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BA OF OA ON AN AONOF O FOO OAY OO

J

Global Products Manager for Screw Compressors

Frick Division of York International

Wynesboro, Pennsylvania

Joseh W Pillis is Global ProductsManager for Screw Comesso with theFrick Division of York nteational inWaynesboro Pennsylvania He has workedin scew comressor design alicationand service for 19 years with FrickComany and as a Fuel nection TestEngineer for for years with Mack TrucksM Pillis has ublished a varie oftechnical aers fr A AR and NST and current serves on technical

committees for AE and R. holds numeus US andinteational atents in screw comressor technology

M Pillis has a BS degree (Mechanical ngineering 19)m rginia Polytechnic nstitute and State Universiy is aregisted Professional gineer in the States of Mland andw

ABSTRT

Rotary screw compressors e widely used today in chemicaland petrochemical rening, gas processing, and in the oil patchCommon applications include reigeration using hydrocarbons,hydrouorocarbons (HFCs), and ammonia reigerants, vaporrecover, gas gathering,

and

compression of el gas, natural gas,landll gas, tail gas, C, and helium Simle in concept, the screwgeometry is suciently dicult to visualze that many peopleusing screws today have only a vague idea how they actually wokAn understanding of the basics of their operation will help inapplying them correctly, avoiding nuisance problems in operation,and achieving the best overall system designs

CONSTRUCTION

A typical oil ooded twin screw compressor consists of maleand female rotors mounted on bearings to x their position in arotor housing that holds the rotors in closely tolerancedintersecting cylindrical bores (Figure 1) The rotors basic shape isa screw thread, with varying numbers of lobes on the male andfemale rotors The driving device is generally connected to themale rotor with the male driving the female though an oil lm Inlow and medium pressure gas compression, four or ve lobed malerotors generally drive six or seven lobe female rotors to give afemale rotor speed that is somewhat less than the male speedSome designs connect the drive to the female rotor in order toproduce higher rotor speeds, thus increasing displacementHowever, this increases loading on the rotors in the area of torquetransfer and can reduce rotor life For this reason, female drive isnot used on larger compressors

Screw compressors are available in a wide variety of matealsFor most reigeration applications, the housings are made of greycast iron, though nodulr or ductile ron is available A variety ofcast steel housings are also available including LCB, LC2, or LC3 for improved ductility at lower application temperatures Rotor

147

{gure 1 Scw Comessor Basic Geomet

mateial is noally mld crbon steel Heat tatment of the rotorsis normally not required Bearings can be ball and roller type inantiction being equipped compressors, or conventional sleevebearing materials in journal bearing equipped compressors

Neoprene or BunaN rings and seals are common, with a widevety of optional elastomers for special applications Shat sealdesigns are of the conventional mechanic types, with single,double, and wet and dry tandem aangements usually available

OIL INJCTIO

Nonoil injected or dry screw compressors have been used on avariety of gas compression applications since the 1930s Theproliration of engineered synthetic oils and coalescing oilsepators in the last 20 years has caused a signicnt shi awayfrom

dry screws to e use of oil iection on many applications

Most common gas compression screws applied today use oiliection into the compression ea for lubrication, sealing ofleakage paths, and cooling Injected oil quantities e approximately10 to 20 gamin per 100 hp The use of such lge oil quantitiestransfers most of the heat of compression to the oil and allowsdischarge temperatures to be very low even at high compressionratios Running single stage at 20: 1 compression ratio on methanewould yield 650F dischge temperatures in nonooded types ofcoressors th oil ooding, the screw discharge temperaturedoes not exceed 230 Running screws at 20: 1 or even higher,single stage, though not energy ecient compared with two stagesystems, will not hm the compressor Many such systems arerunning today Oil iection quantity and oil temperature can bevaried in order to maintain the discharge temperature above the dewpoint of water or hydrocarbons in the gas stream This is essential toavoid dilution with hydrocbons or accumulation of water in thehigh side oil sump on gas streams saturated with water

FUNDAMENLS OF OPERTION

A screw compressor is best descrbed as a positive displacementvolume reduction device Its action is analogous to a reciprocatingcomressor (recip) moe than ny other common compressor typeIt is helpl to refer to the equivalent recip process to visualize howcompression proceeds in a screw Gas is compressed by pure rotarymotion of the two intermeshing helical rotors Gas travels arondthe outside of the rotors, starting at the top and traveling to thebottom, whle it is ansfeed axially om e suction end to thedischarge end of the rotor area

