cag i air drying selection guide

Compressed Airand Gas Drying

CONTENTS

I. Why Do Compressed Air And Gas Need Drying? 1

II. Applications Requiring Clean, Dry Air 1

III. How To Measure Moisture Content 2

IV. Selecting The Right Dryer 3

V. Types Of Compressed Air Dryers 4

Refrigerant Type

Regenerative Desiccant Type

Heat of Compression Type

Single Tower Deliquescent Type

Membrane Type

VI. Specifying A Compressed Air Dryer 7

VII. What Is A Compressed Air Drying System? 9

VIII. Additional Literature 10

Appendices 11-13

IIIWHY DO COMPRESSED AIRAND GAS NEED DRYING?

MOISTURE IS ALWAYS PRESENT. All atmospheric air contains some water vaporwhich will begin to condense into liquid water inthe compressed air or gas system when the air orgas cools past the saturation point, i.e., the pointwhere it can hold no more water vapor.*

The temperature at which this happens is knownas the dew point.** This dew point becomes all-important in determining how much drying is needed.

Condensation in the compressed air system wouldoccur at the inlet air saturation temperature if tem-perature remained constant as air was compressed.However, since there is a rise in temperature duringactual compression, condensation generally isavoided within the compressor. Later, as com-pressed air is discharged and cooled in an aftercool-er, condensation begins to occur. The condensedmoisture must be removed by a separator and trap.The air leaving the aftercooler normally is saturatedat the aftercooler discharge temperature.

For many years, problems from moisture in airlines were tolerated. To prevent freezing, alcoholswere injected into the lines and electric heaterswere used during cold periods. Filters were used toseparate moisture and other contaminants but didnot completely solve the problem.

The increased use of compressed air and the devel-opment of many new and more sophisticateddevices and controls have accelerated the need forclean dry air. Hence, drying technology advanced,and dryers came into general use.

MOISTURE IS DAMAGING.Moisture in compressed air used in a manufacturingplant causes problems in the operation of pneumaticair systems, solenoid valves and air motors. Thismoisture causes rust and increased wear of movingparts as it washes away lubrication.

Moisture adversely affects the color, adherence, andfinish of paint applied by compressed air.

Moisture jeopardizes process industries wheremany operations are dependent upon the properfunctioning of pneumatic controls. The malfunc-tioning of these controls due to rust, scale, andclogged orifices can result in damage to product orin costly shutdowns. Additionally, the freezing ofmoisture in control lines in cold weather commonlycauses faulty operation of controls.

Corrosion of air or gas operated instruments frommoisture can give false readings, interrupting orshutting down plant processes.

APPLICATIONS REQUIRINGCLEAN, DRY AIR

PLANT AIR

In almost every operation, clean, dry compressed airwill result in lower operating costs. Dirt, water andoil entrained in the air will be deposited on the innersurfaces of pipes and fittings, causing an increase inpressure drop in the line. A loss of pressure is a lossof the energy used to compress the air and a reducedpressure at the point of use results in a loss of performance efficiency.

Liquid water accelerates corrosion and shortens theuseful life of equipment and carry over of corrosionparticles can plug valves, fittings and instrumentcontrol lines. When water freezes in these compo-nents, similar plugging will occur.

VALVES AND CYLINDERS

Deposits of sludge formed by dirty, wet and oily airact as a drag on pneumatic cylinders so that the sealsand bearings need frequent maintenance. Operationis slowed down and eventually stopped. Moisturedilutes the oil required for the head and rod of an aircylinder, corrodes the walls and slows response. Thisresults in loss of efficiency and production.

Moisture flowing to rubber diaphragms in valvescan cause these parts to stiffen and rupture.Moisture also can cause spools and pistons to pit.

In high-speed production, a sluggish or stuckcylinder could create costly downtime. A clean, dry air supply can prevent many of these potentialproblems.

AIR POWERED TOOLS

Pneumatic tools are designed to operate withclean, dry air at the required pressure. Dirty andwet air will result in sluggish operation, more frequent repair and replacement of parts due tosticking, jamming and rusting of wearing parts.Water also will wash out the required oils, result-ing in excessive wear.

A decrease in pressure at the tool caused by restrict-ed or plugged lines or parts will cause a reductionin the efficiency of the tool.

Clean, dry air at the required pressure will enablethe production worker to start operating immedi-ately at an efficient level, with no time lost to purgelines or drain filters and will help to maintain pro-ductivity and prolong tool life.

