Industrial heat pumpsMarch 2011
ContentsExecutive summary .2Background .2Coefficient of performance .3Ammonia heat pump applications .3Traditional heat pump.4Chiller/heat pump .4Scavenging heat pump .4Example heat pump applications .5140 F Poultry .5160 F Beef .5190 F Dairy .6Economic analysis approach .7Heat pump system design .8Heat pump compressor . 10Heat exchanger considerations . 12Operational considerations . 12References . 15Contributors . 15Executive summaryIndustrial refrigeration systems discard a significant quantityof heat to the atmosphere. Heat pumps can capture thiswasted heat efficiently and use it to cut the fossil fuels burnedto heat water. These industrial heat pumps are environmentally friendly and economical, allowing operators to makethe most of their energy resources.Ammonia heat pumps offer an owner a comprehensivegreen solution. They are environmental as the refrigerant isa natural refrigerant that has an ozone depletion potential(ODP) of zero and a global warming potential (GWP) lessthan 1.Ammonia heat pumps provide conservation, by conservingthe energy removed from the refrigeration system and transforming it so that it can be re-used and applied in a plant’swater heating requirements. This source of energy is renewable as the heat is naturally occurring within food productsand will be an available energy source for as long as we mustrefrigerate our food.Ammonia heat pumps generate cash because the heat thatis transformed from useless energy to useful energy providessavings. Even though the energy cost for each unit of energyis more for the heat pump’s electric power supply, the heatpump saves cash because the effective Coefficient of Performance (COP), a ratio energy output to energy input, of theheat pump system is far superior to traditional water heatingmethods which cannot reach a COP greater than 1.2A secondary benefit, but of significant value, is that the heatpump diverts load away from evaporative condensers, allowing them to operate at lower pressures and conserving waterand water treatment resources.This paper addresses ammonia heat pump system challenges. Selecting the right heat pump compressor that balances first cost with operating efficiency and maintenancecosts will ensure the best results from heat pump systems.The technology exists today to overcome the compressorchallenges and deliver required temperatures using a naturaland efficient refrigerant. With zero ozone depletion and aglobal warming potential less than one, ammonia is a naturalchoice for environmentally focused organizations, however,it is the efficiency of the refrigerant that makes it a naturalchoice in economic evaluations.BackgroundIndustrial food and beverage processors burn fossil fuels toheat water for processing and equipment sanitation. Theyalso need mechanical refrigeration systems for processingand for cold storage. These refrigeration systems consumeelectricity. The heat absorbed by the refrigeration systems isusually discarded to the atmosphere as wasted heat. There issignificant potential to harness the discarded heat and use itto produce hot water, thus reducing the fossil fuels used.Heat pumps can capture rejected heat and put it to use forbeneficial heating purposes. Recent developments in industrial screw compressors now enable heat pumps to deliverhigh quality hot water from 140 F (60 C) to 190 F (88 C).Capturing the rejected heat of refrigeration systems is not anew concept. Although many means of heat recovery wereused to pull out as much heat as possible from the dischargegases, ammonia heat pumps have not been viable untilrecently because of compressor limitations. Reciprocatingcompressors cannot deliver the water temperatures requiredby most processes and are employed mainly to supplementfossil fuel heating. To deliver the required temperatures,most heat pumps use Freon refrigerants with commercialrefrigeration compressors. These systems have deliveredattractive performance in northern Europe. While effortsare made to be environmentally friendly by selecting environmental Freon refrigerants such as R-134a (ODP 0, butGWP 1430 source: UNEP), a natural refrigerant such asammonia would be a preferred environmental solution.Heat pumps using ammonia refrigerant are extremelyefficient and can deliver superior COP for similar operatingconditions. Ammonia is a natural refrigerant with no ODP
