Devices including a rotatory compressor are popular worldwide. However, internal pressures in the housings in almost all these rotatory compressors are high. This is a result of advantages such as an energy efficiency and a cost of a high-pressure rotatory compressor, miniaturization, and oil controls. On the other hand, in the point of being environmental friendly to the earth, more attentions are paid to the use of natural refrigerants, such as CO2 and HC refrigerants. In addition, a plan of using HC refrigerants in a rotatory compressor is developing.
However, CO2 has a quite high operation pressure. Therefore, a housing with high internal pressure of a rotatory compressor needs to withstand a pressure of more than 100 MPa and a thickness of a wall of the iron housing needs to be more than 7 mm, which causes significant problems for the production and cost. In addition, since R290 refrigerants of HC series have a strong flammability, the amount of refrigerants sealed in the refrigerating system must be limited. Due to those described above, as to rotatory compressors having a high-pressure housing, it is expected to develop a rotatory compressor which has a thin housing wall, with small amount of sealed refrigerants, and the housing of which being the low-pressure side. Moreover, for a low-pressure rotatory compressor using CO2 (carbonic acid gas) or HC refrigerants (hydrocarbonic acid gas), as the refrigerants has a strong solubility (dissolution) in the lubricating oil, viscosity of the oils may be further significantly reduced.                Reference 1: U.S. Pat. No. 2,988,267 ROTARY COMPRESSOR LUBRICATING ARRANGEMENT (1961).        Reference 2: patent application publication No. JP1998-259787, rotatory sealed compressor and refrigerating device.        
USPTO Backgrounds
USPTO Backgrounds