INSTRUMENT AIR

Control air supplied to transmitters, relays, inte-grators, converters, recorders, indicators or gaugesis required to be clean and dry. A small amount ofmoisture passing through an orifice can cause

* The maximum water vaporthe air can hold dependsupon the temperature andpressure. The amount ofvapor the air actually doescontain - relative to themost it can contain is rela-tive humidity (the ratio ofthe quantity of water vaporpresent to the quantitypresent at saturation at thesame temperature).

** Dew Point. The tempera-ture at which water vaporin the air starts to con-dense or change fromvapor to a liquid or a solidstate. (Dew points may beexpressed at an operatingpressure or at atmosphericpressure. Operating pres-sure should be specifiedwhen using pressure dewpoint. The relationshipbetween pressure andatmosphere dew point isshown on Chart No. 3 inthe appendix.

COMPRESSED AIR AND GAS DRYING 1

III

malfunction of the instrument and the process itcontrols. Moisture and resultant corrosion particlesalso can cause damage to instruments and plugtheir supply air lines.

Pneumatic thermostats, which control the heatingand air conditioning cycles in large and smallbuildings, also require clean, dry air.

Instruments and pneumatic controllers in powerplants, sewage treatment plants, chemical andpetrochemical plants, textile mills and general manufacturing plants, all need clean, dry air forefficient operation.

The Instrument Society of America (ISA) has pub-lished ISA-S7.3 Quality Standard for Instrument Air.

PRESERVATION OF PRODUCTS

When used to mix, stir, move or clean a product, airmust be clean and dry. Oil and water in compressedair used to operate knitting machinery will cause thetiny latches on the knitting needles to stick. Whenused to blow lint and thread off finished fabrics, con-taminants in the air may cause product spoilage.

If air is used to blow a container clean before pack-aging, entrained moisture and oil may contaminatethe product. Moisture in control line air can causethe wrong mixture of ingredients in a bakery, theincorrect blend in liquor, water-logged paint, orruined food products.

In some printing operations, air is used to lift orposition paper, which will be affected by dirty, wetair and any water on the paper will prevent properadhesion of the inks.

In pneumatic conveying of a product such as papercups or cement, dry air is essential.

TEST CHAMBERS

Supersonic wind tunnels are designed to simulateatmospheric conditions at high altitudes wheremoisture content is low. These chambers use largevolumes of air, which must be dried to a very lowdew point to prevent condensation in the tunnelair stream.

BREATHING AIR

The air coming from an air compressor, whetherlubricated or oil free type, is not suitable for breath-ing. Treatment of the air is required before the aircan be considered suitable for breathing and certainhealth and safety standards must be met.

In industrial plants, air may be supplied to respira-tors, hoods and helmets and for applications suchas sand blasting. Occupational Safety and HealthAdministration (OSHA) standard OSHA:1910:13dapplies and requires drying, filtration and treat-ment to meet specific levels, including carbonmonoxide, with an alarm system.

The Compressed Gas Association (CGA) StandardCommodity Specification G-7.1, Grade D, common-ly is specified for plant breathing air systems.

Medical air for hospitals must meet National FireProtection Association (NFPA) Standard NFPA-99.

Air Quality Classes encompassing pollutants havebeen established in an International Standard ISO8573-1. For moisture content, these are as follows:

CLASS MAXIMUM PRESSURE °C DEW POINT °F

1 -70 -100

2 -40 -40

3 -20 -4

4 +3 +38

5 +7 +45

6 +10 +50

7 not specified

HOW TO MEASUREMOISTURE CONTENT

Obviously there are times when it is desirable toknow with varying degrees of accuracy the moisturecontent of the compressed air. Methods are availablewhich will give you readings, which vary fromapproximations to precise measurements.

Moisture Indicating Desiccants. A typical exampleis silica gel, which when treated with a solution ofcobaltous chloride*, is dark blue in color when dry.As moisture is adsorbed, the color changes to pinkat approximately a 0ºF dew point.

Dew Point Cup. This apparatus consists of a smallpolished stainless steel cup placed in a containerinto which you pass the sample air or gas. The temperature of the polished surface is lowered byimmersing dry ice (solid carbon dioxide) in an ace-tone solution contained in the cup. The temperatureat which fog appears on the cup is the dew point ofthe sample.

