The Characteristics of Tangential Leakage in Scroll Compressors for Air-Conditioners

Transcription

1 Prde University Prde e-pbs nternational Compressor Engineering Conference School of Mechanical Engineering 2 The Characteristics of Tangential Leakage in Scroll Compressors for Air-Conditioners N. K. Cho Korea University Y. Yon Korea University B. C. Lee L. G. Electronics M. K. Min Korea University Follow this and additional works at: Cho, N. K.; Yon, Y.; Lee, B. C.; and Min, M. K., The Characteristics of Tangential Leakage in Scroll Compressors for Air- Conditioners (2). nternational Compressor Engineering Conference. Paper This docment has been made available throgh Prde e-pbs, a service of the Prde University Libraries. Please contact epbs@prde.ed for additional information. Complete proceedings may be acqired in print and on CD-ROM directly from the Ray W. Herrick Laboratories at Herrick/Events/orderlit.html

2 The Characteristics of Tangential Leakage in Scroll Compressors for Air-conditioners Nam-Ky Cho\ Yong Yon\ Byng-Chl Lee 2, and Man-Ki Min 3 1 HV AC & Refrigeration Laboratory, Gradate School of Korea University 2 Digital Appliance Laboratory, LG Electronics Co. Ltd. 3 Department of Mechanical engineering, Korea University Abstract Althogh the leakage loss occpies a considerable portion of the total loss in a scroll compressor, stdies on the leakage phenomena nder the actal operating conditions are insfficient becase of the complicated shape of a scroll compressor. Therefore in this present stdy, we exected two researches for obtaining basic data abot the tangential leakage phenomena at radial clearance. Firstly, experiments were performed to fmd the characteristics of the tangential leakage between pockets in a scroll compressor nder actal operating conditions. The qantities of tangential leakage between pockets cold be measred with orbiting angle, operating condition, and radial clearance between the fixed and the orbiting scroll. Especially, the flow coefficient was yielded from the experimental vales. The flow coefficient vale of tangential leakage in a scroll compressor was always constant nder the condition of constant vale of oil concentration regardless to the shape of leakage path, the radial clearance, and the pressre condition. Later, simlations which employed this flow coefficient were condcted to examine the inflence of tangential leakage on the performance of a scroll compressor nder varios conditions. Throgh this analysis, we were able to fmd ot the qantity of tangential leakage nder actal operating conditions and the inflence of tangential leakage on the performance of a scroll compressors. Nomenclatre A c k m; m leak.tan mideal m m l R Leakage area(m 2 ) Flow coefficient Specific heat ratio Theoretical mass of refrigerant on completion of sction (kg) Mass of tangential leakage to sction pocket dring the sction process(kg) deal mass flow rate of refrigerant {kg/sec) Mass flow rate of refrigerant by leakage loss (kg/sec) Mass flow rate to test compressor inlet (kg/sec) Leakage mass flow rate at high-pressre part (kg/sec) Leakage mass flow rate at middle-pressre part (kg/sec) Leakage mass flow rate at low-pressre part (kg/sec) Bypass mass flow rate at Pd pocket bypass valve (kg/sec) Bypass mass flow rate at P1 pocket bypass valve (kg/sec) Bypass mass flow rate at Pz pocket bypass valve (kg/sec) Pressre of pstream (Pa) Pressre of down stream (Pa) Gas constant of refrigerant (kj/kg mol K) Temperatre of pstream (C) 87