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14 PROCEEDINGS OF THE 28TH TURBOMACHINERY SYMPOSIUM

Suction Process

Suction gas is drawn into the compressor to ll the void wherethe male rotor rotates out of the female ute on the suction end ofthe compressor Suction charge lls the entire volume of eachscrew thread as the unmeshing thread proceeds down the length ofthe rotor This is analogous to the suction stroke in a reciprocatingcompressor as the piston is drawn down the cylinder (Figure 2)

Dsch

(gure 2 Suction Process

The suction charge becomes trapped in two helically shapedcylinders formed y the screw threads and the housing as thethreads rotate out of the open suction port The volume trapped inboth screw threads over thei entire length is dened as the volumeat suction (Vs)

In th

recipanalog,

th piston reaches the bottomof the stroke and the suction valve closes trapping the suctionvolume (Vs) (igure 3)

Figure 3 Maximum Suction Volume Ted

The displacement per revolution of the recip is dened in termsof suction volume by the bore times the stroke times the number ofcylinders The total displacement of the screw compressor is thevolume at suction per thread times the number of lobes on thedriving rotor

Comression

The male rotor lobe will begin to enter the trapped female uteon the bottom of the copressor at the suction end, forming theback edge of the trapped gas pocket The two separate gascylinders in each rotor are joined to form a "V shaped wedge of

gas, with the point of the "V at the intersection of the threads onthe suction end (Figure 4) Further rotation begins to reuce thetrapped volume in the "V and compress the trapped gas Theintersection point of the male lobe in the female ute is like thepiston in the recip that is staing up the cylinder and compressingthe gas ahead of it (Figure 5)

hv

Figue 4 Coression

Figure 5 Continuation of Comression

Discharge Pcess

)

In the recip compressor, the discharge process strts when thedischge valve rst opens. As the pressure in the cylinder exceedsthe pressure above the valve, the valve lifts, allowing thecompressed gas to be pushed into the discharge manifold Thescrew compressor has no valves Only the location of the dischgepot deteines when compression is over (Figure 6) The volumeof gas remaining in the "V shaped trapped pocket at dischargeport opening is dened as the volume at dischge (Vd)

A radial discharge port is used on the outlet end of the slidevalve and n axial port is used on the dischge end wall These twoports provide relief of the inteal compressed gas and allow it tobe pushed into the dischge housing Positioning of the dischargeports is very important as this controls the amount of intealcompression

In e recip, the discharge process is complete when the pistonreaches the top of the compression stroke and the discharge valvecloses The end of the discharge process in the screw occurs as etrapped pocket is lled by the male lobe at the outlet end wall ofthe compressor (Figure 7 The recip always has a small amount ofgas (cleance volume) that is le at the top of the stroke to expandon the next sucion stroke, taking up space that could have been

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BASICS OF OPERATION AND APPLICATION OF OIL FLOODED ROTA SCREW COMPRESSORS 149

Figu Beginning f Dschae.

used to draw in more suction charge At the end of the dischrgeprocess in the screw, no cleance volume remains All compressedgas is pushed out the discharge ports This is a signicant factorthat helps the screw compressor to be able to rn at much highercompression ratios than a recip

uVv

gure End f Discharge.

Vlume Rati

(

In a reciprocating compressor, the dischrge valves open whenthe pressure in the cylinder exceeds the pressure in the dischargemnifold Because a screw compressor does not have valves, thelocation of the discharge port detemines the maximum dischrgepressure level that will be achieved in the screw threads before thecompressed gas is pushed into the discharge pipe

Volume ratio is a design charactestic of all screw compressorsbecause the compressor is essentially a volume reduction deviceThe comparison of the volume of trapped gas at suction (Vs) to thevolume of trpped gas remaining in the compression chamberwhen it opens to dischage (Vd), denes the inteal volume

reduction ratio of the compressor This volume index, or "Vi,determines the inteal pressure ratio of the compressor, and therelationship between them can be approximated as follows:

i=Vsd

where:

Vi = Volume ratio or indexV s = Volume at suctionV d = Volume at dischge

Pi=Vi

(1)

