Important Parameters for Small, Twin-Screw Refrigeration Compressors

This paper deals with open-shaft twin-screw refrigeration compressors in the 10 175 m3/h (10 100 CFM) range. Applications for such compressors are, for example,external combustion engine heat-pumps and transport refrigeration. The twin-screw compressor concept is discussed from following view-points; operational mode and rotor housing design drive arrangement bearings lubricants cooling volume ratio capacity control economizer performance INTRODUCTION The twin-screw compressor is well established in the industrial refrigeration and air-conditioning field. To soae extent it is also applied for bus air-conditioning and larger commercial air-conditioning. However, for smaller capacities the use of twin-screw compressors has been limited. Up till now the small twin-screw compressor has often been a scaled-down industrial refrigeration compressor or a converted air compressor. Also "the manufacturing state-of-the-art" has not been ready for large scale production of small high precision rotors. To-day, however, the situation is different. 1129 OPERATIONAL MODE AND ROTOR HOUSING DESIGN The compressor can be designed for oil-flooded or oilreduced operation (reference 1). Oil-flooded operation is preferred at high pressure ratios and low rotor tip speed. Oil-reduced operation is preferred at low pressure ratios and high rotor tip speed. The compressor housing or at least the discharge end plate should have similar theraal characteristics as the rotors. If this is not the case, a less favourable operational mode aust be selected, viz. liquid refrigerant flooding (compare reference 1). DRIVE ARRANGEMENT Since the compressor is quite small (40 100 u rotor dial it is important to keep the rotor tip speed at a reasonably high level. This can be done with belts, gears or female rotor drive. ROTORS The rotors can be designed for oil-reduced as well as for oil-flooded operation (reference 1). The rotor production method/material should be selected for low cost and mass production. Since the rotors are so small the interlobe clearances are critical for the performance. The rotor shaping methods available to-day are: single index milling bobbing grinding extrusion (for aluminium, close to final shape) injection moulding (for polymers, to final shape) The polymer rotors have not yet been fully evaluated for high pressure refrigeration duty but results so far look pro•isinq. BEARINGS Antifriction bearings are preferred for this compressor size. The bearings should be able to operate with oil-mist lubrication as well as forced oil lubrication. Suitable types are cylindrical, tapered roller and ball bearings.