1、5.1CHAPTER 5COMPONENT BALANCING IN REFRIGERATION SYSTEMSRefrigeration System . 5.1Components. 5.1Selecting Design Balance Points 5.2Energy and Mass Balances . 5.3System Performance 5.4HIS chapter describes methods and components used in bal-Tancing a primary refrigeration system. A refrigerant is a
2、fluidused for heat transfer in a refrigeration system. The fluid absorbsheat at a low temperature and pressure and transfers heat at a highertemperature and pressure. Heat transfer can involve either a com-plete or partial change of state in the case of a primary refrigerant.Energy transfer is a fun
3、ction of the heat transfer coefficients; tem-perature differences; and amount, type, and configuration of theheat transfer surface and, hence, the heat flux on either side of theheat transfer device.REFRIGERATION SYSTEMA typical basic direct-expansion refrigeration system includes anevaporator, whic
4、h vaporizes incoming refrigerant as it absorbs heat,increasing the refrigerants heat content or enthalpy. A compressorpulls vapor from the evaporator through suction piping and com-presses the refrigerant gas to a higher pressure and temperature. Therefrigerant gas then flows through the discharge p
5、iping to a con-denser, where it is condensed by rejecting its heat to a coolant (e.g.,other refrigerants, air, water, or air/water spray). The condensed liq-uid is supplied to a device that reduces pressure, cools the liquid byflashing vapor, and meters the flow. The cooled liquid is returned tothe
6、evaporator. For more information on the basic refrigeration cycle,see Chapter 2 of the 2009 ASHRAE HandbookFundamentals.Gas compression theoretically follows a line of constant entropy.In practice, adiabatic compression cannot occur because of frictionand other inefficiencies of the compressor. Ther
7、efore, the actualcompression line deviates slightly from the theoretical. Power to thecompressor shaft is added to the refrigerant, and compressionincreases the refrigerants pressure, temperature, and enthalpy.In applications with a large compression ratio (e.g., low-temperature freezing, multitempe
8、rature applications), multiplecompressors in series are used to completely compress the refriger-ant gas. In multistage systems, interstage desuperheating of thelower-stage compressors discharge gas protects the high-stagecompressor. Liquid refrigerant can also be subcooled at this inter-stage condi
9、tion and delivered to the evaporator for improved effi-ciencies.An intermediate-temperature condenser can serve as a cascadingdevice. A low-temperature, high-pressure refrigerant condenses onone side of the cascade condenser surface by giving up heat to a low-pressure refrigerant that is boiling on
10、the other side of the surface.The vapor produced transfers energy to the next compressor (orcompressors); heat of compression is added and, at a higher pres-sure, the last refrigerant is condensed on the final condenser surface.Heat is rejected to air, water, or water spray. Saturation temper-atures
11、 of evaporation and condensation throughout the system fixthe terminal pressures against which the single or multiple compres-sors must operate.Generally, the smallest differential between saturated evaporatorand saturated condensing temperatures results in the lowest energyrequirement for compressi
12、on. Liquid refrigerant cooling or subcool-ing should be used where possible to improve efficiencies and min-imize energy consumption.Where intermediate pressures have not been specifically set forsystem operation, the compressors automatically balance at theirrespective suction and discharge pressur
13、es as a function of their rel-ative displacements and compression efficiencies, depending onload and temperature requirements. This chapter covers the tech-nique used to determine the balance points for a typical brine chiller,but the theory can be expanded to apply to single- and two-stagesystems w
14、ith different types of evaporators, compressors, and con-densers.COMPONENTSEvaporators may have flooded, direct-expansion, or liquidoverfeed cooling coils with or without fins. Evaporators are used tocool air, gases, liquids, and solids; condense volatile substances; andfreeze products.Ice-builder e
15、vaporators accumulate ice to store cooling energyfor later use. Embossed-plate evaporators are available (1) to cool afalling film of liquid; (2) to cool, condense, and/or freeze out vola-tile substances from a fluid stream; or (3) to cool or freeze a productby direct contact. Brazed- and welded-pla
16、te fluid chillers can beused to improve efficiencies and reduce refrigerant charge.Ice, wax, or food products are frozen and scraped from somefreezer surfaces. Electronic circuit boards, mechanical products, orfood products (where permitted) are flash-cooled by direct immer-sion in boiling refrigera
17、nts. These are some of the diverse applica-tions demanding innovative configurations and materials thatperform the function of an evaporator.Compressors can be positive-displacement, reciprocating-piston, rotary-vane, scroll, single and double dry and lubricant-flooded screw devices, and single- or
