter, sintered filters of metal, ceramic, plastic).
In the filtration of compressed air, chiefly two filtra-tion types are used:surface filtration and depth filtration.
Surface filtration
In surface filtration, sieving is the main function ofthe separation mechanism. If the dirt particles arelarger than the defined pores, they are separatedon the surface of the filter material.
33
Depth filtration
In depth filtration, fleece fibres are used as the fil-ter material; these are made up of an entangle-ment of very fine single fibres. These filter materi-als do not function only as a sieve; instead, dirt par-ticles that are significantly smaller than thedistance between the fibres are removed. A com-bination of several separation mechanisms is res-ponsible for this:• the pushing action itself• adsorption• electrostatic discharge• diffusion• to a low degree, the function of the sieve• bonding due to �van der Waal� forces
On most pressure filter types, a combination ofsurface and depth filtration is effective.
Figure 24: High-performance filter set with 99.99% effectbased on 0.01 µm particle size.1: end cap of plastic or aluminium;2: borium-silicate fibreglass layer;3: steel outer support sleeve;4: PVC-coated Styrofoam sleeve
High-performance filtration
In high-performance filtration of compressed air,the borium-silicate fibre material is the mostpopular of depth filter materials. Using these fil-ters, the content of residual oil can be reduced to0.01 mg/m3. If active charcoal layers are additio-nally used, a residual oil content of less than 0.005mg/m3 can be attained.
Familiarity with operating conditions and com-pressed air quality requirements are necessaryfor the filter material and for chosing adequate fil-ter types and systems.
The main criteria for selecting the correct filtersize are the temperature at the installation location,the delivered amount (volume flow) and the ope-rating pressure of the compressed air .
The volume flows for filters given in the manufac-turers� specifications are always based on a cer-tain pressure. If your operating pressure changes,the maximum throughflow amount through the fil-ters will also change. The amount that the throughflow is decreased or increased can beeasily determined using conversion factors.Thesecan usually be found in the manufacturers� docu-ments.
At a temperature of +30 degrees C, five times as much oil passes through a filter than at +20 degrees C. If the temperature changes from+20 degrees C to +40 degrees C, the amountincreases by 10 times. For this reason, you mustpay attention that micro and submicro filters areinstalled in a location where the compressed airtemperature is as low as possible.
In Germany, the Waste Removal Law (AbfG) is bin-ding for the removal of soiled filter elements,which must be classified as hazardous waste.
34
Condensate Removal
Condensate drains
Compressed air condensate from after-coolers, fil-ters, dryers and pipe systems is aggressive, gene-rally contains much oil and a large amount of dirtparticles. Removing this condensate from thecompressed air system poses certain problemsfor the user. Float-type condensate drains canstick; therefore they no longer remove the con-densate. Under some circumstances, they perma-nently blow out the expensive compressed air.
The functioning of time-controlled solenoid valvesis not always reliable. Due to the removal, which isonly set to certain times, not to the actual conden-sate deposition, they may cause significant lossesof compressed air and thus high energy costs.
For these reasons, electronic level-controlled con-densate drains have become popular in com-pressed air technology. These condensate drainscollect the condensate in containers with nomoving parts and thus work without wear.
Figure 25: Electronic level-controlled condensate drain.
In electronic level-controlled condensate drains,the condensate drips through an inlet opening in acontainer. If the container is filled to the maximum,a capacitative level sensor sends an impulse to asolenoid valve. This opens the outlet pipe and thecondensate exits. The valve only remains open aslong as there is still condensate. Therefore, nocompressed air can escape.
In special models, condensate removers can alsobe used for aggressive condensates (such asthose that occur in oil-free dry-running compres-sors), for operating pressures up to 63 bar and inexplosion-endangered areas.
Oil-water separators
The exiting condensate contains residual amountsof oil that can range from 1000 to 10,000 mg/l.Legal regulations prescribe preparation of oil-containing water "according to the generally recognised technical rules" (Paragraph 7a of theGerman Water Supply Law). Accordingly, thinningto decrease the harmfulness is not permitted.The-refore, oil-containing condensate must be prepa-red in such a manner that the oil content of thewater flowing out of the separation device doesnot exceed the permitted values.
According to the German ATV (Technical Waste-water Association), Worksheet A115, the maxi-mum value is 20 mg/l. However, the drainage sta-tute of the responsible community is alwaysauthoritative; in certain cases, the maximum valuemay be lower (10 or 5 mg/l). So the local regu-lations will always have to be followed
There are two basic possibilities to satisfy thewater supply statute: removal or preparation. Itmust be pointed out that removal of oil-containingcondensate involves high costs.
