Compressed Air Facts:

Compressed Air Facts:I Compressed air applicationsII ThermodynamicsIII Measurement technology for compressed airIV Production of compressed air for industry, trade and commerceV ControlVI Compressed air treatmentVII Air distributionVIII System optimisationIX Tools

Compressed Air Applications

Characteristics of compressed airCompressed air is an energy form offering an unri-valled range of applications and combining speed,power, precision and safe handling. These charac-teristics make compressed air irreplaceable in manycases. However, there are applications in whichcompressed air is in competition with other forms ofenergy such as electricity or hydraulics which are ofinterest. Here, the principle of economic efficiencydemands a precise cost-benefit analysis. The rela-tively high costs of producing compressed air alwayshave to be weighed against factors such as workingspeed, reliability, maintenance costs etc. The bestavailable technology should be taken as a base.Compressed air applications have made tremendousprogress in recent years with regard to energy effi-ciency.

The versatility of compressed air becomes very clearif one looks at application examples:

Costs Benefits

Energyinput

Heatrecovery

Workingspeed

Maintenancecosts Flexibility

Compressed-air energy

Electro-energy

Fig. 1: Qualitative cost-benefit comparison taking into accountrelevant parameters

Working and energy airAs an important field of application for compressed-air, pneumatics has shown a two-digit growth rate foryears. More and more new patents are being filed oncompressed-air cylinders, air engines and com-pressed-air valves.

Facts ApplicationsPage 2 of 4

Fig. 2: Automation using compressed air

The speed, precision, flexibility and miniaturisation ofthese components play an important role.

Without compressed air the degree of automationessential for the competitiveness of German compa-nies would not be possible.

Fig. 3: Robots operated using compressed air

Life today cannot be imagined without a multitude ofproducts which can only be produced using com-pressed air.

Fig. 4: Plastic bottles

Another special feature of compressed-air devices istheir possible application in explosion-proof areas.

For example, compressed-air hoists ensure thatthere are no sparks caused in varnishing plants.

Fig. 5: Explosion-proof mechanical hoist

To equate compressed air only with old-fashionedapplications does not correspond to the state-of-the-art. For example, cleaning work benches using com-pressed air is no longer up-to-date. In many cases, ahand brush would also do the job. However, if com-pressed air is still being used for such a task then itis recommended to use optimised jets which achievea maximum cleaning effect with minimal air con-sumption.


Fig. 6: Air-jet loom

Active airYou talk about 'active air' if compressed air is beingused as a transport medium. Current applicationexamples are transporting bulk goods, shooting theshuttle back and forth in looms, applications in airbearings or the recently rediscovered pneumaticpost.

Several advantages of compressed air can beshown based on the example of air bearings. Laserguns for aiming GEO satellites have to be positionedexactly and automatically guided. In order to achievethe necessary precision of ±1/3600 degrees, theoptical system is air cushioned. The air bearingsallow completely smooth and infinitely variable tele-scope movements for high measurement accuracyand shield it from vibrations. Without compressedair, such modern methods of geodesy would hardlybe practicable.

Process airIf compressed air is directly incorporated as a proc-ess medium in certain processes, then it is calledprocess air. Common areas of application are dryingprocesses, the aeration of clarifiers or air for fer-mentation processes.

Fig. 7: Fermenting and bottling

Industrial VacuumIndustrial vacuum technology is closely related tocompressed air. Several applications can be solvedusing either compressed air or a vacuum. Using anindustrial vacuum it is possible to pack, dry, stretch,perform suction, hoist, position and many others.More and more sectors are recognising the merits ofvacuum applications.

The electronic industry can be cited as an example,where production depends on absolute precisionwith the largest possible output. In accordance with"clean production", extremely precise, very smallvacuum pumps ensure the exact handling of elec-tronic circuit boards under clean-room conditionsand their fitting with microchips. The stable, con-trolled vacuum "grasps" the chip and positions it atexactly the right place on the printed circuit board.

Fig. 8: Circuit board production

Pressure rangesDifferent applications require different pressures. It isvery rarely economically justifiable to compress tothe highest pressure required and then subsequentlyreduce the pressure again. Therefore it is necessaryto categorise the pressure ranges and apply corre-spondingly suitable generation systems.• Vacuum and blower applications

These range from rough vacuums up to the excesspressure range of approx. 1 bar. These pressurelevels can be generated very economically usingrotary valve vacuum pumps, rotary piston blowersand side channel fans.There is the possibility to generate industrial vac-uums using compressed air but in almost everycase this would represent a misuse of compressedair. Specialised vacuum pumps operate on only afraction of the energy input necessary for com-pressed air.


• Low pressure applicationsLow pressure applications refer to those in therange from 2 up to 2.5 bar excess pressure. Ro-tating positive-displacement compressors are gen-erally used here, but turbo compressors are alsofeasible for extremely large amounts.Specifically in low pressure applications which makedo with much lower excess pressures than the clas-sical 6 bar, it can frequently be observed that theseappliances are connected to the 7-bar grid. At thepoint of use, the pressure is then simply reducedaccordingly. In such cases it should be a matter ofurgency to check whether the introduction of aseparate low pressure supply could raise economicefficiency.

• Standard pressure applicationsThere is a broad range of compressors availablefor standard pressure applications which areserved from a 7-bar grid. The specifications re-garding the amount and quality of air determinewhich combination of compressors operates mosteconomically

• High pressure applicationsOscillating positive-displacement compressorssuch as piston compressors or membrane com-pressors have their field of application in the twoand three figure bar range. Radial turbo compres-sors may also be an economic choice when largeamounts of air are involved.It is quite common that a few high pressure con-sumers can be supplied very economically via thestandard compressed air network with decentral-ised booster compressors close to the high pres-sure consumers.

Correct pressureEvery consumer of compressed air requires a certainworking pressure in order to be able to deliver anoptimal performance. For example, for tools whichare powered with only 5 instead of the necessary6 bar, the on-load speed falls by 25 % even though theidling speed only drops by 5 %. Regular checks aretherefore indispensable to see whether the requiredworking pressure is available, especially under full loadconditions. Pressure losses due to an insufficient pipingcross-section or bottlenecks can only be detected if thecompressed air is actually flowing. Excess operatingpressures do not bring any gains in performance. Theyonly increase the consumption of compressed air andthe wear and tear of the equipment

Quality of compressed-airThere is a similar picture for insufficiently processedcompressed-air. Particles, damp and oil afflict com-pressed-air equipment and increase their fault liabil-ity. Increased wear and tear and output losses arestill relatively small problems compared with totalfailure which may lead to loss of production. Buteven if the compressed-air equipment is operatingtrouble-free, impurities can enter processes via in-sufficiently conditioned compressed-air which mayresult in the loss of whole production batches.

ConclusionWhoever selects his compressed-air applicationswith care, adjusts the compressed-air system ac-cordingly and consistently monitors the parametersrelevant for economic efficiency and operatingsafety, has certainly made a decision in favour of amodern and efficient energy supply.

The campaign “Druckluft effizient“ aims to motivate the operators of compressed-air systems to optimise their systems and save substantialcosts. It is conducted by the German Energy Agency (dena), the Fraunhofer Institute Systems and Innovation Research (FraunhoferISI; project management), and the Federation of the Engineering Industries (VDMA) with support of the Federal Ministry for Economicsand Labour (BMWA) and the following industrial enterprises:

Atlas-Copco BEKO Technologies BOGE Kompressorendomnick-hunter Energieagentur NRW Gardner Denver WittigGASEX Gebr. Becker Ingersoll-RandKaeser Kompressoren Legris – TRANSAIR METAPIPESchneider Druckluft systemplan, Karlsruhe Thyssen Schulte – MULTIPLASTultra air ultrafilter International ZANDER Aufbereitungstechnik

Further information can be found at www.druckluft-effizient.de Druckluft effizient, Fraunhofer ISI, Karlsruhe/Germany, October 2003

ThermodynamicsCompressed air is used in industry as an energysource like electricity from the wall outlet. The effortand expense necessary for producing, treating anddistributing the compressed air is frequently over-looked. In order to provide a better understanding,the basic physical correlation are explained here andtypical misunderstandings are pointed out.

CompositionCompressed atmospheric air is usually implied by theterm 'compressed air'. The major components of un-polluted air are nitrogen (78 vol-%) and oxygen(21 vol-%) as well as small amounts of other gases(1 vol-%) (Fig. 1).

Oxygen 21 %

OtherGases

1 %

Nitrogen 78 %

Fig. 1: Composition of dry atmospheric air

Water is also contained in atmospheric air in the formof water vapour, the amount of which varies stronglydepending on temperature, volume and geographicalconditions. For this reason, the share of water is usu-ally given separately from the other components.

PressureThis is the main parameter of compressed air whichis usually expressed in the units bar or PSI (PSI =pound/(Inch)2; 1 bar = 105 Pa = 105 N/m2 = 14.504 PSI).Absolute pressure (PSIA) is the pressure measuredfrom a base of absolute zero. It is required for alltheoretical observations as well as in vacuum and fantechnology.Gauge pressure (PSIG) is the practical referencevalue and is determined based on atmosphericpressure. Absolute pressure and gauge pressure aregiven in the same units. Therefore, when looking atpressure values, care must be taken to determinewhether absolute or gauge pressures are involved. Inpractice, gauge pressures are usually meant sincepressure sensors mostly display gauge pressures,i.e. the difference between absolute and atmosphericpressure (see Fig. 2). To avoid confusion, it may besensible to show the reference in pressure figuresusing an index.

Facts ThermodynamicsPage 2 of 4

Atmospheric pressure

Suction pressure

Absolute pressure PSIA

100 %vacuum 0 %

98 %

50 %

1 ba

r (a)

1 ba

r (a)

2 ba

r (a)

2 ba

r (a)

3 ba

r (a)

3 ba

r (a)

4 ba

r (a)

4 ba

r (a)

1 ba

r (ü)

1 ba

r (ü)

2 ba

r (ü)

2 ba

r (ü)

3 ba

r (ü)

3 ba

r (ü)

20 mba

r

500

mba

r

0 ba

r (ü)

0 ba

r (ü)

Gauge pressure PSIG

0 ba

r (a)

Fig. 2: Gauge, absolute and suction pressure

Water contentThe maximum water vapour absorption capacity of airis described by the saturation pressure ps. How muchwater can be absorbed is solely a function of thetemperature. The absorption capacity increases withincreasing temperature (Fig. 3).

If the air is cooled, therefore, there is always the dan-ger that the water vapour contained will be con-densed out and that condensate will be formed.

