process compressors theory - air and gas

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Build Your WorkingKnowledge of Process Compressors

How good are

you at selecting

medium-power

reciprocating

compressors?

Edward H. Livingston,

Howden CompressorsIncorporated

The role of compressors in the chem-ical process industries (CPI) is crit-

ical since they are used to circulategas through a process, enhanceconditions for chemical reactions, provideinert gas for safety or control systems,recover and recompress process gas, andmaintain correct pressure levels by either adding and removing gas or vapors from a process system.

The chemical engineer involved with process design and equipment selectionmust have a working knowledge of compressors, since they are the most mechani-cally complex machinery used in the CPI.This working knowledge must not only berelated to the thermodynamics of the gas being compressed, but also to the type of compressor to be used for a particular

process. The latter has become more impor-tant as the result of the passage of the CleanAir Act Amendments of 1990, legislationwhich places limitations on emissions from

process equipment. Fugitive emissions fromsuch sources as compressors, pumps,valves, and piping systems and connectionsmust be reduced over the coming years inorder to comply with EPA Equipment Leak Regulations.

Brief background

The principal types of compressorsfound in the CPI are reciprocating, turbo(centrifugal and axial), and rotary flowdesigns. Within some of the types are varia-tions. Nevertheless, in all cases, compres-sors are used to convert energy from oneform to another.

1. Reciprocating machines are those inwhich gas is moved by the linear motion of a piston within a confining cylinder. The

work done enhances the pressure and densi-ty. Flow through the cylinder is controlled

by valve actions. Examples of reciprocatingmachines include piston compressors, lubri-cated and nonlubricated, and metaldiaphragm compressors.

2. Turbomachinery, or dynamic compressors, are those in which a dynamic headis imparted to the gas by means of highspeed impellers rotating in a confining case.This category includes axial-flow, radial,centrifugal and fan-blower compressors.

3. Rotary machines are those in whichgas is moved by the positive displacementof two rotating lobes or by oscillating vanesconfined in an eccentric cylinder.

4. Ejector machines are those in whichgas is moved by kinetic energy inducedthrough high-velocity nozzles.

This article will primarily deal with recip-rocating, positive displacement compressorswith emphasis placed on applications andmachines having installed power of 200 kW or less. However, some comments with respect tolarger compressors will be made due to their importance in process applications.

Before we discuss specific compressor types in detail, let’s look at typical applica-tions of the units.

Oil refinery processes

Both positive displacement and dynam-ic compressors are used in the refining of crude oil. Crude oil feed-stock contains polluting compounds such as sulfur, chlo-rides and salts. Refining processes extractthese pollutants and convert them intoneeded byproducts, thus reducing emis-sions into the atmosphere. Hydrogen sul-fide and carbon dioxide are being treated bychemical absorption systems such as

CHEMICAL ENGINEERING PROGRESS . FEBRUARY 1993 . 27



AIR AND GAS FLOW

methyldiethanolamine (MDEA) witha high absorption efficiency.

Hydrogenation processes occur inthe initial distillation column, as wellas hydrotreating, hydrocracking, and

catalytic reforming units.

Hydrotreating

Hydrotreating removes objection-able elements such as sulfur, nitrogen,oxygen, and halides from feedstock by reacting them catalytically withhydrogen. In this application, threecompressors are frequently used: therecycle compressor which takeshydrogen-rich gas from the hydrogenseparator and recycles it to the

process front end, the make-up compressor that adds hydrogen to the process at the front end, and the ventgas compressor which handles hydro-carbon gas mixtures of molecular weights from 20 to 40.

The size of the recycle and make-up compressors depends on thehydrotreater capacity. In general,recycle compressor flow rates varyfrom 25,500 to 127,500 m³/h and rec-iprocating or centrifugal compressors

may be used since compression ratiosare low. The make-up compressor flow rates vary from 5,950 to 29,750m³/h, and while the flow rates cansupport centrifugal design compres-sors, the compression ratios are toohigh for centrifugal design and thusreciprocating designs are used.