and negligible GWP, and is widely used in large industrialrefrigeration applications. Compressors are now availablethat can operate in conditions with high discharge pressuresand large differential pressures that are required for ammoniaheat pump applications. These compressors make it possibleto exploit the full potential of the attractiveness that ammonia heat pumps represent.to the water heating). The consumed energy is the motorenergy, some of which is dissipated by the oil cooler, and therest is absorbed in the ammonia gas flow. Because of thisfact, the effective COP must be greater than 1 by definition:Coefficient of performanceThe effective COP will depend on the heat pump inlet pressure and the discharge pressure that is required to deliverthe needed water temperature, just as it depends on suctionand discharge pressures in traditional refrigeration applications. As shown in Table 1, there are several hot watertemperatures required for the various applications in refrigeration for the food and beverage industries, but the COP islikely to range between 2 and 6 while a COP between 7 and10 is possible for low compression ratio duties. The information in Table 1 provides some sample COP’s on single stageammonia heat pump systems.The coefficient of performance (COP) is unit-less indexmeasure to compare two solutions, such as a comparisonbetween a traditional boiler system and that of a heat pump.It is defined as the energy delivered divided by the energyconsumed (note that heat and power energy must beconverted to the same units to make the index unit-less).For fossil fuel heating solutions, the theoretical maximumpossible COP would be equal to 1, though this is not possibleto reach in practical application. Most boilers cite efficiencies between 0.80 and 0.90, though there are some highlyefficient designs that are above 90%. This cited efficiency isfor a brand new boiler and efficiency will deteriorate throughscaling and wear. Also, there are significant transmissionlosses in steam systems from loose fittings, service work,and condensation in the line that make the applied COPmuch lower than the original design of the boiler.With a heat pump that uses refrigeration heat as its source(scavenging heat pump), the delivered energy is equal to theabsorbed refrigeration heat plus the motor energy consumedby the heat pump compressor (note this requires that oilcooling and inlet de-superheating heat is usefully appliedCOP (refrigeration heat motor energy) / motor energy refrigeration heat/ motor energy 1Ammonia heat pumps are much more efficient thantraditional fossil fuel boilers. With an effective COP of 6compared with a boiler system with a COP of 0.8, the heatpump provides 7.5 times more heat energy for each unit ofenergy input.Ammonia heat pump applicationsThere are three typical categories of applications for industrial ammonia heat pumps that can replace fossil fuel burning systems. All three have significant opportunities to takeadvantage of the latest compressor capabilities to delivervalue often sought but previously unobtainable.Potential single stage heat pump COPs for hot water applicationsIndustryApplicationWater TemperatureRequiredDurationAttainableHeat Pump COPBeef ottle Warming145ºFContinuous7.6DairyHTST 155ºFIntermittent6.7GeneralBoiler Preheat175ºFContinuous5.3Pork ProcessingSanitation150ºFIntermittent7.2Poultry ProcessingSanitation160ºFIntermittent6.3Poultry ProcessingScalding and Feather Removal140ºFContinuous8.2Seafood ProcessingCooking Oil Heating150ºFContinuous7.2Seafood ProcessingSanitation160ºFIntermittent6.3Based on 95ºF Host Condensing and 55ºF entering water supply temperatureTable 13
Traditional heat pumpThe first is a traditional heat pump using sources such asground, air or water. While traditional heat pumps are themost widely applied through residential and commercialapplications, industrial heat pumps offer a much larger scale.Industrial heat pumps applied to traditional heat pump applications deliver large scale heating requirements such asthose applied for community heat in northern Europe. Theselarge capacity heating systems absorb heat from large bodiesof water, a large and consistent heat source, to continuouslytransform the “cold” low-grade heat source into the hightemperature heating resource for the community. Thesesystems offer an economically viable heating solution, butrequire long piping circuits to deliver the heat throughout thecommunity. These long piping runs suffer heat and temperature degradation so high grade heat is required, muchhigher than many compressor designs can deliver. Even so,with compressor solutions now available to operate at thesehigh pressures for high grade heating, this renewable energysource is attractive for its economic returns and the use of arenewable energy resource that does not deplete the ozoneor have appreciable global warming impact. Although theseprojects are high profile and quite an interesting subject ontheir own, this is not the focus of this paper.Chiller/heat pumpAnother heat pump application applies the compressorsimultaneously to beneficial cooling and heating needs.The compressor discharge temperature is raised so thatone compressor holds evaporator temperature for coolingand delivers heat through a heat pump system for