Refrigerant Evaporation Dew Point Instrument.This device permits direct gauge reading of the dewpoint temperature of the sample, as determined bythe appearance of a fog on the surface of a highlypolished vessel in which a refrigerant is being evap-orated. Dew point temperatures can be determinedat either system or atmospheric pressure.

Fog Chamber Instrument. This device relies on the relationship between pressure and temperatureduring adiabatic expansion. A sample of gas enters asmall chamber where it is compressed, then rapidlyvented to atmospheric pressure. The point at whichfog appears establishes a relationship with theamount of compression, and from this the dew point may be calculated.

2 COMPRESSED AIR AND GAS DRYING

IV

Capacitance Instrument. This analyzer consists oftwo strips of metal, which form the electrodes of acapacitor. Water vapor passing across the electrodescauses a change in electric impedance and this isused as a measurement of dew point temperature.

Hygroscopic* Cells. These analyzers use a sensingelement which contains a moisture adsorbing mate-rial. A change in the moisture content of the ele-ment is detected by an electronic network and isused as a measurement of dew point temperature.

Infrared Analyzer. This instrument uses two beamsof infrared radiation. One beam travels through acomparison cell and the other through the samplecell. The difference in absorption in the radiation isused as a means of measurement of the dew pointof the sample.

Frequency Oscillator Analyzer. This moisture analyzer uses the change in frequency of a hygro-scopically coated quartz crystal as means of dewpoint measurement.

SELECTING THERIGHT DRYER

Before looking at the several types of dryers avail-able, we need to look at what to consider in decid-ing which dryer is best for the specific requirements.

KNOW THE SPECIFIC USES OF THE

COMPRESSED AIR

The selection of an air dryer is done best by theprofessional who knows or learns the particularend uses, the amount of moisture which each usecan tolerate and the amount of moisture whichneeds to be removed to achieve this level.

Air which may be considered dry for one applica-tion, may not be dry enough for another. Dryness is relative. Even the desert has moisture. There isalways some moisture present in a compressed airsystem regardless of the degree of drying. Differenttypes of dryers, therefore, are available with vary-ing degrees of pressure dew point ability.

To specify a dew point lower than required for anapplication is not good engineering practice.(Naming a pressure dew point is how to state thedegree of dryness wanted.) It may result in morecostly equipment and greater operating expense.

KNOW THE TEMPERATURES

To determine whether or not the compressed air willremain sufficiently dry, we must know the end use ofthe air and the temperature at which it must work.

In an industrial plant where the ambient tempera-ture is in the range of 70°F or higher, a dryer capa-ble of delivering a pressure dew point 20°F lowerthan ambient, or 50°F, may be quite satisfactory.

Summer temperatures do not require a very lowdew point whereas winter temperatures may dic-tate a much lower dew point. In winter, the temper-ature of the cooling medium, air or water, usually islower than in summer, resulting in a variation ofthe air temperature to the dryer. This will affect thesize of the dryer needed, since the same dryer mustwork in both summer and winter temperatures andrelative humidities.

Many chemical processing plants, refineries, andpower plants distribute instrument and plant airthroughout the facility with lines and equipmentlocated outside the buildings. In such plants twodifferent temperature conditions exist at the sametime in the same system. Also, a dryer, which maybe satisfactory for high daytime temperatures maynot be satisfactory for lower nighttime tempera-tures. In areas where freezing temperatures areencountered, a lower pressure dew point may berequired. In general, the dew point should be speci-fied 20°F lower than the lowest ambient tempera-ture encountered in order to avoid potential con-densation and freezing. To specify a winter dewpoint when only summer temperatures will beencountered, can result in over-sizing the equip-ment and increased initial and operating costs.

For plant air and instrument air, primary considera-tions in specifying a dryer are condensation andfreezing. In a system where a lot of internal pipecorrosion could occur, high humidity in the airstream should be avoided.

KNOW THE ACTUAL PERFORMANCE

While many dryers have a standard rating of 100°F saturated inlet air temperature and 100 PSIGoperating pressure, it is important to check on theactual performance of the units obtained in actualplant operating conditions.

KNOW EACH USE

In addition to plant and instrument air applications,there are many other uses requiring moistureremoval to a low dew point. For example, railroadtank cars which carry liquid chlorine are padded(charged) with compressed air to enable pneumaticunloading. Chlorine will combine with water vaporto form hydrochloric acid; therefore, the compressedair must have a minimum moisture content to prevent severe corrosion.

