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Development of Accumulator-less High-efficiency CO2 Scroll
Compressor for Heat-pump Water Heater
Kiyoshi Sawai (Matsushita Electric Industrial Co., Ltd.), Noboru Iida
(Matsushita Electric Industrial Co., Ltd.) Takashi Morimoto (Matsushita
Electric Industrial Co., Ltd.), Yoshiyuki Futagami (Matsushita Electric
Industrial Co., Ltd.), Akira Hiwata (Matsushita Electric Industrial Co.,
Ltd.)
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.
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.
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 (R410A), 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 (A) 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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