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TECHNICAL REPORTS Rotary Compressor for a CO2 Heat Pump Water Heater 1. Introduction Authors: Hideaki Maeyama* and Shinichi Takahashi* Table 1 Specification of the CO2 compressor "ECO-CUTE" is a natural refrigerant heat pump water heater that uses CO2. The CO2 refrigerant is highly efficient in water heating applications. However, because the operating pressure is very high and CO2 has peculiar characteristics that are different from con- ventional refrigerants, we had to overcome some diffi- cult technological challenges regarding the compressor used for this refrigerant. This report discusses these major technological challenges and the actual meas- ures taken in the development of the Mitsubishi rotary compressor for CO2 refrigerant. 2. Rotary Compressor The rotary compressor mechanism consists of a rolling piston that rotates eccentrically in its cylinder and a vane installed in a vane slot in the cylinder that recip- rocates along the slot causing the vane to move along the periphery of the rolling piston. As a result of the vane’s movement, suction and compression chambers are formed inside the cylinder and the volume of each chamber changes in accordance with the rotation of the crankshaft to provide compression. Figure 1 shows a cross-sectional view of the Mitsubishi CO2 compressor and Table 1 shows the major specifications thereof. Compression system Displacement Refrigerant type Motor Application Single rotary 4.5 cm3 R744(CO2) Brushless DC motor (joint-lapped motor) HP water heater Suction Discharge 3. Characteristics of CO2 Refrigerant 3.1 Environmental and safety characteristics (2) (1) CO2 does not destroy the ozone layer. (Ozone depletion potential: ODP = 0.)*1 (2) CO2 refrigerant has a low global warming potential. (Global warming potential: GWP = 1)*2 (3) CO2 refrigerant is nonflammable. (4) CO2 refrigerant has low toxicity. 3.2 Operating characteristics of CO2 as a refrigerant (1) The operating pressure of CO2 is high. (2) The speed of sound in CO2 gas is very high. (3) The pressure of CO2 rises to a high level in re- sponse to volumetric changes. (4) The density of the CO2 refrigerant gas is high. CO2 refrigerant has excellent environmental and safety characteristics, as listed above. However, the use of CO2 as a refrigerant involves some peculiar characteristics compared with conventional refrigerants. 4. Influence of CO2 Refrigerant on a Com- pressor and Countermeasures The influence of the characteristics of CO2 men- tioned above on a CO2 compressor is taken into con- sideration as described below. 4.1 Pressure-resistant structure The most remarkable characteristic of CO2 refrig- erant is that it has a high operating pressure. To cope with this, the compressor enclosure’s pressure resis- tance requirements must be studied. Figure 2 shows the operating pressure levels of the R410A Freon refrigerant used for room air-conditioners and similar equipment and that of natural CO2 refriger- ant used in the ECO-CUTE, together with the specifica- tions of the enclosures for the respective refrigerants. As shown in Fig. 2, the operating pressure of CO2 Upper (main) bearing Motor holder Brushless DC motor (joint-lapped motor) Rotary compression mechanism Rolling piston Crankshaft Lower (sub) bearing Fig. 1 CO2 rotary compressor *Shizuoka Works Mitsubishi Electric ADVANCE December 2007 9PDF Image | Heat Pump with Natural Refrigerants 3041
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