3 ntrodction The higher standard of living makes not only a high performance of air-conditioner bt also lower noise and vibration important. A scroll compressor, which meets these needs is bilt in the room and packaged air-conditioners. The scroll compressor attains the lower noise and redced vibration by virte of low driving torqe variation. Also the scroll compressor has many merits; lower pressre loss dring the sction process, little re-expansion, and a simplicity of strctre. Bt the scroll compressor has a difficlty in machining the scroll wrap compared with a reciprocating compressor and a rotary compressor. And special mechanism is needed for preventing the leakage of high-pressre refrigerant. The back-pressre mechanism and the tip-seal mechanism are applied for preventing the radial leakage at axial clearance and the variable radis crank mechanism is sed for the tangential leakage at radial clearance. Becase leakage loss occpies a considerable portion of the total loss in a scroll compressor, researches on the mechanism for redction of leakage have been made.<'h 5 > Bt stdies on the leakage nder the actal operating conditions are insfficient becase of the complicated shape of a scroll compressor. Therefore, in this research we made a test-compressor which has a little changes in real compressor and realized a real leakage nder the actal driving condition. And we developed the indirect flow measrement method becase of impossibility of measring the leakage directly. Tangential leakage mass flow rate in radial clearance is measred by this method and flow coefficient is picked ot from the experimental vales. Using this flow coefficient simlation was condcted to examine the inflence of tangential leakage on the performance of a scroll compressor nder varios conditions. Experiment An experiment was performed to find ot the characteristics of the tangential leakage between pockets in a scroll compressor nder actal operating conditions. Fig.l is a schematic of experimental apparats. The experimental apparats is consisted of three parts; an air-conditioner part, a mass flow measring device parts and a test compressor part. The air-conditioner part is a device to spply high pressre and temperatre of refrigerant to a test compressor. Heat transfer rate of the condenser with revoltion of fan controls discharge pressre and the area of expansion valve controls sction pressre. The mass flow measring device part is made for measring the flow rates at many parts with one mass flow meter. And the test compressor has the same strctre as real scroll compressor except a driving motor which is removed. Fig.2 is a sectional view of the test compressor. The radial clearance is controlled by changing the eccentricity of crank pin, and the axial clearance became nearly zero by pe1fect contact between the tips of orbiting scroll and the fixed scroll. So we can obtain the only tangential leakage at the radial clearance. The indirect flow measring method is described at Fig.3. Pressre in a pocket is controlled by adjsting bypass ( md, m 1, m2) valve to set pressre conditions. And the leakage flow rates between the pockets can be calclated by the continity eqation. Then, the pressre condition at each pocket for the experiment is arranged at Table 1. n a pressre condition, the vale of P1 and P2 increase with the orbiting angle bt the vale of P, and Pd is constant becase P, and Pd is always adjsted to the sction pressre and the discharge pressre, respectively. And in an orbiting angle, P1 and P2 have the same vale becase the scroll compressor has fixed compression ratio. Theoretical analysis At the radial clearance of the scroll compressor, the leakage of high-pressre refrigerant de to the difference of pressre between two pockets can be theoretically obtained by sing the flow coefficient eqation of steady state one-dimensional compressible flid, considering the refrigerant as a single phase isentropic nozzle flow. Generally, the flow rate increases with the mcrease of pressre difference. Bt in case of the fixed pstream temperatre and pressre and the redcing downstream pressre, choking phenomenon occrs. When the 88

4 radial leakage path is considered as a converging nozzle, the leakage at the clearance is governed by Eq.(l). n the Eq.(l), the factors which inflence on the qantity of leakage are leakage area, pressre ratio between pstream and downstream (Pr/Pp), and conditions(pp, Tp) of pstream. CAPpJ 2....!±.l k ( P dn ) ( 2 ) k 1 2k [ ( Pdn) k _ ( Pdn) (k-l)rt, Pp Pp Pp :2 k+1 m= CAP k+! k ( 2 )k= ( P dn ) ( 2 ) k 1 RT k+ 1 Pp ( k+ 1 (1) n the present research, the flow coefficient can be acqired by ptting the experimental vales,that is the mass flow rate, temperatre, and the leakage area at the radial clearance into the Eq.(l ). Then, sing the acqired flow coefficient the inflence of the tangential leakage on the performance of the scroll compressor sch as volmetric efficiency, indicated work, and so on. Generally, the definition of the volmetric efficiency of a compressor is like the Eq.(2), the ratio of the flow rate in the actal mass flow rate of the cycle to the ideal mass flow rate. mideal (2) For a convenience of the analysis on the effect of tangential leakage, if we assme that there is no leakage at the axial clearance and no heat transfer in the sction process, and we define m; and m1eak,m as theoretical mass of refrigerant on completion of sction and mass of tangential leakage to sction pocket dring the sction process respectively, the Eq.(2) can be converted into the Eq. (3). So we intended to analyze inflence of the tangential leakage by sing the Eq.(3). m; - m leak. tan m; (3) Reslt and Discssion Reslts of Experiment The radial clearance was measred with thickness gage of 1Jlm order. Table 2 shows the actal measred clearance. The clearance of the sample B is 5tLm larger than that of sample A Fig.3 describes that the tangential leakage increases with the radial clearance shown at Table 2. t can be confirmed by experiment that the qantity of the leakage is proportional to the radial clearance as known as Eq.(l). Fig.4 shows the leakage mass flow rate with discharge pressre and orbiting angle in the case of the sample A Becase the change of the discharge pressre has no effect on the compression process de to the fixed compression ratio of the scroll compressor, m2s and ml2 before discharge starting angle(orbiting angle 8 ) are not affected by the change of discharge pressre. Bt, after discharge starting angle, P 1 pocket (high pressre pocket) and Pd pocket are merged, and then the pressre of P1 pocket is eqal to the pressre of Pd pocket. Therefore, rnl2 after discharge starting angle and md! increase with the rising discharge pressre de to the 89