(2)

where:

Pi = Inteal pressure ratiok = Specic heat ratio of the gas being compressed

Only the suction pressure and the internal volume ratiodetermine the inteal pressure level in the trapped pockt beforeopening to the dischge port However, in all systems, the suctionand dischae pressures are determined by the needs of the process,not by the compressor

If the inteal volume ratio of the compressor is too high for a

given set of operating conditions, the dischage gas will be kepttrapped in the screw threads too long d be raised above thedischrge pressure in the piping This is called over compressionand is represented in the pressurevolume curve in Figure 8 In thiscase, he gas is compressed above dischge pressure and when theport opening occurs, the higher pressure gas in the screw threadexpands out of the compressor into the discharge line This takesmore energy than if the compression had been stopped sooner whenthe inteal pressure was equal to the system dischge pressure

Ov

0 ytem

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150 PROEEDINGS OF THE 28TH TURBOMAHINERY SYMPOSIUM

rl

Under CopssoP-V Diagam

C >PI

Figur 9 Undr Comrssion P-V Diagm

order to provide accurate balancing of the compressor cpacity tothe gas compression needed in the system Common capacitycontrol methods

Slide valve controlling discharge port Slide valve controlling discharge port and volume ratio

Slide valve not controlling discharge po

Plug valves

able speed

Slide valves cotrolling the dischae port are a very commotype of capacity control device used in screw compressors They popul because they can give innitely adjustable control ofcpacity, oen om percent to percent This type of slidevve woks by opening a recirculation passage i the highpressure cusp, which allows a portion of the trapped gas in the"V shaped compression chamber to be recirculated back to thesuction cavity bere it begins compression (Figure This

method oers good eciency at part load r two reasons First,the recirculated gas only has to overcome a slight pressure dropin order to bypass back to suction, since the recirculation slotopens to the trped pocket before compression has started,avoiding a precompression loss Secod, as the slide valvemoves, the radial discharge port is also beig moved s thetrpped volume at suction is decreased, he dischge portopening is also delayed, thus maintaining approximately thesame volume ratio at part load as at ll load for optimum ptload eciency

RecirculationPassage

Slide Valve RulatingCapacity and

Discharge Port Location

Figur 10 Slid Val Caaci Control Mchanism

A compressor designed to control cpacity and volume ratio isshown in Figure In this design, a movable slide stop isadjustable in the same bore as e slide valve, and the dischargeport position and the recirculation slot position can both beadjusted This allows an innite number of adjustable positions forboth valves, which provides volume ratio and capacity adjustmentom ll load to approximately percent load, with continuingcapacity adjustment down to percent load This aangementors the potential for improved energy eciency at ll and ptload when compared with a coventional slide valve

M m

hP L

Figur 11 Slid Val Caaci and Volum Ratio Controlchanism

Slide valves that do not control the dischge po come inseveral vieties The most common is a round slide valveintersecting with slots in the rotor bore (Figure This type ofunloader still gives good reduction of capacity, but ot as good areduction in part load power because it does not maintain thevolume ratio during unloading There can also be some leakageacross e slots in th rotor bore, which can hurt performance at allloads These devices are lower in cost than coventional slidevalves and used in some smaller compressors

lug Mves inTi Br

circultinsg

lide Valve egulatinapacity bu ntiscarge orocatin

Fgur 12 Hrringbon Slot Slid Val Caaci ControlMchanism

Plug valves are radial or axial devices, which lift to open arecirculatio passage om the trapped pocket back to suctionThey will typically give unloading in steps of 7 percent,

percent, and percent of full load, as each progressive plug isopened These devices also do not give part load volume ratiocorrection like the rst slide valves Thus, pt load eciency iscomparable to slide valves that do not regulate the discharge portPlug valves also tend to be lower cost and simple in control method(Figure

Variabl Sd

riable speed is occasionally used as a method of capacitycontrol with screws This can be provided with speed controlledengines, steam turbines, or variable equency electric drivesCompressor power does not decrease linearly with speed

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BASCS OF OPRATON AND APPLCATON OF OL FLOODD ROTARY SCW COMPRSSORS 151

ucoGas

-

Figur 13 Two St Plug Val Unloading Mchanism

reductio, but rather decreases as a nction of rotor tp speed adoperatig compression ratio. I geeral, the compessor part loadeciency will be slightly better at low comression ratio ad

sigicatly better at hgh compression ratio wth reduced speedcompared to slide valve cotrol, but this is bere tag intoaccount the losses in the driver a t reduced speed.