Condensate may also occur if the saturation pressureis exceeded during compression. If humid, atmos-pheric air is compressed at constant temperature, thepartial pressure of the water vapour also increasescorresponding to the increase in overall pressure. Ifthe saturation partial pressure at this temperature isexceeded due to compression, condensate is pre-cipitated. Since the air leaves the compressor with amuch higher temperature, the condensate is onlyprecipitated if the compressed air is cooled downbeneath the pressure dew point. Below this tem-perature, condensate is precipitated continuously,i. e. in the aftercooler as well. Approx. 60-80 % of the

total amount of condensate are formed here. A fur-ther intentional separation and drying of the com-pressed air takes place subsequently in the drier orunintentionally in the pipes.

If air with a relative humidity of 60 % and a tempera-ture of 15 °C is compressed to a pressure of 7 barand subsequently cooled again down to 25 °C, 30 gof condensate are obtained per cubic metre com-pressed air.

Further information on this topic can be found in thefacts "Treatment".

Power demand for compressionWhen describing changes in the state of air (com-pression, expansion, cooling) thermodynamically, aircan be regarded as a perfect gas in the temperatureand pressure range relevant for compressed air. Theperfect gas equation describes the relation betweenthe pressure (p), volume (V) and temperature (T) of agas.

The following applies:TRmVp i ⋅⋅=⋅

or with reference to the amount of substance n

TRnVp ⋅⋅=⋅

with R as the universal gas constant with the valueR = 8.3144 J/(mol K). It is then valid that the productfrom the pressure and volume of the air is proportionalto the temperature. The perfect gas equation can beused to describe the changes in state occurring.

The two most important kinds ofstate changes are the isothermal(pressure change at constanttemperature) and the adiabatic(isentropic) (pressure changewithout heat exchange with thesurroundings).

For isothermal changes, thefollowing applies:

p1V1 = p2V2

with R and T = const.0

102030405060708090

100110120130

-35 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 35 40 45 50 55

Temperature [°C]

Satu

ratio

n pr

essu

re p

D [m

bar]

Wat

er c

onte

nt [

g/kg

] [g/

Nm

3]

pD [mbar] Water content gH2O / kg dry airWater content gH20 / Nm3 dry air

Fig. 3: Saturation pressure and water content of air


The specific work for compressionresults from the work for changingthe volume

1

211

2

112 ln

υυυυ ⋅⋅−=⋅−= � pdpw

The following applies to adiabaticchanges:

2

22

1

11

TVp

TVp ⋅=⋅

with R = const.

For temperature:

κκκ

υυ

1

2

1

)1(

1

2

2

1

−−

��

��

�=�

�

��

�=

pp

TT

and for the specific work

( )� � −⋅=⋅=⋅=2

112

2

112, TTcdTcdpw ppt υ

For air in the relevant range for compressed air, theadiabatic exponent κ has a value of κ = 1.4 kJ/(kg K).

The theoretical energy demand for compressing air isthus dependent on the compression ratio and thetype of change of state. Whereas the isothermalcompression results in the lowest specific work, theactual state characteristics during compression(polytropic compression) are closer to reversibleadiabatic compression.

These optimimum values are not achievable in prac-tice, since the compression process is afflicted withlosses. Good compressed air systems are character-ised by specific capacities which are approx. 45 %above the theoretically possible ones of adiabaticcompression (Fig. 4). It should be noted that the spe-cific energy required decreases with increasing sys-tem size. The specific performance data given incor-porate all electrical and mechanical losses duringcompressed air production. They are not directlycomparable with the rated power listed on the name-plate of the drive motor of the compressor. The spe-cific power consumption of a compressed air systemshould lie within the good range. The lower limit ofthe good range is described by the adiabatic com-pression which represents an ideal case and there-fore cannot be achieved by real compressors.

Further information on compressed air production canbe found in the facts "Production".

Pressure lossesAfter production and treatment, the compressed airhas to be distributed in a network to the user points.As well as the pressure losses occurring duringtreatment, other losses occur during distribution dueto the pipe resistance which represent a loss of en-ergy. The loss due to friction is much greater in tur-bulent flows than in laminar flows (Fig. 5).

Laminar Flows Turbulent Flows

Fig. 5: Laminar and turbulent flows

Whether a laminar flow occurs in a pipe depends mainlyon the velocity of flow. The influence of pipe roughnessis negligible and can be ignored, more decisive are thechanges in pipe diameters at joints. Turbulent flows inthe whole of the distribution system are predominantin compressed air systems. The degree of turbulenceincreases with increasing flow velocity. The greaterthe velocity of flow, the greater the flow losses.

0

1

2

3

4

5

6

7

8

9

10

11

12

13

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Pressure ratio p1/p0

Spec

ific

perf

orm

ance

[kW

/ (m

3 /min

)]Thermodynamically impossible

Upper limit: for ideal adiabatic compression

Lower limit: for ideal isothermal compression

Good rangePoor range

Fig. 4: Specific power demand for compressed air production


The flow velocity results from the relation of volumeflow and cross-sectional area for incompressibleflows.

AV�=υ

Pipe diameters which are too small result in high flowrates and high pressure losses in the piping. To re-strict these losses, the flow rate in compressed airdistribution should be preferably smaller than 6 m/s.Further information on distribution can be found inFacts "Distribution".

Measuring compressed airAlthough compressed air is a high quality and expen-sive energy source, usually neither the compressedair consumption nor the energy demand for its gen-eration and treatment is recorded. Measuring andrecording the consumption is, however, a key ele-ment for optimising the costs and energy use in thefield of compressed air. Further details can be foundin the facts "Measurement technology".

More information can be found in the fact sheets onother topics. These facts aim to supply initial informa-tion but cannot replace the problem-specific advicegiven by specialists.




Measurement Technologyfor Compressed AirIn the field of compressed air, measuring pressureprovides the data basis for rating the correct pres-sure head of pressure differences in the distributionsystem as well as for controlling and regulating thecompressors. Before a compressed air system isdimensioned or optimised, the volume flow shouldbe measured. If especially high compressed airquality is required, the relevant measurements pro-vide the basis to ensure compressed air quality aswell as to optimise compressed air processing.

Pressure measurement ordifferential-pressure measurement

Measuring pressure under flow conditions is mainlydone to control and regulate compressors or com-pressor systems as well as to assess compressedair systems.

Differential-pressure measurement is used in addi-tion to monitor the functional performance and eco-nomic efficiency of the air processing systems suchas filters.

Membrane pressure switchIn many of the compressors and compressor sys-tems used today, membrane pressure switches rec-ord the pressure and pass on the data measured inthe form of an electrical switch signal.

Please note:

• the ageing of the mechanical components impairsthe repeatability.

• Membrane pressure switches require a high dif-ferential gap before they react and need a lot ofroom.

Contact manometerUp until the 1990s, it was considered state-of-the-artto use mechanical contact manometers for differen-tial-pressure measurements, e. g. to monitor filters orto control compressor systems.

Please note:

• in order to achieve sufficient resolution, the optimalmeasuring range should be close to the operatingrange.

• Electrical connections result in mediocre repeat-ability and complicated adjustments of max. fourusable contacts.

Facts Measurement TechnologyPage 2 of 5

Electronic pressure sensorThe compressors of modern compressor systemsshould be controlled based on the pressure meas-urements of electronic pressure sensors which con-vert the pressure values into analogue signals.

Please note:

• pressure sensors with an output signal of 4 to20 mA help to avoid cable breaks.

• If the maximum of the measuring range is close tothe range of parameters to be controlled, a higherresolution should be aimed at.

• These very robust and reliable systems are char-acterised by their high repeatability as much astheir compact construction.

Measuring the volume flow rateMeasuring the volume flow rate is used to detect thecapacity of compressors both with regard to the totalair consumption of a plant as well as with a view toindividual air consumption of local production facili-ties.

It must be taken into account that the volume flowdata of compressors and air consumers refer to theoperational environment, but that the measurementis made in the system conducting the pressure. It istherefore necessary to convert the measured data tothe operational environment.

In order to obtain an absolutely exact result, not onlythe volume flow rate, the temperature and the pres-sure of the compressed air, but also the atmosphericpressure, the atmospheric temperature and the hu-midity of the inlet air have to be determined (see Fig.1). This is indispensable for performance testingcompressors.

Inlet temperature T1

Inlet pressure p1

Inlet humidity Frel1

Outlettemperature T2

Outletpressure p2

Outletvolume V2

Power input in kW

12

1221 pT

TpVV×

××=

Fig. 1: Measuring the inlet volume flow rate

Volume flow rate measurements for in-plant ac-counting or planning a compressor system do notjustify the cost of parallel measurements of the am-bient temperature, humidity and atmospheric pres-sure. However, a backward projection should be

made based on average pressure and temperatureconditions at the installation site.

Temperature and pressure compensationPressure and temperature are rarely constant in acompressed air system. Therefore when measuringair consumption, the pressure and temperatureshould be determined as well as the volume flowrate, so that a correct backward projection of themeasured operating state can be made based onthe ambient conditions (see gas equation, Fig. 1).This is essential for an accurate measurement.

Without temperature and pressure compensationIf a volume flow rate measurement is made withoutparallel pressure and temperature measurement andwithout backward projection of these factors to therelaxed state, it is only possible to determine thevolume flow at actual temperature and pressureconditions. Otherwise pressure and temperaturefluctuations occurring during the measurement resultin errors when projecting backwards to the ambientstate.

Direct measurement of the volume ormass flow rateMeasuring the dynamic pressure makes it possibleto determine the volume flow rate with a high degreeof accuracy. A venturi nozzle can be used or alter-natively a dynamic pressure sensor (see Fig. 2).

Fig. 2: Measuring dynamic pressure

dP

v1 v2 v11

2

pstatpstatPdyn +

Measurement withpressure sensor

Venturi nozzle

Measurementwith orifice


Please note:• In order to reach sufficient resolution, the optimum

measurement range should be close to the oper-ating range.

• Electrical connections result in mediocre repetitiveaccuracy and complicated adjustments of the max.four usable contacts.

• The correct lengths of the inlet and outflow zonesare important as is the insertion of the measuringdevice into the distribution system andthe exact geometrical data of the pipe.

• Attention!: contamination hazard.• If the flow rate falls below 10 per cent of the maxi-

mum measured value, this results in lower meas-urement accuracy.

Volumetric measurementVolumetric measurements are highly accuratemeasurements which are used, e. g. to determinethe capacity of compressors. The most importantmeasuring devices are rotary displacement gasmeters and turbine gas meters. Whereas the rotarydisplacement gas meter should be applied in ameasurement range from 10 to 90 % of its max.volumetric flow rate, the turbine gas meter is veryaccurate in the lower measurement range as well.

Please note:

• these measurement devices are complex me-chanical components requiring intensive monitor-ing.

• Not overload-proof (danger in depressurised com-pressed air system).

Calorimetric measurementSo-called hot-wire anemometers can measure thevolume flow rate as a function of the mass flow ratein a compressed air pipe by relating the heat re-moved to the volumetric flow rate (see Fig. 3).

