Compressed Air and 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 WHY DO COMPRESSED AIR APPLICATIONS REQUIRING * The maximum water vapor I II the air can hold depends AND GAS NEED DRYING? CLEAN, DRY AIR upon the temperature and MOISTURE IS ALWAYS PRESENT. PLANT AIR . The amount of vapor the air actually does All atmospheric air contains some water vapor In almost every operation, clean, dry compressed air contain - relative to the which will begin to condense into liquid water in will result in lower operating costs. Dirt, water and most it can contain is rela- the compressed air or gas system when the air or oil entrained in the air will be deposited on the inner tive (the ratio of gas cools past the saturation point, i.e., the point surfaces of pipes and fittings, causing an increase in the quantity of water vapor where it can hold no more water vapor.* pressure drop in the line. A loss of pressure is a loss present to the quantity of the energy used to compress the air and a reduced present at saturation at the The temperature at which this happens is known same temperature). pressure at the point of use results in a loss of as the dew point.** This dew point becomes ** Dew Point. The tempera- all-important in determining how much drying performance efficiency. ture at which water vapor is needed. Liquid water accelerates corrosion and shortens the in the air starts to con- useful life of equipment and carry over of corrosion dense or change from Condensation in the compressed air system would vapor to a liquid or a solid particles can plug valves, fittings and instrument occur at the inlet air saturation temperature if tem- state. (Dew points may be perature remained constant as air was compressed. control lines. When water freezes in these compo- expressed at an operating However, since there is a rise in temperature during nents, similar plugging will occur. pressure or at atmospheric actual compression, condensation generally is pressure. Operating pres- VALVES AND CYLINDERS sure should be specified avoided within the . Later, as com- Deposits of sludge formed by dirty, wet and oily air when using pressure dew pressed air is discharged and cooled in an aftercool- act as a drag on pneumatic cylinders so that the seals point. The relationship er, condensation begins to occur. The condensed and bearings need frequent maintenance. Operation between pressure and moisture must be removed by a separator and trap. is slowed down and eventually stopped. Moisture atmosphere dew point is shown on Chart No. 3 in The air leaving the aftercooler normally is saturated dilutes the oil required for the head and rod of an air the appendix. at the aftercooler discharge temperature. cylinder, corrodes the walls and slows response. This For many years, problems from moisture in air results in loss of efficiency and production. lines were tolerated. To prevent freezing, alcohols Moisture flowing to rubber diaphragms in valves were injected into the lines and electric heaters can cause these parts to stiffen and rupture. were used during cold periods. Filters were used to Moisture also can cause spools and pistons to pit. separate moisture and other contaminants but did not completely solve the problem. In high-speed production, a sluggish or stuck cylinder could create costly downtime. A clean, The increased use of compressed air and the devel- dry air supply can prevent many of these potential opment of many new and more sophisticated problems. devices and controls have accelerated the need for clean dry air. Hence, drying technology advanced, AIR POWERED TOOLS and dryers came into general use. Pneumatic tools are designed to operate with clean, dry air at the required pressure. Dirty and MOISTURE IS DAMAGING. wet air will result in sluggish operation, more Moisture in compressed air used in a manufacturing frequent repair and replacement of parts due to plant causes problems in the operation of pneumatic sticking, jamming and rusting of wearing parts. air systems, solenoid valves and air motors. This Water also will wash out the required oils, result- moisture causes rust and increased wear of moving ing in excessive wear. parts as it washes away lubrication. A decrease in pressure at the tool caused by restrict- Moisture adversely affects the color, adherence, and ed or plugged lines or parts will cause a reduction finish of paint applied by compressed air. in the efficiency of the tool. Moisture jeopardizes process industries where Clean, dry air at the required pressure will enable many operations are dependent upon the proper the production worker to start operating immedi- functioning of pneumatic controls. The malfunc- ately at an efficient level, with no time lost to purge tioning of these controls due to rust, scale, and lines or drain filters and will help to maintain pro- clogged orifices can result in damage to product or ductivity and prolong tool life. in costly shutdowns. Additionally, the freezing of moisture in control lines in cold weather commonly INSTRUMENT AIR causes faulty operation of controls. Control air supplied to transmitters, relays, inte- Corrosion of air or gas operated instruments from grators, converters, recorders, indicators or gauges moisture can give false readings, interrupting or is required to be clean and dry. A small amount of shutting down plant processes. moisture passing through an orifice can cause