Droplets of moisture in wind tunnel air at high-testing velocities may have the effect of machinegun bullets, tearing up the test models.

Air used for low temperature processing (for exam-ple, liquefaction of nitrogen or oxygen) can form iceon cooling coils, thus requiring defrosting. Thelower the moisture content of the air, the longer theperiods between defrosting shutdowns.

* The EEC has re-classifiedcobalt chloride, as of July1, 2000, as a potential car-cinogen by inhalation.Manufacturers and usersshould check with OSHAbefore using or specifyingthis product.

** Hygroscopic: Any materialwhich picks up moisture.


V

For these and similar temperature applications, compressed air must not only be free of liquid phasewater but must also have a minimum content ofvapor phase water. Usually specified for theserequirements are dew points in the range of -40°F to-100°F at pressure.

With these characteristics in mind, what types ofdryers are available?

TYPES OF COMPRESSEDAIR DRYERS

Different methods can be used to remove the mois-ture content of compressed air. Current dryer typesinclude the following:

■ Refrigerant type -cycling -non-cycling

■ Regenerative desiccant type-Heatless (no internal or external heaters)-Heated (internal or external heaters)-Heat of Compression-Non-regenerative single tower

■ Deliquescent

■ Membrane

Each of these dryer types will be discussed in somedetail.

REFRIGERANT TYPE DRYERS

Although it does not offer as low a dew point as canbe obtained with other types, the refrigerant typedryer has been the most popular, as the dew point

obtained is acceptable ingeneral industrial plantair applications. Theprinciple of operation issimilar to a domesticrefrigerator or home airconditioning system. Thecompressed air is cooledin an air to refrigerantheat exchanger to about35°F, at which point thecondensed moisture isdrained off. The air isthen reheated in an air toair heat exchanger bymeans of the incomingair which also is pre-cooled before enteringthe air to refrigerant heatexchanger. This means

that the compressed air leaving the dryer has a pres-sure dew point of 35 to 40°F. A lower dew point isnot feasible in this type of dryer as the condensatewould freeze at 32°F or lower.

In a non-cycling refrigerant dryer, the refrigerantcirculates continuously through the system. Sincethe flow of compressed air will vary and ambienttemperatures also vary, a hot gas bypass valve orunloader often is used to regulate the flow of therefrigerant and maintain stable operating conditionswithin the refrigerant system. In most designs, therefrigerant evaporates within the air to refrigerantheat exchanger (evaporator) and is condensed aftercompression by an air or water to refrigerant heatexchanger (condenser). A typical schematic diagramis shown in Figure 1.

This design provides rapid response to changes inoperating loads. While older refrigerant type airdryers have used CFC refrigerants such as R12 andR22, newer designs are in compliance with theMontreal Protocol and use chlorine free refrigerantssuch as R134A and R407C. The properties of thesenewer refrigerants require careful attention to therefrigeration system design, due to differences inoperating pressures and temperatures.

Cycling type refrigerant dryers chill a mass sur-rounding the air passage in the evaporator. This massmay be a liquid such as glycol or a metal such as alu-minum block, beads or related substance, which actsas a heat sink. The compressed air is cooled by theheat sink which has its temperature controlled by athermostat and shuts off the refrigerant compressorduring reduced loads, providing savings in operatingcosts but at higher initial capital cost.

Advantages of Refrigerant Type Air Dryers include:• Low initial capital cost.• Relatively low operating cost.• Low maintenance costs.• Not damaged by oil in the air stream

(Filtration normally is recommended).

Disadvantages of Refrigerant Type Air Dryersinclude:• Limited dew point capability.

Advantages of Direct Expansion Control include:• Low and precise dew point.• Refrigerant compressor operates continuously.

Disadvantages of Direct Expansion Controlinclude:• No energy savings at partial and zero air flow.

Advantages of Cycling Control include:• Energy savings at partial and zero air flow.

Disadvantages of Cycling Control include:• Dew point swings.• Reduced life of refrigerant compressor from

starting and stopping.• Increased size and weight to accommodate the

heat sink mass.• Increased capital cost.


