ASHRAE HVAC SYSTEMS AND EQUIPMENT IP CH 9-2012 APPLIED HEAT PUMP AND HEAT RECOVERY SYSTEMS.pdf

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1、9.1CHAPTER 9APPLIED HEAT PUMP AND HEAT RECOVERY SYSTEMSTerminology . 9.1APPLIED HEAT PUMP SYSTEMS 9.1Heat Pump Cycles 9.2Heat Sources and Sinks 9.2Types of Heat Pumps 9.4Heat Pump Components. 9.6Industrial Process Heat Pumps 9.9APPLIED HEAT RECOVERY SYSTEMS. 9.14Waste Heat Recovery 9.14Water-Loop He

2、at Pump Systems 9.15Balanced Heat Recovery Systems 9.18TERMINOLOGYALANCED heat recovery. Occurs when internal heat gainBequals recovered heat and no external heat is introduced to theconditioned space. Maintaining balance may require raising thetemperature of recovered heat.Break-even temperature. T

3、he outdoor temperature at whichtotal heat losses from conditioned spaces equal internally generatedheat gains.Changeover temperature. The outdoor temperature thedesigner selects as the point of changeover from cooling to heatingby the HVAC system.Coefficient of performance (COP). The ratio of heat t

4、rans-ferred at the condenser of a heat pump to the energy used to powerthe heat pump.Break-even COP. The COP at which the cost per Btu for a heatrecovery heat pump system equals the cost per Btu of the system itis replacing.External heat. Heat generated from sources outside the condi-tioned area. Th

5、is heat from gas, oil, steam, electricity, or solarsources supplements internal heat and internal process heat sources.Recovered internal heat can reduce the demand for external heat.Internal heat. Total passive heat generated within the condi-tioned space. It includes heat generated by lighting, co

6、mputers,business machines, occupants, and mechanical and electrical equip-ment such as fans, pumps, compressors, and transformers.Internal process heat. Heat from industrial activities andsources such as wastewater, boiler flue gas, coolants, exhaust air,and some waste materials. This heat is normal

7、ly wasted unlessequipment is included to extract it for further use.Pinch technology. An energy analysis tool that uses vector anal-ysis to evaluate all heating and cooling utilities in a process. Com-posite curves created by adding the vectors allow identification of a“pinch” point, which is the be

8、st thermal location for a heat pump.Recovered (or reclaimed) heat. Comes from internal heatsources. It is used for space heating, domestic or service water heat-ing, air reheat in air conditioning, process heating in industrialapplications, or other similar purposes. Recovered heat may bestored for

9、later use.Stored heat. Heat from external or recovered heat sources that isheld in reserve for later use.System coefficient of performance. Ratio of heat recovery sys-tem output to entire system energy input, including compressor,pumps, etc.Usable temperature. Temperature or range of temperatures at

10、which heat energy can be absorbed, rejected, or stored for use withinthe system.Waste heat. Heat rejected from the building (or process) becauseits temperature is too low for economical recovery or direct use.APPLIED HEAT PUMP SYSTEMSA heat pump extracts heat from a source and transfers it to a sink

11、at a higher temperature. According to this definition, all pieces of re-frigeration equipment, including air conditioners and chillers withrefrigeration cycles, are heat pumps. In engineering, however, theterm heat pump is generally reserved for equipment that heats forbeneficial purposes, rather th

12、an that which removes heat for coolingonly. Dual-mode heat pumps alternately provide heating or cooling.Heat reclaim heat pumps provide heating only, or simultaneousheating and cooling. An applied heat pump requires competent fieldengineering for the specific application, in contrast to the use of a

13、manufacturer-designed unitary product. Applied heat pumps in-clude built-up heat pumps (field- or custom-assembled from com-ponents) and industrial process heat pumps. Most modern heatpumps use a vapor compression (modified Rankine) cycle or ab-sorption cycle. Any of the other refrigeration cycles d

14、iscussed inChapter 2 of the 2009 ASHRAE HandbookFundamentals are alsosuitable. Although most heat pump compressors are powered byelectric motors, limited use is also made of engine and turbinedrives. Applied heat pumps are most commonly used for heatingand cooling buildings, but they are gaining pop

15、ularity for efficientdomestic and service water heating, pool heating, and industrialprocess heating.Applied heat pumps with capacities from 24,000 to150,000,000 Btu/h operate in many facilities. Some machines arecapable of output water temperatures up to 220F and steam pres-sures up to 60 psig.Comp

16、ressors in large systems vary from one or more reciprocat-ing or screw types to staged centrifugal types. A single or centralsystem is often used, but in some instances, multiple heat pump sys-tems are used to facilitate zoning. Heat sources include the ground,well water, surface water, gray water,