Fig. 3: Calorimetric volume flow rate measurement

Please note:

• Without temperature and pressure compensation,deviations from the design point in temperature,humidity and pressure fluctuations have a stronginfluence on the result.

Coriolis mass flow rate measurementIs based on exploiting the controlled generation ofthe coriolis force. These forces occur where transla-tory (straight) and rotary (rotating) movements over-lap. The magnitude of the forces depends on themasses moved and their velocity and thus on themass flow rate (see Fig. 4).

ω = Angular velocityt A, B = SensorsF = Coriolis force y = Amplitude∆ϕ = Phase shift t = Time

Fig. 4: Coriolis mass flow measurement

OthersIn addition to the classical methods of measuringvolume flow rate, there are several new measure-ment systems available today.

Karman Vortex StreetThe volume flow rate measurement is based on theKarman vortex street (see Fig. 5).

Fig. 5: Karman vortex street

An exactly defined body fixed in a compressed airsystem generates a vortex and thus vibrations whichcan be recorded. They vary analogue to the changesin the volume stream shed from the diverting body.

This test set-up has similar features to dynamicpressure measurement systems


Please note:

• Vibrations caused by the pipeline construction caninfluence the measurement result.

Ultrasonic measurementUltrasonic measurement devices such as thosecommon in gas and water engineering, have not yetbeen widely used in compressed air systems (seeFig. 6).

Ultrasonic transducerSonic beams

Focusing surface

Fig. 6: Ultrasonic flow measurement

Indirect measurementsWhereas the direct measurements already de-scribed can be applied centrally and locally to meas-ure the air consumption in companies and to deter-mine the performance data of compressors, indirectmeasurements with the aid of the compressors serveto determine air consumption values and character-istics of complete compressed air systems.

Digitally recording the load time of compressorsDiscontinuously regulated compressors are con-nected to a data logger which records the full load,no-load and downtime of the compressors (seeFig. 7).

Optocoupler

digital Data logger

analogP-Transmitter

40-20 mA

Fig. 7: Digital load time recording

After these data have been entered into a computer,the capacities of the individual compressors and thetotal air consumption value of the plant can besimulated.

Please note:

• One advantage of this indirect measurementmethod compared to direct measurements is that itnot only provides information on air consumptionvalues but also data on the efficiency and runningperformance of the compressors.

• Easy to set up.• Min. measurement phase 1 sec to include con-

sumption peaks.

Other methodsSimple air consumption measurement or load meas-urements of compressors can also be determined byreading the counter for load operating hours andmeasuring the time needed to drain the boiler.

Please note:

• Very personnel-intensive and rather vague.

Measuring leaks using pressure measurements

Using a pressure sensor which can be easily inte-grated into the compressed air system the pressurecan be measured and recorded over a longer periodof time at short intervals. In order to do so, the sys-tem does not have to be separated, a coupler or aone inch connection are sufficient.

The pressure curves are subsequently processedusing a mathematical method so that the contractorknows exactly how high the share of leaks and howlarge the load capacity share (in per cent) is for eachindividual measurement point. This is done by cal-culating the pressure losses and their gradients,which result in a perfect curve using a mathematicalalgorithm. The perfect curve is then compared withthe actually measured curves (see Fig. 8).

7.0

7.2

7.4

7.6

7.8

8.0

1:40:00 1:40:28 1:40:57 1:41:32 1:42:01 1:42:30

Time

Pres

sure

0%

20%

40%

60%

80%

100%

Pressure Share of useful air use

Fig. 8: Leckage measuring during plant operation


The results are the relative shares of the load ca-pacity or the leaks at each point in time. If the flowrates or compressor. operating times are recorded atthe same time, the relative values can be convertedinto absolute losses.

Please note:

• The advantage of the method is that it is possibleto calculate the leaks during operation. It is there-fore particularly suitable for plants operating incontinuous production.

Measuring leaks by drainingcompressed air receiver

A simplified leak measurement is also possible viathe air receiver. The pressure here is increased tothe maximum required in the system and the time ittakes for a pressure drop of 1 to 2 bar due to leaks ismeasured (see Fig. 9).

7 9

Sealedinlet

Amount of leaks

Tools not in use

tppV EAB )( −×

= VL�

time Measuringtpressure final Receiverppressure initial Receiverp

volume Receiver Vleaks of AmountV

E

A

B

L

=====�

Fig. 9: Measuring leaks by draining compressed air receiver

Air quality measurements under ISO 8573

The manner of taking samples is particularly impor-tant for exact air quality measurements.

If there is a turbulent stream in a compressed airpipe and especially if boundary flows are present,the sample must be taken at a point at which it canbe ensured that it contains a representative andutilisable mix of all the components of the com-pressed air. This can only be guaranteed with a so-called isokinetic sample (see Fig. 10).

1. Sampling probe in main pipe2. Adjustable socket to fix probe3. Compressed air pipe cross section “D”4. Screw-in depth min. „3 x D“5 Direction of flow6. Min. length of inlet zone = 10 x D

Fig. 10: Isokinetic sampling

For the individual groups of contaminants, e. g.

• ISO 8573-2: Oil aerosol content• ISO 8573-3: Water content• ISO 8573-4: Particle content• ISO 8575-5: Oil vapour and hydrocarbon content• ISO 8573-6: Gaseous contaminants• ISO 8573-7: Microbiological contaminantsthe measuring systems described in each of thenorms should be introduced downstream of thesampling point.

The air qualities are classified under ISO 8573-1.




Production of Compressed Airfor Industry, Trade and Commerce

What type of compressor?In practice, piston, screw, and turbo com-pressors are the main types. In addition,there are membrane, sliding vane, helical,rotary tooth and rotary piston compres-sors.

Recipro-cating

COMPRESSORSPositivedisplacementcompressors

Turbocompressors

Radial AxialOscillating

Helicalcompressor

Sliding vane Rotary toothcompressor

~~ ~~

Rotary piston

Membrane

Screw

Rotating

Double shaftSingle shaft

Fig. 1: Types of compressors

Compression principlePiston compressors

Reciprocating compressors function according to thepositive displacement principle. When movingdownwards, the piston sucks air in from the atmos-phere via the suction valve. When the upstroke be-gins, the suction valve closes. The air is expelled via

20

…

40

bar

16

14

12

10

8

6

4

2

10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 160 170 180 190 … 4000 m³/min

Piston

Membrane

Rotary tooth

Rotary piston

Sliding vane

Screw

TurboTS

SVRPRTMP

T

S

RP

RTM

P

Fig. 2: Potential power of compressor types

Facts ProductionPage 2 of 5

the discharge valve. Piston compressors are multi-ple-cylinder (higher capacity) or multiple stage (highpressures)

Fig. 3: Piston compressor

Screw compressors

Screw compressors function according to the posi-tive displacement principle. Two parallel rotors withdifferent profiles counter-rotate within a commonhousing. There are screw compressors with capaci-ties up to 1000 kW. They are powered directly orwith transmission or V-belts.

Screw elements Compression principle

Fig. 4: Screw elements and compression principle

Injection-cooled screw compressors work in a single-stage up to 15 bar and in two stages up to 20 barmax. pressure. Oil-free screw compressors work upto 3 bar in a single-stage of compression and up to10.5 bar in two-stages with intercooling. The mainand auxiliary rotors are driven by a synchromeshgear in oil-free screw compressors to avoid contact.

Turbo compressorsTurbo compressors are dynamic, the rotating ele-ment (called impeller) has blades to accelerate thegas to be compressed.

Fixed control devices on the blades transform thekinetic energy into pressure energy. Turbo compres-sors usually have large capacities and compress oil-free. They compress up to 2 bar in a single stageand up to 7 bar in two stages. Twenty-stage com-pression is possible.

Turboimpeller

Three-stagecentrifugalcompressor

Fig. 5: Turboimpeller and centrifugal compressor

Pressure ranges of screw compressors

Air- or water-cooled

Dry running

one-stage two-stage

up to 3,5 barup to 500 kW

up to 10 barup to 1000 kW

one-stage

up to 13 barup to 160 kW

up to 14 bar up to 600 kW

up to 20 bar up to 200 kW

Fluid-cooled

OilWater

one-stage two-stage

Rotating compressors

Fig. 6: Pressure ranges of screw compressors

Performance testing ISO 1217Appendix CPerformance testing for screw compressors underISO 1217 is described in Appendix A. Appendix Bdescribes the performance testing of the compres-sion stages, while Appendix C is to be used forcomplete screw compressor systems.

Volume flow rateThe volume flow rate (quantity delivered) of com-pressors is measured in accordance with the givenmeasurement method at max. pressure at the com-pressed air outlet of the entire system and projectedbackwards to the inlet conditions.

Inlet conditions:Inlet temperature +20 CInlet pressure 1 barRelative humidity 0%Cooling water temp. +20 C


Inletfilter Throttle

Compressorelement Oil separator

Aftercooler Waterseparator

Fan withmotor

Drive motor Control panel

PC

VF

VF

Marks for the measurement points recommended in ISO 1217, C:VF = Volume flow ratePC = Power consumptionRV = Reference point volume flow rate

All other measuring/reference points do not comply with the ISO norm as they do nottreat the compressor as a complete unit.

RVRV

RV RV VF

PC

M

VF VF

M

Fig. 7: Performance testing under ISO 1217

Internal enginelosses included inengine efficiency

Yield power:mechanical powerin kW deliveredto shaft

Losses from drivingthe cooling fan

transmission lossesdue to belts/gears

Brake horsepower:required mechanical powerin kW at the compressor‘s driving shaft

Engine rating:mechanical shaft powerin kW produced byengine at 100 % load.Shown on engine nameplate.

Total electricalpower input

Electrical power inputat fan motor, if separatemotor exists

0 0 0kWhkWhkWhkWh

Fig. 8: Power flow in compressors

Power consumptionThe electrical power consumption means the totalpower consumption of all motors (drive and fan)taken from the electrical mains.

Specific power requirementsThe tolerances of the specific power requirementpermitted (electrical power consumption divided byquantity delivered) are fixed in the performancetesting standards.

ISO 1217: 1996 (PN2 CPT)

Volume flow rateat given conditions

Volume flowrate

Specificpowerinput

Power inputno-load

operation*)

below 0.5 m3/min +/- 7 % +/- 8 % +/- 20 %

0.5 – 1.5 m3/min +/- 6 % +/- 7 % +/- 20 %

1.5 – 15 m3/min +/- 5 % +/- 6 % +/- 20 %

above 15 m3/min +/- 4 % +/- 5 % +/- 20 %

The tolerances shown contain the manufacturing tolerances of the compressor incl. themeasurement tolerances for the values obtained during the inspection.*) if povided by manufacturer

Table 1: Specific power requirements under ISO 1217

Compressor rooms and compressorassembly (VDMA 4363)The heat produced during compression – which isalmost all the energy entering the compressor fromthe mains – has to be discharged. The permissibletemperatures in the compressor room are fixed inthe German Engineering Federation VDMA standardsheet 4363. They range between +5 °C and +40 °C.If the temperature is too low, there is the danger thecompressor's safety devices will freeze. If it is toohigh, there can be problems with components over-loading.

