The twin helical screw compressor Part 2: a mathematical model of the working process

Part 1 of this paper describes the physical characteristics, applications and analytical procedures currently relevant to the twin helical screw compressor. Part 2, presented here, describes a mathematical model of the working process of the machine. Use is made of a geometric model developed by Tang [2] which describes all the essential features as functions of the male rotor angle of rotation. The following effects are taken into account: internal leakage through six paths, gas-oil heat transfer, injected liquid refrigerant flashing in the compression cavity, dissolved refrigerant flashing from the oil injected into the compression cavity, dissolved refrigerant flashing in the end casings from oil supplied to the bearings and the draining of flashed refrigerant gas from the end casings to the low-pressure regions of the main casing. Internal friction at the principal rubbing surfaces is taken to cause heating of the oil and is modelled by making use of extensive measured data and manufacturers' information. The hotter oil influences the compression process via a changed gas-oil heat transfer rate. The gas-oil heat transfer coefficient and the internal leakage coefficients are determined from measured data for mid-range conditions and applied over a range of conditions. The model output is compared with measured data; its current accuracy and possible improvement are commented upon.