ASHRAE HVAC SYSTEMS AND EQUIPMENT SI CH 43-2012 LIQUID-CHILLING SYSTEMS.pdf

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1、43.1CHAPTER 43LIQUID-CHILLING SYSTEMSGENERAL CHARACTERISTICS. 43.1Principles of Operation 43.1Common Liquid-Chilling Systems . 43.1Selection. 43.3Control . 43.3Standards and Testing 43.5General Maintenance. 43.5RECIPROCATING LIQUID CHILLERS . 43.5Equipment 43.5Performance Characteristics and Operati

2、ng Problems. 43.6Method of Selection . 43.7Control Considerations 43.7Special Applications 43.7CENTRIFUGAL LIQUID CHILLERS . 43.7Equipment 43.7Performance and Operating Characteristics . 43.9Selection . 43.10Control Considerations 43.11Auxiliaries 43.11Special Applications. 43.12Operation and Mainte

3、nance 43.12SCREW LIQUID CHILLERS . 43.13Equipment 43.13Performance and Operating Characteristics . 43.13Selection . 43.14Control Considerations 43.14Auxiliaries 43.14Special Applications. 43.15Maintenance. 43.15IQUID-CHILLING systems cool water, brine, or other sec-Lsecondary coolant for air conditi

4、oning or refrigeration. Thesystem may be either factory-assembled and wired or shipped insections for erection in the field. The most frequent application iswater chilling for air conditioning, although brine cooling for low-temperature refrigeration and chilling fluids in industrial processesare al

5、so common.The basic components of a vapor-compression, liquid-chillingsystem include a compressor, liquid cooler (evaporator), con-denser, compressor drive, liquid-refrigerant expansion or flow-control device, and control center; it may also include a receiver,economizer, expansion turbine, and/or s

6、ubcooler. In addition, aux-iliary components may be used, such as a lubricant cooler, lubri-cant separator, lubricant-return device, purge unit, lubricant pump,refrigerant transfer unit, refrigerant vents, and/or additional con-trol valves.For information on absorption equipment, see Chapter 18 of t

7、he2010 ASHRAE HandbookRefrigeration.GENERAL CHARACTERISTICSPRINCIPLES OF OPERATIONLiquid (usually water) enters the cooler, where it is chilled by liq-uid refrigerant evaporating at a lower temperature. The refrigerantvaporizes and is drawn into the compressor, which increases thepressure and temper

8、ature of the gas so that it may be condensed at thehigher temperature in the condenser. The condenser cooling mediumis warmed in the process. The condensed liquid refrigerant thenflows back to the evaporator through an expansion device. In theexpansion device, some of the liquid refrigerant changes

9、to vapor(flashes) as pressure drops. Flashing cools the liquid to the saturatedtemperature at evaporator pressure. The following modifications(sometimes combined for maximum effect) reduce flash gas andincrease the net refrigeration per unit of power consumption.Subcooling. Condensed refrigerant may

10、 be subcooled belowits saturated condensing temperature in either the subcooler sec-tion of a water-cooled condenser or a separate heat exchanger.Subcooling reduces flashing and increases the refrigeration effectin the chiller.Economizing. This process can occur either in a direct-expansion (DX), an

11、 expansion turbine, or a flash system. In a DXsystem, the main liquid refrigerant is usually cooled in the shell ofa shell-and-tube heat exchanger, at condensing pressure, from thesaturated condensing temperature to within several degrees of theintermediate saturated temperature. Before cooling, a s

12、mall portionof the liquid flashes and evaporates in the tube side of the heatexchanger to cool the main liquid flow. Although subcooled, the liq-uid is still at the condensing pressure.An expansion turbine extracts rotating energy as a portion ofthe refrigerant vaporizes. As in the DX system, the re

13、maining liquidis supplied to the cooler at intermediate pressure.In a flash system, the entire liquid flow is expanded to interme-diate pressure in a vessel that supplies liquid to the cooler at saturatedintermediate pressure; however, the liquid is at intermediate pressure.Flash gas enters the comp

14、ressor either at an intermediate stage ofa multistage centrifugal compressor, at the intermediate stage of anintegral two-stage reciprocating compressor, at an intermediate pres-sure port of a screw compressor, or at the inlet of a high-pressurestage on a multistage reciprocating or screw compressor

15、.Liquid Injection. Condensed liquid is throttled to the interme-diate pressure and injected into the second-stage suction of the com-pressor to prevent excessively high discharge temperatures and, inthe case of centrifugal machines, to reduce noise. For screw com-pressors, condensed liquid is inject