Depending on the on-site conditions, air-cooled com-pressors can be used up to approx. 250 kW capac-ity.


If there is no possibility to remove heat using airbecause the volume required is too large, the heatmust be removed using water. The operating costsof water-cooled compressors are about 30 % higherthan air-cooled ones.

Ventilation of compressor roomsSupportet convection (with fan, without ducts)

� Low investment costs� Minimal technical input� Automatic space heating in winter

� Only applicable in small/mediumcompressors

� Room temperature increase by∆ t = 5-10 K, therefore increased volumeof cooling air requested

� Risk if inlet air is warm.

Note:

Fig. 9: Natural ventilation for small capacities

Ventilation via duct

� Average investment costs� Average technical input� Cooling air heated by ∆ t = 25 K,

therefore small volume ofventilation air necessary

� Compressor room only heatedslightly

� Heating possible due to hingedvent

� Noise reduction.

Fig. 10: Ducted discharge air in larger compressors

Air coolingThe simplest kind of heat removal is via cooling air.The cool air has to enter the compressor and theheated air then exit it. The volume required has to besupplied by the user. The cooling air can be fed inand discharged through free openings. If this naturalventilation, which is usually found in small compres-sors, is not sufficient, then either the injection or thedischarge has to be supported using a fan. If this isalso no longer sufficient, then the air sup-plied/discharged has to be fed via a duct. An addi-tional fan is necessary in long ducts to bridge pres-sure losses in the duct. Special controls permit amixed air operation in winter. Warm air from thecompressor room is mixed with cold air drawn fromoutside through a louver. Drawing cool air in fromoutside through ducts is also recommended if the airin the compressor room is not clean.

Water coolingIt can be difficult to supply the necessary amount ofcooling air if large amounts of heat have to be dis-charged, i.e. in large compressors, or if severalcompressors are positioned in one room. The ma-chines then have to be cooled using water. Ofcourse, the operator has to have a supply of coolingwater at hand. Fresh water cannot be used on ac-count of the high costs involved. Compressors caneasily be connected to open or closed circuit coolingwater systems. Before the decision is made for watercooling, it has to be ensured that the compressor'scooler is correctly designed for the water quality.Aggressive cooling water requires coolers made ofresistant material.

Another point is often overlooked: in spite of watercooling, the heat emitted by the individual compo-nents in the compressor still has to be removed.Cooling air is still required for this, although a rela-tively small amount.

Heat recoverySpace heating

The most economic type of heat recovery is to ex-ploit compression heat for space heating. Prerequi-site is an air-cooled compressor over which thecooling air can be channelled. This kind of heat re-covery is economic because all the heat, includingthat emitted in the compressor, can be used. Theheated air has to be transported via a system ofducts. Care should be taken to keep the distancesas short as possible. Firstly, long distances meanpressure losses in the duct, which in turn can onlybe compensated using an additional fan, and sec-ondly, heat losses occur if the air is in the duct for along time. Insulated ducts are an alternative butwould also involve higher investment costs.

It must be borne in mind that only winter months canbe used for the amortisation time of heat recoverythrough space heating. In summer, the waste heat isdischarged outside through a hinged vent.

Heating waterFor screw compressors with oil injection, the oil re-moves approx. 72 % of the electrical energy sup-plied. This energy can be recovered. It is irrelevantwhether the screw compressor is air- or water-cooled. To recover the thermal energy, the oilpasses through a heat exchanger which can heatwater by 50 K up to 70 °C. The heat exchanger isusually a plate-type heat exchanger which is capableof very high heat utilisation, can be housed com-pactly and makes these high water temperaturespossible.


It must be noted that water is only heated if thecompressor is operating to capacity. Since this is notalways the case, and thus hot water will not alwaysbe produced, heating water using heat recovery canonly supplement the heating circuit. The amortisationof heat recovery in this case is only possible in thewinter months.

Hot water productionIf the plates are defective, there may be a break-through in the plate heat exchangers used to heatwater so that water and oil mix. To avoid oil-pollutedwastewater, a safety heat exchanger is used whenheating sanitary or process water. The pressure of acarrier fluid between the oil and the water sidechanges if oil breaks through.

A signal is sent through a pressure switch to turn offthe system. In this system, water can be heated byabout 35 K to approx. 55 °C. In contrast to the pro-duction of heating water, an amortisation is possibleover the whole year.

Cold waterWarm water

Oil circulationCompressed air

Fig. 12: Heating sanitary or process water by an oil-injectedscrew compressor




ControlControls in compressor systemsare used for both compressedair production and compressedair treatment. This fact sheetdeals with the controls whichmatch compressed air produc-tion to consumption (see Fig. 1).

Internal and mastercontrollersWithin compressor systems, adistinction is made betweeninternal and master control ofthe compressors. Internal con-trols are responsible for adjust-ing the respective compressorcomponent to the air consumption required and toprevent overloading by an optimal coordination ofthe internal control processes. Since modern com-pressor systems are usually made up of severalindividual compressors, the task of the master con-troller is to operate the individual systems to capacityand to coordinate and monitor their use according tothe actual air consumption.

Types of internal controlFor internal types of control, a distinction is madebetween discontinuous and continuous controls.

Discontinuous controlThe full load-idle-stop/start control is currently one ofthe most common controls in drives without variablespeed control. If the operating pressure reaches theset lower pressure limit pmin, then the compressor isswitched on and delivers compressed air. When pmax

Controllers

cascade

serial load dependent valves full load-idle-

stop/start

pressure band

speed

continuous

full load full-loadshutdown

discontinuous

pulsed directcurrent

frequencyconverter

suction throttlespin throttle

master internal

Fig. 1: Control of compressed air systems

Facts ControlPage 2 of 6

is reached, the compressor is not switched off butinto idling mode by venting. If pmin is reached duringthe no-load period, the compressor then returns tofull load operation. For low air consumption, thecompressor is shutdown after a certain idling period(Fig. 2).

100

Motor power in %

pmax

pmin

Idle

Shutdown

Full-load

20406080

Time

Pressure

t1

t1

Fig. 2: Range of application peak load compressor

Note:

• Fast reaction• High switching frequency without overloading the

motor• If poor load, high energy consumption during idling.In no-load control with optimised idling time, thefollow-up time is varied depending on the pressurefluctuations over time and the motor size and thushelps to make considerable cost savings in idlingmode, especially in base load machines (Fig. 3).

Motor power in %

A BC D E F GHIK L MNOPQ RS TU V W

t3t3t8

t1

t2

t1

t4t5

t3 t6 t1 t7

pmax

pmin

Idle

Shutdown

Full-load

20406080

100

Pressure

Time

Fig. 3: Range of application base load compressor

Note:

• Lowest possible no-load share• Good energy efficiency• Longer reaction time.Systems with discontinuous control have in commonthat they are controlled via pressure limits pmax andpmin.

Measuring transducerThe pressure limits required in mechanical pressureswitches are sometimes up to one bar apart, butpressure differences can be reduced to 0.2 bar to-day using modern pressure sensors.

Note:

• Energy saving through small ∆p• High repeatability• Low pressure fluctuations• No universal interchangeability.

Continuous ControlMotor speed control

The most common ways to regulate speed in mod-ern compressors are either to use a frequencyinverter or direct current modulation. In both casesthe systems are started at a pressure limit pmin. Themotors then progress along a characteristic curve toa speed which is specified by the ratio of actualpressure to control pressure.If the air consumption exceeds the control range ofthe machine, the system is either shutdown orswitched to idling mode depending on the sequencer(Fig. 4).

A B C D E F GH I K L

t1

t3t2 t1

pmax

pmin

Idle

Shutdown

Full-load

20406080

100

Pressure

Motor power in %Time

PR

Fig. 4: Range of application peak load compressor

Note:

• Good controllability• Fast reaction• Constant pressure +/- 0.1 bar• Good energy efficiency in the control range be-

tween 40 and 80 %• Low energy efficiency at load > 80 %, < 40 %• High investment costs• Back coupling to electric gridThe characteristic curve of the controller, the motorand the air end in the partial load range is decisivefor the efficiency of the control mode (Fig. 5).


6.5

7.5

8.5

9.5

10.5

15 25 35 45 55 65 75 85 95

Capacity use in %

kW /

(m3 /m

in)

Fig. 5: Specific performance of a speed-controlled compressor

Suction throttle control

Machines with suction throttle control are normallycompressors with a full load-idle-stop/start controland an additional control device. This is set to acertain control pressure. If this pressure is reached,the inlet valve of the compressor is either closed oropened depending on the plus-minus deviation fromthe control pressure. In positive displacement com-pressors, this actually only involves a reduction ofthe volume flow rate which only has a negligibleinfluence on the performance of the compressor(Fig. 6).

Power

Volume flow100 % 50 % 10 %

80 %70 %

20 %

100 %

Idling

Fig. 6: Control of the volume flow rate using a suction throttle

Note:

• Low costs• Large control range 100 % to 10 %• Extremely poor energy efficiency.

Turbine bypass control system

Controls are characterised as turbine bypass con-trols in which the compressor discharges com-pressed air into the atmosphere and thus adapts theoutput to the actual air consumption.

This type of control is used in low pressure systems(e.g. fans) or also in dynamic compressors.

This control is also used in dynamic compressors toinfluence the performance but this is only possible ina relatively small control range (Fig. 7).

Control range

Blow off range

Pres

sure

Volume flow

Fig. 7: Turbine bypass control system

Note:

• Linear performance in the control range• Control range normally approx. 20-30 % without

turbine bypass (higher energy loss).

Master controllerAmong master controllers, a distinction is made be-tween cascade and pressure band regulation.

Cascade controlThe best known type of coordination is the so-calledpressure cascade; in such setups, every compressoris assigned a particular Schaltbereich by the mastercontroller (Fig. 8).

Compressor 1Compressor 2

Compressor 3Compressor 4

Pres

sure

rang

e

Compressor 5

Fig. 8: Cascade control

Note:

• Pressure band, avoidable energy consumption asa result (per bar approx. 6-10 % excess energyconsumption)

• No consideration of current air consumption• Recommended only up to a maximum of 4 com-

pressors.