REGENERATIVE DESICCANT TYPE DRYERS

These dryers use a desiccant, which adsorbs thewater vapor in the air stream. A distinction needs tobe made between adsorb and absorb. Adsorb meansthat the moisture adheres to the desiccant, collectingin the thousands of small pores within each desic-cant bead. The composition of the desiccant is notchanged and the moisture can be driven off in aregeneration process by applying dry purge air, bythe application of heat, or a combination of both.Absorb means that the material which attracts themoisture is dissolved in and used up by the mois-ture as in the deliquescent desiccant type dryer.

Regenerative desiccanttype dryers normallyare of twin tower con-struction. One towerdries the air from thecompressor while thedesiccant in the othertower is being regen-erated after the pres-sure in the tower hasbeen reduced toatmospheric pressure.Regeneration can be

accomplished usinga time cycle or ondemand by meas-uring the tempera-ture or humidity inthe desiccant tow-ers or by measur-ing the dew pointof the air leavingthe on-line tower.

In the heatless regener-ative desiccant type, no internal or external

heaters are used. Purge air requirement can rangeup to 18% of the total air flow. The typical regenera-tive desiccant dryer at 100 psig has a pressure dewpoint rating of -40°F but a dew point down to -100°Fcan be obtained. A typical schematic diagram isshown in Figure 1.

Heat reactivated regenerative desiccant dryers mayhave internal or external heat applied by heaters. Inthe internal type, steam or electricity may be usedin heaters embedded in the desiccant bed. Thisreduces the amount of purge air required for regen-eration to about 5%. The purge air plus normalradiation is used to cool the desiccant bed afterregeneration to prevent elevated air temperaturesgoing downstream.

In externally heated regenerative desiccant dryers,the purge air is heated to a suitable temperatureand then passes through the desiccant bed. The

amount of purge air is approximately 5-10% of theair flow through the dryer. The purge air can beeliminated if a blower is used for circulation ofatmospheric air through the desiccant bed.

To protect the desiccant bed from contaminationfrom oil carry-over from the air compressor, a coa-lescing filter is required upstream of the dryer. Toprotect downstream equipment from desiccant dustor “fines”, a particulate filter downstream of thedryer also is recommended.

Advantages of Regenerative Desiccant Type Dryersinclude:• Very low dew points can be achieved without

potential freeze-up.• Moderate cost of operation for the dew points

achieved.• Heatless type can be designed to operate pneumat-

ically for remote, mobile or hazardous locations.

Disadvantages of Regenerative Desiccant TypeDryers include:• Relatively high initial capital cost.• Periodic replacement of the desiccant bed

(typically 3-5 years).• Oil aerosols can coat the desiccant material,

rendering it useless if adequate pre-filtering isnot maintained.

• Purge air usually is required.

HEAT OF COMPRESSION TYPE DRYERS

Heat of Compression Type Dryers are RegenerativeDesiccant Dryers which use the heat generated dur-ing compression to accomplish desiccant regenera-tion, so they can be considered as Heat Reactivated.There are two types, theSingle Vessel Type andthe Twin Tower Type.

The Single Vessel Heat ofCompression Type Dryerprovides continuous dry-ing with no cycling orswitching of towers. Thisis accomplished with arotating desiccant drumin a single pressure vesseldivided into two separateair streams. One airstream is a portion of the hot air taken directly fromthe air compressor at its discharge, prior to theaftercooler, and is the source of heated purge air forregeneration of the desiccant bed. The second airstream is the remainder of the air discharged fromthe air compressor after it passes through the airaftercooler. This air passes through the drying sec-tion of the dryer rotating desiccant bed where it isdried. The hot air, after being used for regeneration,


Hot unsaturated air used for regeneration

Cold saturated

airDry air

Hotsaturated

air

passes through a regeneration cooler before beingcombined with the main air stream by means of anejector nozzle before entering the dryer.

The Twin Tower Heat of Compression Type Dryeroperation is similar to other Twin Tower HeatActivated Regenerative Desiccant Dryers. The dif-ference is that the desiccant in the saturated toweris regenerated by means of the heat of compressionfrom the hot air leaving the discharge of the aircompressor. The total air flow then passes throughthe air aftercooler before entering the drying tower.Towers are cycled as for other RegenerativeDesiccant Type Dryers.

Advantages of Heat of Compression Type Dryersinclude:• Low electrical installation cost.• Low power costs.• Minimum floor space.• No loss of purge air.

Disadvantages of Heat of Compression TypeDryers include:• Applicable only to oil free compressors.• Applicable only to compressors having a

continuously high discharge temperature.• Inconsistent dew point.• Susceptible to changing ambient and inlet air

temperatures.• High pressure drop and inefficient ejector nozzle

on single vessel type.• Booster heater required for low load (heat)

conditions.

