ASHRAE REFRIGERATION IP CH 1-2010 HALOCARBON REFRIGERATION SYSTEMS《卤化碳制冷系统》.pdf

1、1.1CHAPTER 1HALOCARBON REFRIGERATION SYSTEMSRefrigerant Flow 1.1Refrigerant Line Sizing 1.1Discharge (Hot-Gas) Lines 1.19Defrost Gas Supply Lines. 1.21Receivers 1.21Air-Cooled Condensers 1.23Piping at Multiple Compressors 1.24Piping at Various System Components. 1.25Refrigeration Accessories 1.28Hea

2、d Pressure Control for Refrigerant Condensers 1.32Keeping Liquid from Crankcase During Off Cycles 1.33Hot-Gas Bypass Arrangements 1.34EFRIGERATION is the process of moving heat from oneR location to another by use of refrigerant in a closed cycle. Oilmanagement; gas and liquid separation; subcooling

3、, superheating,and piping of refrigerant liquid and gas; and two-phase flow are allpart of refrigeration. Applications include air conditioning, com-mercial refrigeration, and industrial refrigeration.Desired characteristics of a refrigeration system may include Year-round operation, regardless of o

4、utdoor ambient conditionsPossible wide load variations (0 to 100% capacity) during shortperiods without serious disruption of the required temperaturelevelsFrost control for continuous-performance applicationsOil management for different refrigerants under varying load andtemperature conditionsA wid

5、e choice of heat exchange methods (e.g., dry expansion,liquid overfeed, or flooded feed of the refrigerants) and use of sec-ondary coolants such as salt brine, alcohol, and glycolSystem efficiency, maintainability, and operating simplicityOperating pressures and pressure ratios that might require mu

6、lti-staging, cascading, and so forthA successful refrigeration system depends on good piping designand an understanding of the required accessories. This chapter cov-ers the fundamentals of piping and accessories in halocarbon refrig-erant systems. Hydrocarbon refrigerant pipe friction data can befo

7、und in petroleum industry handbooks. Use the refrigerant proper-ties and information in Chapters 3, 29, and 30 of the 2009 ASHRAEHandbookFundamentals to calculate friction losses.For information on refrigeration load, see Chapter 22. For R-502information, refer to the 1998 ASHRAE HandbookRefrigerati

8、on.Piping Basic PrinciplesThe design and operation of refrigerant piping systems should(1) ensure proper refrigerant feed to evaporators; (2) provide prac-tical refrigerant line sizes without excessive pressure drop; (3) pre-vent excessive amounts of lubricating oil from being trapped in anypart of

9、the system; (4) protect the compressor at all times from lossof lubricating oil; (5) prevent liquid refrigerant or oil slugs from en-tering the compressor during operating and idle time; and (6) main-tain a clean and dry system.REFRIGERANT FLOWRefrigerant Line VelocitiesEconomics, pressure drop, noi

10、se, and oil entrainment establishfeasible design velocities in refrigerant lines (Table 1).Higher gas velocities are sometimes found in relatively shortsuction lines on comfort air-conditioning or other applicationswhere the operating time is only 2000 to 4000 h per year and wherelow initial cost of

11、 the system may be more significant than low oper-ating cost. Industrial or commercial refrigeration applications,where equipment runs almost continuously, should be designedwith low refrigerant velocities for most efficient compressor perfor-mance and low equipment operating costs. An owning and op

12、erat-ing cost analysis will reveal the best choice of line sizes. (SeeChapter 36 of the 2007 ASHRAE HandbookHVAC Applicationsfor information on owning and operating costs.) Liquid lines fromcondensers to receivers should be sized for 100 fpm or less to ensurepositive gravity flow without incurring b

13、ackup of liquid flow. Liq-uid lines from receiver to evaporator should be sized to maintainvelocities below 300 fpm, thus minimizing or preventing liquidhammer when solenoids or other electrically operated valves areused.Refrigerant Flow RatesRefrigerant flow rates for R-22 and R-134a are indicated

14、in Fig-ures 1 and 2. To obtain total system flow rate, select the proper ratevalue and multiply by system capacity. Enter curves using satu-rated refrigerant temperature at the evaporator outlet and actualliquid temperature entering the liquid feed device (including sub-cooling in condensers and liq