COMPRESSED AIR AND GAS DRYING 1 malfunction of the instrument and the process it The Compressed Gas Association (CGA) Standard controls. Moisture and resultant corrosion particles Commodity Specification G-7.1, Grade D, common- also can cause damage to instruments and plug ly is specified for plant breathing air systems. their supply air lines. Medical air for hospitals must meet National Fire Pneumatic thermostats, which control the heating Protection Association (NFPA) Standard NFPA-99. and air conditioning cycles in large and small Air Quality Classes encompassing pollutants have buildings, also require clean, dry air. been established in an International Standard ISO Instruments and pneumatic controllers in power 8573-1. For moisture content, these are as follows: plants, sewage treatment plants, chemical and CLASS MAXIMUM PRESSURE °C DEW POINT °F petrochemical plants, textile mills and general 1 -70 -100 manufacturing plants, all need clean, dry air for efficient operation. 2 -40 -40 3 -20 -4 The Instrument Society of America (ISA) has pub- 4 +3 +38 lished ISA-S7.3 Quality Standard for Instrument Air. 5 +7 +45 PRESERVATION OF PRODUCTS 6 +10 +50 When used to mix, stir, move or clean a product, air 7 not specified must be clean and dry. Oil and water in compressed air used to operate knitting machinery will cause the HOW TO MEASURE III tiny latches on the knitting needles to stick. When MOISTURE CONTENT used to blow lint and thread off finished fabrics, con- taminants in the air may cause product spoilage. Obviously there are times when it is desirable to know with varying degrees of accuracy the moisture If air is used to blow a container clean before pack- content of the compressed air. Methods are available aging, entrained moisture and oil may contaminate which will give you readings, which vary from the product. Moisture in control line air can cause approximations to precise measurements. the wrong mixture of ingredients in a bakery, the incorrect blend in liquor, water-logged paint, or Moisture Indicating Desiccants. A typical example ruined food products. is silica gel, which when treated with a solution of cobaltous chloride*, is dark blue in color when dry. In some printing operations, air is used to lift or As moisture is adsorbed, the color changes to pink position paper, which will be affected by dirty, wet at approximately a 0ºF dew point. air and any water on the paper will prevent proper adhesion of the inks. Dew Point Cup. This apparatus consists of a small polished stainless steel cup placed in a container In pneumatic conveying of a product such as paper into which you pass the sample air or gas. The cups or cement, dry air is essential. temperature of the polished surface is lowered by TEST CHAMBERS immersing dry ice (solid carbon dioxide) in an ace- Supersonic wind tunnels are designed to simulate tone solution contained in the cup. The temperature atmospheric conditions at high altitudes where at which fog appears on the cup is the dew point of moisture content is low. These chambers use large the sample. volumes of air, which must be dried to a very low Refrigerant Evaporation Dew Point Instrument. dew point to prevent condensation in the tunnel This device permits direct gauge reading of the dew air stream. point temperature of the sample, as determined by the appearance of a fog on the surface of a highly BREATHING AIR polished vessel in which a refrigerant is being evap- The air coming from an , whether orated. Dew point temperatures can be determined lubricated or oil free type, is not suitable for breath- at either system or . ing. Treatment of the air is required before the air can be considered suitable for breathing and certain Fog Chamber Instrument. This device relies on health and safety standards must be met. the relationship between pressure and temperature during adiabatic expansion. A sample of gas enters a In industrial plants, air may be supplied to respira- small chamber where it is compressed, then rapidly tors, hoods and helmets and for applications such vented to atmospheric pressure. The point at which as sand blasting. Occupational Safety and Health fog appears establishes a relationship with the Administration (OSHA) standard OSHA:1910:13d amount of compression, and from this the dew applies and requires drying, filtration and treat- point may be calculated. ment to meet specific levels, including carbon monoxide, with an alarm system.

