三维滚动涡旋压缩机的发展-外文文献.pdf

SPECIAL ISSUE PAPER193 Development of a three-dimensional scroll compressor H Sato1,M Fujitani2,H Kobayashi2,H Mizuno2,andT Itoh1 1Nagoya Research and Development Center, Mitsubishi Heavy Industries, Ltd, Nagoya, Japan 2Air-Conditioning and Refrigeration Systems Headquarters, Mitsubishi Heavy Industries, Ltd, 3-1 Asahi Nishibiwajima-cho Kiyosu, Aichi 452-8561, Japan The manuscript was received on 6 December 2007 and was accepted after revision for publication on 16 September 2008. DOI: 10.1243/09544089JPME189 Abstract: Scrollcompressorhasbeenemployedinvariousappliancesduetoitsadvantagessuch as high effi ciency and low vibration. For the purpose of further performance improvement in scroll compressor, the authors have developed a new conceptual three-dimensional compres- sion mechanism (three-dimensional scroll) by adding an axial compression to the conventional radial compression. By realizing three-dimensional compression, which has been impossible for the conventional scroll, higher effi ciency, higher reliability, and smaller size are achieved. Since the three-dimensional scroll has steps in scroll tip and end plate, the key point of effi ciency improvementinthethree-dimensionalscrollistominimizethegasleakageinthesteps.Through cylinder pressure measurements and visualization tests, characteristics of the leakage in the stepsareobtainedandtheoptimumclearancerangesaredetermined.Basedonthis,theauthors have developed high effi ciency, small size, and lightweight three-dimensional scroll compressor for commercial air-conditioner. The developed three-dimensional scroll compressor archived 35 per cent smaller size, 26 per cent lighter weight, and 5.5 per cent improvement of effi ciency compared with the conventional one. Keywords: compressors, design, refrigeration, control, performance, development, simulation 1INTRODUCTION Ademand for saving energy has been extensively increasing from the viewpoint of environmental con- servation against global warming. Since most energy in refrigerating and air-conditioning appliances is consumed by the compressor, its effi ciency improve- ment is indispensable for saving energy. Meanwhile, these appliances are often restricted in their installa- tionspace.Therefore,miniaturizationofeachcompo- nent including the compressor is required to enhance the fl exibility of installation. Today, scroll compressor, which has advantages such as high effi ciency and low vibration, has been used for various appliances to meet the demand for saving energy and its use is expected to spread increasingly. For the purpose of further performance Corresponding author:NagoyaResearchandDevelop- ment Center, Mitsubishi Heavy Industries, Ltd, 1-Takamichi Iwatsuka-cho Nakamura-ku, Nagoya 453-8515, Japan. email: hajime_satomhi.co.jp improvement in scroll compressor, the authors have developed a new conceptual three-dimensional com- pression mechanism (three-dimensional scroll) that adds an axial compression to the conventional radial compression. This article describes the effi ciency improvement technology in the three-dimensional scroll and the development of the three-dimensional scroll com- pressor for commercial air-conditioner. 2STRUCTURE OFTHETHREE-DIMENSIONAL SCROLL 2.1Features of the three-dimensional scroll Figure1showsthedevelopedthree-dimensionalscroll compressorforcommercialair-conditionerandapho- tographoftheorbitingscroll.Refrigerantgasfl owsinto thecompressorthroughthesuctionpipeplacedonthe side body. Then, it reaches the compression chamber getting compressed from the outer side to the inner side. The compressed refrigerant is discharged from the centre of the fi xed scroll. JPME189IMechE 2008Proc. IMechE Vol. 222 Part E: J. Process Mechanical Engineering 194H Sato,M Fujitani,H Kobayashi,H Mizuno,andT Itoh Fig.1Three-dimensional scroll compressor Fig.2Sectional view of the conventional and the three-dimensional scroll Figure 2 shows a schematic diagram of the sectional view of the conventional and the three-dimensional scroll. The wrap height of the conventional scroll is constant throughout the compression process, and therefrigerantiscompressedtwo-dimensionallyfrom the outer side to the inner side as the compression chamber continuously becomes smaller in volume. Forthethree-dimensionalscroll,incontrast,theouter wrap is higher than the inner one by installing steps in the scroll tip and the end plate. Therefore, three- dimensional compression, radial and axial direction, becomes possible. The three-dimensional scroll has the following fea- tures. 