pical pulse width modulated viable equecy drives willhave a loss in eciency of approximately percet at full load.The will also cause an additioal loss of motor eciecy of 2.5percent ue to the ects of osinusoidal pwer on the motorThis gives a full load eciency loss of abou percet thsome dives, this loss may stay at the same relatve horsepowerlevel as the dve speed is reduced, ectively becoming a larerpercentage loss at lower speeds. If a compressor will operate at pload and at high compresson ratio for many hours per yer, thecost of the drive may be justied. Hoever, if a compressoroperates e ll load for a high pecentage of time, or operatespmaril at low compression ratios, it is unlikely that there s any

advantage ith a variable speed drive, or that the cost of the drivecan be justied aaist slide valve conrol. Tpcal curves fordrences in power cosumption re shown i Figures 14 and 15.These curves e calculated assumin drive and motor losses are axed percetage of power, ot xed k at reduced speed.

KW Ug 4 le ave v a ee, p a are Pre

G bmn

ap

Figur 1 Part Load Powr Consumtion at Low CR, VariablSd Vrsus Slid Val Control

Whether variable speed is justied as a t load method, oeust cosider te loa prole ad oeratig cotions tat aeepected a particular aplicatio.

D Slide Vae vs. Vari Speed, psia Dscharge Ress

5

Q( 5

ariSpd

:tJi

: i=t

5 L--- 6 8

NH Flow l'nn

Figur 15 Part Load Powr Consumtion at gh CR VariablSd Vrsus Slid Val Control

riable speed cotrol with screws should ot be implementedwithout cosulng the compresso maufacturer. Thee are lowerspeed limits for compressors below which bearings may fail due tonadequate bearing lubricatio Large compressors will have lowerminimum speeds than small compressors. Miimum speedcosideraios wi typically limit unloadig by speed control to percent to percet of ll load, dependig on compressor size

Many small compressors ma be able to accommodate drivespeeds above the put lie equency, but separator limts, oilcooler sze, and other package limitatios must be nvestigated Itis also possible to ll a compressor with oil and caus e ilure, if thespeed is reduced below an acceptable rage wth the compressorunloaded. May of these limits not published but should beinvestigated ey in a variable speed proposal or study.

OIL SYSTEMS SEPRION AND COOLING

As poited out, oil iection perrms ma useful functions ithe screw compressor; however, sice oil is no desirable i otherpts of the process sstem, oi ooded screws brig with them theneed for oi sepators. Oe type of oil separator design is seen iFgure . The mixture of discharge gas and oil leaving thecompressor is directed against one head of the oil eparator, where

it experieces a chage in direction and a large reducio n velocity.The larger oil pticles are draw to the oil sump b grvi, with thesmallest prticles or oil smoke being cied ino the coaescglers. Here, these sml particles impact o the nteal bers in helters and coalesce ito larger oil droplets that c then be collectedi the dry end sump and retued to a low-pressure ea i thecompressor. Besides removing oil from the gas stream, the separatoralso gives the oil i the main sump time for any codesed gas toabsorb heat and vpoze or eained bubbles o se to the suaceof the oil, givig higher qualty oil for reiection in the compressor.

Figur Scrw Comrssor il Sarator Function

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15 PROCEDINGS OF THE 28TH TURBOMACHINERY SYMPSIUM

Since most of the heat of compression is ansfeed to the oilduring compression, an oil cooling system must remove this heat. Thethree most common systems are watercooled, thermosyphoncooled,or liquid inection. Aircooled oil coolers are also used, but not aspopular as the above three systems and will not be addressed here

WatrCoold il Cooling

Refeing to Figure 7 the hot oil leaves the oil sepatorthrough a strainer into the oil pump. The oil is pumped through ashell nd tube or plae type heat exchger where he heat is

reected to circulating water, or glycol. The cool oil is then lteredand returned to the compressor for reinection. Primarydisadvantages of this system involve the initial cost andmaintence of the water or glycol system, and sks of tubeupture or plugging if proper water condition is not maintined. Itis generally prefeed to leave wter ow to the cooler at a xedrate and use a temperature regulted mixing vlve on the oil side ofthe cooler to blend hot nd cold oil to the desired reiectiontemperature. This will help to minimize waterside fouling of thetubes by maintaining a higher minimum water velocity. The use ofclosed loop glycol elimnates fouling risks.

