Tokyo-Engineers around the world have been seeking more environmentally friendly refrigerants to replace hydrofluorocarbons (HFCs). One candidate is carbon dioxide (CO 2 ). In addition to its environmental advantages, it is non-flammable and inexpensive. However, using CO 2 in a vapor compression refrigeration cycle requires working pressures ranging from 3 to 9 MPa, substantially higher than the pressures required for conventional refrigerants. This has limited consideration of CO 2 to large-scale facilities. Now, in what they claim to be a world's first, Sanyo engineers have brought the benefits of CO 2 refrigeration to small applications with a rotary two-stage compressor.
Sanyo engineers wanted to use a rotary compressor because of its simplicity and compact size, and to adapt existing manufacturing technology and production lines. Rotary compressors are best suited for low compression ratios. To achieve the large pressure difference needed to use CO 2 as the working gas, therefore, Sanyo decided on a two-stage compression cycle. Two rotary compression units mount on a single drive shaft but with a 180°phase difference. Low-pressure gas is taken into the first stage compression unit and compressed to a pressure of 5 to 6.5 MPa. This gas is directed both into the compressor case and a piping loop outside the case.
The two channels merge outside the case where the gas is directed into the second stage compression unit. This high-pressure gas is then discharged to the refrigeration cycle. The first stage discharge gas volume equals the second stage intake volume, with allowance for leakage and temperature changes. The CO 2 compressor yields an isentropic efficiency of 80% or higher in the operating frequency range of 50 to 80 Hz.
Sanyo keeps the case maintained at the intermediate pressure, rather than the usual discharge pressure, to avoid having to strengthen the case. Potential applications: a heat pump for a hot water heater, home refrigerators, and car air conditioners.