35
Example: A compressor station with an airflow of20 m3/min generates up to 60 m3 condensate aswaste per year.The average disposal costs for thistype of hazardous waste are currently (early 1995)ca. 1700 DM/m3 and will certainly increase. Theoperator of the compressor system will then haveyearly disposal costs of hundreds of thousands ofDM.
The less expensive alternative is condensate pre-paration using an oil-water separator. Here, thecondensate is fed into a separation container; thedirt particles carried by the condensate are col-lected in removable containers.The condensate isseparated in the separation container using grav-ity and transported through a filter combination ofan oleophile pre-filter and active charcoal adsorp-tion. The oil that slowly collects at the surface istransported via an oil overflow into an over-flow-free canister.
Figure 26: Function schematic of an oil-water separator.
The utilisation duration of the active charcoal stron-gly depends on the degree of dispersion andemulsification of the oil in the water. In turn, theseare determined mainly by the compressordesign, the oil type used and the removal of con-densate.
Normal oil-water separators are not able to sepa-rate stable emulsions. Stable emulsions can occurdue to high compression temperatures, poorlydemulsifying compressor oils as well as emulsion-enhancing chemical substances in the suction air.These stable emulsions require a special type ofpreparation.
36
Compressed Air Distribution
Compressed air receivers
Compressed airoperated machines and toolsrequire a continuous air flow for trouble-free ope-ration. This is attained by using correctly propor-tioned compressed air receivers. The receiversare prime coated, laquered or internally andexternally galvanised; they can be vertical or hori-zontal (see Figure 27).
Figure 27: Vertical compressed air receiver.
They fulfil the following duties:
• Compressed air storage
The compressor builds up a storage volume in thereceiver; this balances out varying compressedair consumption in the network, thus reducing thenumber of on/off cycles of the compressor.
• Pulsation damping
Displacement compressors, especially pistoncompressors, generate a pulsing compressed airflow that is damped by the volume of the receiver.
• Condensate removal
Due to cooling of the compressed air on the recei-ver walls, a portion of the condensate precipitates,collects on the receiver bottom and can then beremoved from there without problems.
The permitted number of times that the compres-sor can be switched on and off depends on theperformance of the electromotor (see Table 3).
Table 3: Switching frequency of compressors depending onthe performance of the drive motor.
37
Nominalperformance
in kW
Permittedmotor frequency
per hour
4 - 11 55 - 40
15 - 30 30 - 15
37 - 75 12 - 6
90 - 250 5 - 2
Determining the receiver size
The following formula provides an approximationfor determining the receiver size. In the case ofmulti compressor systems, this refers to the peakload compressor:
where:V = compressed air container volume in m3
Voleff= volume flow in m3/h (ISO 1217)pa = ambient pressure in bar(a)zs = switching frequency (per hour)∆p = switching pressure difference in bar
Example:Voleff= 240 m3/h = 4 m3/minpa = 1 barzs = 15h-1
∆p = 2 bar
In case that a standard compressed air receiver inexactly the calculated size would not exist, oneselects the next larger size.This formula appliesfor compressors whithout idling function, i.e.piston compressors. Compressor units with inte-grated idling function, like screw type units, usual-ly can operate into a smaller receiver. It is advisa-ble, however, to consider a certain compressedair buffer volume at fluctuating air consumption,which is the usual case in industrial air networks.The above mentioned formula can usally beapplied also for screw compressor installations.
IMPORTANT: The bigger fluctuations of com-pressed air consumption and the more such fluc-tuations differ to above from the compressed airflow of the compressor(s), the bigger the receivervolume should be.
Legal regulations for compressed airreceivers
(6th ordinance to the German Machine SafetyCode, dated June 25, 1992; Pressure receiverOrdinance dated June 25, 1992) Pressure recei-vers of pressure volume product of 200 or moreare to be inspected before commissioning; pres-sure receivers of pressure volume product of 1000or more are also to be inspected repeatedly thereafter by professionals of the correspondinginspection authorities.
38
ps
aeff
Z
PVolV
∆×××
=4
31
3
0,22154
1240
4m
barh
barhm
Z
PVolV
PS
aeff =××
×=
∆×××
= −
Pipelines
In the case of a central compressed air supply, it isrequired that a pipeline system be installed; thissupplies the individual consumers with compres-sed air.The job of a pipeline network is to providecompressed air to the consumers
• in a sufficient quantity,• with the required pressure,• at the required quality,• with a smalllest possible pressure drop,• safely• and inexpensively.