Hydrocracking

Hydrocracking produces gasolinefrom heavy feedstocks. In many cas-es, hydrocracking and cat reformingunits work in unison. Hydrocrackingtakes place at higher pressures, 6.9MPa to 13.8 MPa and has a high con-centration of hydrogen with a fixed- bed catalyst. Two compressors areused for this application: the first isthe recycle compressor which takeshydrogen-rich gas from the separator and recycles that gas to the processfront end where it mixes with the liq-uid feedstock, and the second is the

make-up compressor which addshydrogen to the process after the sep-aration and before the recycle com-

pressor. The recycle compressor is

usually a barrel-type centrifugal compressor since the compression ratio islow, while the make-up compressor isa multistage balanced opposed recip-rocating compressor because the

compressor must boost the gas from1.4 MPa to 13.8 MPa.

To reduce emissions within therefinery, hydrocarbon gases are col-lected, recompressed and used else-where. Hydrogen collected from thereciprocating compressor packinggroup and from the barrel-type compressor mechanical seal are oftenrecycled back into the process bysmall, positive displacement, recipro-cating compressors.

Polymerization

Compressors are required to feedgases at elevated pressures into reac-tors or compress gases to a pressurethat will permit liquefaction after which the liquid is pumped directlyinto the reactor. Typical gases com-

pressed include ethylene, hydrogen,hydrogen chloride, methyl chloride, phosgene, propane, and butane.Depending on the polymer and the

process used, pressures can rangefrom 0.1 MPa to 380 MPa

Higher flow rate

applications requir e

centr if ugal or axial f low

machines.

The original low density polyethyl-ene process required gas pressures of 200 MPa to 320 MPa Three positivedisplacement compressors were usedin series to compress ethylene from0.5 Pa to 240 MPa The combined power requirements of these threecompressors exceeded 7,500 kW.

The operating pressures of the lin-ear linear low density process have been reduced to 1 to 2.1 MPa, howev-

er, compression equipment isrequired to keep the gas circulating in

the reactor and fluidize the polymer particles.

Some specialized co-polymers stilluse pressures of 200 to 320 MPa. The

flow rates are small in comparison tostandard, full-size plants. Feed compres-sors for additives operate to 320 MPa,and power is in the range of 75 kW.

High density polyethylene is often

co-produced with polypropylene in a pressure range of 1 to 3.5 MPa. Thecatalyst is fed to the reactor with thegas is flashed, separated, and recycled.

Electronicsand semiconductors

Gases are produced for the manu-facture of electronic components andsemiconductors. Purity of the gases isvital and ultrapure systems are used inall phases of manufacturing. Addingto the purity requirements is the han-dling problem associated with strongoxidizers, flammable, pyrophoric,and highly toxic compounds.

Oxidation processes are used for the formation of protective silicondioxide coating on wafer surfaces.This is accomplished in a diffusionfurnace in an oxygen atmosphere.

Protective atmosphere doping usesnitrogen in the purity range of of 99.9999% for the manufacture of

microelectronic components to protectthe material as well as being a carrier gas for dopants. Chip manufacturing involves ultrahigh-purity argon for sili-con crystal growing, oxide removal(etching) and doping of wafers for desired chemical composition.

Manufacturing of integrated cir-cuits and semiconductors use ultra-high purity gaseous chemicals for dop-

i n g etching, epitaxy, and ionimplantation. Gaseous chemicals suchas arsine, phosphine, silane or chloros-lanes, diborane, halocarbons, hydro-gen selenide, hydrogen sulfide, and sul-fur hexafluoride are used in a mixtureof diluent gases like argon, helium,nitrogen, and hydrogen.

Hydrogen recovery

Even though the cost of hydrogenis relatively low, hydrogen is recov-ered for safety reasons. The hydrogenvapor from liquid storage tanks is

recovered by a compressor rather thanventing it to the atmasphere. Thisreduces the possibilities of auto-igni-tion, a situation that could occur when



vapor is vented by a relief device.

High-pressure piston compressors use packing vents todispose of effluent leaks.

Small compressors recover,recompress, and inject the gas

into the process stream.

Reactor gas feeds

Hydrogen. Hydrogen isrequired for full or partialhydrogenation of fats and oils by convert ing unsaturatedradicals of fatty glycerides tohighly or completely saturat-ed glycerides, pharmaceuticalintermediates, and final prod-ucts. Compressors are usedfor initial hydrogen feed and

recycle of hydrogen in the process.

