Dynamic Model and Performance of Absorption Heat Pump in Shut-down Process

36Journal of Dong Hua University (Eng. Ed.) Vol. 19 No.4 (2002)Dynamic Model and Performance of Absorption Heat Pump inShut- down ProcessWANG Lei(王磊)Cllge of Ervironment Science and Engineeing, Dong Hua Univesiy, Shanghai , 2001LU Zhen(陆震)Insiue of Energy and Power, Shanghai Jiaotong Univrsity, Shanghai, 20030The dynamic model of LiBr absorption heat pump ittemperature and solution concentration in these components anshut down process is established. The simulation resultsanalyzed in detail. The purpose of studying the shut-down processshow good agreement with the experiments. The dynamicof LiBr absorption heat pump is to reduce the shut-down time andperformance of high-pressure generator, low-pressurethe energy consumption without crytallization. The emphasis isgenerator and heat exchanger are analyzed in detail. Thethe change of solution in some components.proper shut-down mode of the heat pump is presented,which, in consideration of solution parameters, has aDynamic Model of Absorption Heat Pumpgreat effect on the possibility of crystallization of sormecomponents.1 The shut-down modeKeywords absorption heat punp，shut-down process， LiBr/The shut-down process of a dirsct-fired absorption heat pumpH2Oof double effct refrigeration cycle with reverse series solutioncircuit is simulated. The shut-down mode is performed in thefollowing onder: stop of the bumer first, after some time, shuttingIntroductionoff the circulating solution pump and the refrigerant pump, shuttingoff the cooling water purmp and the chlled water pump, tum ofAbsorption heat purmp is widely used in many fields. It hasthe overall power at last. The dynamic prformance is referred tothe advantage of not using CFCs refrigerant and having no ham tothe process in which the state of heat pump changes from steady tothe environmnent. LiBr- H2O solution, as the working pair in thethe shut off.absorption heat pump， has the shortcoming of the possible of2 Model of high pressure generator (HPG)crytallization. If the temperature of the solution is lower than theThe shut-down process in HPG can be considered two stages.saturated temperature, the solubility decreases and the crystal ofIn the first stage, alhough the heat is not supplied, the vapor islithium bromide separates out of the solution， then thesill generated owing to whose own heat capacity. In the secondcrysalization occurs. This phenomenon takes place with highstage, no vapor is generated.probability under shut-down process. As the heat soure is notSolution mass balance equation:supplied, the highly concentrated solution has a tendency ofcrystallization when it is cooled. To avoid this phenormenon, whenoe= ma- meo- mw(1)the bumer is stopped in shut-down process, the circulating solutionpump and the refrigerant pump should keep running，so that themhy is greater than 0 in the first stage, equals to 0 in the secondsolution in the heat pump is circulated and the concentratedstage.solution in some components is diluted. The shut-down processLiBr mass balance equation:can not be stopped until the solution in the machine is mixedd(G. x)= mu. xhi一mhw' Sh(2)sufficiently. The running time is usually determined byd:experience, which is quite uncertain. Therefore, it is importantThe average solution concentnation in the HPG is substiwuted by theto sudy the shut-down performnance for reducing the running timeoutlet solution concentration.and the energy consumption. Zhuo21， Stephan'3 J discussed theVapor mass balance equation:different shut-down mode of the heat ransfomer. Anand-4JdGmanalyzed the stages of the shut-down process. In this paper, thedr=maw-mmo(3)dymamic performance under shut-down process is investigated bysimulation and experiment. The dynamic model of the heat pumpmhno中国煤化工x beween LPG and HPG.has been established and the simulation results show goodIn thin the second stage, mmwoagreement with experiments. The changes of pressure, soluionvariesMYHCNMHGThis paper is supported by the UTC RongHong foundation.Recived Dec.28, 2001Jourmnal of Dong Hua University (Eng. Ed.) Vol. 19 No.4 (2002) 37Solution energy balance equntion:Model of solution heat exchangermai"hai = mho.hw+ mho" h +d(G . hoe)+ Q.