Figure 28: Schematic of a compressed air pipeline network.1: Ring line;2: main line;3: stub;4: condensate drain;5: consumer connection
Constructing a pipeline network
It is recommended that a compressed air pipelinenetwork be divided into individual segments (see Figure 28).The compressed air network begins in the com-pressor station with the main line, which connectsthe compressor with the dryer, compressed airreceiver and filters.The pressure decrease via themain line should not exceed 0.04 bar, not inclu-ding filters, armatures, etc.
The distributor line connects to the main line; thissupplies the consumers with compressed air as astub/ring line.Stubs are lines leading from a distributor lineacross the room/hall ending at a specific position.They have the advantage that they require lesspipeline material than ring lines, based on thepipe length. On the other hand, individual seg-ments of ring lines, which form a closed distribu-tor ring, can be blocked off while still guarante-eing the supply of compressed air to other areas.A ring line requires lower nominal tube sizes.
The pressure drop through the distributor lineshould not exceed 0.03 bar. In the case of moistu-re condensation, the pipeline should have a maxi-mum tilt of 5 degrees to the lowest point so that thecondensate can be collected and removed - thisequals a drop of ca. 9 mm per 1 m pipe length.
The pipeline can be attached to the wall usingpipe holders or by hanging it from the ceilingusing a threaded control rod or loop. For repairs orreconstruction in segments, it is recommendedthat sufficient blocking equipment be planned inorder to sequester pipe sections without affectingthe entire system.
Connection pipelines branch off from the distribu-tor line; these provide the direct supply of the con-sumers. Armatures and connection accessoriesare used for this purpose (see page 32 ff).
39
2
1
5 4
3
The pressure decrease via the connection linesshould not exceed 0.03 bar. In order to keep theconnection lines as free from condensate as pos-sible, it is recommended that the pipes from eachconnection line be bent upwards at 180 degrees("goosenecks"). Although this is no longer requi-red in the case of already dried compressed air,the pipelines should be installed this way anywaydue to safety reasons.
For industrial use, the German pipe size DN 25(equal to one inch) or greater is always recom-mended for the connection lines because thereare barely any cost advantages for material andassembly for smaller sizes. Therefore, consumersthat require up to 1800 l/min compressed air at anominal pipe line length of up to 10 m can be sup-plied without excessive pressure decrease.
Table 4 shows the relationship of the pressuredecrease and the volume flow at a pipe diameterof 25 mm and a nominal length of 10 m.
Table 4:Relationship of pressure drop and volume flow
A pressure decrease occurs due to
• too low a pipeline cross-section• flow obstacles in pipelines• roughness of the walls • leaks.
Leaks in the distributor line or on the connectionsto the consumer mean a high cost factor. The lea-king positions act like nozzles from which airescapes with enormous speed. Since outflowingair does not pose a direct danger, however, it isusually not treated with the same attention as, forexample, a leak in a water pipe.
The increasing required volume flow caused byleaks leads to higher energy costs in the genera-tion of compressed air.Table 5 provides an idea ofthe scope of energy costs caused by leaks.
Table 5: Energy costs caused by leaks at 8000 operatinghours/year and 0.2 DM/kWh.
40
Volume flow in l/min Pressure drop in bar
600 0.005
1,200 0.02
1,800 0.04
Holediameter
(mm)
Max.leak flow at7 bar (l/s)
Energy costs(DM/year)
1 1.2 800
2 5 3,200
3 11.2 7,100
4 19.8 12,500
6 44.6 28,000
10 124 79,000
The amount of leaking in a compressed air systemcan be most easily measured by emptying thecompressed air receiver.This shows in what amo-unt of time the pressure drops by, for example, 1bar. During the measurement, the receiver is nolonger supplied with compressed air.
Assuming that the compressed air flows out iso-thermally, the amount of leaking in a compressedair system can be approximated according to thefollowing formula:
where
VL = Amount of leaking in l/minVR = Pressure receiver contents in lPS = Receiver starting pressure in barPF = Receiver final pressure in bart = Measuring time in min
Example:
VR = 1000 lPS = 8 barPF = 7 bart = 2 min
Pipeline dimensions
During construction of a new compressed airsystem, the pipeline dimensions are of primaryimportance.
In order to obtain favourable dimensions, the follo-wing conditions are to be precisely determined:
• set-up of the individual consumers
• number of consumers
• type of consumers
• compressed air consumption of the variousconsumers
41
t
PPVV FSR
L
)( −×=
min/500min2
)78(1000l
lVL =−×=
Determination of the compressed airconsumption
Operation timeMost compressed air machines and devices arenot operated continuously. Therefore, it is impor-tant to determine the operation time as referenceinformation for the total consumption. It is given asa factor or percentage.