10 0

10

1

0.1

Inlet Flow, Actual m³ /hour

Figure 1, Range of CPI Compressors

Hydrogen from either gas plants or tube trailers is boosted by positivedisplacement piston and diaphragmcompressors to the reaction feed pres-sures required.

, Pulsation ampener

, Intercooler

Carbon d i ox ide is a solvent atsupercritical conditions. High purity

carbon dioxide is compressed to thecritical pressure conditions and isthen recycled after separation.

Nitrogen is sparged into reactors toreduce dissolved oxygen, blanketssensitize compounds against oxida-

tion and contamination, and purges process reactors and piping systems atshutdown and startup. Crankcase (Frame)

Refri gerant gases such as chloro-difluoromethane are fed to reactors for manufacture of fluoropolymers such as

polytetralluoroethylene (PTFE) .

n Figure 2. Pi ston design.

Gas separation

required by the plant. These compressors are sized to meet peak

demands for the gas (gases) and arefrequently specified with a standbyunit of 50 or 100% capacity.

Cryogenic separation of atmospher- Membrane separation can produce

ic gases results in the highest purity higher purity gas streams and are used

levels in comparison to membrane sep- in upgrading hydrocarbon gases.

aration or Pressure Swing Adsorption Processing pressure drops may

(PSA). If the final product can tolerate require the treated gas to be com-lower gas purity levels, PSA produc- pressed for recycle. In most cases cen-

tion of nitrogen and oxygen can result trifugal compressors are applicablein cost reductions of 20-60%. because of the low compression ratio.

Conventional air compressors feed

the PSA unit. Depending on the pres-sure requirements, reciprocating pis-ton compressor can be installed to

boost the PSA outlet pressure to that

Nitrogen boosting

Under certain conditions, nitrogenis required to maintain flow of liquid

inder

from storage tanks or tank cars.Typically, bone dry nitrogen is intro-

duced under pressure to induce flow.Chlorine tank cars frequently usethis method during cold weather andthe importance of using an oil-free,nonlubricated compressor is appar-ent. Hydrocarbons from lubricants

could carryover and react with thechlorine, while water vapor (150 ppm or more) would cause a highlycorrosive condition.

Nitrogen also acts as a blanketinggas to prevent fire or explosion condi-

tions and creates an oxygen free envi-ronment to enhance long-term storage,especially for perishable products.

Nitrogen is used to balance pres-



AIR AND GAS FLOW

sures in high-speed mechanical sealsto prevent leakage of gas to the envi-ronment. The system also permitsmonitoring of seal performance by

pressure decay in the seal. Small, pos-

itive displacement, reciprocatingcompressors are used to pressurizethe seal and maintain purge flow.

Compressor types

Compressor selection is based onthe process operating variables andhow those variables fit the designranges of the types of available com-

pressors. Figure 1 illustrates the rangeof compressors used in the CPI.

Positive displacement pistoncompressors are normally selectedfor applications where the inlet flowrate is no greater that 6,800 m³/h.Design discharge pressures canrange from 0.5 MPa to 380 MPa. Thelatter for small displacementmachines assure low density poly-ethylene process. Diaphragm com-

pressors, a specialized version of positive displacement piston compressors, are limited to single cylin-der inlet flows up to 204 m³/h.

Centrifugal compressors range frominlet flows of 850 m³/h to 340,000 m³/hwith case pressures to 70 MPa for smallcentrifugal units and considerably low-er pressure for large units, and axialflow compressors range from 34,000m³/h to 1 ,020 ,000 m³/h with case pres-sures generally limited to 1 to 2 MPafor all sizes.

Engineering considerations

and economicsFor the higher flow rate applica-tions, the process engineer has fewalternatives, with selections being lim-ited to either centrifugal or axial flowmachines. However, in the lower flowrange, several choices are available:

positive displacement piston anddiaphragm compressors, or hybridmachines that combine piston anddiaphragm cylinders on one crankcase.General arrangements of these com-

pressors are shown by Figures 2through 4.

