The Smallest And Most Lightweight Throughthe Industry, High

Transcript

The Smallest and Most Lightweight High-Efficiency 6 - H P D C Tw i n R o t a r y Compressor in the Industry YOSHIZUMI FUJITA*1 MAKOTO FUJITANI*2 KOBAYASHI*1 SHIGEKI MIURA*1 IKUO ESAKI*1 TOSHIYUKI GOTO*3 HIROYUKI Consistent with the current industrial market demands for increased capacity of small compressors, Mitsubishi Heavy Industries, Ltd. (MHI) has developed the smallest and most lightweight DC twin rotary compressor for R410A refrigerant with a maximum capacity of 6 HP. The company approached this design challenge by defining various capacity increase limitations utilizing its original technologies and then successfully reducing the crankshaft deflection under heavy-load operating conditions. At the same time, MHI has succeeded in producing a highly reliable and efficient compressor, which is the smallest and most lightweight unit in the industry, by maintaining the oil circulation ratio during operation and increasing the refrigerant circulation volume to levels comparable with those found in conventional-sized compressors. small compressors. 1. Introduction More ef f icient compressors are required to reduce their annual energ y consumption, resulting in energ y savings. There is market demand for increased capacity of compressors smaller than conventional ones with size reduction of the outdoor unit to save on resources and for ease of installation. MHI has developed the smallest and most lightweight high-efficiency DC twin rotary compressor for R410A refrigerant that is a maximum capacity of 6 HP. This paper described the technology, which satisfies mutually contradictory requirements that include increased capacity, increased reliability, and improved efficiency for 2. Outline of the twin rotary compressor 2.1 External appearance Figure 1 shows the external dimensions of our new 6-HP twin rotary compressor. This compressor provides 39% space savings and 56% weight reduction as compared to our competitors’ 6 -HP constant-speed scroll compressors, and 11% space savings and 22% weight reduction as compared to our competitors’ 6-HP DC twin rotary compressors. These achievements contribute toward improving the compressor’s mountability on the outdoor unit while reducing the overall weight. Space volume ratio : Competitor’s compressor 1.00(1.45) : Competitor’s compressor 0.69(1.00) : Newly developed compressor 0.61(0.89) Weight ratio 1.00 (1.78) 0.56 (1.00) 0.44 (0.78) Chamber diameter φ186 Height: 440 Height: 376 Chamber diameter φ156 Chamber diameter φ133 (Units: mm) Fig. 1 Comparison of external dimensions and weight *1 Air-Conditioning & Refrigeration Systems Headquarters *2 Production System Innovation Planning Dept. *3 Nagoya Research & Development Center, Technical Headquarters 19 Mitsubishi Heavy Industries, Ltd. Technical Review Vol. 45 No. 2 (Jun. 2008) Discharge pipe Terminal Suction pipe Flow of refrigerant Accumulator Chamber above motor Motor stator Motor rotor Crankshaft Chamber below motor Upper muffler Upper bearing Displacement (compression volume) Upper cylinder Separator plate Lower cylinder Lower bearing Suction Rotor Shaft pin Lower muffler Oil sump Discharge Blade Fig. 2 Cross-sectional view of the twin rotary compressor 3.0 2.5 2.4 Rigidity 2.0 2.0 1.6 0.4 0.0 0 1.5 Reference value for standard crankshaft (=1.0) 1.2 0.8 Load on upper shaft pin Maximum 2.5 × increase 20% reduction 60 Rotor rotation angle 1.0 180 240 Load on lower shaft pin Standard crankshaft Crankshaft with increased rigidity 0.5 Deflection 120 Crankshaft rigidity (–) Crankshaft deflection ratio (–) 2.8 300 0.0 360 Deflection Large (de .) Small The load on the upper/lower shaft pins changes in both magnitude and direction while the rotor makes one turn. Therefore, this analysis was performed at several rotor angles. =0(de .) Fig. 3 Crankshaft rigidity and deflection Fig. 4 Finite element analysis of the crankshaft The data shown in Fig. 3 were validated by detailed finite element analysis. 2.2 Internal structure Figure 2 shows the internal structure of the twin rotary compressor. T he compressor capacit y is propor tional to the displacement (compression volume). The size of the compressor cylinder is determined by the diameters of the cylinder and motor, and is also inf luenced by the crankshaft diameter. Therefore, to increase the capacity of small compressors, it is necessary to increase the rotor displacement without changing these diameters. generated by any crankshaft def lection. Therefore, we adopted a configuration that helps to effectively increase t he cra nksha f t r ig idit y in t he load di rect ion w it hout increasing its diameter to reduce crankshaft def lection during high-load operation. Figure 3 shows an analysis of the crankshaft deflection and rigidity during one turn of the rotor. Increases in local contact pressure caused by the increased displacement were prevented when deflection of the crankshaft with increased rigidity was reduced by more than 20% as compared to a conventional crankshaft. Based on this result, we performed detailed finite element analysis (Fig. 4) and conducted a confirmation test using actual units to verify their reliability. 