2 COMPRESSED AIR AND GAS DRYING Capacitance Instrument. This analyzer consists of Summer temperatures do not require a very low * The EEC has re-classified two strips of metal, which form the electrodes of a dew point whereas winter temperatures may dic- cobalt chloride, as of July capacitor. Water vapor passing across the electrodes tate a much lower dew point. In winter, the temper- 1, 2000, as a potential car- cinogen by inhalation. causes a change in electric impedance and this is ature of the cooling medium, air or water, usually is Manufacturers and users used as a measurement of dew point temperature. lower than in summer, resulting in a variation of should check with OSHA Hygroscopic* Cells. These analyzers use a sensing the air temperature to the dryer. This will affect the before using or specifying element which contains a moisture adsorbing mate- size of the dryer needed, since the same dryer must this product. work in both summer and winter temperatures and ** Hygroscopic: Any material rial. A change in the moisture content of the ele- which picks up moisture. ment is detected by an electronic network and is relative . used as a measurement of dew point temperature. Many chemical processing plants, refineries, and Infrared Analyzer. This instrument uses two beams power plants distribute instrument and plant air of infrared radiation. One beam travels through a throughout the facility with lines and equipment comparison cell and the other through the sample located outside the buildings. In such plants two cell. The difference in absorption in the radiation is different temperature conditions exist at the same used as a means of measurement of the dew point time in the same system. Also, a dryer, which may of the sample. be satisfactory for high daytime temperatures may not be satisfactory for lower nighttime tempera- Frequency Oscillator Analyzer. This moisture tures. In areas where freezing temperatures are analyzer uses the change in frequency of a hygro- encountered, a lower pressure dew point may be scopically coated quartz crystal as means of dew required. In general, the dew point should be speci- point measurement. fied 20°F lower than the lowest ambient tempera- ture encountered in order to avoid potential con- SELECTING THE IV densation and freezing. To specify a winter dew RIGHT DRYER point when only summer temperatures will be Before looking at the several types of dryers avail- encountered, can result in over-sizing the equip- able, we need to look at what to consider in decid- ment and increased initial and operating costs. ing which dryer is best for the specific requirements. For plant air and instrument air, primary considera- tions in specifying a dryer are condensation and KNOW THE SPECIFIC USES OF THE freezing. In a system where a lot of internal pipe COMPRESSED AIR corrosion could occur, high humidity in the air The selection of an air dryer is done best by the stream should be avoided. professional who knows or learns the particular end uses, the amount of moisture which each use KNOW THE ACTUAL PERFORMANCE can tolerate and the amount of moisture which While many dryers have a standard rating of needs to be removed to achieve this level. 100°F saturated inlet air temperature and 100 PSIG Air which may be considered dry for one applica- operating pressure, it is important to check on the tion, may not be dry enough for another. Dryness actual performance of the units obtained in actual is relative. Even the desert has moisture. There is plant operating conditions. always some moisture present in a compressed air KNOW EACH USE system regardless of the degree of drying. Different In addition to plant and instrument air applications, types of dryers, therefore, are available with vary- there are many other uses requiring moisture ing degrees of pressure dew point ability. removal to a low dew point. For example, railroad To specify a dew point lower than required for an tank cars which carry liquid chlorine are padded application is not good engineering practice. (charged) with compressed air to enable pneumatic (Naming a pressure dew point is how to state the unloading. Chlorine will combine with water vapor degree of dryness wanted.) It may result in more to form hydrochloric acid; therefore, the compressed costly equipment and greater operating expense. air must have a minimum moisture content to prevent severe corrosion. KNOW THE TEMPERATURES To determine whether or not the compressed air will Droplets of moisture in wind tunnel air at high- remain sufficiently dry, we must know the end use of testing velocities may have the effect of machine the air and the temperature at which it must work. gun bullets, tearing up the test models. In an industrial plant where the ambient tempera- Air used for low temperature processing (for exam- ture is in the range of 70°F or higher, a dryer capa- ple, liquefaction of nitrogen or oxygen) can form ice ble of delivering a pressure dew point 20°F lower on cooling coils, thus requiring defrosting. The than ambient, or 50°F, may be quite satisfactory. lower the moisture content of the air, the longer the periods between defrosting shutdowns.

