ASHRAE HVAC SYSTEMS AND EQUIPMENT SI CH 38-2012 COMPRESSORS.pdf

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1、38.1CHAPTER 38COMPRESSORSPOSITIVE-DISPLACEMENT COMPRESSORS 38.1Performance. 38.2Abnormal Operating Conditions, Hazards, and Protective Devices. 38.4Motors 38.6RECIPROCATING COMPRESSORS 38.7ROTARY COMPRESSORS. 38.11Rolling-Piston Compressors 38.11Rotary-Vane Compressors 38.13Single-Screw Compressors.

2、 38.14Twin-Screw Compressors. 38.18ORBITAL COMPRESSORS . 38.24Scroll Compressors 38.24Trochoidal Compressors 38.26CENTRIFUGAL COMPRESSORS 38.27Isentropic Analysis . 38.29Polytropic Analysis. 38.30Application. 38.34Mechanical Design. 38.35Operation and Maintenance 38.36Symbols 38.37COMPRESSOR is one

3、of the four essential components of theA basic vapor compression refrigeration system; the others are thecondenser, evaporator, and expansion device. The compressor circu-lates refrigerant through the system and increases refrigerant vaporpressure to create the pressure differential between the cond

4、enser andevaporator. This chapter describes the design features of several cat-egories of commercially available refrigerant compressors.There are two broad categories of compressors: positive displace-ment and dynamic. Positive-displacement compressors increaserefrigerant vapor pressure by reducing

5、 the volume of the compres-sion chamber through work applied to the compressors mechanism.Positive-displacement compressors include many styles of com-pressors currently in use, such as reciprocating, rotary (rolling pis-ton, rotary vane, single screw, twin screw), and orbital (scroll,trochoidal).Dy

6、namic compressors increase refrigerant vapor pressure bycontinuous transfer of kinetic energy from the rotating member tothe vapor, followed by conversion of this energy into a pressure rise.Centrifugal compressors function based on these principles.There are many reasons to consider each compressor

7、 style. Somecompressors have physical size limitations that may limit theirapplication to smaller equipment; some have associated noise con-cerns; and some have efficiency levels that make them more or lessattractive. Each piece of equipment using a compressor has a certainset of design parameters (

8、refrigerant, cost, performance, sound,capacity, etc.) that requires the designer to evaluate various com-pressor characteristics and choose the best compressor type for theapplication.Figure 1 addresses volumetric flow rate of the compressor as afunction of the differential pressure (discharge press

9、ure minus suc-tion pressure) against which the compressor is required to work.Three common compressor styles are represented on the chart.Positive-displacement compressors tend to maintain a relativelyconstant volumetric flow rate over a wide range of differential pres-sures, because this compressor

10、 draws a predetermined volume ofvapor into its chamber and compresses it to a reduced volumemechanically, thereby increasing the pressure. This helps to keepthe equipment operating near its design capacity regardless of theconditions. Centrifugal compressors dynamically compress the suc-tion gas by

11、converting velocity energy to pressure energy. There-fore, they do not have a fixed volumetric flow rate, and the capacitycan vary over a range of pressure ratios. This tends to make centrif-ugal-based equipment much more application specific.POSITIVE-DISPLACEMENT COMPRESSORSTypes of positive-displa

12、cement compressors classified by com-pression mechanism design are shown in Figure 2.Compressors also can be further classified as single-stage ormultistage, and by type of motor drive (electrical or mechanical),capacity control (single speed, variable speed, single speed withadjustable compression

13、chamber volume), and drive enclosure (her-metic, semihermetic, open). The most widely used compressors (forhalocarbons) are manufactured in three types: (1) open, (2) semiher-metic or bolted hermetic, and (3) welded-shell hermetic.Open compressors are those in which the shaft or other movingpart ext

14、ends through a seal in the crankcase for an external drive.Ammonia compressors are manufactured only in the open designbecause of the incompatibility of the refrigerant and hermetic motormaterials. Most automotive compressors are also open-drive type.Hermetic compressors contain the motor and compre

15、ssor in thesame gastight housing, which is permanently sealed with no accessfor servicing internal parts in the field, with the motor shaft integralThe preparation of this chapter is assigned to TC 8.1, Positive Displace-ment Compressors, and TC 8.2, Centrifugal Machines.Fig. 1 Comparison of Single-

16、Stage Centrifugal, Reciprocating, and Screw Compressor Performance38.2 2012 ASHRAE HandbookHVAC Systems and Equipment (SI)with the compressor crankshaft and the motor in contact with therefrigerant. Hermetic compressors normally have the motor-com-pressor pump assembly mounted inside a steel shell,