3. Size reduction 3.1 Bearing reliability The increase in displacement increases the load applied to the crankshaft. There is a risk of abnormal bearing wear or seizure due to the increase in local contact pressure Mitsubishi Heavy Industries, Ltd. Technical Review Vol. 45 No. 2 (Jun. 2008) 20 Inner diameter of the cylinder 1.2 (2) Geometrical limit Cylinder width ratio (−) (1) Limit line within which the chamber diameter of the compressor can be smaller. (2) Limit line within which the compression section can be assembled geometrically. (3) Limit line within which reliability of the blade can be assured. (4) Limit line within which the reliability of the bearing can be secured. (See Section 3.1 for details.) Blade Upper bearing 1.1 Cylinder width 1.0 (3) Reliability   limit   (blade) Lower bearing (4) Reliability limit (bearing) Satisfied zone 0.9 (1) Limit for size reduction 0.8 0.9 1.0 1.1 Cylinder inner diameter ratio (−) 1.2 Fig. 5 Zone satisfied by the rotary compressor specifications compressor decreases as the refrigerant flow rate increases with the capacity of the compressor (volumes of the motor adopting/receiving chambers: Fig. 2), thus increasing the oil circulation ratio. A reduction in the volume of oil inside the compressor not only adversely affects the reliability of the compressor itself, but also inhibits heat transfer in the heat exchanger, thus reducing the efficiency of the system. To address this problem, MHI optimized the configuration of the inner/outer circumference passages of the motor. These serve as refrigerant gas passages connecting the motor adopting chamber and the motor receiving chamber. As a result, the oil circulation ratio for MHI’s compressor is much less than those of our competitors’ 4 to 5-HP compressors, which have almost the same chamber diameters, as shown in Fig. 6. Our oil circulation ratio is approximately the same as those of our competitors’ 6 to 8-HP compressors, which have larger chamber diameters. An appropriate combination of cylinder inner diameter and width as the main specifications of a rotary compressor must fall within an area surrounded by the limit lines (the satisfied zone) shown in Fig. 5. MHI was able to increase the capacity of its small compressors by relaxing the reliability limit for the bearing by effectively increasing the crankshaft rigidity, and by clearly defining the values for other limits through implementation analyses and confirmation tests using actual units. 3.2 Reduction of oil circulation ratio Refrigerant oil in the bottom section of the compressor serves to lubricate the sliding parts and to seal the leak clearances of the compression section. As the oil circulation ratio increases, the volume of the refrigerant oil taken out of the compressor together with the refrigerant gas also increases. In particular, when the capacity of a small compressor increases, the oil separation efficiency inside the 1.0 Oil circulation ratio (wt%) 0.9 0.8 Competitor’s compressor (chamber diameter 130, 4 to 5 HP) 0.7 Newly developed compressor (chamber diameter 133) 0.6 0.5 0.4 Competitor’s compressor (chamber diameter 156, 6 to 8 HP) Significant reduction (4 to 5 HP) Equivalent (6 HP) 0.3 0.2 0.1 0.0 0.4 0.6 0.8 1.0 1.2 Refrigerant flow ratio (−) 1.4 1.6 Fig. 6 Comparison of lubricant discharge volume Mitsubishi Heavy Industries, Ltd. Technical Review Vol. 45 No. 2 (Jun. 2008) 21 Rated efficiency: efficiency related to COP Annual performance factor: efficiency related to annual energy consumption Efficiency ratio (%) 106 Up to approx. 5% (excellent) : Rated efficiency : Annual performance factor 104 102 Approx. 2% (excellent) Up to approx. 1% (excellent) 100 98 Competitor’s level (=100%) 4 HP 5 HP 6 HP Newly developed compressor 133 133 133 Competitor’s compressor 130 130 156 (Chamber diameter) Fig. 7 Comparison of compressor efficiency 4. Efficiency improvement technology Detailed deflection analysis of the crankshaft, as described in Section 3.1, and effective reduction of crankshaft deflection led an optimized leak clearance for the compression section. In addition, by reducing the oil circulation rate, as described in Section 3.2, the sealing performance of leak clearance in the compression section was improved over a wide range of operating conditions. A high level of efficiency over a range of speeds was achieved through use of a highly efficient DC neodymium motor. Thus, the development of a 4- to 6 -HP series compressor based on the above technology resulted in a unit that is more efficient than those of our competitors for all HP settings, as shown in Fig. 7. Fig. 8 External appearance of THACOM (Chachoengsao, Thailand) 5. Conclusion T he 6 - H P D C t w i n r ot a r y c omp r e s s or for R 410 A refrigerant developed by MHI is highly reliable and efficient, and is the smallest and most lightweight unit in the industry. This compressor has already been put into commercial production at Thai Compressor Manufacturing Co., Ltd. (TH ACOM, Fig. 8), which was established in 1989 as a Thai-Japan joint venture company. In addition to supplying compressors for MHI units, THACOM also serves as a key site for marketing compressors as independent units. Yoshizumi Fujita Makoto Fujitani Hiroyuki Kobayashi Shigeki Miura Ikuo Esaki Toshiyuki Goto Mitsubishi Heavy Industries, Ltd. Technical Review Vol. 45 No. 2 (Jun. 2008) 22