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INTRODUCTION

Compressors used for industrial, commercial, and domestic applications consume approximately 17% of the world's electrical power output. The majority of these are of the positive displacement type, of which the present world production rate is in excess of 200 million units per year. Most of these are required for compressed air and refrigeration systems. Although reciprocating compressors still dominate this market, many other types have a substantial share of it. Among these, screw compressors have a growing role, especially where the power requirement is high and machine sizes are relatively large.

Apart from their use in refrigeration and air-conditioning systems, a significant number of screw compressors are used in building engineering, food processing, and pharmaceutical industries and for metallurgical and pneumatic transport applications.

Screw compressors are essentially simple volumetric machines in which the moving parts all operate with pure rotational motion. This enables them to operate at higher speeds with less wear than most other types of compressors. Consequently, they are up to five times lighter than their reciprocating counterparts of the same capacity and have a nearly ten times longer operating life between overhauls. Furthermore, their internal geometry is such that they have a negligible clearance volume, and leakage paths within them decrease in size as compression proceeds. Thus, provided that the running clearances between the rotors and between the rotors and their housing are small, they can maintain high volumetric and adiabatic efficiencies over a wide range of operating pressures and flows. Specialized machine tools now enable the most complex rotor shapes to be manufactured with tolerances of the order of 5 μm or less at an affordable cost. The use of these in screw compressor manufacture, together with advances in rolling element bearings in which the rotors are retained, have led to great improvements in performance and an increasing percentage of all positive displacement compressors sold and currently in operation to be of this type. Consequently, as pointed out by Fleming et al. (1998) in a review of contemporary design, modeling, and optimization of screw compressors mainly for refrigeration, the development of these machines is one of the great success stories of the last quarter of the twentieth century, given that they were harldy used prior to that time.