1. Higher compression ratio is obtained by radial and axial compression. 2. The strength of scroll wrap is improved and higher reliability is obtained by decreasing the height of inner wrap, which receives a heavy load. 3. Largercapacityisobtainedwithoutextensionofthe outer diameter of scroll by increasing the height of outer wrap, and thus the three-dimensional scroll has smaller size and lighter weight. 2.2Compression mechanism and leakage clearances in the three-dimensional scroll Thecompressionmechanismofthethree-dimensional scroll is shown in Fig. 3. The three-dimensional scroll has steps in the scroll tip (tip step) and the end plate (bottom step).When these steps are not engaged (see Figs 3(b) and (d), compression chambers across the step have the same pressure. Therefore, no leakage Proc. IMechE Vol. 222 Part E: J. Process Mechanical EngineeringJPME189IMechE 2008 Development of a three-dimensional scroll compressor195 Fig.3Compression mechanism of the three-dimensional scroll occurs in the steps. On the other hand, when these are engaged (see Figs 3(a) and (c), seal lines are formed by the engagement of both steps. Figure 3 also shows the enlarged view of the step and the sectional view in the longitudinal direction of the orbiting scroll. Leakage clearances in the step (hereafter called step clearances) can be broadly clas- sifi ed into tip clearance and side clearance, and the gas leakage occurs from the high-pressure chamber to the low-pressure chamber through these clearances. Therefore, the key point of effi ciency improvement in the three-dimensional scroll is to minimize the gas leakageinthestepsbyoptimizationofstepclearances. 3EFFICIENCY IMPROVEMENT OF THREE-DIMENSIONAL SCROLL 3.1Optimization of clearances in the steps As mentioned above, it is important for three- dimensional scroll to reduce the gas leakage in the steps. To investigate leakage characteristics in the stepclearances,cylinderpressuremeasurementswere conducted and indicative effi ciencies were obtained fromPV diagram.Figure4showsanexampleofmea- suredPV diagram.Thisshowsthatthepressurecurve follows about the same line as the ideal one when the step clearance is small. However, when it becomes large the pressure curve moves in a direction away from the ideal curve due to increase in gas leakage. Figure 5 shows the variation of indicative effi ciency iobtained from the PV diagrams against step clear- ances where Fig. 5(a) is the result when the side clearance is fi xed and the tip clearance is varied and Fig.4PV diagram Fig. 5(b) is when the tip clearance is fi xed and the side clearance is varied.The indicative effi ciency ratio is defi ned as the proportion of the result at /0= 1. From Fig. 5(a), it can be seen that the indicative effi ciency is improved with decreasing tip clearance. However,itplateauedintheregionwherethetipclear- ance is small because the clearance is fi lled with oil when it is suffi ciently small, and thus the gas leak- age is decreased. As can be seen in Fig. 5(b), it has the same tendency when the side clearance is varied. Theseresultsindicatethatthereductionofleakageloss can be obtained by setting the step clearances within the range between the minimum value determined from the tolerance of profi le, thermal deformation, and pressure deformation, and the maximum value determined from the permitted limit of effi ciency. JPME189IMechE 2008Proc. IMechE Vol. 222 Part E: J. Process Mechanical Engineering 196H Sato,M Fujitani,H Kobayashi,H Mizuno,andT Itoh Fig.5 Variation of indicative effi ciency against step clearances 3.2Sensitivity of leakage in the step clearances Inthenextstep,sensitivityofleakageinthestepclear- ances is considered. Quantities of leakage fl ow in the tip clearance and the side clearance are examined byexperimentalandanalyticalapproach,respectively. Thesensitivityofleakageineachclearanceisobtained by the following processes. 1. PV diagrams are drawn by cylinder pressure mea- surementsvaryingtipandsideclearancesindepen- dently. 2. Considering mass and heat balance, leakage analy- sesareconducted.Here,leakagefl owG issupposed to be governed by the equation of nozzle fl ow as follows G = C A ? ? ? ? 2 1P11 ? P2 P1 ?2/ ?P 2 P1 ?+1/? whereC isthefl owcoeffi cient,A istheareaofclear- ance, P1and P2are the pressure at inlet and outlet, respectively, 1 is the density of fl uid at inlet, and is the specifi c heat ratio. 