COMPRESSED AIR AND GAS DRYING 3 For these and similar temperature applications, In a non-cycling refrigerant dryer, the refrigerant compressed air must not only be free of liquid phase circulates continuously through the system. Since water but must also have a minimum content of the flow of compressed air will vary and ambient vapor phase water. Usually specified for these temperatures also vary, a hot gas bypass valve or requirements are dew points in the range of -40°F to unloader often is used to regulate the flow of the -100°F at pressure. refrigerant and maintain stable operating conditions With these characteristics in mind, what types of within the refrigerant system. In most designs, the dryers are available? refrigerant evaporates within the air to refrigerant heat exchanger (evaporator) and is condensed after compression by an air or water to refrigerant heat TYPES OF COMPRESSED V exchanger (condenser). A typical schematic diagram AIR DRYERS is shown in Figure 1. Different methods can be used to remove the mois- This design provides rapid response to changes in ture content of compressed air. Current dryer types operating loads. While older refrigerant type air include the following: dryers have used CFC refrigerants such as R12 and Refrigerant type R22, newer designs are in compliance with the -cycling Montreal Protocol and use chlorine free refrigerants -non-cycling such as R134A and R407C. The properties of these Regenerative desiccant type newer refrigerants require careful attention to the -Heatless (no internal or external heaters) system design, due to differences in -Heated (internal or external heaters) operating and temperatures. -Heat of Compression Cycling type refrigerant dryers chill a mass sur- -Non-regenerative single tower rounding the air passage in the evaporator. This mass Deliquescent may be a liquid such as glycol or a metal such as alu- minum block, beads or related substance, which acts Membrane as a heat sink. The compressed air is cooled by the Each of these dryer types will be discussed in some heat sink which has its temperature controlled by a detail. thermostat and shuts off the refrigerant compressor during reduced loads, providing savings in operating REFRIGERANT TYPE DRYERS costs but at higher initial capital cost. Although it does not offer as low a dew point as can be obtained with other types, the refrigerant type Advantages of Refrigerant Type Air Dryers include: dryer has been the most popular, as the dew point • Low initial capital cost. obtained is acceptable in • Relatively low operating cost. general industrial plant • Low maintenance costs. air applications. The • Not damaged by oil in the air stream principle of operation is (Filtration normally is recommended). similar to a domestic Disadvantages of Refrigerant Type Air Dryers refrigerator or home air include: conditioning system. The • Limited dew point capability. compressed air is cooled Advantages of Direct Expansion Control include: in an air to refrigerant • Low and precise dew point. heat exchanger to about • Refrigerant compressor operates continuously. 35°F, at which point the condensed moisture is Disadvantages of Direct Expansion Control drained off. The air is include: then reheated in an air to • No energy savings at partial and zero air flow. air heat exchanger by Advantages of Cycling Control include: means of the incoming • Energy savings at partial and zero air flow. air which also is pre- Disadvantages of Cycling Control include: cooled before entering • Dew point swings. the air to refrigerant heat • Reduced life of refrigerant compressor from exchanger. This means starting and stopping. that the compressed air leaving the dryer has a pres- • Increased size and weight to accommodate the sure dew point of 35 to 40°F. A lower dew point is heat sink mass. not feasible in this type of dryer as the condensate • Increased capital cost. would freeze at 32°F or lower.

