Different Operational Modes for Refrigeration Twin-Screw Compress

7/30/2019 Different Operational Modes for Refrigeration Twin-Screw Compress

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Purdue University

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International Compressor Engineering Conference School of Mechanical Engineering

1986

Dierent Operational Modes for RefrigerationTwin-Screw Compressors

L. Sjoholm

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Sjoholm, L., "Dierent Operational Modes for Refrigeration Twin-Screw Compressors" (1986).International Compressor EngineeringConference. Paper 518.hp://docs.lib.purdue.edu/icec/518
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DIFFERENT OPERATIONAL MODES FOR REFRIGERATION TWIN-SCREWCOMPRESSORS

Lars Sjoholm svenska Roto r Maskiner AB (SRM)Box 15085s-104 65 Stockholm , sweden

ABSTRACTThree different operational modes for the refrigerationtwin-screw compressor are discussed: oil-free, oil-reduced andoil-flooded.By oil-reduced operation is meant the case when the o il /gas ratio is about 1 \ by weight and the oil is circulated inthe refrigeration sys tem . In the oil-reducedmode the rotorswork without timing gears - as in the oil-flooded mode.Advancements in profile design and rotor manufacture have madethis possible.Performance tes t results are shown and the differentoperational modes are also discussed from application pointof-view.

INTRODUCTIONToday's twin-screw compressor is the result of developments that started in the early 1930's.I t was firs t introduced into the refrigeration industry inthe late 1950's. Since then i t has been adapted to many otherapplications and the capacity range has been widened consideriably.At present the twin-screw compressor is manufac tured incapacities from 10 to 10 000 m3/h (6 - 6000 CFM) forrefrigeration and air-conditioning applications and up to60 000 m3/h (35 000 CFM) for general purpose.

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BACKGROUND

The twin-screw compresso r consists of two mating ,

helically grooved rotors "male and female" in a statjonaryhousing with in let and outlet ports. The flow of gas in therotor g rooves is main ly axial. Common lobe com b ina t ions are4-6, 5 -6 and 5-7.Ro to r Profiles

Helical rotor design has evo lved over the years startingwith an asymmetric , basically point-generated rotor profile1935, used in compresso rs with timing gears (dry compression).The symmetric "circular" rotor profile was introduced1947, mainly because i t was easier to manufacture than thepreceding profile. Later, i t proved to be possible to operateoil-flooded compresso rs without timing gears using this profile.The asymmetric , point- and line-generated profilewasintroduced 1967, as the result of studies regarding profileoptimization. Perfo rmance was im proved and fem ale rotor drivebecame feasible with this profile.During the 1980's the helical rotor design has spread outto a large family of rotor profiles and lobe combinations,designated for different applications and manufacturingmethods/rotor materials. The rotor profile is normallyasymmetric and line-generated (Fig. 1). Lobe combination, rotorprofile, "b lowho le" , length of sealing line, quality of themesh seal, torque transfer between rotors as well as clearancescan be optimized for each set of conditions, such as pressures,temperatures, speeds, operational mode etc. Optimum rotor t ip speed is 15-40 m/s (50-130 ft/s) for oil-flooded operation,

25-70 m/s (80-230 ft /s) for oil-reduced operation and 60-120

m/s (200-400 ft/s) for oil-free operation.

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FIG. 1: ROTORS TO A TWIN-SCREW COMPRESSOR

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Rotor C ontac tThe contact between non-synchron ized rotors is mainly ofrolling nature, at a contact band on each rotor's pitchcircle. Very l i t t le sliding is encoun te red and virtually no rotor wear occurs. With the latest profile designs, with speci-ally tailored contact conditions, oil-reduced operation has become possible.

Rotor TorqueOn themale rotor, the internal gas forces always create atorque that acts in a direction opposite to the direction of

rotation. This has been called "positive or b rak ing torque.

On the female rotor, the average torque can be chosenpositive, negative or ze ro . N egative torque means that theinternal gas forces tend to drive the rotor. If the average torque is near zero, the rotor is subjected to torque reversalswhen going th rough i ts phase angles, which under certainconditions can cause instability problem s. This has to beavoided and a stabilizing female rotor torque of sufficientmagnitude can easily be handled by the mesh, due to the goodgear action.Male rotor drive: The torque transmitted from male rotorto female rotor is norm ally 5 - 25 per cent of input torque.Female rotor drive: The torque transmitted from femalerotor to male rotor is normally 50 - 65 per cent of inputtorque.

