ASHRAE REFRIGERATION SI 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 1.18Defrost Gas Supply Lines. 1.20Receivers 1.21Air-Cooled Condensers 1.23Piping at Multiple Compressors 1.24Piping at Various System Components. 1.25Refrigeration Accessories 1.28Pre

2、ssure 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, sup

3、erheating,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 outdoo

4、r 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 wide cho

5、ice 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 multi-s

6、taging, 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 befound i

7、n 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 HandbookRefrigeration.Pi

8、ping 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 the s

9、ystem; (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, noise, a

10、nd 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 the

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

12、g 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 0.5 m/s or less to ensurepositive gravity flow without incurring backup o

13、f liquid flow. Liq-uid lines from receiver to evaporator should be sized to maintainvelocities below 1.5 m/s, 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 in Fig-

14、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 liquid-suc

15、tion 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 reducedapproximately

16、0.5% for each 1 K increase in superheat in the evap-orator.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, but

17、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 the he

18、at 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 because

19、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, Refr

20、igerant Piping.Table 1 Recommended Gas Line VelocitiesSuction line 4.5 to 20 m/sDischarge line 10 to 18 m/s1.2 2010 ASHRAE HandbookRefrigeration (SI)loss of compressor capacity and increased power usage. Excessiveliquid line pressure drops can cause liquid refrigerant to flash,resulting in faulty ex

21、pansion 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 temperature.Pr

22、essure 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 5C saturated evaporator tem-perature

23、with a 40C 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 1 Kor less for each segment of the discharge, suction

24、, 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 thatcorresponding

25、 to about a 0.5 to 1 K change in saturation tempera-ture. 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 thetotal loss of the liquid line. Loss caused by risers is approximately11.3 kPa per metre of liquid lift. Total loss is the sum of all frictionlosses plus pressure loss from liquid risers.Example 1 illustrates the process of determining liquid-li

28、ne sizeand checking for total subcooling required.Example 1. An R-22 refrigeration system using copper pipe operates at5C evaporator and 40C condensing. Capacity is 14 kW, and the liquidline is 50 m equivalent length with a riser of 6 m. Determine the liquid-line size and total required subcooling.S

29、olution: From Table 3, the size of the liquid line at 1 K drop is 15 mmOD. Use the equation in Note 3 of Table 3 to compute actual tempera-ture drop. At 14 kW,Refrigeration systems that have no liquid risers and have theevaporator below the condenser/receiver benefit from a gain in pres-sure caused

30、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 condenser)

31、 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, where h

32、 isthe liquid height in the vessel. Gas pressure does not add to thevelocity unless gas is flowing in the same direction. As a result, bothgas and liquid flow through the line, limiting the rate of liquid flow.Fig. 1 Flow Rate per Ton of Refrigeration for Refrigerant 22Fig. 1 Flow Rate per Kilowatt

33、of Refrigeration for Refrigerant 22Fig. 2 Flow Rate per Ton of Refrigeration for Refrigerant134aFig. 2 Flow Rate per Kilowatt of Refrigeration for Refrigerant 134aTable 2 Approximate Effect of Gas Line Pressure Drops on R-22 Compressor Capacity and PoweraLine Loss, K Capacity, % Energy, %bSuction Li

34、ne0 100 1001 96.8 104.32 93.6 107.3Discharge Line0 100 1001 99.2 102.72 98.4 105.7aFor system operating at 5C saturated evaporator temperature and 40C saturated con-densing temperature.bEnergy percentage rated at kW (power)/kW (cooling).Actual temperature drop = (50 0.02)(14.0/21.54)1.8= 0.46 KEstim

35、ated friction loss = 0.46(50 0.749) = 17.2 kPaLoss for the riser = 6 11.3 = 67.8 kPaTotal pressure losses = 67.8 + 17.2 = 85.0 kPaSaturation pressure at 40C condensing (see R-22 properties in Chapter 30, 2009 ASHRAE HandbookFundamentals)= 1534.1 kPaInitial pressure at beginning of liquid line 1534.1

36、 kPaTotal liquid line losses 85.0 kPaNet pressure at expansion device = 1449.1 kPaThe saturation temperature at 1449.1 kPa is 37.7C.Required subcooling to overcome the liquid losses = (40.0 37.7) or 2.3 KHalocarbon Refrigeration Systems 1.3Table 3 Suction, Discharge, and Liquid Line Capacities in Ki

37、lowatts for Refrigerant 22 (Single- or High-Stage Applications)Nominal LineOD, mmSuction Lines (t = 0.04 K/m)Discharge Lines(t = 0.02 K/m, p = 74.90)Liquid LinesSaturated Suction Temperature, C See note a40 30 20 5 5Saturated SuctionTemperature, CVelocity =0.5 m/st =0.02 K/mp = 749Corresponding p, P

38、a/m196 277 378 572 731 40 20 5TYPE L COPPER LINE12 0.32 0.50 0.75 1.28 1.76 2.30 2.44 2.60 7.08 11.2415 0.61 0.95 1.43 2.45 3.37 4.37 4.65 4.95 11.49 21.5418 1.06 1.66 2.49 4.26 5.85 7.59 8.06 8.59 17.41 37.4922 1.88 2.93 4.39 7.51 10.31 13.32 14.15 15.07 26.66 66.1828 3.73 5.82 8.71 14.83 20.34 26.