5 increase of the density and the pressre difference. On the other hand, becase the pressres of the P 1 and the Pd pocket are same after discharge starting angle, there is meaningless in mdl. Flow Coefficient To obtain the flow coefficient, we sed the reslt of choking experiment. The choking experiment S to measre the leakage flow rate between Pt and Pd pockets nder the condition of the fixed pressre of Pd pocket(2lkgr/cm 2 G) and the redced pressre of P 1 pocket(from 2kgtlcm 2 G to 6kgr/cm 2 G). The reslt of the choking experiment is shown at Fig.5. The flow coefficients are able to be obtained by ptting the experimental vales into the Eq.(l ), and the vales of flow coefficient were.115 and.16 for sample A and sample B respectively. As we can see at Fig.5, the reslts of theoretical calclation sing the average (.11) of two flow coefficients coincide with experimental vales within the limit of 5 % error. Then, the theoretical reslt from calclation ptting the obtained flow coefficient (.11 ), the leakage area, pressre and temperatre into Eq.(l) is shown at Fig.6. This reslt has a good agreement with experimental vale in Fig.4 within the limit of 1 % error. So we cold conclde that the flow coefficient of tangential leakage is always constant nder the condition of constant oil concentration regardless to shape of leakage path, radial clearance, and pressre condition. Effect of. Tangential Leakage The simlation sing the flow coefficient were condcted to analyze the inflence of the tangential leakage on the performance of a scroll compressor nder the varios operating conditions. The condition for simlation was the same as the standard compressor performance test condition of AR (American Refrigeration nstitte), and the parameters are leakage factors, sch as radial clearance, discharge pressre, sction pressre, and sction gas temperatre. Fig.7 shows the leakage mass flow rate with the orbiting angle at each radial clearance(opm, 2pm, and 3 pm), and Fig.8 shows the volmetric efficiency with the radial clearance. According to the increase of the radial clearance, the leakage flow rate also increases linearly. This reslt makes the volmetric efficiency decrease with the increase of tangential clearance. Fig.9 shows the leakage mass flow rate with the orbiting angle at the radial clearance of 2JLm at each discharge pressrc(l.866mpa, 2.161MPa, and 2.455MPa). Before discharge starting angle, there is no difference in the leakage flow rate. So the volmetric efficiency, which is defined as Eq.(3), has little difference. After discharge stmting angle, however, the leakage flow rate has a great difference owing to the discharge pressre. The leakage flow rate increases with the increasing discharge pressre becase of the increase of pstream density. The variation of discharge pressre has no effect on the volmetric efficiency, bt the difference of the leakage flow rate cold inflence on the power consmption. Fig.O shows the indicated work ratio with discharge pressre. When the discharge pressre increases, the leakage flow rate increases, so the power consmption increases also. lhe variation of sction pressre cases the variation of the leakage flow rate. n Fig. the leakage flow rate with orbiting angle at each sction pressre (.4936MPa,.5916MPa,.6897MPa). Before discharge starting angle, there is a large difference in the leakage flow rate. When the sction pressre is high, there is mch leakage flow rate de to the high pstream density. So it cold be thoght that the volmetric efficiency wold be low. When the theoretical mass of refrigerant on completion of sction (mi in Eq.(3)) increases at the high sction pressre, however, the volmetric efficiency is not nearly affected by the variation of sction pressre as shown in Fig.l2. As an example, in the case that the sction pressre increases from.4936mpa to.5916mpa, the volmetric efficiency increases.1%. Fig.l3 shows the leakage mass flow rate with orbiting angle at the radial clearance of 2pm at each sction gas temperatre (7.2C, 18.3C, 35C). The qantity of leakage flow rate becomes higher over all the orbiting angle at a low sction gas temperatre becase the density becomes higher at the low temperatre and same 81