17、which is sealedby welding.A semihermetic compressor (also called bolted, accessible, orserviceable) is a compressor of bolted construction that is amenableto field repair. The seal in the bolted joints is provided by O rings orgaskets.PERFORMANCECompressor performance depends on an array of design c

18、ompro-mises involving characteristics of the refrigerant, compressionmechanism, and motor. The goal is to provide the following:Greatest trouble-free life expectancyMost refrigeration effect for least power inputLowest applied costWide range of operating conditionsAcceptable vibration and sound leve

19、lA useful measures of compressor performance is the coefficientof performance (COP). The COP is the ratio of the compressorsrefrigerating capacity to the input power. For a hermetic or semiher-metic compressor, the COP includes the combined operating effi-ciencies of the motor and the compressor:COP

20、 (hermetic or semihermetic) = The COP for an open compressor does not include motor effi-ciency:COP (open) = Because capacity and motor/shaft power vary with operatingconditions, COP also varies with operating conditions.Power input per unit of refrigerating capacity (W/W) is used tocompare differen

21、t compressors at the same operating conditions,primarily with open-drive industrial equipment.Ideal CompressorDuring operation, pressure and volume in the compression cham-ber vary as shown in Figure 3. There are four sequential processes:first, gas is drawn into the compression chamber during the s

22、uctionprocess (12); next is compression (23); and then higher-pressuregas is pushed out during the discharge process (34), followed by thenext cycle.The capacity of a compressor at a given operating condition is afunction of the mass of gas compressed per unit time. Ideally, massflow is equal to the

23、 product of the compressor displacement per unittime and the gas density, as shown in Equation (1):= sVd(1)where= ideal mass flow of compressed gas, kg/ss= density of gas entering compressor (at suction port), kg/m3Vd= geometric displacement of compressor, m3/sThe ideal refrigeration cycle, discusse

24、d in detail in Chapter 2 ofthe 2009 ASHRAE HandbookFundamentals, consists of four pro-cesses, as shown in Figure 4:12: isentropic (reversible and adiabatic) compression23: desuperheating, condensing, and subcooling at constant pressure34: adiabatic expansion41: boiling and superheating at constant p

25、ressureThe following quantities can be determined from the pressure-enthalpy diagram in Figure 4 using m, the mass flow of gas fromEquation (1),Qo= mQrefrigeration effect= m(h1 h4)(2)Po= mQwork of compression= m(h2 h1) = mwoi(3)where woi= specific work of isentropic compression, J/kgQo= ideal capaci

26、ty, WPo= ideal power input, WFig. 2 Types of Positive-Displacement Compressors (Classified by Compression Mechanism Design)Capacity W,Input power to motor W,-Capacity W,Input power to shaft W,-WinWout-Power input to shaft, WCompressor capacity, W-=Fig. 3 Ideal Compressor CyclemmCompressors 38.3Actua

27、l CompressorIdeal conditions never occur, so actual compressor performancediffers from ideal performance. Various factors contribute todecreased capacity and increased power input. Depending on com-pressor type, some or all of the following factors can have a majoreffect on compressor performance.Pr

28、essure drops in compressorThrough shutoff valvesThrough suction accumulatorAcross suction strainer/filterAcross motor (hermetic compressor)In manifolds (suction and discharge)Through valves and valve ports (suction and discharge)In internal mufflerThrough internal lubricant separatorAcross check val

29、vesHeat gain by refrigerant fromCooling the hermetic motorInternal heat exchange between compressor and suction gasPower losses because ofFrictionLubricant pump power consumptionMotor lossesValve inefficiencies caused by imperfect mechanical actionInternal gas leakageOil circulationReexpansion (clea

30、rance losses). The gas remaining in the com-pression chamber after discharge reexpands into the compressionchamber during the suction cycle and limits the mass of fresh gasthat can be brought into the compression chamber.Over- and undercompression. Overcompression occurs whenpressure in the compress

31、ion chamber reaches discharge pressurebefore finishing the compression process. Undercompressionoccurs when the compression chamber reaches the dischargepressure after finishing the compression process.Deviation from isentropic compression. In the actual compres-sor, the compression process deviates

32、 from isentropic compressionprimarily because of fluid and mechanical friction and heat transferin the compression chamber. The actual compression process andwork of compression must be determined from measurements.Compressor Efficiency, Subcooling, and SuperheatingDeviations from ideal performance

33、are difficult to evaluate indi-vidually. They can, however, be grouped together and considered bycategory. Their effect on ideal compressor performance is charac-terized by the following efficiencies:Volumetric efficiency vis the ratio of actual volumetric flow toideal volumetric flow (i.e., the geo