r[

(

DRE

i

j . 1

I

-

i

Fgur Scrw Comrssor with atr Coold il Cool

osyhon il Cooling

Theosyphon oil cooling is used pmarily in reigerationapplications in industal plants where the copressed gas iscondensed to a liquid as pt of the cycle. A typicl thermosyphonsystem is shown in Figures and The theosyphonsystem is similar to the watercooled system, except the water isreplced by regerant boiling on the tube side of the oil heatexchnger. The themosyphon system is bsiclly a oodedevaportor fed by the gravity hed of liquid regerant i athermosyphon receiver, elevated above the level of the heatexchaner. As hot oil enters the shell side of the het exchnger,reigerant boils in the tube side with the bubbles rising in retuline back to the thermosyphon receiver. Vapor generated by thisprocess is vented back to the condenser inlet, where it gives up heatand retus with the system liquid. In eect, the theosyphon oilcooler is a gravityflooded evaporator, with its evaporatingtemperre set by the pressure t the condenser.

Thermosyphon systems are popular because they requirevirtully no mintenance nd they do not degrde compressorperfomance.

Liquid Injction il Cooling

Liquid inection is another oil cooling system tht is usedpimily in reigertion applications where the compressed gas isliqueed s part of he cycle. Liquid injection cools the oil by

-k -r9

-'j!

j

8VER

i:.

I OR

-

.1Figur 18 Scrw Corssor with osyhon il Cool

OIL IN

gsF

36#/F3

OIL OU

RIVR

Figur 19 oshon il Cooling with Cossor NotRunning

I IN

g2.5/T3

105" I UT

TS RECEIVER f

Figur 20. Toyhon il Cooling with Comrssor Running

direct inection of condensed liquid into a lowpressure screwthread, p way down the compession process, as seen in Figure The oil and the discharge gs re controlled to the desiredtemperature by a theal expansion lve. The valve controls theinected liquid ow to mintain the temperture of a ermal bulbinstalled in the compressor dischrge line.

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BASCS OF OPETION AND APPLCATON OF OL FLOODED ROTARY SCREW CMPRESSORS 153

r--q

FL

n- r - I!\ ;

i

I

UQOA)

l..i Il.......................... U

EASIO

1-

:

.

Figu 21 Scrw Comrssor wih Liquid Rigrant Injction ilCooling

Some of the ijecte liqui expas though the expasio valveito the scew theas equiig aitioal powe to compess it.All the emaiig liqui mixes with the oil a is caie ow theischge lie with the dischae gas. The iecte liqui emais

i the compesso less ha sec. As heat tas takes timemuch of the coolig takes place i the dischage lie ad the oilsepato whee the liqui has time to absob heat a beevpoat. Some of the liqui mixe wih the oil will leak thoughiteal compesso cleaaces to the suctio. Oil lee to suctiohas little ect. Howeve the liqui will expad ito the suctio asvapo with a lage icease i specic volume educig theamout of suctio chge that ca be aw ito the compesso.This causes a uctio i capacit with liquid ijectio withlge eect at high compessio atios.

Liqui iectio coolig is attactive because of low iitial costad low maiteace; howeve the powe ad capacit pealtiesmke it uattactive fo most applicatios with high heat ofcompessio. Liqui iectio is also ot ecommee fsstems with eve tempo coitios of high suctio a low

hea whee iaequate ietial acoss the expasio valvecauses ratic liqui fee.

ECONOMIZERS AND SIDELOADS

Sice the pessue i a scew compesso is gauall iceasigalog the legth of the oto it is possible to locate holes ow theoto boe at a pessue betwee suctio a dischge. Aecoomize po is a hole locate at a xe volume atio osuctio that ca be use as a seco suctio pot o thecompesso. Aitioal gas ca be aw ito the scew theawhile it is ope to the ecooize pot aisig the pessue i thethe with the combie massow beig compesse togethe toischge.