6 pressre. Fig.14 represents the volmetric efficiency with sction gas temperatre. n the case of the lower sction gas temperatre, the volmetric efficiency becomes higher. Althogh the leakage flow rate increases according to the decreasing sction gas temperatre, the rate of increase of the theoretical mass of refrigerant on completion of sction is higher than that of increase of the leakage flow rate as shown in Eq.(3), thereafter the volmetric efficiency increases. Conclsions n this present stdy, the indirect flow measring method was developed in order to measre the leakage flow rate at the clearance of scroll compressor nder the actal operating conditions, and the simlation was condcted sing the flow coefficient obtained by experiment. Finally, following conclsions were obtained abot the inflence of the tangential leakage on the scroll compressor. (1) From the reslt of choking experiment, the flow coefficient was obtained, and this vale was always constant,.11, regardless to the shape of leakage path, the radial clearance, and the pressre condition at a given oil concentration. (2) n a scroll compressor, the increasing radial clearance makes the linear increase of tangential leakage flow. This cases the volmetric efficiency decrease as a reslt. (3) The load variation affects the tangential leakage, bt has little effect on the volmetric efficiency. ( 4) The variation of sction gas temperatre has an effect on the leakage flow and performance of a scroll compressor. n the case of lower sction gas temperatre, the leakage flow rate becomes higher, bt volmetric efficiency becomes lower owing to the increase of theoretical mass of refrigerant on completion of sction. Acknowledgement We wold like to express or great thanks to LG electronics Co. Ltd., for their financial spport m carrying ot this research and their permission to pblish this work. References (l) Jean-Lc Caillat, Shimao Ni, Michael Daniels, 1988, A compter model for scroll compessors, Proc. of nt. Compressor Engineering Conf. at Prde, pp (2) Makoto Hayano, Hirotsg Sakata, Shigemi Nagatomo, Hiroaki Mrasaki, 1988, An analysis of losses in scroll compressor, Proc. of nt. Compressor Engineering Conf at Prde, pp.l (3) Tstom naba, Masahiro Sgihara, Toshyki Nakamra, Tadashi Kimra, Etso Morishita, 1986, A scroll compressor with sealing means and low pressre side shell, Proc. of nt. Compressor Engineering Conf at Prde, pp (4) Tadashi Yanagisawa, Cheng Min Chi, Mitshiro Fkta, Takashi Shimiz, 199, Optimm operating pressre ratio for scroll compressor Proc. of nt. Compressor Engineering Conf. at Prde, pp (5) Noriaki shii, Kenichi Bird, Kiyoshi Sano, Mamor Oono, Shinji lwamra, Takayki Otokra, 1996, Refrigerant leakage flow evalation for scroll compressor, Proc. of nt. Compressor Engineering Conf at Prde, pp (6) N. K. Cho, Y. Yon, B. C. Lee, M. K. Kim, M. K. Min, 1999, A Stdy on the Characteristics of Tangential Leakage in Scroll Compressors for Air-conditioner, Proc. of the SAREK'99 Winter Annal Conf., pp.l