Piston compressorsAll piston compressors have a

First Stage

Diaphragm Cylinder

Second Stage

Drive Motor Baseplate Crankcase (Frame)

n Fi gure 3. Diaphragm design.

Inlet PulsationDampener

\ Last Stage Crankcase (Frame) \Diaphragm Cy l i nde r 1

Baseplate First Stage _ I Second Stage

Piston Cylinder Piston Cylinder

n Fi gure 4. Hybri d design.

Crosshead

\

Suction Valve Pockets

Packing Case w/Valves

I / I

Crankcase Compression Cylinder

n Figure 5. A simpli fi cation of compressor crankcase elements.

crankcase that converts rotary motionto linear motion: a crosshead for guid-ing the motion of the piston, a pistonfitted with seal rings, a cylinder inwhich gas compression takes place,and one or more suction valves andone or more discharge valves that reg-ulate the flow of gas into and out of the compression cylinder. A simpli-

fied diagram of these elements isshown in Figure 5.

Air compressors should not beconsidered as process gas compres-sors. There are significant design dif-ferences inherent in the air compres-sors and the improper use of an air compressor in a process gas applica-tion could have severe consequences,



such as excessive leakage, fire, or explosion. For example, the air com-

pressor piston is sealed from thecrankcase by piston rings, usually of cast iron. Since gas can leak into the

crankcase, the simple air compressor should be limited to air or nitrogeneven if the crankcase is pressurized.

Process-gas piston compressorsare designed with a distance piece toeliminate gas leakage into the atmosphere and crankcase. The piston ismounted on a piston rod which is con-nected to a crosshead. The piston rodis relatively small in diameter, andtherefore can be sealed within a pack-ing case with one or more sets of packing. Figure 6 illustrates a typical

packing case. Special purged packingcases and distances pieces are fittedon these compressors to further mini-mize gas leakage.

In the lubricated process-gas compressor fluid is injected into the compression cylinder to provide lubrica-tion for the piston rings and thecompressor valves. Packing is bothlubricated and cooled by injecting thesame lubricant into the packing set(s).The lubrication is provided by either aseparate, crankshaft driven lubrica-tion pump, or an auxiliary motor dri-ven lubricator.

Nonlubricated compressor opera-tion prevents the entrainment of oil in

DistancePacking Rings

.Vent to Flare

Cylinder

’ Purge Connection

n Figure 6. A typical packing case.

the gas being compressed. Types of service for nonlubricated compressorsinclude oxygen compression, food pro-cessing, container manufacturing, breweries, chemical, and specialty gas plants where oil contamination of theend product cannot be tolerated. Nonlubricated reciprocating compres-sors vary in design, but fundamentallythe piston is driven through a crossheadfrom the crankshaft. The piston rod issealed by a stuffing box with packingrings. The compression cylinders aremounted to distance pieces which iso-

late them from the crankcase thus pre-venting oil carryover. The cylindersmay be mounted horizontally, vertical-ly, at an angle, or in combinations of these on multistage, multithrowcrankcases. Nonlubricated cylindersare normally limited to pressures of 25to 41 MPa. A typical non-lubricated

piston cylinder is shown in Figure 7.For the successful operation of non-

lubricated piston compressors, non-metallic piston rings are required.Materials such as PTFE with fillershave proved to be the most efficient.

Valve FlangeCylinder

Valve Hold-down suction a’ve

Rider (Guide) Ri Piston Ring Discharge Valve

F igure 7. A typical lubri cated piston cylinder.

Suction Valve Discharge Valve&Retainer & Retainer

Gas Plate

Contour

Diaphragm Group

O-Ring Seals

Head Integrity O-Ring

Head IntegrityDetection Port

nFigure 8. Moti on of the displacing element causes the

diaphragm to move into the compression chamber to

reduce volume and thereby incr ease gas pressure.

Piston Seals

Piston Crankcase



AIR AND GAS FLOW

Not only are the wear ratesextremely low on the rings madeof these materials, but cylinder wear is also reduced. It is vitalthat these materials be uniform

throughout the entire cross sec-tion so that wear rates and sealing properties are continuous for thelife of the ring.

Another type of oil-free pistoncompressor is one in which alabyrinth profile is used.