3. Flow coeffi cients Ctipand Csideare determined by comparing analytical results with PV diagrams from cylinder pressure measurements. In this study, 45 cases of cylinder pressure measure- ments(tipclearance:threecases,sideclearance:three cases, and operating condition: fi ve cases) and corre- spondinganalysesweremade.Asaresult,itwasfound that fl ow coeffi cients Ctipand Csidehave a relationship as follows Cside= 1.7 Ctip This indicates that the quantity of leakage fl ow par sectionalareainthesideclearanceislargerthanthatin thetipclearance.Thedifferenceisattributedtothedif- ference of longitudinal shapes of each clearance. The side clearance is composed of two circular walls (tip step and bottom step). Therefore, the length of side clearance in the direction of leakage fl ow is shorter than the wrap thickness, whereas the length of tip clearance is equal to the wrap thickness. 3.3 Visualization of leakage fl ow in the step clearance Visualization tests were performed to examine the behaviour of leakage fl ow in the step clearance. A pro- totype compressor that can observe the behaviour of leakagefl owinthestepclearancewasmade.Thiscom- pressor equipped a sight glass in the end plate of fi xed scroll, and the orbiting motion of the orbiting scroll andthebehaviourofleakagefl owwerevisualizedwith a high-speed video camera. Photographs of leakage fl ow are shown in Fig. 6. In eachphotograph,thehigh-pressurechamberisonthe right side and the low-pressure chamber is on the left side. Figures 6(a), (b), and (c) show the variation with the oil circulation ratio (OCR). Focusing on the side clearance marked with circles, it is found that there is no oil in the clearance and the gas leakage occurs through the clearance when the OCR is small (see Fig. 6(a). On the other hand, when the OCR increases as shown in Figs 6(b) and (c), the clearance is fi lled with oil and an oil-fl ow along the bottom step is also observed. Figures 6(d) and (e) show the variation with the magnitude of clearance setting the OCR at a con- stant value. The clearance is fi lled with oil when it is small as in Fig. 6(d). However, when the clearance is large as in Fig. 6(e), it is no longer fi lled with oil and the gas leakage occurs. This indicates that the required OCR to seal the step clearances depends on Proc. IMechE Vol. 222 Part E: J. Process Mechanical EngineeringJPME189IMechE 2008 Development of a three-dimensional scroll compressor197 Fig.6 Visualization of leakage fl ow in step clearance the magnitude of clearance, and it is also important to set the oil content in the cylinder at a proper value. Bytheabove-mentionedapproaches,thestepclear- ances in the developed three-dimensional scroll com- pressor were optimized keeping the OCR in the same level as the conventional one. 3.4Reduction of thrust bearing loss The coeffi cient of friction in the thrust bearing is con- sidered rather higher than that in the journal bearings in scroll compressor due to the difference of lubrica- tion condition 1. Therefore, the thrust bearing loss occupies a large part of the total mechanical loss and it is necessary for performance improvement to decrease the thrust bearing loss. The thrust bearing loss Wthrustis given as follows Wthrust= thrustFthrust(2 N) where thrust is the coeffi cient of friction of the thrust bearing, Fthrustis the thrust gas force, is the orbit- ing radius, and N is the rotation speed. Assuming that thelubricationconditionofthethrustbearingandthe operating condition are constant, the thrust bearing loss depends only on the thrust gas force and orbiting radius. Figure 7 shows a schematic diagram of thrust bear- ing in the conventional and the three-dimensional scroll. By introducing three-dimensional scroll, the wrap height can be set at a higher value, and the outer diameter can be decreased compared with the con- ventional scroll, which has the same capacity. Accord- ingly, the area of end plate which receives cylinder pressure can be decreased and thus the thrust gas force is reduced. Moreover, the orbiting radius can be also set at a smaller value and the sliding distance is decreased. For the developed three-dimensional scroll com- pressor described in the next section, the scroll diam- eter and the orbiting radius are decreased by 9 and 20 percentrespectively,comparedwiththeconventional scroll. As a result, a substantial reduction of thrust bearing loss is archived. 