18、multistage centrifugals.They can be operated in series or in parallel with each other, inwhich case special controls may be required.Drivers for compressors can be direct hermetic, semihermetic, oropen with mechanical seals on the compressor. In hermetic and semi-hermetic drives, motor inefficiencie
19、s are added to the refrigerant asheat. Open compressors are driven with electric motors, fuel-powered reciprocating engines, or steam or gas turbines. Intermedi-ate gears, belts, and clutch drives may be included in the drive.Cascade condensers are used with high-pressure, low-temperature refrigeran
20、ts (such as R-23) on the bottom cycle, andhigh-temperature refrigerants (such as R-22, azeotropes, and re-frigerant blends or zeotropes) on the upper cycle. Cascade condens-ers are manufactured in many forms, including shell-and-tube,embossed plate, submerged, direct-expansion double coils, andThe p
21、reparation of this chapter is assigned to TC 10.1, Custom EngineeredRefrigeration Systems.5.2 2010 ASHRAE HandbookRefrigeration (SI)brazed- or welded-plate heat exchangers. The high-pressure refrig-erant from the compressor(s) on the lower cycle condenses at agiven intermediate temperature. A separa
22、te, lower-pressure refrig-erant evaporates on the other side of the surface at a somewhat lowertemperature. Vapor formed from the second refrigerant is com-pressed by the higher-cycle compressor(s) until it can be condensedat an elevated temperature.Desuperheating suction gas at intermediate pressur
23、es where mul-tistage compressors balance is essential to reduce discharge temper-atures of the upper-stage compressor. Desuperheating also helpsreduce oil carryover and reduces energy requirements. Subcoolingimproves the net refrigeration effect of the refrigerant supplied to thenext-lower-temperatu
24、re evaporator and reduces system energy re-quirements. The total heat is then rejected to a condenser.Subcoolers can be of shell-and-tube, shell-and-coil, welded-plate, or tube-in-tube construction. Friction losses reduce the liquidpressure that feeds refrigerant to an evaporator. Subcoolers areused
25、 to improve system efficiency and to prevent refrigerant liquidfrom flashing because of pressure loss caused by friction and thevertical rise in lines. Refrigerant blends (zeotropes) can take ad-vantage of temperature glide on the evaporator side with a direct-expansion-in-tube serpentine or coil co
26、nfiguration. In this case,temperature glide from the bubble point to the dew point promotesefficiency and lower surface requirements for the subcooler. Aflooded shell for the evaporating refrigerant requires use of onlythe higher dew-point temperature.Lubricant coolers remove friction heat and some
27、of the super-heat of compression. Heat is usually removed by water, air, or adirect-expansion refrigerant.Condensers that reject heat from the refrigeration system areavailable in many standard forms, such as water- or brine-cooledshell-and-tube, shell-and-coil, plate-and-frame, or tube-in-tubeconde
28、nsers; water cascading or sprayed over plate or coil serpentinemodels; and air-cooled, fin-coil condensers. Special heat pump con-densers are available in other forms, such as tube-in-earth and sub-merged tube bundle, or as serpentine and cylindrical coil condensersthat heat baths of boiling or sing
29、le-phase fluids.SELECTING DESIGN BALANCE POINTSRefrigeration load at each designated evaporator pressure, refrig-erant properties, liquid refrigerant temperature feeding each evapo-rator, and evaporator design determine the required flow rate ofrefrigerant in a system. The additional flow rates of r
30、efrigerant thatprovide refrigerant liquid cooling, desuperheating, and compressorlubricant cooling, where used, depend on the established liquidrefrigerant temperatures and intermediate pressures.For a given refrigerant and flow rate, the suction line pressuredrop, suction gas temperature, pressure
31、ratio and displacement, andvolumetric efficiency determine the required size and speed of rota-tion for a positive displacement compressor. At low flow rates, par-ticularly at very low temperatures and in long suction lines, heatgain through insulation can significantly raise the suction tempera-tur
32、e. Also, at low flow rates, a large, warm compressor casing andsuction plenum can further heat the refrigerant before it is com-pressed. These heat gains increase the required displacement of acompressor. The compressor manufacturer must recommend thesuperheating factors to apply. The final suction
33、gas temperaturefrom suction line heating is calculated by iteration.Another concern is that more energy is required to compressrefrigerant to a given condenser pressure as the suction gas gainsmore superheat. This can be seen by examining a pressure-enthalpydiagram for a given refrigerant such as R-