39、24 27.89 29.70 44.57 131.035 6.87 10.70 15.99 27.22 37.31 48.03 51.05 54.37 70.52 240.742 11.44 17.80 26.56 45.17 61.84 79.50 84.52 90.00 103.4 399.354 22.81 35.49 52.81 89.69 122.7 157.3 167.2 178.1 174.1 794.267 40.81 63.34 94.08 159.5 218.3 279.4 297.0 316.3 269.9 1415.079 63.34 98.13 145.9 247.2

40、 337.9 431.3 458.5 488.2 376.5 2190.9105 136.0 210.3 312.2 527.8 721.9 919.7 977.6 1041.0 672.0 4697.0STEEL LINE10 0.47 0.72 1.06 1.78 2.42 3.04 3.23 3.44 10.66 15.9615 0.88 1.35 1.98 3.30 4.48 5.62 5.97 6.36 16.98 29.6220 1.86 2.84 4.17 6.95 9.44 11.80 12.55 13.36 29.79 62.5525 3.52 5.37 7.87 13.11

41、 17.82 22.29 23.70 25.24 48.19 118.2 32 7.31 11.12 16.27 27.11 36.79 46.04 48.94 52.11 83.56 244.4 40 10.98 16.71 24.45 40.67 55.21 68.96 73.31 78.07 113.7 366.6 50 21.21 32.23 47.19 78.51 106.4 132.9 141.3 150.5 187.5 707.5 65 33.84 51.44 75.19 124.8 169.5 211.4 224.7 239.3 267.3 1127.3 80 59.88 90

42、.95 132.8 220.8 299.5 373.6 397.1 422.9 412.7 1991.3 100 122.3 185.6 270.7 450.1 610.6 761.7 809.7 862.2 711.2 4063.2 Notes:1. Table capacities are in kilowatts of refrigeration.p = pressure drop per unit equivalent length of line, Pa/mt = corresponding change in saturation temperature, K/m2. Line c

43、apacity for other saturation temperatures t and equivalent lengths Le3. Saturation temperature t for other capacities and equivalent lengths Let = Table t 4. Values based on 40C condensing temperature. Multiply table capacities bythe following factors for other condensing temperatures.CondensingTemp

44、erature, CSuctionLineDischarge Line20 1.18 0.8030 1.10 0.8840 1.00 1.0050 0.91 1.11aSizing is recommended where any gas generated in receiver must return up condensate line tocondenser without restricting condensate flow. Water-cooled condensers, where receiver ambienttemperature may be higher than

45、refrigerant condensing temperature, fall into this category.bLine pressure drop p is conservative; if subcooling is substantial or line isshort, a smaller size line may be used. Applications with very little subcool-ing or very long lines may require a larger line.Table 4 Suction, Discharge, and Liq

46、uid Line Capacities in Kilowatts for Refrigerant 22 (Intermediate- or Low-Stage Duty)NominalType LCopper LineOD, mmSuction Lines (t = 0.04 K/m)DischargeLines*LiquidLinesSaturated Suction Temperature, C70 60 50 40 30Corresponding p, Pa/m31.0 51.3 81.5 121 22812 0.09 0.16 0.27 0.47 0.73 0.74See Table

47、315 0.17 0.31 0.52 0.90 1.39 1.4318 0.29 0.55 0.91 1.57 2.43 2.4922 0.52 0.97 1.62 2.78 4.30 4.4128 1.05 1.94 3.22 5.52 8.52 8.7435 1.94 3.60 5.95 10.17 15.68 16.0842 3.26 6.00 9.92 16.93 26.07 26.7354 6.54 12.03 19.83 33.75 51.98 53.2867 11.77 21.57 35.47 60.38 92.76 95.0679 18.32 33.54 55.20 93.72

48、 143.69 174.22105 39.60 72.33 118.66 201.20 308.02 316.13130 70.87 129.17 211.70 358.52 548.66 561.89156 115.74 210.83 344.99 583.16 891.71 915.02Notes:1. Table capacities are in kilowatts of refrigeration.p = pressure drop per equivalent line length, Pa/mt = corresponding change in saturation tempe

49、rature, K/m2. Line capacity for other saturation temperatures t and equivalent lengths Le3. Saturation temperature t for other capacities and equivalent lengths Let = Table t 4. Refer to refrigerant property tables (Chapter 30 of the 2009 ASHRAE Hand-bookFundamentals) for pressure drop corresponding to t.5. Values based on 15C condensing temperature. Multiply table capacities by thefollowing factors for other condensing temperatures.CondensingTemperature, CSuction Line Discharge Line30 1.08 0.7420 1.03 0.9110 0.98 1.090