4FEATURES OFTHE DEVELOPED THREE-DIMENSIONAL SCROLL COMPRESSOR The main changes from the conventional compressor to the developed one are listed below. 1. The newly developed three-dimensional scroll was employed. 2. Bypass ports are installed to avoid over compres- sion. 3. A higher effi ciency motor was adopted. The main dimensional data of the developed three- dimensional scroll compressor for 10PS commercial air-conditioner and the conventional one, which has the same capacity, are shown in Table 1 and the outlines of both compressors are shown in Fig. 8. The developed three-dimensional scroll compressor JPME189IMechE 2008Proc. IMechE Vol. 222 Part E: J. Process Mechanical Engineering 198H Sato,M Fujitani,H Kobayashi,H Mizuno,andT Itoh Fig.7Schematic diagram of thrust bearing Table 1Dimension comparisons between the conven- tional and the three-dimensional scroll Developed three-dimensionalConventional scrollscroll Cooling capacity28kW28kW Ratio of orbiting radius 3D/conv 0.801 Ratio of wrap height L3D/Lconv 1.241 Ratio of scroll diameter D3D/Dconv 0.911 Fig.8Outlineofthedevelopedthree-dimensionalscroll compressor and the conventional one archived 35 per cent reduction of volume and 26 per cent reduction of weight by introducing three- dimensional scroll. Figure 9 shows the effi ciency improvement of the developed three-dimensional scroll compressor. Figure 9(a) shows the variation of effi ciency against operating pressure ratio. In this fi gure, the motor effi - ciency was eliminated from the total effi ciency to focus on the performance of the mechanical part. The compression ratio of the three-dimensional scroll can be set at a higher value than the conventional scrollandare-compressionlossduetotheshortageof compression ratio is decreased. As a result, substan- tial improvement of effi ciency on high-pressure-ratio condition is obtained. This indicates that the three- dimensional scroll has a special advantage for appli- ances operating under high compression ratio such as heat pump for cold area and refrigeration in addition to air-conditioning. On the other hand, an over com- pression loss on the low-pressure-ratio condition can beavoidedbyinstallationofbypassports.Theworking mechanismofthebypassportsisshowninFig.9(b).In case the operating pressure ratio is below the built-in pressure ratio, the pressure of intermediate compres- sionchambersexceedsdischargepressureandanover compression loss occurs. By installing bypass ports on intermediate compression chambers, the refrig- erant that reaches discharge pressure is released to the discharge chamber through the bypass ports, and the pressure of intermediate compression chambers, which is communicated with the bypass ports, is kept at discharge pressure. Figure 10 shows loss classifi cations based on the cylinder pressure measurements on rated condi- tion (Pd/Ps= 3.4) and high-pressure-ratio condition (Pd/Ps = 6.3). Loss ratio is defi ned as the percent- age of total loss of the conventional compressor. The following improvements are made for the developed three-dimensional scroll compressor. Proc. IMechE Vol. 222 Part E: J. Process Mechanical EngineeringJPME189IMechE 2008 Development of a three-dimensional scroll compressor199 Fig.9 Effi ciency improvement of the developed three-dimensional scroll compressor Fig.10 Loss classifi cations 1. Reduction of indicative loss by the optimization of compression ratio with the three-dimensional scroll and the minimization of the leakage loss in thesteps(12percentonratedconditionand26per cent on high-pressure-ratio condition). 2. Reduction of mechanical loss by miniaturization of mechanicalpartswiththethree-dimensionalscroll (15 per cent on rated condition and 12 per cent on high-pressure-ratio condition). 3. Reduction of motor loss by introducing a high effi ciency motor (15 per cent). Bytheimprovementsmentionedabove,5.5percent improvementoftotaleffi ciencyonratedconditionand 12.5 per cent on high-pressure-ratio condition were archived. 5CONCLUSIONS Theeffi ciencyimprovementtechnologiesinthethree- dimensionalscrollwereinvestigatedandthefollowing conclusions were obtained. 1. The indicative effi ciency plateaus in the region wherethestepclearanceissmallduetotheoilseal. 2. Thefl owcoeffi cientinthesideclearanceis1.7times larger than that in the tip clearance. 3. Visualizationtestsgavetheconfi rmationthatfi lling the step clearance with oil decreases the leakage. 4. Thethrustbearinglossisreducedbydecreasingthe outer diameter and the orbiting radius of scroll. Based on t