4 COMPRESSED AIR AND GAS DRYING REGENERATIVE DESICCANT TYPE DRYERS amount of purge air is approximately 5-10% of the These dryers use a desiccant, which adsorbs the air flow through the dryer. The purge air can be water vapor in the air stream. A distinction needs to eliminated if a blower is used for circulation of be made between adsorb and absorb. Adsorb means atmospheric air through the desiccant bed. that the moisture adheres to the desiccant, collecting To protect the desiccant bed from contamination in the thousands of small pores within each desic- from oil carry-over from the air compressor, a coa- cant bead. The composition of the desiccant is not lescing filter is required upstream of the dryer. To changed and the moisture can be driven off in a protect downstream equipment from desiccant dust regeneration process by applying dry purge air, by or “fines”, a particulate filter downstream of the the application of heat, or a combination of both. dryer also is recommended. Absorb means that the material which attracts the moisture is dissolved in and used up by the mois- Advantages of Regenerative Desiccant Type Dryers ture as in the deliquescent desiccant type dryer. include: • Very low dew points can be achieved without Regenerative desiccant potential freeze-up. type dryers normally • Moderate cost of operation for the dew points are of twin tower con- achieved. struction. One tower • Heatless type can be designed to operate pneumat- dries the air from the ically for remote, mobile or hazardous locations. compressor while the desiccant in the other Disadvantages of Regenerative Desiccant Type tower is being regen- Dryers include: erated after the pres- • Relatively high initial capital cost. sure in the tower has • Periodic replacement of the desiccant bed been reduced to (typically 3-5 years). atmospheric pressure. • Oil aerosols can coat the desiccant material, Regeneration can be rendering it useless if adequate pre-filtering is accomplished using not maintained. a time cycle or on • Purge air usually is required. demand by meas- HEAT OF COMPRESSION TYPE DRYERS uring the tempera- Heat of Compression Type Dryers are Regenerative ture or humidity in Desiccant Dryers which use the heat generated dur- the desiccant tow- ing compression to accomplish desiccant regenera- ers or by measur- tion, so they can be considered as Heat Reactivated. ing the dew point There are two types, the of the air leaving Single Vessel Type and Hot unsaturated air Hot the on-line tower. the Twin Tower Type. used for regeneration saturated In the heatless regener- air ative desiccant type, no internal or external The Single Vessel Heat of heaters are used. Purge air requirement can range Compression Type Dryer up to 18% of the total air flow. The typical regenera- provides continuous dry- tive desiccant dryer at 100 psig has a pressure dew ing with no cycling or point rating of -40°F but a dew point down to -100°F switching of towers. This can be obtained. A typical schematic diagram is is accomplished with a shown in Figure 1. rotating desiccant drum in a single Cold Heat reactivated regenerative desiccant dryers may divided into two separate Dry air saturated have internal or external heat applied by heaters. In air streams. One air air the internal type, steam or electricity may be used stream is a portion of the hot air taken directly from in heaters embedded in the desiccant bed. This the air compressor at its discharge, prior to the reduces the amount of purge air required for regen- aftercooler, and is the source of heated purge air for eration to about 5%. The purge air plus normal regeneration of the desiccant bed. The second air radiation is used to cool the desiccant bed after stream is the remainder of the air discharged from regeneration to prevent elevated air temperatures the air compressor after it passes through the air going downstream. aftercooler. This air passes through the drying sec- In externally heated regenerative desiccant dryers, tion of the dryer rotating desiccant bed where it is the purge air is heated to a suitable temperature dried. The hot air, after being used for regeneration, and then passes through the desiccant bed. The