15、uid-suction interchanger, if used).Because Figures 1 and 2 are based on a saturated evaporatortemperature, they may indicate slightly higher refrigerant flow ratesthan are actually in effect when suction vapor is superheated abovethe conditions mentioned. Refrigerant flow rates may be reducedapproxi

16、mately 3% for each 10F increase in superheat in the evapo-rator.Suction-line superheating downstream of the evaporator fromline heat gain from external sources should not be used to reduceevaluated mass flow, because it increases volumetric flow rate andline velocity per unit of evaporator capacity,

17、 but not mass flow rate.It should be considered when evaluating suction-line size for satis-factory oil return up risers.Suction gas superheating from use of a liquid-suction heatexchanger has an effect on oil return similar to that of suction-linesuperheating. The liquid cooling that results from t

18、he heat exchangereduces mass flow rate per ton of refrigeration. This can be seen inFigures 1 and 2 because the reduced temperature of the liquid sup-plied to the evaporator feed valve has been taken into account. Superheat caused by heat in a space not intended to be cooled isalways detrimental bec

19、ause the volumetric flow rate increases withno compensating gain in refrigerating effect.REFRIGERANT LINE SIZINGIn sizing refrigerant lines, cost considerations favor minimizingline sizes. However, suction and discharge line pressure drops causeThe preparation of this chapter is assigned to TC 10.3,

20、 Refrigerant Piping.Table 1 Recommended Gas Line VelocitiesSuction line 900 to 4000 fpmDischarge line 2000 to 3500 fpm1.2 2010 ASHRAE HandbookRefrigerationloss of compressor capacity and increased power usage. Excessiveliquid line pressure drops can cause liquid refrigerant to flash,resulting in fau

21、lty expansion valve operation. Refrigeration systemsare designed so that friction pressure losses do not exceed a pressuredifferential equivalent to a corresponding change in the saturationboiling temperature. The primary measure for determining pressuredrops is a given change in saturation temperat

22、ure.Pressure Drop ConsiderationsPressure drop in refrigerant lines reduces system efficiency. Cor-rect sizing must be based on minimizing cost and maximizing effi-ciency. Table 2 shows the approximate effect of refrigerant pressuredrop on an R-22 system operating at a 40F saturated evaporatortempera

23、ture with a 100F saturated condensing temperature.Pressure drop calculations are determined as normal pressure lossassociated with a change in saturation temperature of the refrigerant.Typically, the refrigeration system is sized for pressure losses of 2For less for each segment of the discharge, su

24、ction, and liquid lines.Liquid Lines. Pressure drop should not be so large as to causegas formation in the liquid line, insufficient liquid pressure at theliquid feed device, or both. Systems are normally designed so thatpressure drop in the liquid line from friction is not greater than thatcorrespo

25、nding to about a 1 to 2F change in saturation temperature.See Tables 3 to 9 for liquid-line sizing information.Liquid subcooling is the only method of overcoming liquid linepressure loss to guarantee liquid at the expansion device in the evap-orator. If subcooling is insufficient, flashing occurs in

26、 the liquid lineand degrades system efficiency.Friction pressure drops in the liquid line are caused by accesso-ries such as solenoid valves, filter-driers, and hand valves, as well asby the actual pipe and fittings between the receiver outlet and therefrigerant feed device at the evaporator.Liquid-

27、line risers are a source of pressure loss and add to the totalloss of the liquid line. Loss caused by risers is approximately 0.5 psiper foot of liquid lift. Total loss is the sum of all friction losses pluspressure loss from liquid risers.Example 1 illustrates the process of determining liquid-line

28、 sizeand checking for total subcooling required.Example 1. An R-22 refrigeration system using copper pipe operates at40F evaporator and 105F condensing. Capacity is 5 tons, and the liq-uid line is 100 ft equivalent length with a riser of 20 ft. Determine theliquid-line size and total required subcoo

29、ling.Solution: From Table 3, the size of the liquid line at 1F drop is 5/8 in.OD. Use the equation in Note 3 of Table 3 to compute actual tempera-ture drop. At 5 tons,Refrigeration systems that have no liquid risers and have theevaporator below the condenser/receiver benefit from a gain in pres-sure