17、solar energy, the air, and inter-nal building heat. Compression can be single-stage or multistage.Frequently, heating and cooling are supplied simultaneously to sep-arate zones.Decentralized systems with water-loop heat pumps are common,using multiple water-source heat pumps connected to a commoncir

18、culating water loop. They can also include ground coupling, heatrejectors (cooling towers and dry coolers), supplementary heaters(boilers and steam heat exchangers), loop reclaim heat pumps, solarcollection devices, and thermal storage. The initial cost is relativelylow, and building reconfiguration

19、 and individual space temperaturecontrol are easy.Community and district heating and cooling systems can bebased on both centralized and distributed heat pump systems. The preparation of this chapter is assigned to TC 6.8, Geothermal HeatPump and Energy Recovery Applications.9.2 2012 ASHRAE Handbook

20、HVAC Systems and Equipment HEAT PUMP CYCLESSeveral types of applied heat pumps (both open- and closed-cycle) are available; some reverse their cycles to deliver bothheating and cooling in HVAC systems, and others are for heatingonly in HVAC and industrial process applications. The followingare the f

21、our basic types of heat pump cycles:Closed vapor compression cycle (Figure 1). This is the mostcommon type in both HVAC and industrial processes. It uses aconventional, separate refrigeration cycle that may be single-stage, compound, multistage, or cascade.Mechanical vapor recompression (MVR) cycle

22、with heatexchanger (Figure 2). Process vapor is compressed to a temper-ature and pressure sufficient for reuse directly in a process.Energy consumption is minimal, because temperatures are opti-mum for the process. Typical applications include evaporators(concentrators) and distillation columns.Open

23、 vapor recompression cycle (Figure 3). A typical applica-tion is in an industrial plant with a series of steam pressure levelsand an excess of steam at a lower-than-desired pressure. Heat ispumped to a higher pressure by compressing the lower-pressuresteam.Heat-driven Rankine cycle (Figure 4). This

24、cycle is usefulwhere large quantities of heat are wasted and energy costs arehigh. The heat pump portion of the cycle may be either open orclosed, but the Rankine cycle is usually closed.HEAT SOURCES AND SINKSTable 1 shows the principal media used as heat sources and sinks.Selecting a heat source an

25、d sink for an application is primarily influ-enced by geographic location, climate, initial cost, availability, andtype of structure. Table 1 presents various factors to be consideredfor each medium. AirOutdoor air is a universal heat source and sink medium for heatpumps and is widely used in reside

26、ntial and light commercial sys-tems. Extended-surface, forced-convection heat transfer coils trans-fer heat between the air and refrigerant. Typically, the surface areaof outdoor coils is 50 to 100% larger than that of indoor coils. Thevolume of outdoor air handled is also greater than the volume of

27、indoor air handled by about the same percentage. During heating,the temperature of the evaporating refrigerant is generally 10 to20F less than the outdoor air temperature. Air heating and coolingcoil performance is discussed in more detail in Chapters 23 and 27.When selecting or designing an air-sou

28、rce heat pump, two fac-tors in particular must be considered: (1) the local outdoor air tem-perature and (2) frost formation.As the outdoor temperature decreases, the heating capacity of anair-source heat pump decreases. This makes equipment selection fora given outdoor heating design temperature mo

29、re critical for an air-source heat pump than for a fuel-fired system. Equipment must besized for as low a balance point as is practical for heating withouthaving excessive and unnecessary cooling capacity during the sum-mer. A procedure for finding this balance point, which is defined asthe outdoor

30、temperature at which heat pump capacity matches heat-ing requirements, is given in Chapter 49.When the surface temperature of an outdoor air coil is 32F orless, with a corresponding outdoor air dry-bulb temperature 4 to10F higher, frost may form on the coil surface. If allowed to accu-mulate, frost

31、inhibits heat transfer; therefore, the outdoor coil mustbe defrosted periodically. The number of defrosting operations isinfluenced by the climate, air-coil design, and the hours of opera-tion. Experience shows that, generally, little defrosting is requiredwhen outdoor air conditions are below 17F a

32、nd 60% rh. This can beconfirmed by psychrometric analysis using the principles given inFig. 1 Closed Vapor Compression CycleFig. 2 Mechanical Vapor Recompression Cycle with Heat ExchangerFig. 3 Open Vapor Recompression CycleFig. 4 Heat-Driven Rankine CycleApplied Heat Pump and Heat Recovery Systems

33、9.3Table 1 Heat Pump Sources and SinksMedium ExamplesSuitability Availability Cost Temperature Common PracticeHeatSourceHeatSinkLocation Relativeto NeedCoincidence with Need InstalledOperationandMaintenance Level Variation Use LimitationsAIROutdoor Ambient airGood, but efficiency and capacity in hea