Rotor LoadsThe rotors are subjected to radial, axial and tilting

loads. (Tilting loads:radial lo ads due to axial loads outside

rotor centre line). The axial load is norm ally ba lanced w ith abalancing piston for larger, h igh pressuremachines (rotord iam ete r above 150 mm (6 inch ) and pressure above 1.1 MPa (160psia)). The balancing force is created by high-pressure gas oroil.Bearings

Radial: Sleeve bearings have been most common for largercom pressors (male rotor diam ete r larger th an 150 mm (6 inch))but antifriction bearings are being used more and more, evenfor larger com pressors; in com pressors for oil-reducedoperation, they aremandatory.

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In smaller compressors , the antifr ic tion type is mostpractical. Antifriction bearings in this context can meancylindrical or tapered roller bearings as well as ballbearings.

Axial: Most common are angular contact ball bearings,but tapered or axial roller bear ings are also used, main ly forsmaller compressors .General: Low -pressu re compresso rs have long, largedisplacement rotors. The space available for the bearings is sufficient as the rotor loads are light. These compresso rs arenormally designed without thrust balancing pistons.High-pressure compresso rs have short and s t if f rotors(shallow grooves) and therefore have space for large bearings.High-pressure compresso rs are normally designed with balancingpistons but smaller compresso rs can often get adequate bearing

l ife without th em .Ro tor Materials

The rotors are normally made of ferrous materials but alsoaluminum and plastics are feasible for certain applications.

OIL-FLOODED OPERATIONIn oil-flooded screw compresso rs the oil is injected at arate of about 1 \ of the displacement volume f low . Part of th isoil is used for lubrication of. bea.rinq:> and shaft seal p.rior toinjection.Historically, the large oil flow was n e ~ d e d for thefollowing reasons:

1) Sealing- but with modern profiles and better rotor manufacturing methods the rotors can form an adequateseal without oil-flooding, at least for low erpressure ratios;2) Coo ling - but for refrigeration systems, liquid refrigerantinjection can be used. The inevitable perfo rm ance loss has with the la test rotor profiles decreasedsignificantly;3) Lubrication - but correct design of rotor contact and torque transmission as well as matched rotor

materials make the compresso r less dependen tof rotor lubrication.

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The type of lubricant is normally a naphtenic base mineraloil but synthetics like polyalphaolefines (PAO) are taking over more and more.

Oil is iniected into intermediate pressure region throughpo'rts, either situated in a slide valve or through stationaryports in the casing.The oil is separated out from the gas by gravity orcentrifugal force. The oil carry-over is about 1 \ by w eightwith those methods. If the refrigeration system does not allow

any oil return, the oil carry-over is designed to be about 50PPM and th is is achieved with coalescing fil te rs .

Figure 2 shows the oil-flooded twin-screw com pressor inthe refrige'ration system .

CONDENSER OIL CoNTENT:

EXPANSIQIIVALVE

EVAPORATOR

OPTICIIIALLIO.REP'.INJECTIONCOMPRESSOR

OIL SUPPLYTO SEARINGSAND IN.JECTIONFIG, 2: THE O I L - ~ L O O D E D T W I N ~ S C R E W COFAESSOR IN TI-E REI""IGERltTIClll SYSTEY