30、 caused by liquid weight and can tolerate larger friction losseswithout flashing. Regardless of the liquid-line routing when flash-ing occurs, overall efficiency is reduced, and the system may mal-function.The velocity of liquid leaving a partially filled vessel (e.g., areceiver or shell-and-tube co

31、ndenser) is limited by the height of theliquid above the point at which the liquid line leaves the vessel,whether or not the liquid at the surface is subcooled. Because liquidin the vessel has a very low (or zero) velocity, the velocity V in theliquid line (usually at the vena contracta) is V2= 2gh,

32、 where h isthe liquid height in the vessel. Gas pressure does not add to theFig. 1 Flow Rate per Ton of Refrigeration for Refrigerant 22Fig. 1 Flow Rate per Ton of Refrigeration for Refrigerant 22Fig. 2 Flow Rate per Ton of Refrigeration for Refrigerant134aFig. 2 Flow Rate per Ton of Refrigeration f

33、or Refrigerant 134aTable 2 Approximate Effect of Gas Line Pressure Drops on R-22 Compressor Capacity and PoweraLine Loss, F Capacity, % Energy, %bSuction Line0 100 1002 96.4 104.84 92.9 108.1Discharge Line0 100 1002 99.1 103.04 98.2 106.3aFor system operating at 40F saturated evaporator temperature

34、and 100F saturatedcondensing temperature.bEnergy percentage rated at hp/ton.Actual temperature drop = 1.0(5.0/6.7)1.8= 0.59FEstimated friction loss = 0.59 3.05 = 1.8 psiLoss for the riser = 20 0.5 = 10 psiTotal pressure losses = 10.0 + 1.8 = 11.8 psiR-22 saturation pressure at 105F condensing (see R

35、-22 properties in Chapter 30, 2009 ASHRAE HandbookFundamentals)210.8 psigInitial pressure at beginning of liquid line 210.8 psigTotal liquid line losses 11.8 psiNet pressure at expansion device = 199 psigThe saturation temperature at 199 psig is 101.1F.Required subcooling to overcome the liquid loss

36、es = (105.0 101.1) or 3.9FHalocarbon Refrigeration Systems 1.3Table 3 Suction, Discharge, and Liquid Line Capacities in Tons for Refrigerant 22 (Single- or High-Stage Applications)Line SizeSuction Lines (t = 2F)Discharge Lines(t = 1F, p = 3.05 psi) Line SizeLiquid LinesSaturated Suction Temperature,

37、 F See notes a and bType LCopper,OD40 20 0 20 40Saturated SuctionTemperature, FType LCopper,ODVel. =100 fpmt = 1FCorresponding p, psi/100 ft0.79 1.15 1.6 2.22 2.91 40 40 p = 3.051/2 0.40 0.6 0.75 0.85 1/2 2.3 3.65/8 0.32 0.51 0.76 1.1 1.4 1.6 5/8 3.7 6.77/8 0.52 0.86 1.3 2.0 2.9 3.7 4.2 7/8 7.8 18.2

38、1 1/8 1.1 1.7 2.7 4.0 5.8 7.5 8.5 1 1/8 13.2 37.01 3/8 1.9 3.1 4.7 7.0 10.1 13.1 14.8 1 3/8 20.2 64.71 5/8 3.0 4.8 7.5 11.1 16.0 20.7 23.4 1 5/8 28.5 102.52 1/8 6.2 10.0 15.6 23.1 33.1 42.8 48.5 2 1/8 49.6 213.02 5/8 10.9 17.8 27.5 40.8 58.3 75.4 85.4 2 5/8 76.5 376.93 1/8 17.5 28.4 44.0 65.0 92.9 1

39、20.2 136.2 3 1/8 109.2 601.53 5/8 26.0 42.3 65.4 96.6 137.8 178.4 202.1 3 5/8 147.8 895.74 1/8 36.8 59.6 92.2 136.3 194.3 251.1 284.4 4 1/8 192.1 1263.2Steel SteelIPS SCH IPS SCH1/2 40 0.38 0.58 0.85 1.2 1.5 1.7 1/2 80 3.8 5.73/4 40 0.50 0.8 1.2 1.8 2.5 3.3 3.7 3/4 80 6.9 12.81 40 0.95 1.5 2.3 3.4 4