SINGLE TOWER

DELIQUESCENT TYPE

DRYERS

The deliquescent desic-cant type dryer uses ahygroscopic desiccantmaterial having a highaffinity for water. Thedesiccant absorbs thewater vapor and is dissolved in the liquidformed. These hygro-scopic materials areblended with ingredientsto control the pH of theeffluent and to preventcorrosion, caking andchanneling. The desic-cant is consumed onlywhen moist air is passingthrough the dryer. Onaverage, desiccant must

be added two or three times per year to maintain aproper desiccant bed level.

Deliquescent dryers normally are designed to give adew point depression from 20ºF to 50ºF at an inlettemperature or 100ºF. This means that with air enter-ing at 100ºF and 100 PSIG, a leaving pressure dewpoint of 80ºF to 50ºF may be obtained (a reduction of20ºF to 50ºF from the inlet temperature). Dew pointsuppression of 15 to 50°F is advertised. This type ofdryer actually dries the air to a specific relativehumidity rather than to a specific dew point.

Advantages of Single Tower DeliquescentDesiccant Type Dryers include:• Low initial capital and installation cost.• Low pressure drop.• No moving parts• Requires no electrical power.• Can be installed outdoors.• Can be used in hazardous, dirty or corrosive

environments.

Disadvantages of Single Tower Deliquescent TypeDryers include:• Limited suppression of dew point.• Desiccant bed must be refilled periodically.• Regular periodic maintenance.• Desiccant material can carry over into down-

stream piping if there is no after-filter and if thedryer is not drained regularly. Certain desiccantmaterials may have a damaging effect on down-stream piping and equipment.

• Some desiccant materials may melt or fusetogether at temperatures above 80°F.

MEMBRANE TYPE DRYERS

Membrane technology has advanced considerably inrecent years. Membranes commonly are used for gasseparation such as in nitrogen production for foodstorage and other applications. The structure of themembrane is such that molecules of certain gases(such as oxygen) are able to pass through (permeate)a semi-permeable membrane faster than others (suchas nitrogen) leaving a concentration of the desiredgas (nitrogen) at the outlet of the generator.

When used as a dryer in a compressed air system,specially designed membranes allow water vapor(a gas) to pass through the membrane pores fasterthan the other gases (air) reducing the amount ofwater vapor in the air stream at the outlet of themembrane dryer, suppressing the dew point. Thedew point achieved normally is 40°F but lower dew points to -40°F can be achieved at the expenseof additional purge air loss.


VI

Advantages of Membrane Type Dryers include:• Low installation cost.• Low operating cost.• Can be installed outdoors.• Can be used in hazardous atmospheres.• No moving parts.

Disadvantages of the Membrane Type Dryersinclude:• Limited to low capacity systems.• High purge air loss (15 to 20%) to achieve

required pressure dew points.• Membrane may be fouled by oil or other

contaminants.• High initial capital cost.

DRYER RATINGS

The standard conditions for the capacity rating inscfm of compressed air dryers, are contained inCAGI ADF 100, Refrigerated Compressed Air Dryers –Methods for Testing and Rating*. These commonly arecalled the three 100s. That is, a dryer inlet pressure of100 psig, an inlet temperature of 100°F and an ambi-ent temperature of 100°F. If the plant compressed airsystem has different operating conditions, this willaffect the dryer rating and must be discussed withthe supplier to ensure compatibility.

SPECIFYING A COMPRESSEDAIR DRYER

The air dryer with certain auxiliary equipmentbecomes a system, which is an important componentof the whole plant compressed air system. Variouscomponents comprising the dryer subsystem shouldbe selected on the basis of the overall requirementsand the relationship of the components to each other.

There are just three main factors to analyze inselecting the appropriate dryer (including size) toprovide your required performance – dew point,operating pressure and inlet temperature.

DEW POINT

Refrigerant dryers provide a pressure dew point of35°F or 50°F at operating pressure based on saturatedinlet air temperature of 100°F.

Regenerative desiccant dryers generally provide a pressure dew point of minus 40°F or lower, atoperating pressure and 100°F saturated inlet airtemperature.

Deliquescent dryers are more sensitive to the satu-rated inlet temperature and, based upon a saturatedinlet air temperature of 100°F, provide a dew pointfrom 64°F to 80°F at operating pressure.