+ QanThe model of solution heat exchanger under shut-downlτprocess is similar to the startup model5. The high temperatureneat exchanger (HTEX) and low temperature heat exchangerThere is no heat radiation in the hearth. The heat transfer between. (LTEX) are both shl-ube heat exchangers. The highthe smoke and the tube bundle of the solution is ignored becausetemperature concentrated solution is cooled at the shell side, andthe heat capacity of smoke is negligible compared to the heatthe low temperature diluted solution is heated at the tube side.Under the shut-down process, the cool flow rate of the highcapacity of the shell and the solution.temperature solution and the heat flow rate of the low temperatureVapor energy balance equation;solution decrease gradually.d(G . hee)t= mm" hm- mheo' hwo(5)Dynamic Performance under Shut-downVapor and soluion pressure equation:ProcessdPre_ dpwi_ dpwe(6)dr dr dr1 Performance of high pressure generatorState equations of vapor and solution should be added for solving(HPG)above equations.When the bumer is stopped, the solution is still beated owing3 Model of low pressure generator (LPG)to the large bheat capacity of the shell and the solution although noThe parameters of LPG are related to HPG, high temperatureflame radiation heat exists. The decreasing of heat flow rate resultseat exchanger ( HTEX)， low temperature heat exchangerin reduction of the rate of vapor generation. The vapor still flows(LTEX), condenser and absorber. The shut-down process in LPGinto LPG as a result of pressure dfference between HPG and LPG.can be divided into three stages. In the first stage, low-pressureThe decreasing of the volume of vapor in the container bringsvapor is generated. When the high-pressure vapor generated inabout the pressure drop. The temperature of the solution and theHPG goes into LPG, the solution is heated, and then low presseshell decreases simultaneously. The lower the pressure is, thevapor is generated. In the second stage, the heat of the high-easier the vapor is generated. Therefore in this case vapor ispressure vapor is not sufficient for generating vapor; hence no low-generated at lower pressure and lower temperature. The inletpressure vapor is generated. In the hird stage, no high-pressuresolution is sill concentrated continuously but the overallvapor goes into LPG.concentration decreases. Not until is the overall temperature lowerThe mass balance equations of solution and vapor, the energythan the balance temperature corresponding to the inletbalance equation of vapor and the pressure balance equation ofconcentration and the pressure of HPG, no vapor is generaled andvapor in LPG are similar to the equations of HPG.the inlet concentration is equal to the outlet.Solution energy balance equation:As shown in Fig. 1，when there is no heat supply, thed( Gghwo)= mlhms- mwhwo- mlhn+ Qhe (7)pressure in the HPG responses rapidly, the delay time is no moredthan 0.5 minute. The pressure decreases sharply from 842 kPa to373 kPa in 10 minutes. Fig.2 shows the change of the outletIn the first stage, mv and Qng are both greater than 0; in thesolution tempernture of HPG. The descend process is more gentle.second stage, my is equal to0, Qhe is greater than 0; in the thirdTo decrease from 150C to 90C, it needs 30 minutes with delaytage, mw and eng are both equal to 0.time of 5 minutes. Fig.3 shows the change of concentration ofMass balance equation of high pressure vapor in the heatingoutlet solution of HPG. The concentration decreases after 5ubes:minutes. Fig 4 shows the change of vapor generating rate of HPG.dGpmui- mao = mam+dr(8)The generating rate decreases rapidly after 2 minutes.00 rVapor energy balance equation in heating tubes:d( Ghn)= (mm- mo)hw- mashaw- Qxe (9)80 Fsimulation50 FVapor pressure equation in heating tubes:-- experimentdpet. d(Peo- p)drFJr(10)中国煤化工心心Pessure does not drop along the tube line. The friction resistanceYHCNMHG304and the local resistance are considered at the outlet of the heatingTime/mintube.Fig.1 Pressure of HPGState equations of vapor and soluion should be added forsolving above equations.Jourmal of Dong Hua University (Eng. Ed.) Vol. 19 No.4 (2002)00 r)0 rsimulation150000texperiment0上一一experiment0 102004(50 L3040Tim/minTime/minFg.2 Tamp. of outle solution of HPGFig.5 Tem. of outlet solution of LPG0.00850 F0.00610 t0.0040.00210Fig.3 Conc. of oullet solution of HPGFig.6 Vapor gererating rate of LPG0.010In shut-down process, the volume and the pressure of highpressure vapor as the heat source decrease. The decreasing of bheat0.009to solution resuts in the generating rate of low-pressure vapor droprapidly. The solution temperature decreases slowly owing to thee 0.004heat capacity of the shell and the solution. When the temperatureof solution is lower than the balance temperature corresponding tothe inlet concentration of solution and the pressure of LPG, the0.0002vapor is not generated again. This phenomenon was seen at thellst minute.3 Performance of solution heat exchangerFig.4 Vapor ganeraing ralte of HPGIn the HTEX, the middle concentrated solution from LPGThe difrence between the simulation and the experiment mayflows on tube side and the highly concentrated solution from HPGbe caused by the position of the inlet and the outlet of the solution.flows on shell side. Fig 7 shows the change of inlet and outletIn the mumerical model, the solution flows from one side to anobhertemperature of solution at shell side. With the decreasing of theof HPG. In the actual situation, the ideal single oriented flow doesoutlet temperature of solution of HPG, the temperature differencenot exist because of the bigger size of the HPG cootainer, comparedbetween the shell side and the tube side decreases, so decreases thewith the inlet and the outlet which are not at same level. There isheat transfer rate. After the bumer stopped for 20 minutes, thvortex in HPG. In the vortex, the heat cannot be taken out by thedifference between inlet and outlet tube side temperabure is 45C.heat convection but only by the conduction, so the decreasing rateof measured temperature is lower than that of simulation and the200simulation-- experimentsolution uniformity is not ideal.之1502 Performance of low pressure generatoroutlet Tem.(LPG)00上.In LPG, the high-pressure vapor from HPG heats the dilutedsolution, which is around the pipes, and then the lowpressurevapor is generated. The high-pressure vapor is condensed in the中国煤化工pipe, and vapor evaporates at the outlet of the pipe, then becomesMYHCNMHGthe mixture of vapor and liquid. Fg.5 shows the change of theoutlet solution temperature of LPG. Fig.6 shows the change of theFig.7 Tem. of inlet & outlet solution of HTEX atvapor volume, which is generated in LPG. The heat supplied toshell sideLPG comes from HPG, so its dynamic response is later than HPG.Journal of Dong Hua University (Eng. Ed.) Vol. 19 No.4 (2002) 39Fig.8 shows the change of inlet and outlet solution termperaure onube side. When the bumer is stopped for 20 minutes, the outletNomenclatureshell side solution concentration is 58. 9%，and its temperature is52.2C which is higher' than the crysalization temperaturem mass flow rnate, kg/s(13.0C) at this concentration. In the experiment process, theG mass of working fluid in a container, kgliquid surface of HPG and LPG is not stable completely, the flowconcentrationrate of the solution getting into HPG and LPG change sometimes,enthalpy, kJ/kgso the temperature of experimental result has some disurb.time, sconvection heat transfer rate, V20 rsimulation - +experimentQn heat capacity of the shell, WQng bheat flow rate from heating twbe, W80 |inlet Tem.pressure, kPa60 tpressure loss, kPaSubscriptsoutlet Tem.20high presure generator010low pressure generatorheating tubesTime/mininletFig.8 Tem. of inlet & outlet solution of LTHEX aoutlettube sidesolutionvaporConclusionsWw waterIn absorption heat pump, the part where aysallization occursReferencesmost probably is the outlet at shell side of LHEX. At this positionhere is the solution with the highest concentration and the lowest[1] Dai Y.Q, Geng H. B.. Lu z. a al. The technology andapplicarion of LiBr absorprion nefrigeration, Mcchanical Industrytempenature. Since the temperature of general environment isPress, Bejing, 1996, 177- 182bigher than 13.0C, the absorption heat pump will not crystallize[2] Zhuo C. Z., Absorprion Heat Trunsfomer with TFE-Pyr as thewhen sutting down the heat pump 20 minutes after the burmerWorking Pair, Ph. D.，Delft Univ. of Technology, Thestopped according to the experiment.Nehcrlands. 1994When the bumer has stopped for 20 minutes, the performance[3] Stephan K., Dynarmnics of a heat transfomer workcing with theofthe heat pump is far from the stable state. If the circulatingmixpure NAOH-H2O, 19th hnuermational congress of refrigeration,solution pump and the refrigerant pump continue running, th1995, IVb, 1254- 1261solution in the heat pump goes on mixing. Afer the bumer stoppedAnand D. K., Transient simoulation of aboption machine, Joumalfor 38 minutes, the inlet and the outlet solution temperature isof solar energy engineering, 1982, 104, 197 - 203nearly equal, which indicates that the solution is mixed[S] LuZ., ZhouJ. s., Fan L.. a al., Startup performance of dirctfired absorption cillr，, Jowmal of Shanghai Jaotong Universiy,completely. This moment the solution temperature is still higher1999, 11, 1514- 1 516than the environment. The beat transfer to the environment goeson, until the two temperatures are the same and the stable state iseached.Acording to the experiment, if the sbut-down process is fortheshort term, the process can be finished 20 minutes after thebumer is stopped, while crytallization will not occur in the bheatpump. The residual refrigerant water can be kept in the water tankof the evaporator, which can reduce the time of next start-up ofheheat pump. If the shut-down is for the long term, such asthrough winter, which means that the environment temperaturemay be lower than 13.0C, the shut- down process can be finished中国煤化工38 minutes after the bumer stopped. Pumping the residualrefrigerant water from the water tank to the absorber is an effectiveMHCNMHGmethod to reduce the shut- down time. In this case, the shut-downtime is related to the volume of the refnigerant water in the tank.