Normally, the leakage volume past the labyrinth does not exceed5% of the rated displacement. Atwo-compartment distance piecefor oil wiping and for holding pis-ton alignment is required.

Diaphragm compressors

Piston compressors anddiaphragm compressors sharemany of the same components:a crankcase, crankshaft, con-necting rod(s), and piston.

Head Integrit yDetection O-Ring , Gas Plate

ProcessO-Ring

DiaphragmGroup

HydraulicO-Ring

Head Integrity Detection Port Oil Plate

n Figure 9. Typical l eak detection ar rangement f or a

diaphragm compressor.

nFigure 10. Typical l eakage moni toring system.

The main difference between piston and diaphragm compres-sors is how the gas is compressed. Unlike other types of

reciprocating piston compres-sors in which the primary dis-

placing element, a piston, con-tacts the gas, the metal diaphragmcompressor completely isolates thegas from the displacing element dur-ing the entire work cycle. The motionof the displacing element is transmit-ted to a hydraulic fluid, and thehydraulic fluid transmits its motion toone or more thin, flexible metal discscalled “diaphragms.” This motioncauses the diaphragm to move into the

compression chamber, reducing thevolume and thereby increasing thegas pressure. See Figure 8.

Vent to Flare

Vent to Flare

ture diaphragm failure is minimal. As

a result, this equipment has becomewidely accepted for all types of cont-amination-free applications in labora-tory, pilot, and plant operations.

Because the diaphragms isolate thegas from the compressor lubricants,the discharged gas is as pure as thegas entering the compression head.The gas only contacts clean, drymetallic surfaces and static elastomer or metallic seals. With improveddiaphragm materials and contour con-figurations, reliabi lity of thesemachines has been demonstrated by

years of service in critical applica-tions. With proper installation andmaintenance, the likelihood of prema

Corrosive gases can be handled inthese compressors because the com-

pression cylinder can be manufac-tured from virtually any machinable

material. Certain limitations do applyand these limitations are related to thediaphragm material. Materials of con-struction commonly used for gas con-tacting parts include 17-4ph, 17-7ph,

304SS, 316SS, 400SS, 20Cb, nickel,carbon and low alloy steels. Designsto handle H2S and conforming to

National Association of CorrosionEngineers (NACE) MR-01-75 can be produced.

The benefit of a no-leakage designis evident and is of greater importance

with the advent of the Clean Air Act.Synthetic Organic Chemical Manufac-turing Industry (SOCMI) factors have

been developed by the EPA for allequipment, including compres-sors. For compressor seals, theemission factor is 0.228 kg/h/sour e e Using standard design and

construction methods, leak ratesfrom a diaphragm compressor arein the order of 1 x 10 - 7 standardcc/s. When extremely low leak rates are required, in the order of 1x 10e8 standard cc/s or less, thediaphragm can be sealed by metal-lic “0” rings or it can be seal-

welded to the gas head.In the event the integrity of the

diaphragm or seal is breached,effluent process gas is retained inthe head assembly detection sys-

tem. With a relatively inexpensivemonitoring system, an anomalycan be detected and corrected

before there is a discharge to theatmosphere. Figure 9 illustrates atypical arrangement for diaphragmcompressors and Figure 10 illus-trates a monitoring system

Hybrid compressors

Hybrid compressors are

unique since they combine nonlubricated piston technology withdiaphragm technology on one

reciprocating frame (crankcase).These compressors find applicationwhen inlet pressures are low, the gasflow rates are relatively high, andthe gas must be compressed to high pressure. Depending on the gas flow

rate, multiple two or three stagediaphragm compressors, or a fivestage piston compressor may other-wise be required. To avoid such a

situation, two or three stages of non-lubricated piston cylinders are usedwith a final stage diaphragm cylin-der. In a single machine, the largecapacity of a piston compressor iscombined with the high pressureand leak-tight performance of adiaphragm cylinder.

Piston and diaphragmcompressor efficiency

The volumetric efficiency of a

positive displacement compressor isthe ratio of the gas handled to thecompressor displacement including



gas compressibility. Several factorsinfluence volumetric efficiency:

• compression ratio;• compressibility factors of the gas

at suction and discharge conditions;

• cylinder clearance volume;• valve action (losses);

• piston ring leakage (piston com-pressors);

• adiabatic or polytropic exponent;

and• water vapor.