34、metric compressor displace-ment).Compression isentropic efficiency oiconsiders only whatoccurs within the compression volume and is a measure of the devi-ation of actual compression from isentropic compression. It isdefined as the ratio of work required for isentropic compression ofthe gas wioto wor

35、k delivered to the gas within the compression vol-ume wa.oi= woi /wa(4)(as obtained by measurement).For a multicylinder or multistage compressor, this equationapplies only for each individual cylinder or stage.Mechanical efficiency mis the ratio of work delivered to thegas (measured) to work input t

36、o the compressor shaft wm.m= wa/wm(5)Isentropic efficiency iis the ratio of work required for isentro-pic compression of the gas woito work input to the compressor shaftwm.i= woi/wm= oim(6)Motor efficiency eis the ratio of work input to the compressorshaft wmto work input to the motor we.e= w/we(7)T

37、otal compressor efficiency comis the ratio of work requiredfor isentropic compression woito work input to the motor = woi/we= oime(8)Actual shaft compressor power is a function of the power input tothe ideal compressor and the compression, mechanical, and volu-metric efficiencies of the compressor,

38、as shown in the followingequation:Pe= Pm/e= Pa/(me) = Poi/(oime)(9)wherePe= power input to motorPm= power input to shaftPoi= power required for isentropic compressionActual capacity is a function of the ideal capacity and volumet-ric efficiency vof the compressor:Q = Qov(10)Total heat rejection is t

39、he sum of refrigeration effect and heatequivalent of power input to the compressor. Heat radiation or usingmeans for additional cooling may reduce this value. The quantity ofheat rejection must be known in order to size condensers.Note that compressor capacity with a given refrigerant dependson satu

40、ration suction temperature (SST), saturation discharge tem-perature (SDT), superheating (SH), and subcooling (SC). Satura-tion suction temperature (SST) is the temperature of two-phaseliquid/gas refrigerant at suction pressure. SST is often called evap-orator temperature; however, in real systems, t

41、here is a differencebecause of pressure drop between evaporator and compressor.Saturated discharge temperature (SDT) is the temperature oftwo-phase liquid/gas refrigerant at discharge pressure. SDT is oftenFig. 4 Pressure-Enthalpy Diagram for Ideal Refrigeration Cycle38.4 2012 ASHRAE HandbookHVAC Sy

42、stems and Equipment (SI)called condensing temperature; however, in real systems, there is adifference because of the pressure drop between compressor andcondenser.Liquid subcooling is not accomplished by the compressor.However, the effect of liquid subcooling is included in compressorratings by some

43、 manufacturers. Note: Air-Conditioning, Heating,and Refrigeration Institute (AHRI) Standard 540 and EuropeanCommittee for Standardization (CEN) European Norm (EN) 12900do not include subcooling.Suction Superheat. In general, no liquid refrigerant should bepresent in suction gas entering the compress

44、or, because it causes oildilution and gas formation in the lubrication system. If liquid carry-over is severe enough to reach the cylinders, excessive wear ofvalves, stops, pistons, and rings can occur; liquid slugging can breakvalves, pistons, and connecting rods. Measuring suction superheatcan be

45、difficult, and the indication of a small superheat (5 K) doesnot necessarily mean that liquid is not present. An effective suctionseparator may be necessary to remove all liquid. In some cases, asmall amount of liquid refrigerant in suction gas may help improvecompressor reliability by effectively r

46、emoving the heat of frictionduring boundary lubrication or reducing the discharge temperature.Some compressors are specifically designed to operate withoutsuction superheat. In this case, special design features are intro-duced to keep liquid from reaching suction valves and cylinders; oilviscosity

47、must also be adjusted to anticipate its dilution with refrig-erant.High suction superheat may result in dangerously high dischargetemperatures and, in hermetic compressors, high motor tempera-tures.ABNORMAL OPERATING CONDITIONS, HAZARDS, AND PROTECTIVE DEVICESTo operate through the entire range of c

48、onditions for which thecompressor was designed and to obtain the desired service life, it isimportant that mating components in the system be correctlydesigned and selected. Suction superheat must be controlled, lubri-cant must return to the compressor, and adequate protection must beprovided agains

49、t abnormal conditions. Chapters 1, 2, 4, and 13 the2010 ASHRAE HandbookRefrigeration provide more informa-tion on protection against abnormal conditions. Chapter 7 of thatvolume gives details of cleanup in the event of a hermetic motorburnout.Compressors are provided with one or more of the followingdevices for protection against abnormal conditions and to co