I the case of a sieloa te aditioal gas geeall comesfom a seco evapoato. The efigeat pessue at the

evapoato must be highe tha e ecoomize pt pessue by asuciet amout to ovecome pessue dop a be able to focegas ito the scew thea. As a example i the ecoomize pot islocate at a pessue of times suctio pessure Figue givesa coparso of how much sieloa capacity woul be available atvaous suction pessues.

An economizer is a special case of a seloa use iefigeatio applicatios. A potio of the lqui fom thecoese is evapoate with the ash gas goig to theecooize prt ode to subcool the remaig coeseliqui to the satuate tempeatue at the pot pessue (Figue This allows a icease i capacity because the liqui goig to themai evapoato is cole. This also allows a icease i oveall

ideload Capacity % f uc ccy

Sidelad Temp& 400-

t .......................... 300 t .......................... 00

....... : ,

:

100b

0S Su T F

F+F,oF

F

Figur 22. Scrw Comrssor Sidload Caaci at VaingSuction and Sidload Tmraturs

eciec because of the themoamic avatage of subcooliga the fact that the compessio of vapo at the ecoomize potis geeally ve eciet.

Cndese

Evapor

ese

t

gur 23 Piing Diagram for Economir and SidloadAlication

Whe the compesso is spiig thee ae cetai souces ofieciec that must alwas be ovecome: ictio i the beigsa at the walls sheig of the oil lms leakage of a cetaiamout of oil fom thea to tha a ctio at the sha seal.The aitio of the ecoomize gas oes ot sigicatl iceasea of these losses a hus e ecoomize ompessio is almostlike it is pfom i a ictioless "lossless compesso.

MAINENNCE

Follow maufactue's ecommede maiteace scheules asequiemets v betwee poucts. Howeve some geealcommets ca be mae that shoul t most applicatios.

Oil aalysis is a impotat pa of a compesso maiteacepogam. It is ost mpotat to watch for water contet (KarlFische test) ad viscosit chag ove time whch woul iicateoil beakow o ilutio. Excessive wate can amagecompessos if allowe to emai i the system. Metals analssmay etect some poblems but usually iicates poblems atherlate i the failure cycle.

Vibatio alysis is the best ethod fo moitog beaigcoitio. It is ve eective with antiictio beaigs givig aiicatio of beag eteioatio i e ve ealy stages of

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154 PROCEEDINGS OF THE 28TH TURBOMACNEY SYMPOSIUM

failure A good vibratio moitoig program or oboard vibratiomoitorig employed with antifrictio bearig compressordesigs ca elimiate the eed for routie disassembly ispectioif properly implemeted ad greatly miimize the risk ofcatastrophic compressor failure

MARKET TRENDS

Screw compressors hve eed a reputatio r reliabilityexibility low maiteace ad good efficiecy i mayidustries Screws have a lge market share i the portable ad

statioary markets idustrial reigeratio aircoditioigad i the oil gas ad petrochemcal reigeratio markets Asgatherig pressures i the gas patch decrease the same agumetsthat have allowed screws to take lge mket shares i otheridustries e becomig obvious to packagers ad operators have see a large icrease i screw compressor usage i atural gasgatherig i the last ve yers

Scew compressors have ectively displaced reciprocatigcompressors i the medium displacemets ad are today the mostwidely used compressor type i their size rage With theitroductio of laer displacemet screws they ecroachigher o the lower ed of e cetrigal sizes

The Motreal Protocol (Motreal Protocol has dictatedthe elimiatio of all chlorie cotaiig regerats with chlorouorocarbos (CFCs) already baed i most coutries ad

hydrochlorouorocarbons HCFCs) slated for phaseout i newequipmet sold aer 2010 Oil ooded screw comressors areuiquely suited to compressio o a wide rage o ew andcovetioal reigerat gases because o positive displacemet adlow discharge temperature Screws have already bee applied omay o the ew alteative bled reigerats with umeroussystems i operatio o R-404a R-507 R-134a R-410a ad R-407c As new regeratio plats are istalled the li cycle o theplats mkes cosideratio of log-term reigerat availability aissue to be cosidered Th exibility o screws gives the advatagethat they do ot have to be redesiged to hadle the ew gases