34、22, which is shown inFigure 2 in Chapter 30 of the 2009 ASHRAE HandbookFunda-mentals. As suction superheat increases along the horizontal axis,the slopes of the constant entropy lines of compression decrease.This means that a greater enthalpy change must occur to produce agiven pressure rise. For a
35、given flow, then, the power required forcompression is increased. With centrifugal compressors, pumpingcapacity is related to wheel diameter and speed, as well as to volu-metric flow and acoustic velocity of the refrigerant at the suctionentrance. If the thermodynamic pressure requirement becomes to
36、ogreat for a given speed and volumetric flow, the centrifugal com-pressor experiences periodic backflow and surging.Figure 1 shows an example system of curves representing themaximum refrigeration capacities for a brine chilling plant. Theexample shows only one type of positive-displacement compress
37、orusing a water-cooled condenser in a single-stage system operatingat a steady-state condition. The figure is a graphical method ofexpressing the first law of thermodynamics with an energy balanceapplied to a refrigeration system.One set of nearly parallel curves (A) represents cooler capacityat var
38、ious brine temperatures versus saturated suction temperature(a pressure condition) at the compressor, allowing for suction linepressure drops. The (B) curves represent compressor capacities asthe saturated suction temperature varies and the saturated con-denser temperature (a pressure condition) var
39、ies. The (C) curvesrepresent heat transferred to the condenser by the compressor. It iscalculated by adding the heat input at the evaporator to the energyimparted to the refrigerant by the compressor. The (D) curves rep-resent condenser performance at various saturated condenser tem-peratures as the
40、 inlet temperature of a fixed quantity of coolingwater is varied.The (E) curves represent the combined compressor and con-denser performance as a “condensing unit” at various saturated suc-tion temperatures for various cooling water temperatures. Thesecurves were cross plotted from the (C) and (D) c
41、urves back to the setof brine cooler curves as indicated by the dashed construction linesfor the 27 and 33C cooling water temperatures. Another set of con-struction lines (not shown) would be used for the 30C coolingwater. The number of construction lines used can be increased asnecessary to adequat
42、ely define curvature (usually no more thanthree per condensing-unit performance line).The intersections of curves (A) and (E) represent the maximumcapacities for the entire system at those conditions. For example,these curves show that the system develops 532 kW of refrigerationwhen cooling the brin
43、e to 7C at 2.8C (saturated) suction and using27C cooling water. At 33C cooling water, capacity drops to483 kW if the required brine temperature is 6C and the requiredsaturated suction temperature is 1.7C. The corresponding saturatedcondensing temperature for 6C brine with an accompanying suc-tion te
44、mperature of 2.8C and using 27C water is graphically pro-jected on the brine cooler line with a capacity of 532 kW ofrefrigeration to meet a newly constructed 2.8C saturated suctiontemperature line (parallel to the 1C and 3C lines). At this junction,draw a horizontal line to intersect the vertical s
45、aturated condensingtemperature scale at 34.2C. The condenser heat rejection is appar-ent from the (C) curves at a given balance point.The equation at the bottom of Figure 1 may be used to determinethe shaft power required at the compressor for any given balancepoint. A sixth set of curves could be d
46、rawn to indicate the powerrequirement as a function of capacity versus saturated suction andsaturated condensing temperatures.The same procedure can be repeated to calculate cascade systemperformance. Rejected heat at the cascade condenser would betreated as the chiller load in making a cross plot o
47、f the upper-cycle,high-temperature refrigeration system.For cooling air at the evaporator(s) and for condenser heat rejec-tion to ambient air or evaporative condensers, use the same proce-dures. Performance of coils and expansion devices such asthermostatic expansion valves may also be graphed, once
48、 the basicconcept of heat and mechanical energy input equivalent combina-tions is recognized. Chapter 2 of the 2009 ASHRAE HandbookFundamentals has further information.Component Balancing in Refrigeration Systems 5.3This method finds the natural balance points of compressorsoperating at their maximu
49、m capacities. For multiple-stage loads atseveral specific operating temperatures, the usual way of control-ling compressor capacities is with a suction pressure control andcompressor capacity control device. This control accommodatesany mismatch in pumping capabilities of multistage compressors,instead of allowing each compressor to find its natural balancepoint.Computer programs could be developed to determine balancepoints of complex systems. However, because applications, compo-nents, and piping arrangements are so diverse, many designers useavailable capacity perform