7 Tables and Figres Table l Experimental conditions for pocket pressre with orbiting angle Orbiting angle ( ) Pressre conditions (PJPd) [kg/cm'g] 5/18 5/21 5/24 5/5/13/18 [PJP:JPdPd] 5/5/13/21 5/5/13/24 5/ /6/14/21 5/6/14/24 5/8/18/18 5/8/21/21 5/8/24/24 5/1/18/18 5/1/ /1/24/24 Table 2 Measred radial clearances according to contact points at the reference angle ( ) Clearances(ttm) Sample A Sample B nner contact point Middle contact point Oter contact point Condenser f----- {-1 Distrib -ter Jl Sight il fffi 1 glass seperator F-t: [-t(-t f. Expansion Mass flow valve meter w: r l Drivin9'----- : compressor Evaporator l Test compressor J Heater H [J c }-- == pressre : ltjj!!ir-mlif&mt.i L L Fig.l Schematic of experimental apparats Test Compressor Fig. 2 Exampled diagram of the indirect flow measring method 812

8 .6.., =====jl.5,.4.3 ';.2 >.1 s til = rl o.oj u-- c 45 Orbiting angle( ) (a).5 Tr======;-----, E.4 (i).3 1 3':,g.2 >.1 ro Sample A Sample B. -'f.>--o--ll j-j il ' SampleA!SSS::J Sample B Orbiting angle( ) (b) c.7.6 SamP.e B.5 o------g-- Samp Ol e 2.4 m --c \.3 o= Ol m.2 m Experiment co.1. -o- Theory --\ Pressre ratio(p P l Fig. 5 Comparison of leakage mass flow rate between experimental and theoretical vales.6, ;::::::=======;;.5 -- High Middle Low - - m -- m -- m.4 m1.. D- m <> mls.3 -o.2 ---o. =, :::::: = ::: ==::: :::: = jil Orbiting angle( ) (c) Fig. 3 Leakage mass flow rate with orbiting angle and radial clearance at (a)inner contact point, (b )middle contact point, and ( c )oter contact point. - --, ,---.--,-'..,.-'-'-'-.,.._.._ Orbiting angle( ) Fig. 6 Leakage mass flow mte with orbiting angle at each discharge pressre by simlation (sample A).6, =========' oo -----o 45 High Middle Low - -md1 o-md, -o-md, - - mo -o- m 12 -o- m,l - - ml --mz. -- mh _ :====:::::::::::. - o o Orbiting angle( ) Fig. 4 Leakage mass flow mte with discharge pressre and orbiting angle at each contact point(sample A) c ! ;:.4 o= >.2 m.... Radial clearance -1J.lm 2fU11 3fUT ' 36 Orbiting angle( ) Fig. 7 Leakage mass flow rate with orbiting angle at each radial clearance 72 ' '. 813

9 ';j -,;; c: ii= ;:: E > :>!. -,;; 99.4 c: 99.2 Ql E 99. c E 98.8 > Radial clearance(jlm) Fig. 8 Volwnetric efficiency with radial clearance Sction pressre(mpa) Fig. 12 Volmetric efficiency with sction pressre.8 O.o7 c.6.e en (]) Ol.3 J!l.2 c (]) _j.1 Discharge pressre Low Middle High /--: -- : _r E=... _--= -: _- :.---- = --;.-_;-:: en.5-2! Ol.4.3 rn.2 -' Ql _j.1 Sction gas temperatre 35.C 18.3C 7.2C,,. 36 Orbiting angle( ) Orbiting angle( ) Fig. 9 Leakage mass flow rate with orbiting angle at each discharge pressre Fig. 13 Leakage mass flow rate with orbiting angle at each sction gas temperatre.q iii 1.1., ffi 'i Discharge pressre(mpa) Fig. 1 ndicated work ratio pressre(radial clearance=2lllll) with discharge.8...-r======;---[------,,.7 Sction pressre Low 99.4 >: c: 99.2 <ll ii= Ql.:: (j) 99. E 98.8 > 98.6 Sction gas temperatre(c) Fig. 14 Volmetric efficiency with temperatre sction gas J------; f Orbiting angle( ) Fig. 11 Leakage mass flow rate with orbiting angle at each sction pressre 814