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OPTIONALOIL COOi..ER

WITHOUT OILRETURN20-200 ppy8Y

OILSEPARATOR


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Performance Test ResultsSmaller CompresSPrCompressor: tw in -screw* Operation oil-floodedRoto rs 5-6 (male-female) lobe combinationAsymmetric, line-generated rotor profile**Male rotor diameter: 113.4 mm (4 .465 inch)Male rotor material: steelFemale rotor diameter: 95.8 mm (3 .772 inch)Female rotor material: nodular ironRotor length: 150 mm (5 .91 inch)Clearances: discharge end: 0.04 - 0.07 mm(0.0016 - 0.0028 inch)interlobe: 0.0 05 - 0.04 mm (0.0 002 - 0 .0016 inch)Rotor shaping method: single-index milling to final shape Bearings: Radial: cylindrical rollersAxial: cylindrical rollersDisp lacem ent: 175.3 m3/h (103.2 CFM) at 3550 RPMMale rotor t ip speed : 21 m/s (69 ft /s) Built-in volume ratio: op tim a l within 2 .0 - 5.7Lubricant : PAO ISO 200Oil flow : 20 - 30 1/m in (5 .3 - 7.9 GPM)Oil temperature: 45oc (113DFJPerformance for R22, see figure 3. (Definition of isentropicefficiency, see reference (1)).

* SRM K318** SRM D-3-13

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EFFICIENCY(%)

90

80

70

60

JOLUMETRIC

5 0 ~ - - r - - - - - ~ - - - - ~ - - - - ~ - - - - - - ~ - - - - ~ - - - - ~ - - - - ~ -2 4 6 8 10 12 14 16PRESSURE R!ITIO FIG. 3; OIL-FLOOOED TWIN-SCREW COMPRESSOR,TEST RESULTS

D I S P L A C E M E N T ~ 175,3 m3/h CI03,Z CFM) AT 3550 RPMR22. COND.TEMP. ' 40"C Ci04"F)

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Larger CompressorCompressor: tw in-sc rew*Operation: oil-floodedRotors : 4-6 (male-female) lobe combinat ionAsymmetri c, line-generated rotor profile**Male rotor diameter: 204 mm (8 .03 inch)Male rotor material: steelFemale rotor diameter : 204 mm (8.03 inch)Female rotormaterial: steelRotor length: 336 .6 mm (13 .25 inch)Clearances: discharge end: 0 .05 - 0.06 mm(0 .0020 - 0 .0024 inch)interlobe: 0.035- 0 .105 mm(0 .0014 - 0.0041 inch)Rotor shaping method: single-index milling to final shapeBearings: Radial: sleeveAxial: angular contact ball (with thrust balancing)Displacement: 1222 m3/h (719 .2 CFM) at 3000 RPMMale rotor tip speed: 32 m/s (105 f t/s) Built-in volume ratio: optimal within 2.2 - 4.8Oil flow : 115 - 138 l/min (30 - 36 GPM) for R71782 - 93 !/min (22 - 25 GPM) for R22Oil temperature: 45oc (1130F)Performance for R22 with PAO ISO 68 lubricant, see figure 4 .Performance for R 717 withmineral oil naphtenic base ISO 66,see figure 5.

SRM K 526** SRM D-2-B

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EFF!C IENCY(:1'.)

80

80

70

60

2 4

VOLUMETRIC

ISENTROPJC

6 8 10 12 14 16PRESSURE RATJO

FIG . 4; OIL-FLOODED TWlN-SCREW COMPRESSOR.'EST RESULTSD ISPLACEMENT" 1222 m3/h (714,2 CFM) AT 3000 RPMR22, COND .'EMP. 40 9 C t l04 FJ

EFFICIENCY( :1'.)

90

80

70

60

so

VOLUMETRIC.....____,

2 4 6 8 10 12 14 16PRESSURE RATIO

FIG . 5. OIL-FLOODED TWIN-SCREW COMPRESSOR,TEST RESULTSDISPLACEMENT = 1222 rn3 /h (714,2 CFM) AT 3000 RPMR717, COND.TEMP. "40 'C CI04'F)

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OIL-REDUCED OPERATIONThe operation can be called oil-reduced when the oil/gasratio is about 1 \ by w eigh t, which corresponds to about 0.03 \of displacement volume. The oil is circulatedwithin the refrigeration system . T h is means that the oil-reduced operationeliminates the need for the entire oil system typical for oilf looded com presso rs .Since there is no oil separator in an oil-reduced system ,the lubricant selected can have high solubility/miscibilitywhich gives good oil-return characteristics. Examples of suchlubricants are alkylated benzenes and fluorocarbon lubricants.The total amount of oil needed for an oil-reducedcom pressor system is norm ally 20 - 40 t im es less than for anoil-flooded system.This means that the money can be spent on amore sophisticated lubricant (longer l ife , better lubricity)when the com pressor is oil-reduced.The bearings are oilmist lubricated. Figure 6 shows theoil-reduced twin-screw compressor in the refrigeration sys tem .