Table 6 in the Appendix of Chapter 2 providesexamples of the operation time of certain com-pressed air consumers.
Example:A mounting machine is in operation for 45 minutesper hour. The operation time is thus
Simultaneity factorThe simultaneity factor is an empirical value. It isbased on the experience that not all consumersare in operation simultaneously when compressed airconsumers of the same type are in use. If themounting machine mentioned in the above examplehas a switch-on duration of 75%,several independently wor-king machines will not always run at the same time.
Table 5 in the Appendix of Chapter 2 shows whichsimultaneity factors can be used in practice for acertain number of compressed air consumers.
Example:Five mounting machines are operated in parallel.Taking into account the switch-on duration of 75%,each machine has a compressed air consumptionof 200 l/min at 6 bar. If all machines always oper-ated simultaneously, the total compressed airrequirement would be 5x 200 l/min = 1000 l/min.However, since a simultaneity factor of 0.83 can beconsidered for five machines operated in parallel,an actual compressed air requirement of ca. 830 1/min results.
Tool wearTool wear consideres losses produced by wearcaused by ageing, leaks and incorrect treatmentof compressed air tools. Wear should be set to amaximum of 5% based on the total air consumpti-on of a tool.
Pipeline diameterPipeline diameters are either determined using anomogram (see Figure 29) or calculated using anapproximation formula:
whered =internal diameter of the pipe in dmVoleff = total volume flow in m3/sL = nominal length of the pipeline in m∆p = pressure drop in barp1 = operating pressure in bar(g)
42
%7575.06045 ==
100106.1
5
1
85,13
÷×∆
×××=
PP
LVold eff
Determination of the internal diame-ter using a nomogram (example 1):
Figure 29 shows a nomogram with which theinternal diameter can also be determined.
Using a nomogram:
• Set the pipe length on line A and the volu-me flow on line B.
• Connect the points with a straight line andlengthen this to axis 1
• Set the system pressure on line E and thepermitted pressure drop on line G.
• Connect the points with a straight line. Thisline crosses line D.
• The pipe diameter that is to be determinedlies at the intersection.
43
44
Pipe length(m)
Free airdelivery(m³/h)
Axis 1
Pipe width(mm)
Axis 2
Systempressure
(bar)
Pressuredrop
(bar abs.)0,03
0,04
0,05
0,07
0,1
0,15
0,2
0,3
0,4
0,5
0,7
1,0
1,5
GFDC
BA
2
3
4
5
7
101520
E
500
400
300
250
200
150
100
70
50
40
30
25
20
10.000
5.000
2.000
1.000
500
200
100
5.000
2.000
1.000
500
200
100
50
20
10
Figure 29: Nomogram for determining the pipeline diameter and pressure drop
Determination of the internal diame-ter using a nomogram (example 2):
Is the nomogram in Figure 29 too unclear for youor the work too difficult? Then see Figure 30. Thisnomogram takes only the most important para-meters into account and is thus more clear.
Using the nomogram:
• Set the air throughflow in the left column: markthe line with the required volume.
• Determine the length of the pipeline and markthe corresponding column.
• The intersection of the line and the columnends in an irregularly outlined area in whichthe correct diameter is located.
Example:- Air flow = 1000 l/min- Length of pipeline = 100 m- Required pipeline diameter = 1"
45
46
Length of pipeline (m)Air flow (l/min of free air)
100
200
300
400
500
750
1000
1500
2000
2500
3000
3500
4000
4500
5000
6000
7000
8000
10 20 30 40 50 75 100 150 200 250 300 350 400 450 500
Pressure drop: ca. 0.1 bar at 8 bar Network pressure
1/4"
3/8"
3/8"
1/2"
3/4"
1"
1 1/4"
1 1/2"
2"
DN 65
DN 80
Figure 30: Nomogram for determining the pipeline diameter and pressure decrease
Additional armatures:
All installed armatures (valves, brackets, elbows,etc.) pose an additional resistance that must betaken into account. The lengths that must beadded to the length of the pipeline are found in thetable.
Example: A G 3/4" shut-off valve has a factor of4.00; theoretically, the pipeline must thus be lengthened by 4 m.