mediumgrade heating requirements. The power consumed by thecompressor is also used to raise the heat delivered by theheat pump system. Therefore the effective COP is doubledas compared to either a cooling only or heating only application. This is useful for applications that require both heatingand cooling if the demands are correlated. Although thereare many opportunities for dual chilling and heating, thispaper will focus on the third application type, host systemscavenging heat pumps.Scavenging heat pumpThis paper focuses on scavenging heat pump systems thatuse ammonia refrigeration systems as the heat source of theheat pump. The technology exists to apply heat pumps tolarge and small refrigeration systems, diverting the refrigeration system discharge around the condenser directly to theheat pump system. The solution can be added to existingsystems as a stand-alone energy efficiency improvementthat delivers quick payback while allowing the owner to cutcarbon emissions.Ideal for retrofit applications, the owner can offset the fossilfuel heating with heat already captured in the refrigerationsystem, cutting heating energy costs directly. Reducingthe load on the condenser of the host system provides theindirect benefit of lowering the refrigeration system headpressure and improving refrigeration system efficiency.Single stage scavenging ammonia heat pump systemExisting AmmoniaRefrigeration SystemHeat PumpScavenging SystemFigure 14
Example heat pump applicationsHot water demands vary among processing facilities, butthree categories are the most common. The three applications are for 140ºF, 160ºF and 190ºF delivered water temperatures. Three sample applications are described below, onefor each temperature level.140 F: Poultry processing scalding and featherremovalTo remove the feathers and scald the chicken carcass skin,140ºF water is required. As part of a continuous process, thisdemand is constant as long as production is taking placemaking this a good application for a heat pump. The sameprocess which generates a consistent demand for heatingrequires significant cooling that provides the source of heatfor the heat pump in the same process. With a direct correlation for cooling demand and heating demand, the riskof inadequate heat source or imbalance between heatingand cooling demand is low. For this reason, the heat pumpcan operate 18-20 hours a day with a fast payback. Withthe modest hot water temperature demand, a simple singlestage heat pump with high COP performance is possible. SeeFigure 1 for a layout of the sample system and the summaryof performance figures below:Required Refrigeration (Heat Source):Heat Delivered:Entering Water Temperature (EWT)Leaving Water Temperature (LWT)640 TR9,522 MBTUH65ºF140ºFWater Flow Rate (GPM)Annual Heat Delivered:Compressor Nominal Swept Volume:HP Consumed:Location:Electric Utility Rate:Natural Gas Utility Rate:MBTUH)Installed Capital Investment:Heat Pump System COP:Gas Boiler System COP:Operating Hours per day:Annual Operating Hours:Annual Electric Op. Expense:Annual Avoided Gas Expense:Simple Payback:25056,603 MMBTU600 CFM538 HPTexas 0.075/kWH 0.022/kWH ( 6.5/ 750,000.7.00.83195,944 178,900. 443,300.2.8 Years160 F: Beef processing equipment sanitationThe sanitation applications represent a significant hot waterrequirement at temperatures easily attainable with an ammonia heat pump, but two concerns arise from this application. The first is that the sanitation takes place betweenprocessing shifts so the heaviest refrigeration demand fromprocessing is at minimum or diminishing levels. Most ofthese facilities have significant cold storage and freezingloads that can provide the source of refrigeration heat. Evenso, the sanitation demand ranges between 4 and 8 hours,limiting the annual operating hours of the heat pump systemand limiting the payback opportunities as a result.Two- stage scavenging ammonia heat pump systemExisting AmmoniaRefrigeration SystemTwo-Stage Heat PumpScavenging SystemFigure 25
6One alternative to overcome both of these issues is to usea storage tank so that the hot water supply can be built upthroughout the day benefitting from the processing refrigeration load and allowing the heat pump system to operate formore hours in a day. This thermal storage strategy requiresmore capital for the storage tank and plant floor space for thetank. Though the former may be justified by the faster payback, available floor space may prohibit the thermal storageapproach. Both approaches are summarized below. Note thatfor simplicity, identical system sizes were used so that theheat delivered by the thermal storage approach is 2.5 timesgreater because of the longer operating hours. A similarresult would be obtained by using a system that is smaller todeliver the same heat. See Figure 1 for a layout of the samplesystem