COMPRESSED AIR AND GAS DRYING 5 passes through a regeneration cooler before being Deliquescent dryers normally are designed to give a combined with the main air stream by means of an dew point depression from 20ºF to 50ºF at an inlet ejector nozzle before entering the dryer. temperature or 100ºF. This means that with air enter- The Twin Tower Heat of Compression Type Dryer ing at 100ºF and 100 PSIG, a leaving pressure dew operation is similar to other Twin Tower Heat point of 80ºF to 50ºF may be obtained (a reduction of Activated Regenerative Desiccant Dryers. The dif- 20ºF to 50ºF from the inlet temperature). Dew point ference is that the desiccant in the saturated tower suppression of 15 to 50°F is advertised. This type of is regenerated by means of the heat of compression dryer actually dries the air to a specific relative from the hot air leaving the discharge of the air humidity rather than to a specific dew point. compressor. The total air flow then passes through Advantages of Single Tower Deliquescent the air aftercooler before entering the drying tower. Desiccant Type Dryers include: Towers are cycled as for other Regenerative • Low initial capital and installation cost. Desiccant Type Dryers. • Low pressure drop. Advantages of Heat of Compression Type Dryers • No moving parts include: • Requires no electrical power. • Low electrical installation cost. • Can be installed outdoors. • Low power costs. • Can be used in hazardous, dirty or corrosive • Minimum floor space. environments. • No loss of purge air. Disadvantages of Single Tower Deliquescent Type Disadvantages of Heat of Compression Type Dryers include: Dryers include: • Limited suppression of dew point. • Applicable only to oil free . • Desiccant bed must be refilled periodically. • Applicable only to compressors having a • Regular periodic maintenance. continuously high discharge temperature. • Desiccant material can carry over into down- • Inconsistent dew point. stream piping if there is no after-filter and if the • Susceptible to changing ambient and inlet air dryer is not drained regularly. Certain desiccant temperatures. materials may have a damaging effect on down- • High pressure drop and inefficient ejector nozzle stream piping and equipment. on single vessel type. • Some desiccant materials may melt or fuse • Booster heater required for low load (heat) together at temperatures above 80°F. conditions. MEMBRANE TYPE DRYERS

SINGLE TOWER Membrane technology has advanced considerably in DELIQUESCENT TYPE recent years. Membranes commonly are used for gas DRYERS separation such as in nitrogen production for food The deliquescent desic- storage and other applications. The structure of the cant type dryer uses a membrane is such that molecules of certain gases hygroscopic desiccant (such as oxygen) are able to pass through (permeate) material having a high a semi-permeable membrane faster than others (such affinity for water. The as nitrogen) leaving a concentration of the desired desiccant absorbs the gas (nitrogen) at the outlet of the generator. water vapor and is When used as a dryer in a compressed air system, dissolved in the liquid specially designed membranes allow water vapor formed. These hygro- (a gas) to pass through the membrane pores faster scopic materials are than the other gases (air) reducing the amount of blended with ingredients water vapor in the air stream at the outlet of the to control the pH of the membrane dryer, suppressing the dew point. The effluent and to prevent dew point achieved normally is 40°F but lower corrosion, caking and dew points to -40°F can be achieved at the expense channeling. The desic- of additional purge air loss. cant is consumed only when moist air is passing through the dryer. On average, desiccant must be added two or three times per year to maintain a proper desiccant bed level.

6 COMPRESSED AIR AND GAS DRYING Water vapor * CAGI ADF 100 can be Out ordered through the CAGI office at (216) 241-7333.

Compressed air Compressed air Out In

Sweep air Water vapor Out Advantages of Membrane Type Dryers include: Regenerative desiccant dryers generally provide • Low installation cost. a pressure dew point of minus 40°F or lower, at • Low operating cost. operating pressure and 100°F saturated inlet air • Can be installed outdoors. temperature. • Can be used in hazardous atmospheres. Deliquescent dryers are more sensitive to the satu- • No moving parts. rated inlet temperature and, based upon a saturated Disadvantages of the Membrane Type Dryers inlet air temperature of 100°F, provide a dew point include: from 64°F to 80°F at operating pressure. • Limited to low capacity systems. Membrane dryers deliver pressure dew points rang- • High purge air loss (15 to 20%) to achieve ing from -40°F to 50°F or higher, if required. The required pressure dew points. dry air flow from a membrane dryer is dependent • Membrane may be fouled by oil or other on the inlet dew point and pressure of the supply contaminants. air and outlet pressure dew point required by the • High initial capital cost. application. Supply air temperature has no appre- DRYER RATINGS ciable impact on capacity.