Membrane dryers deliver pressure dew points rang-ing from -40°F to 50°F or higher, if required. The dry air flow from a membrane dryer is dependenton the inlet dew point and pressure of the supplyair and outlet pressure dew point required by theapplication. Supply air temperature has no appre-ciable impact on capacity.

OPERATING PRESSURE

At higher pressures, saturated air contains lessmoisture per standard cubic foot than lower pres-sure saturated air. Drying air at the highest pressureconsistent with the plant design will result in themost economical dryer operation. Chart 1 shows therelationship between operating pressure and watercontent. Taking air at 100 PSIG as the normal pres-sure, a subsequent decrease in pressure results in asubstantial increase in the water to be removed. Athigher pressures the water content curve tends toincrease at a slower rate.

INLET TEMPERATURE

The temperature of air entering the dryer usually isclose to the temperature at which it leaves the after-cooler. Saturated air at 100°F saturated containsalmost twice the amount of moisture of saturated airat 80°F. For every 20°F increase in the temperature ofsaturated air, there is an approximate doubling ofthe moisture content. Thus it is desirable to operatethe dryer at the lowest feasible inlet temperature.The moisture content of saturated air at a given temperature, in grains per cubic foot, is given inChart No. 2 in the appendix.

With that in mind, a dryer specification is needed.

* CAGI ADF 100 can beordered through the CAGIoffice at (216) 241-7333.


Water vaporOut

Water vaporOut

Compressed airIn

Compressed airOut

Sweep air

HOW TO SPECIFY

To provide a good basis for the purchase of eachtype of compressed air dryer the following samplespecifications are given:

Refrigerant Type

1. Inlet ConditionsFlow scfmOperating Pressure Inlet Temperature

2. PerformanceOutlet Dew PointPressure Drop-Maximum Allowable

3. DesignPressureTemperatureAmbient Temperature-

MinimumMaximum

Air cooled or water cooled

4. UtilitiesElectric

______ Volts ______ Phase ______ Hz. Cooling Water Temperature

(if applicable)

Electric EnclosureNEMA-1 General PurposeNEMA-4 WeatherproofNEMA-7 Explosion ProofNEMA-12 Dust Tight

Regenerative Desiccant Type

1. Inlet ConditionsFlow scfmOperating Pressure Inlet Air Temperature

2. PerformanceOutlet Dew PointPressure Drop

3. DesignPressureTemperatureInlet and Outlet Connections

4. UtilitiesElectric

______ Volts ______ Phase ______ Hz. Steam

______ PSIG ______ TemperatureWater

______ F ______ PSIG

Electric EnclosureNEMA-1 General PurposeNEMA-4 WeatherproofNEMA-7 Explosion ProofNEMA-12 Dust Tight

Single Tower Deliquescent Type

1. Inlet ConditionsFlow scfmOperating Pressure Inlet Air Temperature Ambient Temperature



MinimumMaximum

Membrane Type

1. Inlet ConditionsFlow scfmOperating Pressure Inlet Air Temperature Ambient Temperature



MinimumMaximum


These specifications are of a general nature for most plant and instrument and air applications. Add special requirements along with any additional data regarding the application.

Certainly good suppliers are available who can assess your system and specify the drying equipment, buteven the finest supplier does a better job when the operator or owner or buyer of the apparatus is himselfknowledgeable.

VIIWHAT IS A COMPRESSED AIRDRYING SYSTEM?

While the dryer is the heart of the matter, good dry-ing performance usually requires it to be supportedby auxiliary components in a drying system.

ABOVE: LEFT TO RIGHT: TYPICAL FLOW DIAGRAMS OF DRYING SYSTEMS.Left: Refrigerant. Center: Desiccant.Right: Single Tower Deliquescent.

Filters are shown in the flow diagrams as single com-ponents. Local operating conditions and selection ofequipment will dictate which filters may be requiredand whether a bypass is to be installed for servicing.Also dual filters and block valves may be considered.

To ensure the expected performance and reliability ofthe compressed air system, good attention must beexercised in the selection of all components. A typicalsystem may include several of the following items:

Compressor – to increase the pressure of the air tothat desired in the system. Numerous types andpressure ratings are available. Compressor dis-charge pressure must allow for pressure dropsthrough downstream treatment equipment, pipingand valves.