For compressors of equal size, the compressors aretransposed into base, medium and peak load de-pending on the running time of the compressors orvia an interval timer. Sometimes when switching 4compressors in a pressure cascade using mem-brane pressure switches or contact manometers,pressure spreads of up to 2 bar are required in order toswitch the systems properly. The use of modern pres-sure sensors makes it possible to reduce the pres-sure spread to 0.7 bar for 4 compressors.

Pressure band regulationModern master controllers use the possibility to con-trol an unlimited number of systems using a pres-sure band, the smallest control difference is 0.2 bar(Fig. 9). The advantage of this kind of control is areduction of the maximum pressure in the com-pressed air system and thus a reduction of the pri-mary energy costs and the losses in the compressedair system.

Upper cut of pressureAir main pressureLower cut of pressure

Pressure fluctuation of conventionalbase load selective switching

Pressure fluctuation with band control

6.0

6.5

7.0

7.5

Safety margin Time

Different pressure fluctuations and pressure savingin conventional base load selective switching (cascade control) and modern compressor controlled sequencers (pressure band control)

Fig. 9: Pressure band regulation

Extension possibilities with master controllersExtended pressure band regulation can also selectdifferent compressor sizes depending on the loadand coordinate these with each other should thedemand arise. The correct selection of the compres-sor size prevents the production of so-called controlgaps (Fig. 10). Control gaps can arise at incorrectgrading of the compressors and a discrepancy be-tween amount of air required and compressed airproduced.

1 x 15 m3/min

1 x 15 m3/min1 x 15 m3/min

1 x 7.5 m3/min1 x 7.5 m3/min

1 x 6 m3/min1 x 6 m3/min1 x 9 m3/min

Fig. 10: Ways to split-up compressed air production

In order to improve monitoring and to depict the pro-cesses within a compressed air system, these mas-ter controllers can record not only the compressordata but also the data of each air treatment and dis-tribution system in a compressed air system andthen transmit these data via a suitable control andinstrumentation installation software to a centralisedcontrol centre (Fig. 11).

�HAUPTANTRIEB STEHT, BREMSE IST AKTIVIERT�WERKZEUGWECHSEL VORBEREITEN

�-DP

�Leitebenen -Netzwerk Ethernet

�Vesis

Service center world-wide

�Telefon

�Modem

�Modem

�Leitwarte

Modem

Modem

Telephone Control room

Control level network Ethernet

Mastercontroller

Process-Profibus-DP

Compressors

Fig. 11: Use of control technology for compressor control


Saving potentialAccording to an EU paper, master controllers canachieve an energy saving potential of 12 % on aver-age by lowering the pressure and better coordina-tion. Optimised internal controls can achieve an en-ergy saving potential of 15 % on average by reduc-ing internal losses.

Storing compressed airThe energy of compressed air is stored in the pipesand receivers. Compressed air users often work verydiscontinuously. Producing compressed air usingcompressors has to be reconciled with the discon-tinuous air consumption. Receivers constitute thebackbone/mainstay of the efficiency of a com-pressed air system. They should be chosen to belarger rather than too small. The influence of thereceiver on the efficiency of a system is dependenton the size of the pressure loss between the meas-urement point of the control and the storage location.Usually this should not be larger than 0.1 bar. To-day, a distinction is made between decentralisedand centralised buffers in a compressed air system.

Centralised buffers

The main buffer receiver in a compressed air systemis primarily there to minimise the switching frequencyof compressors. In addition it prevents overlargepressure fluctuations in the system. It should beselected in accordance with the equation shown,although the efficiency of the compressed air systembenefits if a larger receiver is selected than theminimum value calculated in the equation (Fig. 12).

z ≈ 45 for screw compressors (full load; idle) Rule of thumb: (x - x2) ≈ 0.25

Usualz-values/h for

motor switching:Compressor

power

7.5 kW 30 kW110 kW250 kW

3015 8 4

VB = V1 • (x - x2)

z • ∆∆∆∆p

•

VB = Volume of air receiver [m³]V1 = Quantity delivered by switching compressor [m³/h]V2 = Peak consumption minus average consumption [m³/h]x = V2 : V1 = Load factor [m³/h]z = Permissible switching cycle [1/h]∆∆∆∆p = Pressure difference ON/OFF [bar]

•

•

• •

Fig. 12: Dimensioning of centralised compressed air storage

Decentralised buffer

The decentralised buffer often serves to supplycompressed air to users with sudden large and tem-porary demand and to prevent a pressure drop in therest of the air mains. It has to be selected corre-sponding to the running time, the air consumptionand the permitted pressure fluctuations of the de-centralised user (Fig. 13).

VB = Volume of air receiver [m³]V = Air consumption [m³/min]t = Duration of air consumption [min]∆∆∆∆p = Permissible pressure drop [bar]

•

Note: Does not replace the compressor over a longer period

Use as:

� Buffer for short but acute with-drawal of compressed air

� as emergency “power generator”

VB = V • t∆∆∆∆p

•

Fig. 13: Dimensioning decentralised storage




Compressed Air TreatmentThe quality of untreated compressed air is no longersufficient for most applications today and would re-sult in quality reductions in compressed air products.This may mean disturbances of production systemsright up to lost output or unusable products, i. e. aclear and critical reduction of product quality. Thecompressed air application determines the air qualityrequired.

Maximum number of particles/m³Particle size d (µm)

Cla

ss

0.1< d ≤ 0.5 0.5 < d ≤ 1 1 < d ≤ 5

Pressuredew point

(°C)

Remain-ing oil

content(mg/m³)

0 specified according to application and better than Class 1

1 100 1 0 ≤ -70 0.01

2 100 000 1 000 10 ≤ -40 0.1

3 – 10 000 500 ≤ -20 1

4 – – 1 000 ≤ +3 5

5 – – 20 000 ≤ +7 –

Table 1: Purity classes under DIN-Norm ISO 8573-1

The maximum loads with particles, water and oil aredivided into purity classes in the DIN norm ISO8573-1. This allows manufacturers of compressedair products to define the required quality.

Drying compressed airThe different methods of drying compressed air canbe classified as shown in Fig. 1 using the achiev-able pressure dew point and the energy necessaryfor this: depending on the system, the energy de-mand is recorded as compressed air or as electricalenergy.

14

20

10

Ene

rgy

dem

and

due

to d

irect

or i

ndire

ctel

ectri

cal e

nerg

y re

quire

men

t Adsorption dryer

3 (-20 °C)

Refrigeration dryers

Membrane dryers (only for small volume flow rates)

3

1 (-70 °C) 2 (-40 °C) 4 (+3 °C) 5 (+7 °C) 6 (+10 °C)Pressure dew point classes

Refrigeration dryers

Adsorption dryers

(cold-regenerated)

Adsorption dryer

* An indirect electrical energy require-ment results from the additionaldemand for compressed air in order tocompensate the rinsing or regene-ration losses of the system (e.g. cold-regenerating adsorption dryers).

Membrane dryer

Watt/m³

(externally heat-regenerated)

Fig. 1: Methods of drying compressed air

Refrigeration dryersRefrigeration dryers are state-of-the-art today incompressed air systems and just as important asthe compressed air producer itself. Furthermore

Facts TreatmentPage 2 of 7

they represent the most economic process for themajority of applications.

Physical basis:The ability of compressed air to conduct water de-creases with falling temperature. When the tem-perature drops, the water vapour condenses to wa-ter. The refrigeration dryer extracts the water vapourcontained in the compressed air. To achieve this, thecompressed air is cooled in a heat exchanger sys-tem. Water and oil vapour are extracted by conden-sation, oil by coagulation and coalescence. Thecondensate is drained off.

1 Air/air heat exchanger2 Air/refrigerant exchanger3 Condensate trap4 Refrigerant compressor5 Refrigerant liquefier

Compressed air circuitRefrigerant circuit

Compressedair inletT = 35 °C

Compressedair outletT = 27 °C

T = 18 °C

1

54

3

2T = 0 °CT = 0 °C

Fig. 2: How the refrigeration dryer functions

Economic refrigeration drying is divided into twophases. In the first phase, the warm incoming com-pressed air is cooled by the already chilled exitingcompressed air in the air to air exchanger. Approx.70 % of the accumulated water vapour are precipi-tated here. In the second phase, the compressed airflows through a coolant/air heat exchanger. This iswhere cooling to the required pressure dew pointoccurs. The condensate trap is downstream from theheat exchanger. The condensate is separated herefrom the compressed air.

Integrated heat exchanger systems which integrateair to air exchangers, coolant-air exchangers andcondensate traps in one system component aremore energy-efficient due to lower differential pres-sures compared to separate casings.

Fig. 3: Heat exchanger with integrated condensate trap(demister)

Adsorption dryersAdsorption dryers extract the humidity carried in thecompressed air using a desiccant. While adsorptiontakes place in the first container, the desiccant isregenerated at the same time in the second con-tainer. Pressure dew points between -20 and -70 °Ccan be achieved with standard products. There arevarious processes available for the regeneration. Adistinction can be made between cold and warmregenerated adsorption dryers depending on thetype of regeneration involved.

Cold regeneration

For the regeneration, some of the already driedcompressed air is depressurised to atmosphericpressure.+ simple technique+ low investment costs– consumption of compressed air– high operating costs.

Fig. 4: Cold regeneration


Heated regeneration

Regeneration takes place with heated ambient air orheated air from the system.

Blower regenerationIn the heating phase, a blower forces ambient airthrough the heating. The heated air transports thehumidity from the desiccant bed. Ambient air andcompressed air are used for cooling.+ lower operating costs by heating with steam or

electrical energy– compressed air consumption in the cooling

phase.

Heated regeneration without using compressed airBy modifying the set-up and procedure, the desic-cant bed can be cooled using ambient air. Theseadsorption dryers are divided into blowing, suctioncooling or vacuum regeneration systems.+ Lower operating costs by heating with electrical

energy or steam+ no consumption of compressed air in the cooling

phase– higher investment costs– restricted use if ambient air is very humid.

Compressor heat regenerationWhen using oil-free compressors in combination withadsorption dryers, the heat generated during com-pression is used specifically for the regeneration ofthe adsorption dryer. Pressure dew points of -30 °Cand better are guaranteed by suitable compressors.+ Uses the compression heat for regeneration+ no consumption of compressed air– only with oil-free compressors.

Fig. 5: Heated regeneration

Control

All heatless or heated-regenerated adsorption dry-ers are equipped with a time-dependent control.This comes as a manufacturer-specific variant orPLC depending on the extent of control required. Aload-dependent control is an optional supplement.At the dryer outlet, a sensor registers changes ofthe pressure dew point. It automatically adjusts thecycle of the dryer to the load situation. The load-dependent control compensates possible part-loadsituations and reduces operating costs.+ Minimum operating costs even at part-load op-

eration+ continuous pressure dew point measurement

for quality control.

