Built-in Volume Research Articles

Optimum operating pressure ratios for scroll compressors.

A scroll compressor has a built-in volume ratio, and ideally it should be operated under a built-in pressure ratio condition that is a function of the built-in volume ratio. but in actual operation, the compressor shows its maximum efficiency at a pressure ratio that differs from the built-in pressure ratio. In this paper, theoretical performances are presented by analyses accounting of flow resistance at the opening of scroll warps and leakage through wrap clearances. The flow resistance at the discharge opening causes an over compression loss in a built-in pressure operation but reduces a undercompression loss in a high-pressure operation, which indicates that the optimum operating pressure ratio shifts to a pressure ratio higher than the built-in pressure ratio. Leakage accelerated the pressure rise during a compression process so that the optimum pressure ratio becomes lower than it would be in the absence of leakage. Increase in the compressor speed moderates the under compression loss and leads to a higher optimum operating pressure ratio.

Monoscrew - A newly developed refrigerating compressor

A description is given of a new positive displacement compressor design, Monoscrew, in which the compression is achieved in the grooves of a screw shaped rotor, sealed off by the teeth of two gear wheels. Special attention is given to the significance of the built-in volume ratio and its influence on the operating efficiency, and also t̂o the capacity control and the oil system of this machine.

The Thermodynamic Performance of Oil-Flooded Helium Screw Compressors

In recent years the cryogenic community has become interested in the application of hermetic screw compressors for helium refrigeration/liquefaction systems. In the past the screw compressor had not been an attractive compressor for cryogenic systems because of the problems associated with both dry and oil-flooded screw compressors. However, oil separation technology has evolved to a point where oil can successfully be separated from helium gas. The screw compressor has exhibited excellent reliability and performance characteristics for various refrigeration gases, and preliminary test results reveal that the same is true for helium gas. The present paper presents the thermodynamic performace characteristics of 30-kW (40-HP) and 113-kW (150-HP) oil-flooded hermetic screw compressors with various built-in volume ratios operating with helium gas at pressure ratios between 2.0 and 18.0, and for suction pressures of 0.5 atm, 1.0 atm, and 2.0 atm. The thermodynamic parameters of interest are the volumetric efficiency and the isentropic efficiency. It is found that the volumetric efficiency decreases with pressure ratio; however, depending on the built-in volume ratio of the compressor, the maximum value of the isentropic efficiency occurs in a narrow range of values for the pressure ratio at which the optimum isentropic efficiency occurs. The results of the tests indicate that oil-flooded screw compressors may successfully be used for helium service.