Air Treatment

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

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

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Air Treatment