Membrane dryerThe membrane dryer is a supplement and alterna-tive to the traditional refrigeration and adsorptiondryers. It is particularly effective as a point-of-usedryer for smallest compressed air quantities, non-continuous operation or applications without electri-cal energy.

The heart of these membrane dryers are polymerhollow tube membranes which allow the water va-pour to diffuse.

FiltrationThis is used to remove contaminants from the com-pressed air to a large extent.

The main contaminants include oil vapour from oil-lubricated or oil-injected compressors as well assolid particles and hydrocarbons from the ambientair which are then contained in concentrated form inthe compressed air. To guarantee the compressedair quality required today, purification is mandatory.

Due to an increased environmental awareness aswell as stricter measures of health protection atwork, requirements are also made of the emissionvalues of the compressed air expanded after use,specifically with regard to oil vapour, which is emit-ted to the ambient air, e.g. directly from a com-pressed air cylinder or a nozzle.

However, filters also consume energy. Althoughthere is no energy input to a filter, energy is con-sumed by the filter due to the pressure drop (differ-ential pressure) caused which has to be provided bythe compressor located upstream of the filter. Thefollowing rule applies:


The higher the degree offiltration, i. e. the greater thepurity of the filtered air, thehigher the differential pres-sure, i. e. the greater theamount of energy which hasto be supplied by the up-stream compressor.

Filters are therefore necessary, butcost energy and thus money. It isimportant to select the right quality ofpurification depending on the appli-cation involved. ISO 8573-1 or themanufacturer concerned can helpwith the selection.

It makes sense to think carefullyabout the degree of compressed airpurity actually required, in order toindividually select the filter(s) withthe lowest possible differential pres-sure for the applications involved.Fig. 6 shows the saving potentialsconcerned. It shows the energycosts caused by compressors incompensating the pressure dropcaused by the filter. These costs canamount to several thousand euro peryear and may far exceed the pur-chase or replacement costs of theelement. Enormous savings can beachieved by selecting the correctfilter with the lowest possible differ-ential pressures.

Timing the replacement of dirty filters correctly,which have increased differential pressure, isequally important. As shown in Fig. 7, the differen-tial pressure of a new filter element increases veryslowly at first. The longer the element is in opera-tion, the quicker the differential pressure increases.If this element is not replaced, the costs of cover-ing the additional differential pressure are some-times many times higher than the price of a re-placement. As a rule:

Replace elements once a year, at the latestat a differential pressure of 350 mbar

Activated charcoal filters are the exception to thisrule. Here, the following rule applies:

Service life of the elements: max. 1,500operating hours or 3 months, dependingon the inlet temperature and the oil contentsometimes much shorter.

Finally there is the question of the operating safetyof a filter. This criterion depends primarily on thequality of the tools used, the quality of productionand the design features of the filter. The filter con-struction has to be assessed individually. The crite-ria for a filter are summarised below:

Filtration efficiency +Operating safety + Differential pressure =Total operating costs

The sum of these three criteria then determines thetotal operating costs of the filter, breakdown costsdue to insufficient filtration or a failure of the filterare already included.

0

250

500

750

1000

1250

1500

1750

2000

2250

0 0.1 0.2 0.3 0.4 0.5

Pressure drop in bar

Ann

ual e

nerg

y co

sts

in E

uro

110 kW

75 kW

45 kW

30 kW22 kW

11 kW

Com

mpr

esso

r per

form

ance

Operating parameter:6000 hrs/year0.06 Euro/kWh

1 bar ≈ 8 % more energy

Fig. 6: Energy costs due to pressure drops

Costsfilterelement 0.3 bar = 85 % element costs

0.5 bar = 145 % element costs






Differential pressure = operating costsInvestmentcosts

Operation period

Fig. 7: Typical differential pressure; ratio of energy costs to filter element costs


Preliminary separationThe first treatment stage in a compressed air systemis the separation of free condensate from the com-pressed air. To do so, a cyclone separator or a re-ceiver is used at the compressor outlet. The re-ceiver is the simplest system. By reducing the flowvelocity and cooling the compressed air on the largesurface area of the receiver, the condensate is col-lected at the bottom of the receiver and can bedrained. With its vortex, the cyclone separator util-ises mass inertia for separation. Both systems im-prove the performance of the compressed air treat-ment since considerable amounts of condensate areremoved. Neither component replaces compressedair drying since the compressed air is saturated with100 % water vapour after these separators and freewater condenses with each further cooling of the air.

Condensate technologyCondensate is an inevitable by-product of producingcompressed air. This condensate is formed from thehumidity contained in the input air. At compressionand the associated increase in temperature, thishumidity is first present as vapour. Because only aminor fraction of the original volume remains aftercompression, the air becomes oversaturated. Whencooled, the air humidity is precipitated as condensewater. Apart from water and oil, this condensate alsocontains all the other components of the ambient air

sucked in by the compressor. These are concen-trated and result in contamination of the conden-sate.

Consequences of the condensate for the com-pressed air system:

Condensate, irrespective of whether it contains oilor not, results in corrosion in the pipe system anddownstream processes. Whereas oil-free conden-sate has a more acidic effect due to its pH value,oily condensates have the effect of clogging andsticking. The air quality required, even at lowerclasses, can no longer be achieved.

Where is the condensate formed?

Condensate is always formed if the temperature inthe compressed air falls below the pressure dewpoint. This happens in after-coolers, receivers, cy-clone separators, filters, dryers and in the pipesystem. The largest amount of condensate is pre-cipitated at the point of the greatest temperaturedrop after compression.

Trapping condensateDue to the high costs of the resulting damage, re-moving the condensate from compressed air isassigned a very high priority. There are three com-mon ways to trap condensate:

Float control:

The condensate is collected ina storage tank. A float opens avalve when a certain conden-sate volume is reached.

+ low investment– very sensitive to dirt– no monitoring possibilities.

Time-controlled valves:

A valve operated by a timerswitch opens at a fixed interval.

+ large opening diameter+ also available in a high

pressure version– compressed air loss– high energy cost– no monitoring and

operational checks.

Condensate yield per 10standard cubic metre in

Winter

Summer

Spring/Autumn

25 g/m³

Cyclone separatorAftercooler

28 g/m³

53 g/m³

Receiver

3.5 g/m³

6 g/m³

9.5 g/m³

Filter

- -

2 g/m³

3 g/m³

Refrigerationdryer

3.5 g/m³

9.5 g/m³

21.5 g/m³

Fig. 8: Condensate yield according to season


Fig. 9: Time-controlled valve

Electronic level-controlled separator

A sensor located in the condensate collector triggersthe draining of the tank when a set value is reached.

Fig. 10: Level-controlled separator

+ Energy saving+ no compressed air losses+ fault and alarm functions.

Condensate treatmentFrom the legal viewpoint, compressor condensateconstitutes waste which requires particular monitor-ing. The law offers a choice of two possibilities fortreating condensate. Either the specialist disposal byauthorised companies or treatment on site with suit-able and certified treatment technology. Conden-sates occur either as oil/water mixes or stable emul-sions. In practice, these are the main methods.

Static oil/water separator

In this process, the condensate is held for a prede-fined retention time in a separating tank. The lighteroil components rise to the surface. The fine residueand other substances are filtered out in a down-stream activated carbon stage. This method is al-ways sufficient if the condensate is present in adisperse form.+ simple system+ fast amortisation.

Fig. 11: Static oil/water separation system

Emulsion separation systems based on adsorption

With this method, a reaction separating agent isadded to the pre-cleaned condensate. Electrolytescontained in the separating agent break down theoil-water compound and thus split the emulsion.The oil and other components of the condensateare adsorbed by the aluminium oxide and filteredout of the water. Only the residue formed has to betaken for disposal.

Ultrafiltration

With ultrafiltration, the condensate is circulated un-der pressure and filtered through a membrane witha controlled pore width. The oil components areretained and concentrated, while the water is puri-fied and then discharged into the waste water sys-tem without any further filtering. The concentratedemulsion is then disposed of.

In each case, care must be taken when buying ap-pliances and replacement parts that these are li-censed, otherwise expensive individual technicalapproval of the appliances has to be conducted bythe local authorities.


SummaryCompressed air treatment in air mains is state of theart today. The basic demand made of this treatmenttechnology is the reliable and high-level removal ofthe contamination and humidity from the com-pressed air. This contamination leads to quality re-ductions and disturbances up to unusable products.How complex this treatment has to be and whichoperating costs are incurred can be clearly influ-enced by comparing the products found on the mar-ket and selecting the most suitable for a particularapplication.

In the compressed air treatment sector, the mainconcern is to achieve the optimum quality by fulfillingthe specification of the application at optimum en-ergy and operating costs. Increased energy or oper-ating costs result from exceeding or failing to meetthis specification. Figs. 12 and 13 give an overviewof which order and choice of treatment productsachieve which compressed air quality.

The available savings potential per subcomponentcan amount to several thousand euro. Specifically,regular replacement of filter elements within the pre-scribed intervals can achieve obvious savings andthus minimise operating costs.

The serious analysis of the installed or plannedcompressed air system represents an investmentwhich sometimes pays off very quickly.

Residual

0

0

00

1

34

4

Particles

0-1

0-1

0-10-1

0-1

23

4

Compressed air qualityclasses under ISO 8573-1

Humid, contamined air Centrally dried air

4

4

44

4

47

7

CompressorReceiver Refrig.

dryerDFACSF

D(S)F

D(S)FDFAF

DF

SF

AF = activated carbon filterAC = activated carbon adsorberSF = surface filterDF = depth filterD(S)F = depth (sterile) filter

Residualwater oil

Fig. 12: Compressed air quality when using refrigerationdryers

0-1

2

1

1

2

Particles

0

0

1

1

1

Residualoil

0-3

0-3

0-3

0-3

0-3

Residualwater

Compressed air quality classes under ISO 8573-1

Humid, contaminated airCentrally dried air

CompressorReceiver

Adsorptiondryer

DF AF SF

D(S)F

D(S)F

DF

SFAF = activated carbon filterSF = surface filterDF = depth filterD(S)F = depth (sterile) filter

Fig. 13: Compressed air quality when using adsorption dryers




Air Distribution

Energy saving in air distributionAn optimum compressed air distribution is an energypipeline like an electricity cable which transportscompressed air energy with as few losses aspossible, i. e. with the lowest reduction of the

• flow pressure (pressure drop due to narrow pointsin the pipeline)

• the air quantity (leaks) and• the air quality (rust, welding scale, water etc.).

Pipeline systemIn practice, compressed air tubes (main anddistribution lines) are frequently selected withoutsufficient knowledge and without taking energeticissues into consideration with the result that in 80 of100 firms (EU study), often 50 % and more of thecompressed air energy are destroyed before theycan reach the usage points.