16、ed into a port fixed at slightlybelow discharge pressure to provide lubricant cooling.COMMON LIQUID-CHILLING SYSTEMSBasic SystemThe refrigeration cycle of a basic system is shown in Figure 1.Chilled water enters the cooler at 12C, for example, and leaves at7C. Condenser water leaves a cooling tower

17、at 30C, enters thecondenser, and returns to the cooling tower near 35C. Condensersmay also be cooled by air or evaporation of water. This system, witha single compressor and one refrigerant circuit with a water-cooledcondenser, is used extensively to chill water for air conditioningbecause it is rel

18、atively simple and compact.Multiple-Chiller SystemsA multiple-chiller system has two or more chillers connected byparallel or series piping to a common distribution system. MultipleThe preparation of this chapter is assigned to TC 8.1, Positive Displace-ment Compressors, and TC 8.2, Centrifugal Mach

19、ines.43.2 2012 ASHRAE HandbookHVAC Systems and Equipment (SI)chillers offer operational flexibility, standby capacity, and less dis-ruptive maintenance. The chillers can be sized to handle a base loadand increments of a variable load to allow each chiller to operate atits most efficient point.Multip

20、le-chiller systems offer some standby capacity if repairwork must be done on one chiller. Starting in-rush current isreduced, as well as power costs at partial-load conditions. Mainte-nance can be scheduled for one chiller during part-load times, andsufficient cooling can still be provided by the re

21、maining unit(s).These advantages require an increase in installed cost and space,however. Traditionally, flow was held constant through the chillersfor stable control. Today, variable-flow chilled-water systems arefinding favor in some applications. Both variable-flow and primary/secondary hydronic

22、systems are discussed in further detail in Chap-ter 13.When design chilled-water temperature is above about 7C, allunits should be controlled by the combined exit water temperatureor by the return water temperature (RWT), because overchilling willnot cause dangerously low water temperature in the op

23、eratingmachine(s). Chilled-water temperature can be used to cycle one unitoff when it drops below a capacity that can be matched by theremaining units.When the design chilled-water temperature is below about 7C,each machine should be controlled by its own chilled-water temper-ature, both to prevent

24、dangerously low evaporator temperatures andto avoid frequent shutdowns by low-temperature cutout. The tem-perature differential setting of the RWT must be adjusted carefullyto prevent short-cycling caused by the step increase in chilled-watertemperature when one chiller is cycled off. These control

25、arrange-ments are shown in Figures 2 and 3. In the series arrangement, the chilled-liquid pressure drop maybe higher if shells with fewer liquid-side passes or baffles are notavailable. No overchilling by either unit is required, and compressorpower consumption is lower than for the parallel arrange

26、ment atpartial loads. Because evaporator temperature never drops below thedesign value (because no overchilling is necessary), the chances ofevaporator freeze-up are minimized. However, the chiller shouldstill be protected by a low-temperature safety control.Water-cooled condensers in series are bes

27、t piped in a counterflowarrangement so that the lead machine is provided with warmer con-denser and chilled water and the lag machine is provided with colderentering condenser and chilled water. Refrigerant compression foreach unit is nearly the same. If about 55% of design cooling capacityis assign

28、ed to the lead machine and about 45% to the lag machine,identical units can be used. In this way, either machine can providethe same standby capacity if the other is down, and lead and lagmachines may be interchanged to equalize the number of operatinghours on each.A control system for two machines

29、in series is shown in Figure 4.(On reciprocating chillers, RWT sensing is usually used instead ofleaving water sensing because it allows closer temperature control.)Both units are modulated to a certain capacity; then, one unit shutsdown, leaving less than 100% load on the operating machine.One mach

30、ine should be shut down as soon as possible, with theremaining unit carrying the full load. This not only reduces the num-ber of operating hours on a unit, but also leads to less total powerconsumption because the COP tends to decrease below full-loadvalue when unit load drops much below 50%.Heat Re

31、covery SystemsAny building or plant requiring simultaneous operation of heat-producing and cooling equipment has the potential for a heat recov-ery installation.Heat recovery systems extract heat from liquid being chilled andreject some of that heat, plus the energy of compression, to a warm-water c

32、ircuit for reheat or heating. Air-conditioned spaces thusfurnish heating for other spaces in the same building. During thefull-cooling season, all heat must be rejected outdoors, usually by aFig. 1 Equipment Diagram for Basic Liquid ChillerFig. 2 Parallel-Operation High Design Water Leaving Coolers

