Clearances between the Rotors of Helical Screw Compressors: Their Determination, Optimization and Thermodynamic Consequences

Abstract

Although helical screw compressors contain only two moving parts, their shapes are very complicated and must mesh to very fine clearances. The process of determining lobe geometry requires that parts of the male generate parts of the female and vice versa. Once the nominally perfect (zero clearance) profile geometry has been generated, a method for determining clearances is needed which gives an attainable result from the manufacturing and operational viewpoints and gives a thermodynamic efficiency which is at least equal to that of competing machines of the same class. The thermodynamic performance depends principally on the internal leakage rates in the compressor, in particular, the leakage across the contact (sealing) line. This paper presents two ways of setting clearances. They result in different patterns of clearance distribution along the sealing line and have different effects of the thermodynamic performance of the compressor due to the different leakage mass flowrates that they permit. A design technique for obtaining an optimum clearance distribution is discussed. Finally, a thermodynamic model of compressor behaviour is used to predict the performance as influenced by the different clearance distributions, and the predictions are compared with measured values taken from a test compressor. The predictions are shown to be good.