Compressor manufacturers cancontrol clearance volume, valveaction (losses), piston ring leakage,and the compression ratio (by multi-staging). Of great concern to the com-pressor manufacturer is the control of clearance volumes at high-compres-

sion ratios and when gases have a lowspecific heat ratio.

Piston compressor clearance vol-umes can range from 7% to 22% withfixed valve pockets; diaphragm com-pressors are normally designed withclearance volumes between 4% and7% depending on the size of thediaphragm cylinder. For pressure appli-

cations to 300 MPa diaphragm com-pressor clearance volumes may be ashigh as 10% to 12%, due to practicalmanufacturing tolerance limits, partic-ularly in the valve pocket area.

The effect of clearance volume isillustrated in Figure 11. Clearancevolumes of 5%, 10%, and 15% are

given for illustrative purposes. The5% compression slope ABC willattain P2 quicker than the compres-sion slopes of 10%. Likewise, uponre-expansion at the end of the com-pression stroke DEF, the slope issteeper and therefore allows the gas toenter the cylinder sooner during thesuction cycle.

Compression efficiency is con-trolled by the valves and valve pocket

design. For example, the compressor

Figure 11. Compressor efficiency (valve design effect).

designer may concentrate on the clear-ance volume reduction to improve vol-

umetric efficiency and valve lossescould be high, thus affecting compres-sion efficiency. A decrease in com-pression efficiency leads to increasedpower requirements.

Compression efficiency (valvedesign effect) is also illustrated inFigure 11. The pressure increasesalong curve ABC, and when the

cylinder pressure exceeds the line dis-charge pressure P2 at point B, systemenergy unseats the discharge valve.

Pressure spikes to point C, and the gasis released into the discharge piping.The discharge event is a series of pressure waves that will degenerateuntil the piston reaches the end of itsstroke at point D. From top dead cen-ter, unexpelled gas pushes on the pis-ton and expands along curve DEF.Slightly below point E at P1, the pres-sure is further reduced to point F.Here a sufficient differential pressureexists to unseat the suction valve. Gasis drawn in by the piston for theremainder of the stroke until the pis-

ton reaches bottom dead center.

Volumetic efficiency increaseswith a decrease in clearance volumeand a decrease in compression ratio.While the other factors do influencevolumetric efficiency, clearance vol-ume has the most pronounced effect.In the end, a compromise positionmust be taken to balance volumetricand compression efficiency. In gener-al, the design compromises are relat-ed to compression ratio. For highcompression ratios (6 to 15), clear-ance volume is the principal factorand valves are secondary. For the

intermediate compression ratios (3 to6), clearance volume and valvedesign should be balanced. For low

compression ratios (less than 3),valve design is primary.

For a given operating conditionwhere either a piston or diaphragmcompressor can be considered,diaphragm compressors will have ahigher volumetric efficiency sinceclearance volumes are less and ringleakage is not a factor. This will also

permit compression ratios of 15: 1across one cylinder. Most piston com-

pressors limit compression ratios to5: 1 or less in nonlubricated designsdue to the inherent problem of heatremoval and the effect of high tem-peratures on piston rings.

Isothermal efficiency is usuallyhigher in the diaphragm compressor.This is due in part to the large, flat sur-

face area of the cylinder, the proximi-ty of the cooling passages to the com-pressor valves, and the recirculationof the hydraulic fluid on the back side

CHEMICAL ENGINEERING PROGRESS . FEBRUARY 1993 33

Table 1. Relative Cost Factors.Installed Power Lubricated Nonlubricated Diaphragm

to 30 kW 1.0 1.3 1.531 to 50 kW 1.0 1.3 1.951 to 100 kW 1.0 1.4 1.7101 to 200 kW 1.0 1.4 1.6



A I R A N D G A S F L O W

Table 2. Typical selection guidelines.

