Retrofit 'N' Rotors for Efficient Oil-Flooded Screw Compressors

The market for twin screw compressors is highly competitive, especially in compressed air and refrigeration systems, and new designs are continually being introduced which are more efficient and cost effective than their predecessors. However, because of the high cost of development of new machines, manufacturers seek to maintain their existing designs for as long as possible. Closer study of many of the older designs has shown that in the majority of cases, all that is required to bring them up to date is to change the rotor profile to one of more up to date type. This paper describes the design of a family of highly efficient screw compressor rotors based on rack generated profiles which were used to replace standard asymmetric rotors in sizes between 102 and 204 mm and thereby maximise the compressor efficiencies. They were developed with the aid of a software package which takes into account almost every aspect of thermodynamic and geometric modelling. All the definitive dimensions of the original rotors, such as centre distance, outer and root diameters, rotor length and average clearance were retained. However the favourable features of the new 'N' rotors permit their clearances to be reduced, if required, with no increase in manufacturing cost. All the other compressor components remained unchanged and no changes of any kind were required in the housing and bearing design . . Experimental tests showed that in all cases the introduction of the new rotors increased the compressor displacement by more then 6.5 % and raised the adiabatic efficiency by not less than 2.5 %. This led to an increased power input requirement of 2.6 %. However, the existing drive motors were sufficient to meet this additional need. Fifteenth International Compressor Engineering Conference at Purdue University, West Lafayette, IN, USAJuly 25-28, 2000 917