Aftercooler – to remove the heat of compression andcool the air to within 10°F to 15°F of the coolingmedium. The major portion of the water vapor enter-ing the compressor is condensed in the aftercooler.

Separator – to remove liquids condensed by theaftercooler. An automatic trap discharges the liquidsto a drain.

Receiver – to smooth compressor pulsations, andfunction as a reservoir of compressed air. It alsopermits liquid water, oil, and solid particulate matternot removed by the aftercooler-separator to settleout of the air stream. Adequate drain provision, withappropriate treatment, is essential.

A receiver before a dryer is filled with saturated air.A sudden demand for air exceeding the dryer rat-ing, may result in overloading the dryer and ahigher downstream dew point. A receiver placedafter the dryer will be filled with air already driedand can satisfy a sudden demand without impact-ing the dryer performance and presure dew point.


ISO CLASS 1.4.1REFRIGERANT

ISO CLASS 1.2.1DESICCANT

SINGLE TOWER

DELIQUESCENTFIGURE 1

VIII

Mechanical Prefilter– to remove solid and liquidentrainments which remain in the air or whichhave condensed in the piping. All dryers aredesigned for an inlet condition of only saturated air

or gas. Liquids shouldnot be allowed to enterthe dryer.

Coalescing Filter – toremove contaminants,such as the aerosols ofwater and oil, from anair stream by causingthe particles to unite, orcoalesce, so that thedroplets formed may beremoved.

Oil Vapor Filter – toadsorb oil vapors gener-ated by oil lubricatedcompressors. Oil in thecompressed air system isa contaminant and can

be hazardous. It can also contaminate desiccants,thereby reducing their ability to adsorb water.

Air Dryers – to remove water vapor. See Section Vfor a description of the compressed air dryer types.

Afterfilters – to remove dust, pipe scale, and desic-cant particles from the dry compressed air stream.

Drains – All condensate discharges to sewer orwater systems should comply with all Federal,State and local codes.

ADDITIONALLITERATURE

National Fluid Power Association • Glossary of Terms for Fluid Power

American National Standards Institute• ANSI Standard B93.2 – 1971

Compressed Air & Gas Institute • Compressed Air & Gas Handbook• ADF 100 – Refrigerated Compressed Air

Dryers – Methods for Testing and Rating• ADF 200 – Dual Tower Regenerative Desiccant

Compressed Air Dryers – Methods for Testingand Rating

• ADF 300 – Single Tower (Non-Regenerative)Desiccant Compressed Air Dryers – Methodsfor Testing and Rating

• ADF 700 – Membrane Compressed Air Dryers– Methods for Testing and Rating

McGraw Hill Book Company • Adsorption – C. L. Mantell



APPENDIX 1

POUNDS MOISTURE/1000 CU.FT.AT 0LBS GAUGE

PRESSURE – LBS. PER SQ. IN. GAUGE

WATER CONTENT OF AIR AT VARIOUS

TEMPERATURES AND PRESSURES

CHART NO. 1

PO

UN

DS

MO

IST

UR

EP

ER

10

00

CU.F

T. O

FS

AT

UR

AT

ED

FR

EE

AIR

OR

GA

SA

T1

4.7

LB.

AB

S.

AN

DT

EM

P. S

HO

WN


7,000 grains = 1 pound1 pound = 0.833 gallons at 60°F-80°F = .00035 gr/ft3

-100°F = .00007 gr/ft3

APPENDIX 2

MOISTURE CONTENT OF SATURATED

AIR AT VARIOUS TEMPERATURES

CHART NO. 2


To obtain the dew point temperature expected if the gas wereexpanded to a lower pressure proceed as follows:

1. Using “dew point at pressure,” locate this temperature onscale at right hand side of chart.

APPENDIX 3

DEW POINT CONVERSION

CHART NO. 3

2. Read horizontally to intersection of curve corresponding tothe operating pressure at which the gas was dried.

3. From that point read vertically downward to curve corre-sponding to the expanded lower pressure.

4. From that point read horizontally to scale on right hand sideof chart to obtain dew point temperature at the expandedlower pressure.

5. If dew point temperatures of atmospheric pressure aredesired, after step 2, above read vertically downward toscale at bottom of chart which gives “Dew Point atAtmospheric Pressure.”

DEW POINT AT ATMOSPHERIC PRESSURE, °F.

DE

WP

OIN

TA

TP

RE

SS

UR

E,

°F.

cag i air drying selection guide

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