Process and Operating Requirements Compressor Type

Contamination during compression

Limited contamination during compression

Medium corrosive gas

Highly corrosive gas

Hazardous gas (toxic, flammable)

Leaktight

Low suction pressure, low compression ratio

Low suction pressure, high compression ratio

High discharge pressure (to 25 MPa)

High discharge pressure (> 25 MPa)

Lube or NL piston

NL piston or diaphragm

All

Diaphragm

Oiaphragm or NL piston

Diaphragm

Lube or NL piston

Hybrid

NL piston or diaphragm

Lube piston or diaphragm

of the diaphragm group. However,isothermal efficiency of any positivedisplacement compressor is less thana dynamic compressor. Hence, a

comparison of isothermal efficiency between compressor types is a falsecomparison. Overall efficiency. volu-metric and compression will normal-ly favor diaphragm types.

Engineering economics

For comparison purposes, capitalcost factors for lubricated, non-lubri-cated and diaphragm compressors arcgiven in Table 1. These values are rel-ative and are based on manufacturers’

standard materials of constructionand exclude drivers.

Performance cost factors are usu-ally based on such ratios as cost/kW,cost/m³/h, and cost/Nm³/h. Whatever measurement is used, the piston compressor has the most favorable ratiowithin the context of its application.However, relative cost factors shouldnot be the sole criteria for compressor selection assuming that any of thetypes discussed have the pressure anddisplacement capabilities. Process or

environmental constraints can raisethe cost factors of lubricated pistoncompressors above other types.Cleanup and disposal equipment andmonitoring instruments for hazardousgases will rapidly escalate the ratios.

This points out the need to matchapplication. In Table 2, some typicalselection guidelines are given.

Applications requiring compres-sion equipment to boost pressure inthe range of 101 kPa to 21 MPa and ashigh as 40 MPa should consider the

use of hybrid compressors. These

machines combine the high flow of the nonlubricated piston compressor with the leaktight, high-pressurecapabilities of diaphragm compres-

sors. The benefit to the engineer isthat the total package is on one compressor and driver, not multiple compressors and drivers. A limitation of such equipment is the diaphragmcylinder operating speed which must be in the range of 400 to 450 rpm.

Process andequipment considerations

Process c on t a c t i n g materials of construction. Table 3 lists typical

materials of construction for lubricat-ed and non-lubricated piston compressor cylinders.

For temperatures of -25ºC to

-200ºC, type 304SS low carbon,alloyed cast iron, or nickel alloy steelshould be used. These materials will

provide low temperature impactstrength and thus avoid brittle failureof critical component\.

Liners are recommended for cylin-ders since they can be replaced easilyin comparison to complete cylinder.Two types are used--a wet liner or adry liner. Wet liners form the inside pressure boundary of the cylinder andalso the inside of the water cooling

jacket. Because it is a pressure bound-ary, the liner wall must be designed towithstand the internal gas pressureand the external cooling water pres-sure. The liners arc either pressed or shrunk into place. Dry liners are not

pressure boundaries. They arc thinner in section and are pressed into thecylinder. Liner materials include

steel, stainless steel. nodular iron or gray iron. The liner material selectionshould be reviewed with the manufac-turer since there may be limitationsrelating to pressure and piston ringcombinations.

Table 4 lists typical materials of construction for diaphragm compres-sor cylinders.

Diaphragm compressor gas platescan be produced from any machinablematerial. Some limitations do apply because of the special requirements

placed on diaphragm material by thecompressor manufacturers. Certainalloys are not readily available. or areavailable only with a tremendous costimpact. Such materials include highnickel alloys, and so forth.

The diaphragm compressor cylin-

Table 3. Typical piston compressor materials of construction.

Component

Cylinders and Heads

Piston

Liners

Packing

Mater ial Remarks

Gray Iron Pressures to 10 MPaDuctile Cast Iron (DCI) Pressure to 16 MPa

Steel Pressur e to 400 MPa

Stainless Steel Pressure to 25 MPa

Alumin um Large cy li nders , l ow in ert ia

Cast Iron, Steel High pressures, chlor ides

Stainless Steel Corrosive conditions

Gray Iron, DCI Most common

Ni-resist Low temperature, corrosion

Stainless Steel Corrosive conditions

PTFE-Fitted Pressures to 28 MPa

Metal

Pressures > 28 MPa

34 . FEBRUARY 1993 . CHEMICAL ENGINEERING PROGRESS



AIR AND GAS FLOW

E.H. Livingston is President and CEO of HowdenCompressors Incorporated, Langhorne, PA 19047Tel: 215/702-7777, Fax: 215/702-7787

