Rating Technique for Refrigeration Twin-Screw Compressors

The twin-screw compressor is constantly attaJ.ninq hiqher performance due to new rotor profiles, new rotor manufacturing technology and new designs. Besides this, new refrigerants as well as new lubricants can be expected on the market. Therefore there is always a need to update performance data. To m1nimize the number of test points and still receive representative data, there 1s a need of an accurate and compact rating equation system. Certa1n relationships have become clear for representation of volumetric efficiency and power consumption as a result of analysing the behaviour of the tw1n-screw compressor. The relationships include fixed and variable volume ratio, as well as econom1zer operation. Baslc rating equations are described and comparisons between rating equation data and test data are made. INTRODUCTION The curve fitting technique described in tllis paper is mainly developed for the oil-flooded twin-screw refrigeration compressor (ref. 1). The twin-screw compressor is a positive displacement compressor with a built-in volume ratio, no suction or discharge valves, no clearance volume, but with a certain amount of internal leakage. This fact makes it to behave differently from other positive displacement compressors, like the rec1procating compressor. Rating technique for reciprocating compresson is described in reference 2. The performance rating data has to be based on test data and/or performance simulation data (ref. 3, 4, 5). Such performance simulation programs should be used for analysing the compressor itself and they are normally too complicated and the executing time is too long for refrigeration system analysis. The rating equations described in this paper are more suited for refrigeration system design, and analysis, centered around the twin -screw compressor. CAPACITY CALCULATIONS The refrigeration capacity Q (kW) .1s: Q ; NVOL VS NM DH X X WX V where NVOL vs NM v DH volumetric efficency, theoretical compressor displacement (m3/rev), input male rotor speed (rpm), specific volume at suction (m3/kg), enthalpy difference over the evaporator (kJ/kg). (1) Seeing this equation it is natural to curve fit the volumetric efficiency instead of the refrigeration capacity. The volumetric efficiency is normally a straight line when plotted against pressure ratio (for constant discharge pressure). The volumetric efficiency drops when the pressure ratio is increased and the drop is magnified by: low buil t-.in volume ratio, low rotor speed, low viscosity of lubricant-refrigerant mixture.