47
Armature
Shut-off valve
Shut-off slide
Bracket
Pipe elbow r=d
Pipe elbow r=2dT piece
Pipe and armature diameter
Corresponding pipe length in metre
G 3/8" G 1/2" G 3/4" G 1" G 1 1/4" G 1 1/2" G 2" DN 65 DN 80 DN 100
1.00 2.00 4.00 6.00 8.00 10.00 15.00 20.00 25.00 30.00
0.30 0.80 1.50 3.00 4.00 5.00 7.00 9.00 10.00 15.00
0.70 1.00 1.30 1.50 2.00 2.50 3.50 4.00 5.00 7.00
0.10 0.20 0.20 0.30 0.40 0.50 0.60 0.90 1.00 1.50
0.08 0.10 0.12 0.15 0.20 0.25 0.30 0.40 0.50 0.80
0.80 1.00 1.50 2.00 2.50 3.00 4.00 5.00 7.00 10.00
Materials for compressed airpipesSeveral materials can be used within a compres-sed air network. The choice of the material is notonly determined by the cost, but, as with all otherfactors in a compressed air system, depends onseveral points. These are:
• compressed air quality• dimensions of the pipes• pressure• influences from the surroundings• installation effort• material costs• pressure drop• resistance to ageing
Criteria for selecting the material:
This section provides an overview of the advanta-ges and disadvantages of the materials that aremost often used in compressed air pipes:
Steel
• threaded tube: inexpensive, many shapedparts
• seamless: many nominal diameters; but: corro-sion and high flow resistance
• galvanised: resistant to corrosion; but: high flowresistance
• stainless steel: resistant to corrosion, low flowresistance, sealed; but: limited number of sha-ped parts, expensive
Copper• resistant to corrosion, low flow resistance; but:
good technical knowledge required
Plastic
• polyamide (PA)
• polyethylene (PE)
• acrylnitril - butadieri - styrol copolymers (ABS)
• the following applies to all: many shaped parts,no corrosion, generally simple to install; but:high length expansion, lower pressure resili-ence under increasing temperatures
48
Armatures
Compressed air armatures are used in the entiresupply chain, from the compressor to the consu-mer. They are thus very important. The characte-ristic of quality for compressed air armatures isthe rela-tionship between the pressure drop andthe air flow volume.
The following armatures exist (see Figure 31
• maintenance units (1)• cocks (2)• couplings (3)• hoses (4)
Figure 31: Sampling of armatures
Maintenance units
A maintenance unit is a three units combination ofa filter, a pressure reducer and an oiler. Earlier,these were three independent components.Today, one also uses two units combinationswhere the filter and the pressure reducer arecombined as a filter pressure reducer. So-calledcombi-devices also exist, consisting of one part: afilter on the bottom, a pressure reducer in themiddle and an oiler on top.
A compressed air oiler is absolutely required tooperate compressed air tools and cylinders and toguarantee their lubrication. Installation should becarried out as close to the consumer as possiblebecause oil vapour generated in the oiler combi-nes over long distances to form oil droplets.
Rule: normal vapour oilers should be installed at amaximum of 5 m from the consumer, proportionalvapour oilers at a maximum of 10 m. Several con-sumers can also be connected to a correspon-dingly larger oiler.
Ball cocks
Ball cocks are used to block compressed airpipes. They are characterised by a free through-flow without bottlenecks and cause almost noadditional pressure decrease to existing distribu-tor networks. A complete seal is attained by abrass ball turning in a Teflon seal.
Couplings
Couplings are used to provide a sealed connec-tion between two compressed air connections.Safety couplings form a special type of coupling.They can always block the flow from either side.Normal quick couplings have plugged beaks withreturn flow dampers to slowly aerate the decoup-led hose.
49
Hoses
There are two main types of hose: spiral hoses andstandard hoses. Hoses are generally made fromPVC with a cloth inlay. Spiral hoses have a higherpressure drop than normal straight hoses.
The longer a hose connection, the greater thecross-section that should be selected to ensurethat the consumer is satisfactorily supplied withcompressed air.
Non-return valve with distributor prevents the return flow of compressed air into thepressure reducer in case of counter pressure thatmay occur.
50
Figure 32: Example of a modern compressed air sampling station with block construction
Non-return valve with distributor
prevents the return flow of compres-sed air into the pressure reducer incase of counter pressure that mayoccur.
Cleaned, controlled andoiled compressed air
Compressed air oiler incompact block design
Cleaned, controlled and unoiledcompressed air
Filter pressure reducer with mano-meter
in compact design; return controlpossible; independent of pre-pres-sure; for temperatures of -10 to +50degrees C (plastic) or -10 to +90degrees C (metal).
Compressed air inlet
Cock with secondaryaeration
as blocking device