CONDENSEROIL CONTENT:IX BY WEIGHTr--t- -------

tEXP,o,NSION't VALVEItI

LIO.REP.INJECTIONCOWPF!ESSOFI

I EVAPORATOR

- l ~ _ _ _ . l ~ -PIG. a. THE OIL-REDUCED TWIN-SCREWCOWPRE5SOFI IN THE REFRIGERATIONSYSTw

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Oil-reduced operationwithout t ~ m ~ n g gears is possible i f due attention is paid to the tribological conditions in thecontact zone.The performance achievable with an oil-reduced compressoris about the same as for an oil-flooded compressor up to apressure ratio of 8. Arotor profilewith very high sealingquality contributes to this. I t shou ld be pointed out that inthis context the performance figures g iven are valid for the"bare" twin-screw compressor. In the refrigeration system , theoil-reduced compressor is not subjected to pressure drop lossesover an oil separator, resulting in a corresponding increase inthe overall efficiency.In an oil-flooded compressor the main performance loss is due to a high content of refrigerant dissolved in the oil. Thisrefrigerant-rich oil is returned to an intermediate pressureregion by the external oil system and some of this oil leaks toregions with s t i l l lower pressure. When the oil is exposed tolower pressure some of the dissolved refrigerant evaporates.This type of loss is eliminated in the oil-reduced compressor.The discharge temperature is limited by injection of highpressure liquid refrigerantwhen necessary. Two cases of liquidrefrigerant injection can be observed:

1) D ischarge temperature is above saturated condition. Theamount of liquid refrigerant injected is normally lessthan 2 \ of displacement volume. This type of dischargetemperature control is also used for oil-flooded operation.Below a pressure ratio of 8 the quantity needed is smalland the performance deterioration rather insignificant.Below a pressure ratio of about 4 there is no need toinject liquid refrigerant from temperature point-of-view.These values are valid for R22. R12 and R114 are even morefavorable in this respect.

2) D ischarge temperature is at saturation point. Here thecompressor can be said to be f looded with liquidrefrigerant and liquid refrigerant exists in the dischargegas. The amount of high-pressure liquid is normally 0.2 -0.5 \o f displacement volume.A liquid-refrigerant-flooded compressor must be oil-reduced orcompletely oil-free because the oil separation system for anoil-flooded compressor cannot tolerate a saturated condition.

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% EFFICIENCY

90

80

70

60

oc

90

80

70

60

50

40

........ - VOLUMETRIC........

........

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The liquid refrigerant has poor sealing qualities, sincei f i t "seals" i t leaks and when i t leaks, i t will evaporate andcreate losses. This is true unless the liquid refrigerant is subcooled to evaporator tem pera tu re , which canno t be done "freeof charge".

Test results in figure 7 show that the performance alwaysdecreases w ith larger amounts of liquid refrigerant. These testresults indicate that the oil-reduced compressor shows betterperformance w ith supe rhea ted discharge tem pera tu re th an w ith saturated discharge tem pera tu re (liquid-refrigerant-flooded),especially at high pressure ratios.

EFFICIENCY( / . )

90

80

70

60

2

VOLUMETRIC

4 6 8 10 12 14 16PRESSURE RATIOFIG. B OIL-REDUCED TWIN-SCREW COMPRESSOR,'EST RESULTSDISPLACEMENT = 517,5 m3/h ~ 3 0 4 , 6 CFM) AT 2980 RPMR22, CONO.IEMP. ! 40CIC' (104 , F)