The standard conditions for the capacity rating in OPERATING PRESSURE scfm of compressed air dryers, are contained in At higher pressures, saturated air contains less CAGI ADF 100, Refrigerated Compressed Air Dryers – moisture per standard cubic foot than lower pres- Methods for Testing and Rating*. These commonly are sure saturated air. Drying air at the highest pressure called the three 100s. That is, a dryer inlet pressure of consistent with the plant design will result in the 100 psig, an inlet temperature of 100°F and an ambi- most economical dryer operation. Chart 1 shows the ent temperature of 100°F. If the plant compressed air relationship between operating pressure and water system has different operating conditions, this will content. Taking air at 100 PSIG as the normal pres- affect the dryer rating and must be discussed with sure, a subsequent decrease in pressure results in a the supplier to ensure compatibility. substantial increase in the water to be removed. At higher pressures the water content curve tends to SPECIFYING A COMPRESSED VI increase at a slower rate. AIR DRYER INLET TEMPERATURE The air dryer with certain auxiliary equipment The temperature of air entering the dryer usually is becomes a system, which is an important component close to the temperature at which it leaves the after- of the whole plant compressed air system. Various cooler. Saturated air at 100°F saturated contains components comprising the dryer subsystem should almost twice the amount of moisture of saturated air be selected on the basis of the overall requirements at 80°F. For every 20°F increase in the temperature of and the relationship of the components to each other. saturated air, there is an approximate doubling of There are just three main factors to analyze in the moisture content. Thus it is desirable to operate selecting the appropriate dryer (including size) to the dryer at the lowest feasible inlet temperature. provide your required performance – dew point, The moisture content of saturated air at a given operating pressure and inlet temperature. temperature, in grains per cubic foot, is given in DEW POINT Chart No. 2 in the appendix. Refrigerant dryers provide a pressure dew point of With that in mind, a dryer specification is needed. 35°F or 50°F at operating pressure based on saturated inlet air temperature of 100°F.

COMPRESSED AIR AND GAS DRYING 7 HOW TO SPECIFY 4. Utilities To provide a good basis for the purchase of each Electric type of compressed air dryer the following sample ______Volts ______Phase ______Hz. specifications are given: Steam ______PSIG ______Temperature Refrigerant Type Water 1. Inlet Conditions ______F ______PSIG Flow scfm Electric Enclosure Operating Pressure NEMA-1 General Purpose Inlet Temperature NEMA-4 Weatherproof 2. Performance NEMA-7 Explosion Proof Outlet Dew Point NEMA-12 Dust Tight Pressure Drop- Single Tower Deliquescent Type Maximum Allowable 1. Inlet Conditions 3. Design Flow scfm Pressure Operating Pressure Temperature Inlet Air Temperature Ambient Temperature- Ambient Temperature Minimum Maximum 2. Performance Air cooled or water cooled Outlet Dew Point Pressure Drop- 4. Utilities Maximum Allowable Electric ______Volts ______Phase ______Hz. 3. Design Cooling Water Temperature Pressure (if applicable) Temperature Ambient Temperature- Electric Enclosure Minimum NEMA-1 General Purpose Maximum NEMA-4 Weatherproof NEMA-7 Explosion Proof Membrane Type NEMA-12 Dust Tight 1. Inlet Conditions Regenerative Desiccant Type Flow scfm Operating Pressure 1. Inlet Conditions Inlet Air Temperature Flow scfm Ambient Temperature Operating Pressure Inlet Air Temperature 2. Performance Outlet Dew Point 2. Performance Pressure Drop- Outlet Dew Point Maximum Allowable Pressure Drop 3. Design 3. Design Pressure Pressure Temperature Temperature Ambient Temperature- Inlet and Outlet Connections Minimum Maximum

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, but even the finest supplier does a better job when the operator or owner or buyer of the apparatus is himself knowledgeable.

8 COMPRESSED AIR AND GAS DRYING WHAT IS A COMPRESSED AIR VII DRYING SYSTEM? While the dryer is the heart of the matter, good dry- ing performance usually requires it to be supported by auxiliary components in a drying system.

