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TECHNICAL REPORTS Performance Analysis of Scroll Compressors Using CO2 Refrigerant 1. Introduction Authors: Mihoko Shimoji* and Toshiyuki Nakamura** Table 1 Test conditions and stroke volume of We manufactured a prototype of a large-capacity scroll compressor that uses CO2 refrigerant, evaluated the performance of it, and analysed losses using a simplified model based on a basic test. As a result, the compressor input power levels in both the measure- ment and analysis agreed, although with a difference of approx. 3%, in the range of rotational speed from 30 to 100 rps. The availability of the analysis method was thus verified. 2. Specifications of the Prototype For examination of the performance of a CO2 compressor when used for air-conditioning (cooling), the authors manufactured a prototype compressor that uses CO2 refrigerant, which is equivalent to 10HP and based on a mass-produced scroll compressor that uses R410A refrigerant. The stroke volume of this compressor was de- signed to be approximately one-third (24 x 10-6 m3/rev) that of a R410A compressor, since the cooling capacity of CO2 refrigerant per unit stroke volume is about three times that of the R410A refrigerant. The operating pressure of CO2 refrigerant is three times higher than that of R410A refrigerant and fluctu- ates with a large peak-to-bottom difference, resulting in a high operating load. For this reason, the reliability of the compressor was ensured by increasing the strength of the components such as compression part, and also by reinforcing the bearing support structure. In addition, for the prevention of refrigerant leakage, tip seals were applied to the axial leakage gaps in the scroll-wrap tips. Sliders(1) were used for sealing the leakage gaps in the radial direction on the scroll wrapside. (The turning radius of the orbiting scroll was changed during opera- tion and this presses the orbiting scroll against the fixed scroll.) 3. Performance Evaluation of the Proto- type 3.1 Evaluation conditions Table 1 shows the evaluation conditions and stroke volume of the prototype. We evaluated the performance on the assumption that the prototype will be used for air-conditioning (cooling). Item Suction pressure Discharge pressure Compression ratio Suction temperature Rotational speed Stroke volume the prototype Value 4 MPa 10 MPa 2.5 15°C 30rps/60rps/100rps 24x10-6 m3/rev 3.2 Results of the performance evaluation Figure 2 shows the volumetric efficiency and over- all adiabatic efficiency of the prototype under the oper- ating conditions shown in Table 1. The efficiencies shown in Fig. 1 are the results of dividing the efficiency levels at each rotational speed by the efficiency level at 60 rps. The efficiency dropped significantly at low rota- tional speeds: the volumetric efficiency and overall adiabatic efficiency at 30 rps became lower than those at 60 rps by 17% and 16%, respectively. Rotational speed Fig. 1 Efficiency of the prototype In the following chapters, we discuss loss analysis, using the methods described below to clearly identify the relationship between rotational speed and effi- ciency. 4. Loss Analysis The input power to the scroll compressor is divided into theoretical compression power and various types of losses, as shown in Fig. 2. The respective types of losses are calculated based on pressure changes in the compression chamber. We are concerned here only with analysis methods of major losses. Overall adiabatic efficiency *Advanced Technology R&D Center **Air Conditioning and Refrigeration Systems Works Mitsubishi Electric ADVANCE December 2007 17 Volumetric efficiency Efficiency ratio (Efficiency at 60 rps = 1)PDF Image | Heat Pump with Natural Refrigerants 3041
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