and the summary of performance figures below:Location:TennesseeElectric Utility Rate: 0.049/kWHNatural Gas Utility Rate: 0.0243/kWH ( 7.12/MBTUH)Installed Capital Investment: 800,000.Heat Pump System COP:5.9Gas Boiler System COP:0.96Operating Hours per day:20Annual Operating Hours:6,257Annual Electric Op. Expense*: 156,100.Annual Avoided Gas Expense: 476,835.Simple Payback:2.2 Years* Ignores incremental maintenance costs for heat pumpcompressor versus a boiler which are considered to be inconsequential.Direct sanitation heatRequired Refrigeration (Heat Source):675 TRHeat Delivered:10,275 MBTUHEntering Water Temperature (EWT)60ºFLeaving Water Temperature (LWT)160ºFWater Flow Rate (GPM)200Annual Heat Delivered:25,717 MMBTUCompressor Nominal Swept Volume:600 CFMHP Consumed:682 HPLocation:TennesseeElectric Utility Rate: 0.049/kWHNatural Gas Utility Rate: 0.0243/kWH ( 7.12/MBTUH)Installed Capital Investment: 700,000.Heat Pump System COP:5.9Gas Boiler System COP:0.96Operating Hours per day:8Annual Operating Hours:2,503Annual Electric Op. Expense*: 62,435.Annual Avoided Gas Expense: 190,735.Simple Payback:5.5 Years* Ignores incremental maintenance costs for heat pumpcompressor versus a boiler which are considered to be inconsequential.190 F: Dairy HTST pasteurizationThermal storage sanitation heatRequired Refrigeration (Heat Source):Heat Delivered:Entering Water Temperature (EWT)Leaving Water Temperature (LWT)Water Flow Rate (GPM)Annual Heat Delivered:Compressor Nominal Swept Volume:HP Consumed:Two-stageRequired Refrigeration (Heat Source):Heat Delivered:Entering Water Temperature (EWT)Leaving Water Temperature (LWT)Water Flow Rate (GPM)Annual Heat Delivered:LS Compressor Nominal Swept Volume:HS Compressor Nominal Swept Volume:675 TR10,275 MBTUH60ºF160ºF20064,287 MMBTU600 CFM682 HPThe high temperature water demand for fluid dairy HTSTpasteurization (High Temperature Short Time) process issimilar to sanitation in that it is not a continuous demand,though it is required during processing so there is amplerefrigeration to provide the heat source. A thermal storageapproach is needed for reasonable payback periods. Theoperating discharge pressure required to deliver 190ºF waterwill approach 750 psig, which will prohibit the use of mostrefrigeration reciprocating compressors and many screwcompressors. Furthermore, with the high temperature of thedemand, the compression ratio approaches 4.0 with a differential pressure at or above 600 psid. For these reasons, anefficient two-stage system is likely to be more attractive despite the higher first cost capital investment. Also, the highdifferential pressure can be managed easier in two stagesthan in one. The second stage heat pump compressor, whichfaces high suction pressures, must be able to operate at lowinternal compression ratios to remain below the compressor’s discharge pressure limit. The following sums up thetwo-stage solution for this application. See Figure 1 for alayout of the single stage solution and Figure 2 for a layoutof the two-stage solution:650 TR9,670 MBTUH50ºF190ºF14060,507 MMBTU550 CFM300 CFM
Total HP Consumed:907 HPLocation:WisconsinElectric Utility Rate: 0.06/kWHNatural Gas Utility Rate: 0.0341/kWH ( 10.00/MBTUH)Installed Capital Investment: 1,250,000Heat Pump System COP:4.2Gas Boiler System COP:0.85Operating Hours per day:20Annual Operating Hours:6,257Annual Electric Op. Expense*: 254,200Annual Avoided Gas Expense: 711,840Simple Payback:2.7 Years* Ignores incremental maintenance costs for heat pumpcompressor versus a boiler which are considered to be inconsequential.Economic analysis approachIn today’s world, the cost and scarcity of energy resourcesdemand that all projects and operations be viewed throughthe lens of energy conservation and efficient use. As environmental impacts take a central place in the public debate,fossil fuel energy becomes the focus of projects to cut consumption or improve energy efficiency. It seems only naturalthat an owner of a refrigeration system where so much heatenergy is captured in the refrigeration cycle would seek waysto put that energy to use, rather than rejecting the energy toambient without any beneficial use.The payback period of the ammonia scavenging heat pumpsystem depends on the heating demand and the local utilityrates for gas and electricity. The ratios of typical utility ratesin the US are shown in Table 2, showing which markets havethe most favorable utility rates for heat pumps.The other side of the equation is the COP, or heating systemperformance. Lower temperature water demands allow forlower compression ratios and better heat pump COP. Boilersystem efficiencies will not vary greatly, but older boilerswill have lower efficiencies that are further eroded throughoperating wear and tear. Also, the distribution system willhave other steam losses that further