40、.8 6.1 6.9 1 80 11.5 25.21 1/4 40 2.0 3.2 4.8 7.0 9.9 12.6 14.3 1 1/4 80 20.6 54.11 1/2 40 3.0 4.7 7.2 10.5 14.8 19.0 21.5 1 1/2 80 28.3 82.62 40 5.7 9.1 13.9 20.2 28.5 36.6 41.4 2 40 53.8 192.02 1/2 40 9.2 14.6 22.1 32.2 45.4 58.1 65.9 2 1/2 40 76.7 305.83 40 16.2 25.7 39.0 56.8 80.1 102.8 116.4 3

41、40 118.5 540.34 40 33.1 52.5 79.5 115.9 163.2 209.5 237.3 4 40 204.2 1101.2Notes:1. Table capacities are in tons of refrigeration.4. Values based on 105F condensing temperature. Multiply table capacities by the fol-lowing factors for other condensing temperatures.p = pressure drop from line friction

42、, psi per 100 ft of equivalent line lengthCondensingTemperature, F Suction Line Discharge Linet = corresponding change in saturation temperature, F per 100 ft2. Line capacity for other saturation temperatures t and equivalent lengths Le80 1.11 0.79Line capacity = Table capacity 90 1.07 0.88100 1.03

43、0.953. Saturation temperature t for other capacities and equivalent lengths Le110 0.97 1.04120 0.90 1.10t = Table t 130 0.86 1.18140 0.80 1.26aSizing shown is recommended where any gas generated in receiver must return upcondensate line to condenser without restricting condensate flow. Water-cooledc

44、ondensers, where receiver ambient temperature may be higher than refrigerantcondensing temperature, fall into this category.bLine pressure drop p is conservative; if subcooling is substantial or line is short, asmaller size line may be used. Applications with very little subcooling or very longlines

45、 may require a larger line.Table 4 Suction, Discharge, and Liquid Line Capacities in Tons for Refrigerant 22 (Intermediate- or Low-Stage Duty)Line Size Suction Lines (t = 2F)*DischargeLines(t = 2F)* Liquid LinesType LCopper, ODSaturated Suction Temperature, F90 80 70 60 50 40 305/8 0.7See Table 37/8

46、 0.18 0.25 0.34 0.46 0.61 0.79 1.0 1.91 1/8 0.36 0.51 0.70 0.94 1.2 1.6 2.1 3.81 3/8 0.6 0.9 1.2 1.6 2.2 2.8 3.6 6.61 5/8 1.0 1.4 1.9 2.6 3.4 4.5 5.7 10.52 1/8 2.1 3.0 4.1 5.5 7.2 9.3 11.9 21.72 5/8 3.8 5.3 7.2 9.7 12.7 16.5 21.1 38.43 1/8 6.1 8.5 11.6 15.5 20.4 26.4 33.8 61.43 5/8 9.1 12.7 17.3 23.

47、1 30.4 39.4 50.2 91.24 1/8 12.9 18.0 24.5 32.7 43.0 55.6 70.9 128.65 1/8 23.2 32.3 43.9 58.7 77.1 99.8 126.9 229.56 1/8 37.5 52.1 71.0 94.6 124.2 160.5 204.2 369.4Notes:1. Table capacities are in tons of refrigeration.5. Values based on 0F condensing temperature. Multiply table capacities by thefoll

48、owing factors for other condensing temperatures. Flow rates for dischargelines are based on 50F evaporating temperature.p = pressure drop from line friction, psi per 100 ft of equivalent line lengtht = corresponding change in saturation temperature, F per 100 ftCondensingTemperature, F Suction Line

49、Discharge Line2. Line capacity for other saturation temperatures t and equivalent lengths Le30 1.09 0.5820 1.06 0.713. Saturation temperature t for other capacities and equivalent lengths Let = Table t 4. Refer to refrigerant thermodynamic property tables (Chapter 30 of the 2009 ASHRAEHandbookFundamentals) for pressure drop corresponding to t.10 1.03 0.850 1.00 1.0010 0.97 1.2020 0.94 1.4530 0.90 1.80*See section on Pressure Drop Considerations.Table LeActual Le-Actual tTable t