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Development of Accumulator-less High-efficiency CO2 Scroll Compressor for Heat-pump Water Heater

CO[sub]2[/sub] scroll compressor and main technologies.

Fig.1 CO2 scroll compressor and main technologies.

Liquid refrigerant release mechanism.

Fig.2 Liquid refrigerant release mechanism.

Pressure change diagram.

Fig.3 Pressure change diagram.

1. Synopses

The heat-pump water heater using CO2 refrigerant has been rapidly gaining popularity in recent years for its CO2 emission reducing effects and energy-saving characteristics. The compressor plays the role of compressing the refrigerant and sending it out to the refrigeration cycle, and exerts big effects on the cycle COP (coefficient of performance) and reliability. In general, on the suction line of the compressor, an accumulator is installed to prevent the compressor from absorbing liquid refrigerant. In CO2 heat-pump, a large quantity of liquid return to the compressor occurred in the transition period and the accumulator was supposed to be indispensable from the viewpoint of reliability.

Recently, the accumulator-less CO2 compressor has been achieved for the first time by developing several technologies to reinforce reliability in order to improve the cycle performance and noise characteristics. Furthermore, high-efficiency technologies for CO2 were developed and adopted to the present compressor. By using our newly-developed CO2 scroll compressor, a CO2 heat-pump water heater, which achieves energy saving, low noise, and space saving, could be realized.

2. Technologies

Fig.1 shows the cross section of the developed CO2 scroll compressor and main technologies, which have brought an accumulator-less and high-efficiency scroll compressor into practical use.

2.1 Technologies that have achieved the accumulator-less system
When the accumulator is removed, the liquid refrigerant is directly absorbed into the compression chamber. CO2 in the liquid state provides strong power to wash lubricant oil, which causes damage of the sliding section. Therefore, to achieve high reliability of the accumulator-less system, various new technologies were developed as (1) liquid refrigerant release mechanism, (2) orbiting scroll back-up mechanism, (3) rapid oil supply pump, etc.

In this report, the liquid refrigerant release mechanism shown in Fig.2 will be described in detail. At the time of start-up under low ambient temperature, a large quantity of liquid refrigerant returns to the compressor, shown in Fig.3 as the pressure change characteristics in such event. When an accumulator is equipped, the discharge pressure gradually increases but when an accumulator is not equipped, the discharge pressure rapidly increases. Because of the increased pressure, the orbiting scroll slides under strongly pressure against the fixed scroll and there occurs a phenomenon that bottom surface of the orbiting scroll wears. Therefore, at an appropriate position in the vicinity of the wrap-end, release valves for discharging the liquid refrigerant are newly provided. By this mechanism, even when the liquid refrigerant is directly absorbed in the compression chamber, the discharge pressure gradually increases and wear on the bottom surface of the orbiting scroll is not generated and satisfactory sliding is secured.

2.2 Technologies that have achieved high efficiency
In order to achieve high efficiency in the CO2 compressor, it is essential to reduce leakage between the compression chambers. Since the present compressor adopts a high-pressure type shell, it is possible to inject high-pressure oil into the compression chamber. As the oil injection rate increases, leakage loss decreases but on the other hand, but the calorie brought by oil increases, and therefore, the optimum value exists for the oil injection rate. It has been found that the oil injection characteristics of the CO2 refrigerant differs from those of usual refrigerants for air-conditioning (RA410A), and by optimizing the oil injection rate, the high compressor efficiency has been achieved.

3. Summary

The CO2 scroll compressor recently developed has achieved both accumulator-less applications and high efficiency, and by this, (1) compressor noise has been reduced by 4 dB and (2) compressor occupied volume has been reduced by 40%, and at the same time, (3) the compressor has contributed to improved performance (COP 4.21) of the heat-pump water heater. The heat-pump water heater with the present CO2 scroll compressor mounted was produced to market in December 2003 and has been enjoying good reputation.


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