ISO CLASS 1.4.1 ISO CLASS 1.2.1 REFRIGERANT DESICCANT SINGLE TOWER DELIQUESCENT FIGURE 1

ABOVE: LEFT TO RIGHT: Aftercooler – to remove the heat of compression and TYPICAL FLOW DIAGRAMS OF DRYING SYSTEMS. cool the air to within 10°F to 15°F of the cooling Left: Refrigerant. medium. The major portion of the water vapor enter- Center: Desiccant. ing the compressor is condensed in the aftercooler. Right: Single Tower Deliquescent. Separator – to remove liquids condensed by the Filters are shown in the flow diagrams as single com- aftercooler. An automatic trap discharges the liquids ponents. Local operating conditions and selection of to a drain. equipment will dictate which filters may be required Receiver – to smooth compressor pulsations, and and whether a bypass is to be installed for servicing. function as a reservoir of compressed air. It also Also dual filters and block valves may be considered. permits liquid water, oil, and solid particulate matter To ensure the expected performance and reliability of not removed by the aftercooler-separator to settle the compressed air system, good attention must be out of the air stream. Adequate drain provision, with exercised in the selection of all components. A typical appropriate treatment, is essential. system may include several of the following items: A receiver before a dryer is filled with saturated air. Compressor – to increase the pressure of the air to A sudden demand for air exceeding the dryer rat- that desired in the system. Numerous types and ing, may result in overloading the dryer and a pressure ratings are available. Compressor dis- higher downstream dew point. A receiver placed charge pressure must allow for pressure drops after the dryer will be filled with air already dried through downstream treatment equipment, piping and can satisfy a sudden demand without impact- and valves. ing the dryer performance and presure dew point.

COMPRESSED AIR AND GAS DRYING 9 Mechanical Prefilter– to remove solid and liquid entrainments which remain in the air or which have condensed in the piping. All dryers are designed for an inlet condition of only saturated air or gas. Liquids should not be allowed to enter the dryer. Coalescing Filter – to remove contaminants, such as the aerosols of water and oil, from an air stream by causing the particles to unite, or coalesce, so that the droplets formed may be removed. Oil Vapor Filter – to adsorb oil vapors gener- ated by oil lubricated compressors. Oil in the compressed air system is a 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 V for 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 or water systems should comply with all Federal, State and local codes.

ADDITIONAL VIII LITERATURE 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 Testing and Rating • ADF 300 – Single Tower (Non-Regenerative) Desiccant Compressed Air Dryers – Methods for Testing and Rating • ADF 700 – Membrane Compressed Air Dryers – Methods for Testing and Rating McGraw Hill Book Company • Adsorption – C. L. Mantell

10 COMPRESSED AIR AND GAS DRYING APPENDIX 1

WATER CONTENT OF AIR AT VARIOUS TEMPERATURES AND PRESSURES

CHART NO. 1

POUNDS MOISTURE/1000 CU.FT. AT 0LBS GAUGE HOWN . S ATURATED EMP S T OF . AND FT . . CU ABS . LB 1000 14.7 AS AT G OISTURE PER M IR OR A OUNDS P REE F

PRESSURE – LBS. PER SQ. IN. GAUGE

COMPRESSED AIR AND GAS DRYING 11 APPENDIX 2

MOISTURE CONTENT OF SATURATED AIR AT VARIOUS TEMPERATURES

CHART NO. 2

7,000 grains = 1 pound 1 pound = 0.833 gallons at 60°F -80°F = .00035 gr/ft3 -100°F = .00007 gr/ft3

12 COMPRESSED AIR AND GAS DRYING 2. Read horizontally to intersection of curve corresponding to APPENDIX 3 the operating pressure at which the gas was dried. 3. From that point read vertically downward to curve corre- sponding to the expanded lower pressure. DEW POINT CONVERSION 4. From that point read horizontally to scale on right hand side CHART NO. 3 of chart to obtain dew point temperature at the expanded lower pressure. To obtain the dew point temperature expected if the gas were 5. If dew point temperatures of atmospheric pressure are expanded to a lower pressure proceed as follows: desired, after step 2, above read vertically downward to scale at bottom of chart which gives “Dew Point at 1. Using “dew point at pressure,” locate this temperature on Atmospheric Pressure.” scale at right hand side of chart. , °F. RESSURE P EW POINT AT D

DEW POINT AT ATMOSPHERIC PRESSURE, °F.

COMPRESSED AIR AND GAS DRYING 13