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Performance Test ResultsCompressor: tw in-sc rew* Operation: oil-reducedRotors : 4-6 (male-female) lobe combinationAsymmetric, line-generated rotor profile**Male rotor diameter: 163 .2 mm (6 .425 inch)Male rotor material: steelFemale rotor diameter: 163 .2 mm (6 .425 inch)Female rotor material: nodular ironRotor length: 220 mm (8.66 inch)Clearances: discharge end: 0.04 - 0.05 mm(0 .0016 - 0 .0020 inch)interlobe: 0.025- 0.130 mm(0 .0010 - 0.0051 inch)Rotor shaping method: single-indexmilling to final shapeBearings: Radial: cylindrical rollersAxial: angular contact ballDisplacement: 517 .5 m3/h (304 .6 CFM) at 2980 RPMMale rotor tip speed: 25 .5 m/s (83.7 f t/s)Built-in volume ratio: optimal within 2.2 - 5.0Lubricant: alkylated benzene ISO 98Oil/gas ratio : about \ by weightDischarge temperature: 75 - 95oc (167 - 2030F)Injected liquid refrigerant: 0 - 7 1 /m in (0 - 1.8 GPM)Performance for R22, see figure 8.

* SRM K314** SRM D-2-8

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OIL-FREE OPERATIONSince the rotors in a twin-screw compressor are parallel,timing gears are very practical and this was also the originaldesign. The rotors run without contact and the compressorworking chamber does not require any lubricant.Synchronized tw in -screw compressors are historically knownas noisy, high-speed machin es with a limited pressure ratio capability. However, the advancements also on the drycompressor scene may justify a renewed consideration for thistype of machine, e.g. in cases where absolutely no oil can betolerated in the refrigeration system .With new rotor profiles and tem pera tu re -compensa ted clearances, lower tip speeds (resulting in lower noise levels)and higher pressure ratios can be achieved.

CONCLUSIONSThe versatility of the basic twin-screw concept has beendemonstrated by the fact that quite different operational modesare possible. The compressor can be adapted to very specific

needs without loosing i ts favourable characteristics as a heavyduty machine with high perfo rmance.Based on th is, an extrapolation into the future willindicate that:

the oil-flooded twin-screw compressor will continue to havea given place for refrigeration, heat pump and air-conditioning applications;the oil-reduced twin-screw compressor will probably find i t s place in smaller heat-pumps and air-conditioning applications;the oil-free twin-screw compressor will continue to have i t s given place in larger process-refrigeration applications andhas a chance in larger air-conditioning system s and inrefrigeration booster applications.

REFERENCE

1) Lundberg, Aand Glanvall, R.AComparison of SRM and G loboid Type Screw Compressors.Purdue Compressor Technolo gy Conference (1978).

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SINGLE STAGE, OIL-FREE SCREW COMPRESSOR WITH ACOMPRESSION RATIO OF EIGHT

Hidetomo Moril, Katsuhlko Kasuyal, Mitsuru Fuiiwaral,Katsumi Matsubaral, Akira Suzuki2, and Masakazu Aoki2!Mechanical Engineering Research Laboratory, Hitachi,Ltd.,Tsuchiura, Ibaragl, Japan2Ebina Branch, Narashino W orks, Hitachi, Ltd., Ebina,Kanagaw a, Japan

ABSTRACTThe development of a se ries of s i n ~ l e - s t a g e , o i lfree screw a ir compressors with a compression ra t io of8, previously obtainable only with t w o - s t a ~ e compressors,is descr ibed . A new ro to r p ro f i le , which reducesleakage loss to achieve higher effic iency , and desig-ned forease of manufacture, is deta iled . Additionally , a newdesig-n method for the clearance between ro to rs , to compensate for ro to r deformation due to thermalexpansion, is also introduced. This o il- free , screwa ir compressor with a ra t in ? of 37-55 kW has beenmarketed since 1982 and 15-22 kW since 1984.

INTRODUCTIONO il - f r e e a i r compressors hav ing a d ischa rge pressure of approximately 0.8 MPa (8 a ta) , and widely used in e lec tr ica l, food and chemical industries were t ra d i t io n a l ly rec ip roca ting type compressors.Recently , however, the advantages of oi1-free rotaryscrew compressors have been r e c o ~ n i z e d . These includemechanical s implic ity , high re l iab il i ty ,low noise andlow vibration . Two-stag-e, o il- f ree screw a ir compressoruse has predominated in the capacity range above550 m3/h (motor power above 65 kW).Nevertheless, for compressor capacity below th is range,where o il- f re e a ir is needed, th is type of compressorwas not availab le .

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Different Operational Modes for Refrigeration Twin-Screw Compress