The correct planning of a network has direct influ-ence on the performance of the machines and thecosts of producing compressed air. Select the cor-rect diameter taking into account the flow rate re-quired and the permissible pressure drop. The pres-sure drop from the air receiver in the compressorroom to the final connection point should not exceed

0.1 bar. In an optimally designed compressed airnetwork, the pressure loss is split into:

≤ 0.03 bar for the main line≤ 0.03 bar for the distribution≤ 0.04 bar for the connection≤ 0.3 bar for connection equipment

Just as the economic efficiency of thecompressor is documented, the operatingefficiency of air distribution should also bedocumented – missing documentation al-ways means wasted energy.

Main line (ML):Connects the generator system (compressor room)with the distribution system. The main line should besized so as to allow reserves for future extensions.

Distribution line (DL):This distributes the air within a consumption sector.It can be designed as a branch or ring line, or as aring line with integrated branch lines.

Facts Air DistributionPage 2 of 4

Connection line (CL):This is the link between distribution and machines ordispensers/points of use. The joint between the con-nection line and the distribution should be at the topin order to avoid condensate exiting with the air.

Connection equipment:These system components are frequently the criticalpoints of a system and also require careful attention.Couplings, hoses, coils or maintenance units oftenresult in enormous energy wastes due to wrong de-sign. In addition there are many connections herewithin a limited space which can leak.

Explanation of termsdeciding factors

Flow pressure

In spite of decades of aware-ness training by the manufac-turers, the majority of com-pressed air tools are onlydriven with a flow pressurebetween 3 and 5 bar, that is 1to 3 bar too low. The ma-nometers on the controllersand maintenance units beforethe tools show the static pres-sure. But this does not drivethe tools, the dynamic flowpressure does.

Other adverse effects on theflow pressure occur if the pipediameter is too small or the pipesystem has too many bends.Furthermore, when designingthe system, the correspondingequivalent lengths have to beplanned for all connectors.

Air quantity

In compressed air distribution systems which havegrown over the years, are made of widely differingmaterials, with different, suboptimal diameters, moreor less corrosion-free materials and very differenttypes of connection, the rate of leakage can be be-tween 25 and 35 %. Leaks cost a lot of money. Theyare the busiest consumers working 365 day per year.

Air quality

Corrosion- and oxidation-proof premium pipelinesare preferable which have been specially developedfor compressed air applications. A system should beselected so that the quality of air generated by pro-duction and treatment is not impaired by the pipelineeven after a long period of time.

ML= Main lineDL = Distribution lineCL = Connection line

HLNL = 100 mPe = 6 barQ = 200 l/s∆p = 0.03 bar

Ø i = mm

CompressorML

DL

∆p < 0.9 bar

DL

CL

NL = 10 mPe = 6 barQ = 25 l/s∆p = 0.04 bar

Ø i = mm

Ø i = mm

bar030p

sl1002Q

bar6Pe

m1502

NL

.

/

=∆

=

=

=

Fig. 1: Naming of piping segments

Air DistributionPage 3 of 4

Flow pressureat tool(Pe bar)

Air con-sumption

% Measure8.0 125

7.0 111} throttle

controller waste of energy

6.3 bar 100 % optimum performance6.0 96

5.0 77

4.0 61

3.0 44

} increasepressure

disproportionatedecrease inproductivity

Table 1: Annual energy costs due to leaks

Air loss Energy loss CostsHolediameter

mm at 6 barl/s

at 12 barl/s

at 6 barkWh

at 12 barkWh

at 6 bar€

at 12 bar€

1 1.2 1.8 0.3 1.0 144 480

3 11.1 20.8 3.1 12.7 1 488 6 096

5 30.9 58.5 8.3 33.7 3 984 16 176

10 123.8 235.2 33.0 132.0 15 840 63 360

(*) kW x 0.06 € x 8 000 Bh/a

Table 2: Annual energy costs due to leaks

Internal pipediameter

Pressuredrop

Investmentcosts

Energy costs tocompensate thepressure drop

1. 90 mm 0.04 bar � 10 000 € � 150 € p. a.2. 70 mm 0.2 bar � 7 500 € � 600 € p. a.3. 50 mm 0.86 bar � 3 000 € � 3 270 € p. a.

Table 3: Costs resulting from selection of too small diameters

Storage

Another influencing factor for the air quality andquantity is the storage of the compressed air. Com-pressed air storage straight after production alsocalled "central storage" influence the air quality tothe extent that the direct condensate is removed.Furthermore, storage makes it possible to meet re-quests for much larger quantities of air within a shorttime than the compressor could provide immediately.There is also the possibility – depending on the useinvolved – of "decentralised storage" directly at thepoint of use. Further information on storing com-pressed air can be found in the "Druckluft effizient“facts Control and Treatment.

Costs

When comparing the investment costs, the materialand assembly costs of the different pipe systemsshould be compared as there is no generally validformula for the correct pipeline material. For thisreason, priority should be given to the individualdemand case with its respective technical require-ment.

With the exception of stainless steel, the costs ofvarious pipeline materials do not vary widely, thedifferences are so small that they can be negated inthe annual amortisation amounts/depreciationamounts.

However, selecting the correct nominal width is deci-sive. Considerable consequential costs can arise ifthe diameter is too small. Whoever tries to save onacquisition costs here, has to pay for this in conse-quential costs (see Table 3).

Refurbishment of air distributionsystemsGenerally, a pipeline inspection should not be de-layed for economic and ecological reasons. But suchan inspection should also be conducted step for stepand not in haste.

Large saving potentials in compressed air distribu-tion can be determined based on a quick rough di-agnosis as follows:

• air quality• leaks• pressure drops.

Does the air quality meet the requirements?Alongside the type of treatment, this is mainly aquestion of whether the pipe system is corrosion-free. Does the air at the points of use still correspondto the values (produced) at the production outlet?Carbon deposits/water, rust or zinc dust (even if onlyin subsections) often make additional expensivemaintenance work necessary at each extractionpoint as well as a centralised treatment.

Are there leaks in the system?By recording the load at the compressors and com-paring this with the existing extractions, the quantityof leaks can be determined. It is vital to considerboth "opened" and "closed" points of use, sinceleaks at the connecting equipment and in the ma-chines may falsify these measurements.

Facts Air DistributionPage 4 of 4

The impact on the tools of supercharging can alsobe regarded as "leaks". A tool which needs 6 bar,but is charged with 7 or 8 bar wastes considerableadditional amounts of air.

How high is the pressure drop?This can arise due to too small diameters. In net-works which have grown over time, more and morepoints of use have been added to longer and longermain lines without these having been redimensionedin accordance with the requirements. Perhaps, onlythe compressor capacity has been increased. Afterthe diagnosis has been made taking into account allthree criteria, an economically sensible refurbish-ment can be determined. Either, certain parts orsectors should be refurbished or, if all the negativephenomena coincide, a new network may be themost economic solution from the cost/benefit aspect.Such refurbishments often cost much less thanyears of wasting energy – the payback periods arevery short.

An economic concept can be designed by any spe-cialist compressed air company!

Often, a meticulous observation usingmeasurements of the complete systemfrom production and treatment throughdistribution right up to the mechanisms ofthe machines is a time-consuming neces-sity, which, however, pays off lucrativelyfor a company – regardless of the type andsize – quickly and long-term.

The maintenance of the most expensiveenergy source which is also vital to pro-duction should done as diligently as itreally deserves!!!




System OptimisationRoadmap for optimisation

1. Situation audit: assessment of actualstate

2 Engineering – concept3. Examination of complete system4. Reducing interfaces5. Efficiency of compressed air production6. System follow-up – system optimisation7. Outsourcing the compressed air supply8. Organisational changes.

In the age of rationalising industrial plants, the opti-misation of complete systems is a very important toolto increase efficiency. This applies to compressedair technology as well.

Even if at the beginning of an inspection it seemsthat in the past decades not enough emphasis wasplaced on the planning and development of the rele-vant compressed air system and any possible ex-tensions, at closer inspection the problem is usuallyrevealed to be more complex. For example, in thepast, the compressed air system was often consid-ered the responsibility of departments which actuallyhad nothing to do with the technology and was thustreated as a secondary concern. This "uncontrolled

growth" was supported and even reinforced by theadvantage of compressed air – that it is "accident-proof". That compressed air is one of the most ex-pensive energies was often overlooked.

By inspecting the complete system with its diverseimprovement options, quite considerable savingscan be achieved at often very little expense. Severalpoints have to be taken into consideration, althoughthe emphasis here is on supplying ideas and practi-cal assistance. Several ways to optimise a systemwithin the frame of a complete system check arerepresented below.

Situation audit:assessing the actual state

The compressed air system consists of thefollowing sectors:�� Production�� Treatment�� Distribution�� Consumers.

In order to assess the system's condition, an initialoverview of the actual state should be obtained byviewing the plant, room and pipe system diagrams.With the help of the available measurement technol-

Facts System OptimisationPage 2 of 4

ogy (Fig. 1), the relevant parameters such as volumeflow rate, flow pressure, and compressed air quality(temperature, humidity, pressure) can be recorded.In addition, alternative values for the electricity con-sumption of the compressor (load/no-load meas-urements) with subsequent illustration of load pro-files or the measurement of leaks can be carried out(see Facts Measurement technology).

Fig. 1: Measurement technology

Particular attention has to be paid to the predomi-nant system pressure. The most important user isoften located at the end of the network (possiblysupplied via a branch line) and is decisive for thepressure generated in the system. To some extent a"historically developed pressure level" is carried,which has originated primarily from network andplant extensions and which could be reduced atcloser inspection, and through only minor changes inthe network, e.g. through ring closures.

From the individual measurements cited above,valuable information about the system's condition(e.g. inlet and outlet air problems, overloading ofprocessing units, cooling etc.) can be obtained. Thisopportunity should also be used to check the specifi-cations for the compressed air quality required. Allrequirements which deviate from the usual standard(oil-lubricated air processed using refrigeration dry-ers, with simple filters of 1 µm particle size and1 mg/m³ residual oil content, pressure dew point+3 °C), require additional investments and operatingcosts due to the treatment measures then necessi-tated. see Facts Air Treatment).

As soon as the specifications regarding amount,compressed air quality, necessary availability andthe associated redundancy have been fixed, thesystem's durability can be checked for what cancontinue to be used with regard to condition, age,energy efficiency etc.

To assess the compressed air system downstream,it is prudent to estimate the distribution losses occur-ring in a leak check. These should normally lie be-tween 15 and 40 % (rule of thumb).

Ascertaining leaks can either be calculated bymaintaining the pressure in the distribution networkwhile the plant is not operating, or, if this is not pos-sible, be calculated from the pressure curves meas-ured during operation. There is a mathematical ana-lytical method available to do this. To estimate theleak potential during operation, an ultrasonic detec-tion tool is helpful.