His responsibiIities include

management of overall operations,

application and design engineering of pis-

ton and diaphragm type reciprocating com-

pressors he has spent 30 years associated

wi th chemical processing equipment, such

as compressors, pumps, and high pressure

vessels and piping. The author of several

articles on diaphragm compressors and

high pressure equipment, Mr. Livingston

has also lectured in courses on compres-

sors and high pressure equipment for

applications in the chemical and petro-

chemical industries. He received his engi-

neering degree in chemical engineering

from Drexel University. He is an active

member of AIChE, American Society for

Testing and Materials (ASTM), ASMInternational, and National Association of

Corrosion Engineers (NACE).

compression ratios for pistondiaphragm compressors are 5: 1, andfor diaphragm compressors 8: 1 to10: 1. These limitations are based onthe clearance volumes and the heatcharacteristics of each compressor

type.High-pressure differentials can

create higher loads (stresses) on themechanical parts of the compressor.In most cases, this can be solved bymultistaging.

High discharge temperatures are to be avoided to prevent the deteriora-tion of piston rings, packing, andcylinders. Gas discharge temperaturesshould be limited to 200°C.Diaphragm compressors can toleratehigher gas discharge temperatures because of the limited use of non-metallics and a higher rate of heat

rejection.Power savings from multistagingcan range from 5% to 25% depend-ing on the number of stages. Actualsavings will be a function of the

compressor load factor and compressor size.

In conclusion

The compressor types available tothe process engineer are many and the proper selection can be quite difficult.More users are relying on the corn

Table 6. Material and construction guidelines

Ga s Type

Acety lene Explosive

Re ma r ks

Temperature limit, 55 C, low gas

velocity, no copper/copper alloys

Ammonia

Carbon Dioxide

Carbon Monoxide

Corrosive

Corrosive

Toxic

No copper/copper alloys

Corrosive when wet, use 316 SS

Temperature limit 150ºC, carbon

steel or low nickel alloys

Chlorine Toxic Temperature l imit 125ºC,

no hydrocarbon greases/oils

corrosive when wet

Chlorofluorocarbons

Fluorine

Environmental

hazard

Corrosive

Temperature limit 110ºC, leaktight

construction

Fluorinated hydraulic fluid s, 316 SS

or high nickel alloys, degrease all

components

Hydrogen Explosive Leaktight construction, potential

embrittlement above 30 MPa

Hydrogen Sulfide Corrosive Leaktightness, toxic gas, material

hardness limitations, 21 Rc per

NACE public ations

Oxygen Flammable Fluorinated hydraulic fluids , 316 SS

or high nickel alloys, degrease all

components, low gas velocity, limit

. compression ratios

Vinyl Chloride Flammable Temperature limit 90ºC, will

polymerize or l iquify

pressor manufacturers to develop thecomplete gas handling system for a particular process. Still, the processengineer must evaluate and select thetype of compression equipment to beused. During this phase of the pro-

ject, several important points must beconsidered:

1. Are the process conditions accu-rately stated and are there any contin-gencies relating to flow rate and pres-sure identified?

2. Will the process be scaled-up inthe future and, if so, should the compressor specifications reflect immedi-ate needs, future needs or both?

3. Are standard specifications suchas API 618 relevant to the type of equipment to be specified? Should astand-alone specification be written?

4. Can lubrication be tolerated dur-ing compression and if so, mustlubricants be removed from the gas?

5. Can leakage be tolerated?6. Can neither lubrication or leak-

age be tolerated?

7. Is there an actual or potentialcorrosion problem?8. Can the manufacturer service

the equipment?The responses to these questions

are very important and they will helpin the selection process. If there arequestions relating to the type of compressor, controls, process changes,and the like, discuss these issues witha manufacturer or another user. If these steps are not taken, the engineer is faced with difficult equipment andcost comparisons.

P

36 l FEBRUARY 1993 . CHEMICAL ENGINEERING PROGRESS

process compressors theory - air and gas

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