lower the COP of theboiler system. When steam is leaked from the system, thesteam carries with it high temperature vapor which has alarge sensible heat input which is dwarfed by the heat ofvaporization that is wasted when the steam is released. Anaccurate measure of the delivered COP of the boiler systemis important in the economic analysis.Although energy efficiency, cutting waste, and environmental concerns will support interest in heat pump projects, inthe end it will be the economic analysis that determines ifthe project is done. As shown in Table 1, a COP of 2.5-10 isattainable in single stage operation depending on the qualityof heat that is required. The fairly low COP delivered in singlestage solutions for high quality heating applications can beimproved dramatically through the application of two stageheat pump systems. Through heat pump system design anda flexible compressor solution, all the absorbed heat can beapplied into the working fluid for maximum efficiency. Theresult is significantly lower energy consumption. For systemswith consistent high heating requirements, the paybackperiod will be 1-3 years. Even systems with intermittent heat-Regional utility ratesElectricGasElectric/( /kW) ( /kW) Gas RatioRegionEast North Central0.06290.01574.00East South Central0.05340.01334.03Mid w England0.13060.02844.61Pacific Contiguous0.07360.01604.61Pacific Non-Contiguous0.19100.04384.36South Atlantic0.06400.01763.65West North Central0.05370.01363.96West South Central0.06210.01394.48Most/least favorable statesMost FavorableUtility RatioStatesElectric ( /kW)Gas ( 5860.02412.43Montana0.05630.02212.55Least FavorableUtility RatioStatesElectric ( /kW)Gas ( t0.15290.02067.43Alaska0.14420.009814.72Source: www.eia.doe.gov US Energy Information Administration, 2010 ReportTable 27
ing requirements can benefit with 4-6 year paybacks. Thecontinued annuity of energy savings beyond the paybackperiod will often justify the capital expenses by meeting acompany’s economic return requirements, and the environmental benefits further strengthen the justification.Heat pump system designRefrigeration system designers set operating conditionsto increase compressor reliability and longevity by keeping discharge and oil temperatures low. But in a heat pumpthese temperatures are needed to deliver high-grade heat. Acompressors ability to tolerate these conditions reliably candefine its suitability and applicability to heat pump systems.Because the goal is to deliver high temperature heat, highdischarge temperatures are desirable and not cause forconcern. Similarly, compressor designers and system engineers usually limit oil injection temperatures to 120-130 F toensure high viscosity oil is available for bearing and compressor lubrication. With proper bearing and oil selection, the oilinjection temperature can be raised significantly to nearly200 F making it a much more useful heat source. For closedloop, high temperature heating applications where returnwater temperatures are above 130 F, the oil injection temperature can mean the difference between usefully applyingoil cooler heat (for example, 190 F oil injection) and havingto reject it without beneficial use (for example, 130 F oil injection). A compressor with lightly loaded bearings requireslower oil viscosity and allows for higher temperature oilinjection. Besides putting the oil heat to beneficial use, thehigher temperature oil injection allows the discharge gasesto reach higher temperatures, as well, yielding higher qualityheat pump compressor discharge heat content.A final difference in heat pump design from refrigerationdesign is the many heat sources in a heat pump system. Forrefrigeration, the cooling is done in simple heat exchangerorientations of series configuration or occasionally parallel heat exchangers. In heat pump systems where there areHeat pump heat sourcesFigure 38several temperature levels available (oil cooling, desuperheating, condensing, subcooling) the efficient delivery ofheat from the system to the working fluid depends on thecombination of heat exchangers. The performance of yourheat pump system is defined by the efficient design of theheat capture, transfer and delivery making these systemsmore complex.With scavenging heat pump systems, the heat pump sourceis the refrigeration system so there is a ready supply of ammonia vapor with its absorbed heat content. There are norequired “low side” heat exchangers to absorb the sourceheat into the ammonia vapor. However, there are other heatexchangers and the ammonia compressors involved in thesystem design. These are described below.Inlet gas desuperheaterIn a scavenging system, the superheated vapor is supplieddirectly from the compressor discharge of the host refrigeration system. This superheat must be lowered and controlledto allow the compressor to