Another aspect is the higher level control of severalcompressors in a system and thus in a network.Large market innovations have become available inrecent years with integrated processor technology sothat it is always sensible to regard controlling andcontrol technology separately.

According to up-to-date studies, no-load periods, forexample at unregulated screw compressors of up to30 % and electrical power required at no-load op-eration of likewise 30 % of the drive power are citedas starting points for a possible optimisation in con-nection with the use of state-of-the-art controllingand regulation concepts (see Facts Control).

The audit should be completed by a detailed reporton all the work conducted and processes with rele-vant diagrams and illustrations of measurementcurves, Process and Instrumentation Diagram P&ID(any analyses of saving potentials conducted), aswell as formulating suggestions for optimisation.

Engineering – conceptParticular attention must be paid to the overall con-cept when implementing the insights gained from theaudit (effectively taking a wider view): it is obligatoryto comply with frame parameters such as, e.g. legalregulations and possibly a given supply concept(e.g. for condensate).

The energy concept cannot be regarded as a sepa-rate unit, but has to be seen as linked to possibleheat recovery and the synergy effects of other nec-essary energies, such as, e.g. nitrogen demand.Furthermore, it is important to correctly select theindividual components including redundancies to beused in extensions, modernisations and new con-structions in accordance with the overall concept.

With modern integrated control technology (key-words tele-service, remote monitoring and control)the service quality of the system can be boostedquite a lot. This usually involves guaranteeing thelargest compressor unit or supplying the corre-sponding system redundancy. A corresponding reli-


ability can be achieved by additional network con-nections.

Another important point is the overall concept ofmaintenance and service, which essentially co-determine any resulting expenses.

Examination of complete systemWhen examining the in-plant measurement technol-ogy, particular care must be taken to use the tech-nology sensibly. It has to be determined which per-manent measurements of, e.g. energy consumption,leak detection, pressure losses, specific overall per-formance should be made to monitor the systemalongside the "normal" plant measurements such asvolume flow rate, pressure and pressure dew point.Corresponding to the measures involved, it might beprudent to carry out a cost-benefit analysis.

With regard to controlling, it has to be checkedwhether automatic regulation or an infinitely variableone should be installed (see Facts Control).

Note: According to the EU study "Compressed AirSystems in the European Union" an energy savingpotential of approx. 20 % is realisable through em-ploying efficient and higher level control systems.

Reducing interfacesExamining the organisational coordination of thecompressed air system is also important. It shouldbe checked whether it is sensible to consign thecompressed air system its own organisational unit.The resulting cost transparency is one big advan-tage of this and thus better cost control. Up to now,compressed air technology has been knowingly orunknowingly billed at several accounts, which madeit very difficult to check the costs incurred.

This can be changed if someone is put in charge ofa project and thus of the costs in order to improvethe organisation.

When conducting service and maintenance meas-ures it is advantageous if the work can be planned inthe long term. In practice this means preparing thenecessary checklists and maintenance plans inplenty of time in order to ensure maintenance of theplant components without interruption (keyword faultmanagement).

Efficiency of compressed air productionTo determine the efficiency, the m³ costs can be takenas a benchmark for energy/maintenance/capital.Determining the components via the specific outputand service costs is also possible. A cross compari-son with other consumers or projects with subse-

quent optimisation suggestions is to be recom-mended.

After these assessments, estimating the potentialshould be done including the calculation of additionalinternal costs, investment costs, replacement in-vestments, operating costs and service and mainte-nance costs.

The next recommended step is to prepare a balancesheet of the compressed air system. This includesidentifying the specific benchmarks, the degree ofefficiency together with the affiliated network pa-rameters.

The energy efficiency can be further increased by,e.g. examining heat recovery possibilities. Whenoptimising the system, it is also very important tocheck for misuse of compressed air such as, e.g.cooling workers on hot days etc. This is to do withincreasing workers' awareness of the issue.

Finally, the targeted improvements are put into prac-tice.

Fig. 2: Compressed air system

System follow-up – system optimisationHere it is necessary to carry out basic examinationssuch as checking the energy efficiency with alterna-tives and the examination of existing energy formssuch as, e.g. combined heat and power. Linked withthis is the general examination of the existing oper-ating and installation conditions and the mainte-nance friendliness.

It is not sufficient to optimise the system only once.Indeed it is necessary to regularly adjust the systemto changing specifications (consumption, systempressures etc.). The changes in the system arecaused by modifications which are not centrally co-ordinated. It is therefore very important that internal


system modifications should have to be registered oreven better have to be officially authorised first.

In practice in the past different control mechanismssuch as cost controlling and systems to maintainperformance have paid off repeatedly. System prog-nostication is a good tool used to compare today'srequirements with future requirements.

Outsourcing the compressed air supplyThe pros and cons for this course of action have tobe carefully balanced. The contractor's guarantee ofan energy consumption per NM³ compressed air isone argument in favour of outsourcing compressedair. It is therefore in his interests that the systemoperates efficiently. Furthermore, it is ensured thatthe system will be in the hands of a specialist andthat the firm's own staff are not burdened with unfa-miliar tasks.

Arguments against outsourcing are that the key skillsinvolved in optimising compressed air systems,planning new systems and maintaining systems arethen lost to the customer. If the compressed airsystem is reintegrated at some point, these skillshave to be built up again. (See contracting guide-lines.)

Organisational changesIn practice, it has been shown that management ingeneral does not have a high regard for the com-pressed air system. However, those in charge of thesystem are usually to blame for this situation sincethey do not pass on important concerns to the man-agement and thus lead a "wallflower" existence.

The personnel situation has to be examined. If nec-essary, staff have to be trained for the relevanttasks. Another possibility is to appoint someone tobe responsible for compressed air.

ConclusionAs well as the utilisation of individual energy efficientcomponents for the production, treatment, distribu-tion and use of compressed air, the optimum coordi-nation of all the components with one another is alsoof particular significance. The sum of efficient com-ponents does not necessarily result in a reasonableoverall result. The existing optimisation potential isalso considerable.

Professional external help is probably often neces-sary here, but it is also important to ask the rightquestions before commencing the project, and whenplanning and implementing it.




Compressed Air Tools"Before" the toolCompressed air as an energy source for tools has astrong influence on the efficiency of the work per-formed by the tool.

Measures at the workplace to optimise the air supplyof the tool can often contribute to clearly increasingproductivity and lowering energy costs.

Correct system design, from the compressor to thetool, is essential for the efficiency of the overall sys-tem. Many systems have "grown" over time, witholder components which have not always beenadapted to current conditions. Wrongly sized com-pressors or too long running times cause high costsin the same way as pipe losses and leaks (see "cor-rect working pressure").

More detailed information on the design of individualcomponents and their coordination can be found inthe fact sheets "Production", "Control" and "Distribu-tion".

Massive deterioration of productivity because oftoo low working pressure!Compressed air tools are designed for a particularworking pressure (usually 6.3 bar). It should be notedthat this concerns the flow pressure and not the staticpressure often shown at the service station.

The flow pressure can either be measured by a ma-nometer positioned upstream of the tool or by a toolsimulator. Anything below the optimum working pres-sure results in reduced performance of the tool. As anexample, here the cutting performance of an anglegrinder:

Working pressure in bar Material removal in kg/h6.3 5.55.8 4.55.3 4.0

The example shows that even a reduction of 0.5 barin the working pressure results in a clear reduction inproductivity. As a result, not only the necessaryworking time is increased, but also the energy costs.

The air consumption per time unit does fall but thelonger working time still has an effect here.

ExampleThe total costs are listed based on the example of adrill as shown in Figure 1.

Facts Compressed Air ToolsPage 2 of 3

Working pressure in bar Drilling time in s

6.3 2.0

5.8 3.2

Fig. 1: Power drill with air service unit and supply line

This means that the drilling time is increased by 60 %due to the lower pressure. And yet a working pres-sure which is 0.5 bar lower is by no means an excep-tion, but often expensive reality.

In the drill example, the costs would increase as fol-lows:

atefficient drilling time 1 h/dayworking costs 20 €/henergy costs 0.06 €/kWh

additional costs per month result forwork 240.00 €energy 3.60 €

TOTAL 243.60 €i. e. 2,329.20 € per year!

The way to efficient tool use1. Optimisation of the environment

Long hoses = pressure loss!

Consequently, hoses should be kept as short as pos-sible, spiral hoses should be avoided. Where spiralhoses are used, e.g. between mains and balancer,normal hoses could often be used. Attention must bepaid to suitable hose diameters, this can help to avoidunnecessary pressure losses at couplings.

Install loss-free couplings!

The majority of self-venting quick release couplings -especially those made of brass - use up a lot of pres-sure (0.6-1.3 bar flow pressure). The reason is a ballpositioned in the air stream. Modern quick releasecouplings drastically reduce the losses (to approx.0.2 bar) and thus recover their initial cost within ashort period of time.

Fig. 2: Modern quick release coupling

Avoid too much tinkering!

Large diameter tolerances, more couplings than nec-essary, too many nozzles and incorrect hose diame-ters all add up to a large energy "gobbler". Tailor-made installations almost always pay off.

Fig. 3: Energy “gobblers“ in compressed air system

Oil lubrication in the air supply only where necessary!

Turbine-driven tools or those fitted with oil-free multi-disc motors do not need oil lubrication. Oilers causepressure losses. If they are required, the oiler shouldbe positioned 3-5 metres away from the tool.

Facts Compressed Air ToolsPagee 3 of 3

2. Measuring and adjusting the flow pressureAfter optimising the environment, the working pres-sure at the tool will probably be too high. This cannow be reduced using the pressure controller of theair service unit. The tool then operates in the mostefficient state, air consumption is minimised.

Flow pressure at the toolin bar

Air consumptionin %

6.3 1007.0 1108.0 125

3. Adjusting the air mains pressureThe air mains pressure can also often be clearly re-duced. This results in reduced compressor runningtimes and thus massive reductions in energy costs!

Optimisations of the tool's surroundings oftenpay off within the shortest time!

Maintenance of the air pressure systemAfter optimisation has been completed, the efficiencygained has to be retained. Regular maintenance ofthe components is of fundamental importance here.Alongside the draining and cleaning of the filters, careshould be taken to regularly check for leaks.

The supplier of the compressed air system will beable to assist you in making a maintenance plan.

It pays to remember that the condition of the tool itselfhas considerable effect on efficiency.

It is just as important to take into account that everychange in the compressed air system has conse-quences for the pressure relations in the system.

Should this not always be possible because altera-tions occur very frequently, then the entire sytemshould be checked at regular intervals.

SummaryMaking a check of the environment pays off quicklywhere compressed air tools are being used. Incorrectsizing, adjustments and poor maintenance dramati-cally reduce productivity.




Documents

Compressed Air Facts: