Cooling of a Reciprocating Compressor through Oil Atomization in the Cylinder

Transcription

1 Purdue Uiversity Purdue e-pubs Iteratioal Compressor Egieerig Coferece School of Mechaical Egieerig 2010 Coolig of a Reciprocatig Compressor through Oil Atomizatio i the Cylider Rodrigo Kremer Embraco Jader R. Barbosa Federal Uiversity of Sata Cataria Cesar J. Deschamps Federal Uiversity of Sata Cataria Follow this ad additioal works at: Kremer, Rodrigo; Barbosa, Jader R.; ad Deschamps, Cesar J., "Coolig of a Reciprocatig Compressor through Oil Atomizatio i the Cylider" (2010). Iteratioal Compressor Egieerig Coferece. Paper This documet has bee made available through Purdue e-pubs, a service of the Purdue Uiversity Libraries. Please cotact epubs@purdue.edu for additioal iformatio. Complete proceedigs may be acquired i prit ad o CD-ROM directly from the Ray W. Herrick Laboratories at Herrick/Evets/orderlit.html

2 1291, Page 1 Coolig of a Reciprocatig Compressor through Oil Atomizatio i the Cylider Rodrigo KREMER 1,2, Jader R. BARBOSA Jr.* 1, Cesar J. DESCHAMPS 1 1 POLO Research Laboratories for Emergig Techologies i Coolig ad Thermophysics Federal Uiversity of Sata Cataria, Departmet of Mechaical Egieerig Floriaopolis, SC, , Brazil 2 Embraco Compressors Joiville, SC, , Brazil * Correspodig Author (Phoe/Fax: , jrb@polo.ufsc.br) ABSTRACT We preset a experimetal aalysis of the ifluece of atomizatio of lubricat oil i the cylider of a reciprocatig refrigeratio compressor. Durig compressio, oil atomizatio ehaces heat removal from the refrigerat gas, which results i a temperature decrease of the compressor compoets. A prototype was costructed ad tested with R-134a i a fully istrumeted gas-cycle calorimeter. Experimetal results are preseted for the temperature distributio i the compressor compoets. The results are further explored i the light of a calculatio methodology based o macroscopic cotrol volume formulatios of the mass ad eergy coservatio priciples for the compressio chamber coupled with a overall compressor eergy balace. 1. INTRODUCTION A thorough uderstadig of the eergy trasformatios takig place iside compressors is ecessary for improvig their efficiecy ad of the refrigeratio systems of which they are a essetial part. Broadly speakig, the eergy losses i a compressor are divided ito (i) electrical, (ii) mechaical, (iii) thermodyamic ad (iv) cycle losses (Possamai ad Todescat, 2004). Thermodyamic losses ivolve the refrigerat flow iside the compressor. These are mostly associated with valve flows (viscous frictio ad flow reversal), gas leakage through the pisto-cylider gap ad refrigerat superheatig that occurs i the suctio process ad durig vapor compressio. Accordig to Ribas (2007), for a R-134a household refrigeratio compressor at the ASHRAE LBP coditio, aroud 50% of the thermodyamic losses are due to suctio gas superheatig. This superheatig causes a reductio of the volumetric efficiecy ad a icrease of the compressio work per uit mass (Gosey, 1982). May aspects of compressor coolig have bee addressed i the past. Dutta et al. (2001) ivestigated theoretically ad experimetally the effect of liquid refrigerat ijectio i scroll compressors usig R-22. A decrease i the vapor temperature durig compressio was observed. However, the extra work required to compress the vaporized refrigerat gave rise to a decrease i the compressor performace. The experimetal ad umerical studies of Coey et al. (2002) quatified the decrease i power cosumptio (approximately 28%) associated with the atomizatio of water i air reciprocatig compressors. Ooi (2005) evaluated umerically the ijectio of liquid refrigerat i a rotary compressor, takig ito accout the ifluece of the ozzle diameter ad its positio i the compressio chamber. He cocluded that it is more advatageous to iject the liquid shortly before the discharge of the gas to beefit from the latet heat trasfer without the pealty of compressig a volume of gas that is ot used for geeratig coolig capacity. Bojour ad Beja (2006) demostrated the existece of a optimal cofiguratio for the distributio of coolig water i a multi-stage ammoia compressor that miimizes the compressor power. Wag et al. (2007) ivestigated theoretically two performace improvemets for reducig the compressor power for several refrigerats. The first optio was to cool the motor by exteral meas other tha usig the suctio gas. The secod was to combie the processes of isothermal ad isetropic compressio. Their aalysis demostrated that the first optio is more advatageous for low back pressure (LBP) rather tha high back pressure (HBP) applicatios for

3 1291, Page 2 all refrigerats ivestigated. The secod optio showed that the compressio power ca be reduced by up to about 16% depedig o operatig coditios ad workig fluid. I previous work (Kremer et al., 2007, 2008), we preseted a heat trasfer model for the atomizatio of oil droplets i the cylider of R-134a ad R-717 reciprocatig compressors, respectively. Oil atomizatio promotes a icrease i the heat trasfer surface area which ehaces heat removal from the vapor. The resultig coolig effect also cotributes to lowerig the overall thermal profile of the compressor parts ad, cosequetly, the iitial compressio temperature. I quatitative terms, for R-134a, atomizatio of oil at 45 o C with a average flow rate of kg/h gave rise to a maximum temperature reductio of 25 o C at the top dead ceter (TDC). The iitial compressio temperature decreased from 58.8 to 44.6 o C, illustratig the large potetial for reducig losses due to refrigerat superheatig. I this paper, we report o experimetal results of the coolig effect of oil atomizatio i the cylider of a reciprocatig compressor. A prototype was costructed ad tested with R-134a i a fully istrumeted gas-cycle calorimeter. Experimetal results demostrate a sigificat reductio of the temperature levels i the compressor compoets i compariso with the baselie coditio. Parameters such as the coolig capacity, the compressor power ad the COP are compared with the correspodig values predicted by the umerical model. 2. MATERIALS AND METHODS 2.1 Compressor Preparatio The compressor prototype show i Fig. 1 has a split crakcase sealed with flat face bolt flages ad a silicoe rubber gasket. A commercial hollow-coe ozzle (Lechler ) was mouted flush o the wall of the compressio chamber. Due to the ozzle ad housig dimesios ad space restrictios, the distace betwee the ozzle orifice ad the valve plate is approximately 13 mm (see Fig. 1.c). The ozzle is fed by a pressurized 2-mm I.D. oil lie which is coected to the oil separator of the calorimeter loop (see Sectio 2.2). oil lie ozzle housig ozzle thread housig thread 13 mm orifice pisto (a) (c) oil lie (b) Figure 1: Compressor equipped with a oil ozzle. (a) Nozzle ad threaded housig. (b) Compressor compoets ad pressurized oil lie. (c) Positio of the orifice i the cylider. The temperatures of the compressor compoets were measured with 13 T-type thermocouples, as illustrated i Fig. 2. The temperature at which the oil eters the cylider is assumed equal to that of the ozzle housig. The temperature of the gas eterig the compressio chamber is assumed equal to that measured at the suctio chamber. Due to space restrictios, the temperatures of the cylider wall ad of the mai bearig are measured by isertig the thermocouples ito holes drilled i the compressor block so that the tip of the sesor is located at approximately 1mm of the surfaces of the cylider ad bearig, respectively. Although the ucertaity reported by the thermocouple maufacturer is ± 0.2ºC, the oe assumed i this study is ± 1ºC due to errors origiatig from the positioig ad attachmet of the thermocouples to the surfaces. The istataeous gas pressure i the cylider was

4 1291, Page 3 measured with a Kistler 601A absolute pressure trasducer with a samplig frequecy of 60kHz ad a associated ucertaity of ± 0.12 bar. Suctio Ilet Suctio muffler outlet Discharge chamber Nozzle housig Cylider wall Oil lie Crakcase gas Discharge mufflers Discharge outlet Suctio muffler ilet Mai bearig Shell Oil Figure 2: Compressor iteral compoets ad poits of temperature measuremet. The ifluece of the split crakcase, atomizatio ozzle ad pressure ad temperature sesors o the compressor performace has bee evaluated by comparig the refrigerat mass flow rate of the modified compressor prototype, but without oil atomizatio, with that of a compressor without the modificatios. A reductio of the order of 10% i the mass flow rate has bee observed. However, this does ot represet a source of error sice the modified prototype (without oil atomizatio) is the baselie for quatifyig the ifluece of oil atomizatio o the compressor temperature profile. 2.2 Superheated Gas Cycle Calorimeter The compressor was tested with R-134a i a superheated gas cycle calorimeter described schematically i Fig. 3. The compressor discharge pressure was regulated via a had-operated eedle valve (DV). A oil separator was istalled i the discharge lie, ad was kept at a temperature above the saturatio temperature of the refrigerat at the discharge pressure by meas of a electric heater wrapped aroud its exteral surface. Dowstream of DV, at a itermediate pressure betwee suctio ad discharge, a accumulator was istalled to dampe pressure oscillatios. A Coriolis-effect meter (MicroMotio) is used to measure the mass flow rate ad, dowstream of it, aother hadregulated eedle valve (SV) is operated to expad the gas from the itermediate pressure dow to the desired suctio lie pressure. The experimetal error associated with the mass flow measuremet is ±0.15%, accordig to the maufacturer. A fixed ilet temperature of 32ºC is set by a electric heater. The suctio ad discharge pressures are measured with HBM P3MB absolute pressure trasducers (10 ad 50 bar full-scale for the suctio ad discharge pressures, respectively) ad the temperatures are measured with T-type thermocouples. The experimetal ucertaity of the suctio ad discharge pressure measuremets were estimated at ± bar ad ± 0.1 bar, respectively. The compressor power cosumptio was measured with a Yokogawa WT210 electric power trasducer with a estimated error of ± 3%. After beig separated from the discharge gas, the oil is drive through the oil lie via a thermostatic bath for temperature cotrol before beig re-ijected ito the compressor, if valve OV is ope. The experimetal apparatus is fully itegrated with a sigal coditioig ad data acquisitio module. 2.3 Experimetal Procedure The experimets were performed with R-134a at evaporatig ad codesig pressures correspodig to -27 o C ad 42 o C. The room temperature was maitaied at 25 o C. Tests were carried out with ad without oil atomizatio ad, i the latter, the oil ijectio temperatures was 45 o C. The experimetal procedure is as follows. Refrigerat is charged ito the calorimeter uder vacuum of 0.04 mbar eeded to remove moisture ad dissolved gases from the oil. Up to 6 hours are eeded for steady-state ad, i this period, the eedle valves DV ad SV are costatly adjusted to maitai the discharge ad suctio pressures to withi ± 1% of the specified codesig ad evaporatig pressures. I the tests with oil atomizatio, the temperature of the thermostatic bath is also cotiuously adjusted so as to keep the oil atomizatio temperature withi the specified boudaries. The oil flow rate was ot measured directly durig the experimets. However, based o umerical aalyses of the ozzle flow via CFD (Kremer, 2006) ad

5 1291, Page 4 idepedet tests carried out outside the compressor with a costat ozzle pressure drop, with a oil ijectio temperature of 60 o C, the oil flow rate was estimated at approximately 1± 0.2 kg/h. Massflow meter Thermocouple Accumulator DV Pressure trasducer Cotroller Electric heater SV OV Thermostatic bath 45,0ºC Oil separator Compressor Figure 3: Schematic diagram of the superheated gas-cycle calorimeter. The steady-state criterio establishes that, over a uiterrupted 45 mi iterval, the variatio of the temperature readigs should be less tha 1 o C ad the variatio of the measured compressor power ad calculated coolig capacity (i.e., the product of the measured vapor mass flow rate ad latet heat of vaporizatio at -27 o C) should be less tha 1%. Whe this coditio is met, the temperature, mass flow rate ad compressor power data are acquired ad averaged for the ext 10 mi ad the test is eded with the acquisitio ad averagig of 50 pressure-volume cycles. A error propagatio aalysis (Kremer, 2006) yielded ucertaities of ±4 ad ±5% for the coolig capacity ad coefficiet of performace, respectively. At least five tests were carried out for each coditio ad the results reported i this paper cosist of arithmetic averages of the data at each specific coditio. Repeatability tests cofirmed that the dispersio of the data with respect to the ormal distributio was small, with deviatios less tha 3.5% of the average values. 3. MODELING The mathematical model has bee preseted elsewhere (Kremer et al., 2007; 2008), so oly its mai features will be discussed here. The cotrol volume formulatio of the vapor compressio process cosists of mass ad eergy balaces give by (Ussyk, 1984), dm m m s m d m l m sr m dr (1) du dv dm m Q o haw T w T p u m d m l m srh m shsc m drhdc (2) where the istataeous cylider volume is give by, V V dv c 4 3 dv c m N m 0 3 m, Rm R 3 m dn m, (3) (4) where V c ad its time differetial are calculated from a algebraic relatioship for the pisto positio as a fuctio of the crakshaft agle (Ussyk, 1984). The compressio cycle is divided ito time steps of size Δt ad the droplets ijected ito the cylider at a specified time-step (a so-called family ) are assumed idetical with respect to their size, velocity ad temperature. I Eq. (4), N m, is the umber of droplets that were atomized ito the cylider at a

6 1291, Page 5 give istat m (the m th family) ad still are i the cylider at a istat > m. N m, is computed via a populatio balace described i more detail by Kremer et al. (2007) ad Kremer (2006). Sice the vapor pressure of the oil is small, it is assumed that oly sesible heat trasfer takes place betwee the droplets ad the vapor. Temperature gradiets iside the droplets ad thermal iteractio betwee droplets are igored. Thus, the temperature of a droplet i group m at a istat is calculated from, dt m, 3hm, c o po R m T T m, (5) where h m, is the heat trasfer coefficiet betwee the gas ad droplets calculated from the Raz ad Marshall (1952) correlatio. The heat trasfer rate betwee all oil droplets ad the vapor is give by Q 2 o 4R m Nm hm, Tm, T (6) m 0, The i-cylider vapor compressio ad droplet heat trasfer models were icorporated ito a existig overall heat trasfer model that itegrates several compressor compoets (Fagotti et al., 1994). Steady-state eergy balace equatios were writte for each compoet ad solved together with the trasiet i-cylider vapor compressio model. The suctio ad discharge processes were calculated via a oe-degree-of-freedom model with atural frequecy ad dampig coefficiets specific for each valve. The resultig forces o the valves ad their respective flow rates are obtaied with effective force ad effective flow areas derived from umerical simulatios (Matos, 2002). I the oil atomizatio cases, the total flow rate through the discharge valve is calculated assumig a homogeeous two-phase desity based o the discharge mass fractio of each phase (Kremer et al., 2007). The reader is also referred to Kremer (2006) for a discussio o the umerical solutio of the model equatios. 4. RESULTS AND DISCUSSIONS Table 1 shows the temperatures measuremets at the positios idicated i Fig. 2 for the baselie (without oil atomizatio) ad oil atomizatio coditios, respectively. As ca be see, a sigificat temperature decrease due to oil atomizatio at 45ºC is observed, especially at the hottest spots such as the discharge chamber, discharge mufflers ad cylider wall. More importatly, this overall coolig effect is exteded to the compressor parts associated with the suctio gas superheatig (suctio ilet ad suctio muffler), which cotributes to reducig the thermodyamic losses. The P-V diagrams for the cases with ad without oil atomizatio are preseted i Fig. 4. The compressio ad expasio regios behave very similarly i both cases, with the mai differeces appearig i the suctio ad discharge processes, as ca be see from Figs. 5 ad 6. Oil atomizatio icreases the eergy expediture to take i ad discharge the gas, which raises the compressio power ad hece the cycle mea effective pressure. With oil atomizatio, the suctio losses calculated based o the data of Fig. 5 icrease by 17% with respect to the baselie. This icrease ca be attributed to a delay i the suctio valve opeig which gives rise to the more proouced troughs i the pressure sigal durig the suctio process. The delay itself ca be caused by a icrease i valve stictio (Khalifa ad Liu, 1998) due to more oil betwee the valve ad seat, or by chages i the pressure pulsatio patters i the suctio muffler. By the same toke, the discharge losses i the case with oil atomizatio icrease by 57% with respect to the baselie (Fig. 6). The icrease i the discharge pressure ca be associated with (i) a larger flow restrictio durig discharge due to more oil betwee the pisto ad the valve plate, (ii) a larger mass of refrigerat beig discharged due to the lower temperatures alog the suctio path, ad (iii) stictio pheomea i the discharge valve. Figure 7 shows the calculated valve displacemet durig discharge as a fuctio of the crak agle for the baselie ad atomizatio cases. The cylider pressure is also show for compariso purposes. As ca be see, i both cases, the discharge valve opes at degrees from the bottom dead ceter (BDC). However, the pressure i the cylider reaches the discharge value earlier for the oil atomizatio case. Thus, by comparig the pressures at which

7 1291, Page 6 the valve opes i both cases, oe cocludes that there is a delay of approximately 1 degree i the oil atomizatio case. Table 1: Temperatures of the compressor compoets with oil atomizatio. Compoets Temperatures [ºC] Baselie (o atomizatio) Oil atomizatio Differece [ C] Ambiet (room) temperature Suctio ilet Suctio muffler ilet Suctio muffler outlet Cylider wall Discharge chamber Discharge mufflers Discharge outlet Crakcase gas Mai bearig Shell Oil sump Oil lie Nozzle housig Baselie Oil atomizatio - 45 o C Discharge pressure Suctio pressure 1.4 Pressure/Suctio pressure Pressure/Suctio pressure Volume/Displaced volume Figure 4: P-V diagram. 0.6 Baselie Oil atomizatio - 45 o C Volume/Displaced volume Figure 5: P-V behavior i the discharge regio. Figure 8 shows calculated heat trasfer from the vapor as a fuctio of the crak agle for atomizatio of oil at 45 o C, with a average flow rate of kg/h. The heat trasfer durig compressio icreases more sharply tha the baselie ad becomes positive earlier i the cycle showig a strog ifluece of droplet heat trasfer. Durig expasio, heat trasfer is similar i both cases (with ad without oil) as a result of the very low velocity of the droplets remaiig i the clearace volume after discharge. Figure 9 reveals a sigificat reductio of the vapor temperature (maximum of 25 o C at the TDC) with oil atomizatio. The iitial compressio temperature decreases from 58.8 to 44.6 o C, illustratig the large potetial for reducig losses due to refrigerat superheatig. Table 2 compares the experimetal ad calculated coolig capacity, compressor power ad coefficiet of performace for the baselie ad oil atomizatio cases. Although a satisfactory agreemet is observed betwee model ad experimets, the icrease i the coolig capacity due to oil atomizatio i the cylider is overshadowed

8 1291, Page 7 by the icrease i the compressor power, which leads to a reductio i the COP. A umber of pheomea ca be associated with this uwated icrease i the compressor power, amely, the icrease i the oil viscosity due to the compressor coolig, valve losses etc. All of these effects ca be dealt with a improved desig to make the coolig solutio more attractive. Pressure/Suctio pressure Baselie Oil atomizatio - 45 o C Discharge pressure Displacemet [mm] Displacemet - Baselie Displacemet - Oil atomizatio Pressure - Baselie Pressure - Oil atomizatio Pressure/Discharge pressure Volume/Displaced volume Figure 6: P-V behavior i the suctio regio Crak agle [degrees] Figure 7: Discharge valve displacemet. Heat Trasfer to Vapor [kj/kg] Baselie Oil Atomizatio Temperature [ o C] Baselie Oil Atomizatio Crak Agle [degrees] Figure 8: Heat trasfer as a fuctio of the crak agle Volume/Total Displaced Volume Figure 9: T-V diagram. Table 2: Temperatures of the compressor compoets with oil atomizatio. Experimetal Calculated Q e [W] W c [W] COP Q e [W] W c [W] COP Baselie Atomizatio CONCLUSIONS I this paper, we preseted a experimetal aalysis of the ifluece of atomizatio of lubricat oil i the cylider of a reciprocatig refrigeratio compressor. The experimetal results showed a sigificat decrease i the compressor thermal profile due to a ehaced heat removal from the refrigerat gas durig compressio. The results were i satisfactory agreemet with a mathematical model but, although some icrease i the coolig capacity was observed,

9 1291, Page 8 the coefficiet of performace decreased as a result of higher compressor powers i the case with oil atomizatio. Desig strategies could be sought to reduce the thermodyamic losses ad make the coolig solutio more efficiet. NOMENCLATURE h specific ethalpy (kj/kg) T temperature ( o C) l Leakage h heat trasfer coefficiet (W/m 2 K) V volume (m 3 /s) o Oil m mass of vapor (kg) r reflux M mass of a sigle oil droplet (kg) Subscripts s suctio m mass flow rate of vapor (kg/s) c cylider t total M mass flow rate of droplets (kg/s) d discharge w Wall REFERENCES Bojour, J., Beja, A., 2006, Optimal Distributio of Coolig Durig Gas Compressio, Eergy, vol. 31, p Coey, M.W., Stepheso, P., Malmgre, A., Liema, C., Morga, R.E., Richards, R.A., Huxley, R., Abdallah, H., 2002, Developmet of a Reciprocatig Compressor Usig Water Ijectio to Achieve Quasi-Isothermal Compressio, Proc. 16 th It. Compressor Egg. Cof. at Purdue, CD-ROM. Dutta A.K., Yaagisawa, T., Fukuta, M., 2001, A Ivestigatio of the Performace of a Scroll Compressor uder Liquid Refrigerat Ijectio, It. J. Refrig., vol. 24, p Fagotti, F., Todescat, M.L., Ferreira, R.T.S., Prata, A.T., 1994, Heat Trasfer Modelig i Reciprocatig Compressors, Proc. 12 th It. Compressor Egg. Cof. at Purdue, pp Gosey, W.B., 1982, Priciples of Refrigeratio, Cambridge Uiversity Press. Khalifa, H. E, ad Liu, X., 1998, Aalysis of Stictio Effect o the Dyamic Compressor Suctio Valve, Proc. 14 th It. Compressor Egg. Cof. at Purdue, p Kremer, R., 2006, Theoretical ad Experimetal Aalysis of the Ifluece of Oil Atomizatio i Compressio Processes, M.Eg. dissertatio, Federal Uiversity of Sata Cataria (i Portuguese). Kremer, R., Barbosa Jr., J.R., Deschamps, C.J., 2007, Theoretical Aalysis of the Effect of Oil Atomizatio i the Cylider of a Reciprocatig Compressor, Proc. It. Cof. o Compressors ad their Systems, Lodo, paper C Kremer, R., Barbosa Jr., J.R., Deschamps, C.J., 2008, Theoretical Aalysis of the Effect of Oil Atomizatio i the Cylider of a Reciprocatig Ammoia Compressor, Proc. 19 th It. Compressor Egg. Cof. at Purdue, Paper Matos, F.F.S., 2002, Numerical Aalysis of the Dyamic Behavior of Reed-Type Valves i Reciprocatig Compressors, D.Eg. thesis, Federal Uiversity of Sata Cataria (i Portuguese). Ooi, K.T., 2005, The Effects of Liquid Ijectio o the Performace of a Rotary Compressor, Proc. It. Cof. o Compressors ad their Systems, Lodo, p Possamai, F.C., Todescat, M.L., 2004, A Review of Household Compressor Eergy Performace, Proc. 17 th It. Compressor Egg. Cof. at Purdue, CD-ROM. Raz, W.E., Marshall, W.R., 1952, Evaporatio from Drops I ad II, Chem. Eg. Progr., vol. 48, p. 141 ad 173. Ribas Jr., F.A., 2007, Thermal Aalysis of Reciprocatig Compressors, Proc. It. Cof. o Compressors ad their Systems, Lodo, p Ussyk, M.S., 1984, Numerical Simulatio of Hermetic Reciprocatig Compressors, M.Sc. diss., Federal Uiversity of Sata Cataria (i Portuguese). Wag, X., Hwag, Y., Radermacher, R., 2008, Ivestigatio of Potetial Beefits of Compressor Coolig, Appl. Thermal Eg., vol. 28, p ACKNOWLEDGEMENTS The material preseted i this paper is a result of a log-stadig techical-scietific partership betwee the Federal Uiversity of Sata Cataria (UFSC) ad Embraco. The authors are idebted to FINEP ad CNPq through Grat No / (Natioal Istitute of Sciece ad Techology i Refrigeratio ad Thermophysics).