CONCUSION

It is certai that chage will cotinue i the petrochemicalchemical

ad

gas busiessesTh

screw compressor has prove to

be a strog cetral compoet i may of our systems because ofits ieret eciecy saty ad exibility It is certai thatcotiuig iovatio i screw compressor desig ad properapplicatio of screws i ew processes will be a key i improvigsystem desigs for the ture

APPENDIX A

CASE STUDY #-SCREW COMPRESSORS

FOR OW TEMPERURE REFRIGERION

A producer of specialty chemicals had a eed to codese vetgas om a reactor i oe of their plats This vet gas was madeup of umerous compouds icludig phosgee Aside ommiimizig the release of hazardous materials to the atmosphereliquefyig these vapors recovered salable product I order tocodese the vet gas to a liquid it was ecessary to cool it to arather low temperature A mechaical reigeratio system wit arefgerat evaporatig temperature of F was chose as thebest meas to accomplish this

A cascade reigeratio system employig oilooded rotaryscre compressors was selected for this applicatio (Figure A1)o separate workig uids reigerats) are employed with eachuid cotaied i a separate coolig loop The lower temperatureworkig uid absorbs heat om the process ad rejects it to thehigher temperature workig uid i a heat exchager Reigerat23 was chose as the workig uid i the low temperature cooligloop because it operates at positive pressure at the very lowrefrigerat temperature (F) required to cool the vet gas

The R23 vaporizes as it absorbs heat rom the vet gas ad etersthe suctio of the R23 screw compressor at approximately 23 psiaThis compressor boosts the R23 vapor to approximately 183 psiaad discharges to a heat exchager I this heat exchager the R23 vapor codeses as it rejects heat to a secod refrigerat(ammoia) The ammoia i tu vaporizes as it absorbs heatfrom the R23 ad ows to the suctio of a secod screwcomressor The pressure of the amoia vapor is icreased fromapproximately 20 psia to 250 psia i the compressor The heatorigialy absorbed om the vet gas alog with heat of

compressio) is the rejected i a shell ad tube codeser tocoolig tower water as the ammoia vapor codeses

.

RECEIVERNH3COMPR

10

;J

EXPANSIONTANK

-

23COMPR)

1

10

T

gure A-1 s #1 Piing Diagram fo Cascade R23/HReigeaton System

R23 ad ammoia were chose as the lower temperature dhigher temperature workig uids i a cascade aagement toesure that operatig pressures i all pts of the system were aboveatmosphec yet low eough to allow the use of stadard refrigeratioscrew compressors ad compoets ote that the R23 vapor isheated from to 5 before it eters the screw compressor

The rotary screw compressors for this system are directly drive

by electric motors ( 150 hp for the R23 compressor 350 p for theammoia compressor)

I additio to the coolig load associated wi codesig theet gas this system hadles aother refrigertio load at a highertemperature A aqueous calcium chloride solutio is cooled by theammoia regerat i aother heat exchager ad circulated toother process users i the plat

CASE STUDY #2SCREW COMPRESSORS

FOR HYDROCARBON REFRIGERATION

A producer of commodty petrochemicals wished to replace two20yearold refgeratio systems that cooled ethylee dchoride

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BASIS OF OPERATION AND APPICAION OF OI FOODED RO SRE COMPRESSORS 155

in one of their plants Ethylene dichloride (EDC) is an inteediateproduct used in the manufacture of chlorinated and uorinatedcompounds, as a raw material for vinyl chloride monomer, and asa solvent

The existing reigeration systems employed reciprocatingcompressors in a compound aangement (low-stage and hghstage compressors) They used Reigerant-12, which is no longerpermitted in new equipment The plant owners anted the newsystems to use a woking uid that would be aailable for theforeseeable ture, and ultimately selected propylne (CH)

This was the rst time that a hydrocarbon reigerant had beenused at this facility (Figures A-2, A-3, and A-4) Although the plantwas accustomed to handling ammable uids, they wanted to limitthe size of the reigerant charge to 1000 lb of propylene persystem In order to achieve tis, the shell and tube heat exchangersin which the EDC was cooled were of a somewhat unconventionaldesign Whereas the more typical approach would be to evaporatethe propylene reigerant outside the heat exchanger tubes, theseunits were designed with the propylene evaporating inside thetubes

gure A2 Case # Ethylene Dichloride Chiller UtilizingPylene as the Rgeant (During Shping)

Figure A3 Case #2 Case Steel Screw Comressor for PyleneComression.