34、ting mode decrease with decreasing outdoor air temperatureGood, but efficiency and capacity in cooling mode decrease with increasing outdoor air temperatureUniversal Continuous Low Moderate Variable Generally extremeMost common, many standard productsDefrosting and supplemental heat usually required

35、Exhaust Building ventilationExcellent Fair Excellent if planned forin building designExcellent Low to moderateLow unless exhaust is laden with dirt or grease Excellent Very low Excellent as energy-conservation measureInsufficient for typical loadsWATERWell* Ground-water well may also provide potable

36、 water sourceExcellent Excellent Poor to excellent; practical depth varies by locationContinuous Low if existing well used or shallow wells suitable; can be high otherwiseLow, butperiodic maintenance requiredGenerally excellent; varies by locationExtremely stableCommon Water disposal and required pe

37、rmits may limit; may require double-wall exchangers; may foul or scaleSurface Lakes, rivers, oceansExcellent for large water bodies or high flow ratesExcellent for large water bodies or high flow ratesLimited; depends on proximityUsually continuousDepends on proximityand water qualityDepends on prox

38、imityand water qualityUsually satisfactoryDependson sourceAvailable, particularly for fresh waterOften regulated or prohibited; may clog, foul, or scaleTap (city)Municipal water supplyExcellent Excellent Excellent Continuous Low Low energy cost, but water use and disposal may be costlyExcellent Usua

39、llyvery lowUse is decreasing because of regulationsUse or disposal may be regulated or prohibited; may corrode or scaleCondens-ingCooling towers, re-frigeration systemsExcellent Poor to goodVaries Varies with cooling loadsUsually low Moderate Favorable as heat sourceDependson sourceAvailable Suitabl

40、e only if heating need is coincident with heat rejectionClosed loopsBuilding water-loop heat pump systemsGood; loop may need supplemental heatFavorable; may need loop heat rejectionExcellent if designed as suchAs needed Low Low to moderateAs designed As designed VerycommonMost suitable for medium or

41、 large buildingsWaste Raw or treated sewage, gray waterFair to excellentFair; varies with sourceVaries Varies; may be adequateDepends on proximity; high for raw sewageVaries; may be high for raw sewageExcellent Usually lowUncommon; practical only in large systemsUsually regulated; may clog, foul, sc

42、ale, or corrodeGROUND*Ground-coupledBuried or submerged fluid loopsGood if ground is moist; other-wise poorFair to goodif ground is moist; other-wise poorDepends on soil suitabilityContinuous High to moderateLow UsuallygoodLow, particularly for vertical systemsRapidly increasingHigh initial costs fo

43、r ground loopDirect- expan-sionRefrig-erant circulatedin ground coilVaries with soil conditionsVaries with soil conditionsVaries with soil conditionsContinuous High High Varies by designGenerally lowExtremely limitedLeak repair very expensive; requires large refrigerant quantitiesSOLAR ENERGYDirect

44、or heated waterSolar collectors and panelsFair Poor; usually un-acceptableUniversal Highly intermittent; night use requires storageExtremely highModerate tohighVaries Extreme VerylimitedSupplemental source or storage requiredINDUSTRIAL PROCESSProcess heat or exhaustDistillation, molding, refining, w

45、ashing, dryingFair to excellentVaries; often impracticalVaries Varies Varies Generally low Varies Varies Varies May be costly unless heat need is near rejected source*Groundwater-source heat pumps are also considered ground-source heat pump systems.9.4 2012 ASHRAE HandbookHVAC Systems and Equipment

46、Chapter 23. However, under very humid conditions, when smallsuspended water droplets are present in the air, the rate of frostdeposit may be about three times as great as predicted from psychro-metric theory and the heat pump may require defrosting after as lit-tle as 20 min of operation. The loss o

47、f available heating capacitycaused by frosting should be considered when sizing an air-sourceheat pump.Following commercial refrigeration practice, early designs of air-source heat pumps had relatively wide fin spacing of 4 to 5 fins/in.,based on the theory that this would minimize defrosting freque

48、ncy.However, experience has shown that effective hot-gas defrosting al-lows much closer fin spacing and reduces the systems size and bulk.In current practice, fin spacings of 10 to 20 fins/in. are widely used.In many institutional and commercial buildings, some air mustbe continuously exhausted year

49、-round. This exhaust air can be usedas a heat source, although supplemental heat is generally necessary.High humidity caused by indoor swimming pools causes con-densation on ceiling structural members, walls, windows, and floorsand causes discomfort to spectators. Traditionally, outdoor air anddehumidification coils with reheat from a boiler that also heats thepool water are used. This is ideal for air-to-air and air-to-water heatpumps because energy costs can be reduced. Suitable materialsmust be chosen so that heat pump components are resistant to cor-rosion from chlorine and high