33、(Approximately 7C and Above)Fig. 3 Parallel-Operation Low Design Water Leaving Coolers (Below Approximately 7C)Fig. 4 Series OperationLiquid-Chilling Systems 43.3cooling tower. During spring or fall, some heat is required indoors,while some heat extracted from air-conditioned spaces must berejected

34、outdoors.Heat recovery offers a low heating cost and reduces spacerequirements for equipment. The control system must, however, bedesigned carefully to take the greatest advantage of recovered heatand to maintain proper temperature and humidity in all parts of thebuilding. Chapter 9 covers balanced

35、heat recovery systems.Because cooling tower water is not satisfactory for heating coils,a separate, closed warm-water circuit with another condenser bun-dle or auxiliary condenser, in addition to the main water chiller con-denser, must be provided. In some cases, it is economically feasibleto use a

36、standard condenser and a closed-circuit water cooler.Instead of rejecting all heat extracted from the chilled liquid to acooling tower, a separate, closed condenser cooling water circuit isheated by the condensing refrigerant for comfort heating, preheat-ing, or reheating. Some factory packages incl

37、ude an extra condenserwater circuit, either a double-bundle condenser or an auxiliary con-denser.A centrifugal heat recovery package is controlled as follows:Chilled-liquid temperature is controlled by a sensor in the leav-ing chilled-water line signaling the capacity control device.Hot-water temper

38、ature is controlled by a sensor in the hot-waterline that modulates a cooling tower bypass valve. As the heatingrequirement increases, hot-water temperature drops, opening thetower bypass slightly. Less heat is rejected to the tower, condens-ing temperature increases, and hot-water temperature is re

39、storedas more heat is rejected to the hot-water circuit.The hot-water temperature selected affects the installed cost ofthe centrifugal package, as well as on the power consumption whileheating. Lower hot-water temperatures of 35 to 41C result in a lessexpensive machine that uses less power. Higher

40、temperaturesrequire greater compressor motor output, perhaps higher-pressurecondenser shells, sometimes extra compression stages, or a cascadearrangement. Installed cost of the centrifugal heat recovery machineincreases as a result.Another concern in design of a central chilled-water plant withheat

41、recovery centrifugal compressors is the relative size of coolingand heating loads. These loads should be equalized on each machineso that the compressor may operate at optimum efficiency duringboth full cooling and full heating seasons. When the heatingrequirement is considerably smaller than the co

42、oling requirement,multiple packages lower operating costs and allow less expensivestandard air-conditioning centrifugal packages to be used for therest of the cooling requirement. In multiple packages, only one unitis designed for heat recovery and carries the full heating load.Another consideration

43、 for heat recovery chiller systems is thepotential for higher cooling energy use. A standard commercialbuilding water chiller operates with condenser water temperaturesat or below 38C. For heat recovery to be of practical use, it may benecessary for the condenser water to operate at higher temperatu

44、res.This increases the chillers energy consumption. The design engi-neer must examine the tradeoff between higher cooling energy useversus lower heating energy use; the heat recovery chiller systemmay not necessarily be attractive.SELECTIONThe largest factor that determines total liquid chiller owni

45、ng costis the cooling load size; therefore, the total required chiller capacityshould be calculated accurately. The practice of adding 10 to 20% toload estimates is unnecessary because of the availability of accurateload estimating methods, and it proportionately increases costs ofequipment purchase

46、, installation, and the poor efficiency resultingfrom wasted power. Oversized equipment can also cause opera-tional difficulties such as frequent on/off cycling or surging ofcentrifugal machines at low loads. The penalty for a small under-estimation of cooling load, however, is not serious. On the f

47、ewdesign-load days of the year, increased chilled-liquid temperature isoften acceptable. However, for some industrial or commercialloads, a safety factor can be added to the load estimate.The life-cycle cost as discussed in Chapter 37 of the 2011ASHRAE HandbookHVAC Applications should be used to min

48、i-mize overall purchase and operating costs. Total owning cost iscomprised of the following:Equipment price. Each machine type and/or manufacturersmodel should include all necessary auxiliaries such as starters andvibration mounts. If these are not included, their price should beadded to the base pr

49、ice. Associated equipment, such as condenserwater pump, tower, and piping, should be included.Installation cost. Factory-packaged machines are both lessexpensive to install and usually considerably more compact, thussaving space. The cost of field assembly must also be evaluated.Energy cost. Using an estimated load schedule and part-loadpower consumption curves furnished by the manufacturer, ayears energy cost should be calculated.Water cost. With water-cooled towers, the cost of acquisition,water treatment, tower blowdown, and overflow water should beincluded.Maintenan