Oil-ooded rotary screw compressors were chosen to replacethe aging reciprocating compressors Unlike the reciprcatingcompressors, the screw compressors are able to comprss thereigerant vapors from the evaporator conditions of 69 psig,38F to condensing conditions 260 psig, 1 15F in a single stageAn "economizer was employed to improve reigeration cycleeciency

gure A4 Case # Tandem Seal Oil Recove System

The screw compressors for this propylene reigeration servicewere supplied with cast steel casings and tandem mechanical shaftseals Each compressor is directly driven by a 200 hp electicmotor

CASE ST #3ETANE CORESSORS

A compressor system was installed utilizing four 2500 hp oilooded screw compressors for recovering primarily methae oma storage tank at 277 psia and 87F suction, to 435 psia and191F dischrge The compressors operated single stage at 161compression ratio They were equipped with cast steel compressorhousings, cyclig lube oil pumps, and WPII motors The oil wascooled in sepate air-cooled heat exchangers prior to iection ithe copressors

Th

destination as in the Cabbean (Figures A-5, A-6, and A-7)

gue A5 Cse Oil System for Methane Comressors.

CASE ST #4-CLOR ALALI

TEIA CLORINE RECOVER SSTES

A cascade reigeration system was supplied to condensechlorine om a process gas stream at approximately 50F (FigureA-8) Two identical screw compressor systems were designedutilizing a low temperature C reigeration loop condensed byan R-22 refrigeration loop The R-22 loop was designed for 350psig design pressure, and could accommodate R-507 (a hydrouorocarbon (FC)) in the ture, if R-22 (an HCFC slated forelimination by the Montreal Protocol) becomes too expensive overthe life of the equipment

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156 PROCEEDINGS OF THE 28TH TURBOMACHIN ERY SYMPOSIUM

Figue A-6 Case #3 Cast Steel Screw Comressor for thaneComression

Figure Case #3 2500 Electric Drive to

Figure A -8 Case #4 Cascade Reigetion System

The system was designed to utilize secondary oil coalescingechnology on the C reigerant side, in order to maintain the oilcryover to e C evaporator at or below approximately 05ppm The tertiary condensing load is up approximately 50 Ft inelevation, and therefore e liqui C has to be subcooled beforegoing to the evaporator The size of the system is around 7 TR(tons reigeration at 55F, and is driven by a hp electricmotor on the C screw compressor, and hp on the R22 screwcompressor

Each of these systems replaced existing systems utilizing with multistage centrigal compressors in a deep vacuum Thebenets include the following:

With the ecient incremental turndown of the screwcompressor units, the operations people have the exibility oftuing down to approximately 25 percent of design load andachieving reduced power consumption at he low rates

The use of C reigerant in the tertiary chlorine condenserdoes not present a reaction problem Chlorine reacts with mostreigerants However, C is inert in a chlorine stream

The working uids are enviromentally iendly reigerantsthat are readily available and inexpensive

This system does not operate anywhere in a vacuum, thusreducing longterm maintenance costs

The system is easier to operate than the previous system, and is

not depenent upon constant operating conditions, completelyeliminating an possibility of surging,

and

oring the exibilityto operate at any leaving temperature desired

CASE STU #5-

LENE STORAGE ALICION

This aplication is an open loop regeration system attached toa large atmospheric propylene storage cility This cility storespolymer grade propylene, and the reigeration system wasdesigned to recover the ash that results om loading thepropylene at liquid om a

marin

teinal, as well asaccommodating the storage load

Each of these systems utilizes two oil ooded screwscompressing he propylene in two stages om around 5 psia up to257 psia The system is designed with two identic booster andhighstage compressor units used to recover the propylene underloading conditions Each of ese compressors utilizes about hp drive motor In addition to the compound compressor systemoperating during loading times, an additional screw compressorwith a hp motor maintains the vessel pressure single stage,with an economizer cycle during the storage conditions in order tosave energy

REFENCES

Montreal Protocol, "Montreal Protocol on Substances thatDeplete the Ozone Layer, Comlete text on the United ationEnvironmental Protection Website, http:/wwwuneporg/unep/secreta/ozone/montrealht

Pillis, J. W, "Development of a iable Volume Ratio

Screw ompressor, R nnual Meeting