operate reliably. This source ofheat is low grade, defined by the host refrigeration systemcondensing pressure (85 F-95 F SCT). In some cases, thissource of heat cannot be put to use because of the low temperatures involved, but regardless it only represents about4-8% of the heat pump system heat available (See Figure 3).High pressure desuperheaterThe discharge from the heat pump compressors includessignificant superheat above the saturated condensing temperatures. This is the highest temperature available in theheat pump system and can be used to “top off” the deliveredwater temperature to levels slightly above the heat pumpcondensing temperature ( 2-4 F higher). In systems requiring moderate delivered water temperatures, the modestimprovement in COP may not justify the added capital for aseparate heat exchanger so the desuperheat and condensingcan be combined into one exchanger.For high temperature systems the high-pressure desuperheater is needed to reach the highest possible temperature for a given condensing temperature. It also limits therequired compressor discharge and differential pressures byallowing the compressor to operate at a lower condensingpressure. Because it is the highest-grade heat in the systemit can be extremely valuable, even though it only representsabout 4-8% of the heat pump system heat available (SeeFigure 3).
Heat pump condenserThe heat pump condenser delivers 65-75% of the heat tothe working fluid. It is a high-grade heat source since it ison the high side of the heat pump system, and only thehigh-pressure desuperheater can offer higher-grade heat.The heat pump condenser sets the condensing temperatureand defines how much the compressor must work to deliverthe heat. With more condenser heat exchanger surface, theapproach temperature is lowered allowing the compressor to operate at a lower discharge pressure and with lowerconsumed horsepower. The approach temperature will reacha practical limit when cutting compressor power does notjustify the cost for more surface.Compressor oil coolingIn a heat pump, rather than lowering the discharge temperature, the goal is to increase the heat content available with-out compromising compressor performance or reliability.Selecting a compressor that allows higher discharge temperatures and higher oil injection temperatures is importantto heat pump system performance. This source of heat is4-6% of the total heat available (See Figure 3). An oil coolingheat exchanger is a required part of a heat pump and worksto improve system performance.Heat pump subcoolerHigh-pressure liquid must be returned to the host systemfrom the heat pump condenser. Without subcooling, a largeamount of flash gas is generated as the liquid is flashed downto the host system discharge pressure. Once the liquid pressure is reduced to the host system pressure, the liquid is returned to the host high-pressure receiver and the vapor to thevent line. The flashing provides no beneficial heating effect.Pressure and temperature compressor limitsFigure 49
Flash gas can be eliminated with a subcooler. The heatavailable in a subcooler is of moderate grade as it starts atcondensing temperatures, but approaches the host systemdischarge temperature in a sensible cooling process. Theheat exchanger in the heat pump system provides significantheat pump COP improvement because it represents 8-15% ofthe total heat pump system heat available (See Figure 3).The refrigeration system discharge pressures that providethe source of the heat for a heat pump will range from 150225 psig. As the minimum compression ratio for commonammonia screw compressors ranges between 2.8 and 3.4(2.2-2.6 Volume Ratios), a compressor with 150 psig suctionwill deliver between 450 psig and 550 psig, regardless of theintended pressure required by the heating duty (See Table 3).Heat pump compressorWith high suction pressure and a minimum compressionratio, low minimum compression ratios and high dischargepressure limits are desirable attributes for a heat pumpcompressor.The compressor is the heart of the heat pump system. Thecapabilities of the compressor to withstand high dischargepressures, high differential pressures and tolerance of highdischarge and oil injection temperatures have significantimplications in heat pump system design. These challengesmust be taken into consideration when selecting a compressor for an industrial heat pump application.When the heating duty requires even higher temperatures,multi-stage systems may be required wit
of heat to the atmosphere. Heat pumps can capture this wasted heat efﬁ ciently and use it to cut the fossil fuels burned to heat water. These industrial heat pumps are environmen-tally friendly and economical, allowing operators to make the most of their energy resources. Ammonia heat pumps offer an owner a comprehensive green solution.