Obtaining the Optimum Geometrical Parameters of a Refrigeration Helical Screw Compressor

This paper presents a method for optimizing the geometrical pa.rameters of a refrigeration twin-screw compressor. The method optimizes rotor profile and discharge port position etc., by making use of two efficient computer programs; the geometrical parameter calculation program and the performance prediction program. Two new parameters; relative blow hole area and relative contact line le~th, are introduced to compare the blow hole areas and contact line len!!;ths of different rotor profiles. These new parameters, which are .determined hy profiles alone, have an important and direct influence on the performance of the compressor, so that they are suitable for used in an optimization ·process. A technique for reducing the blow hole area by manipulating its shap" in accordance with a logical proc.,dure is described. Calculation and test results arP included and discussed in the paper. INTRODUCTION Altuough many factors influence the performance of a helical screw compressor, they may he grouped into two categoriesas follows: • The operating parameters such as rotational speed, quantity of oil or liquid refrigerant injected etc .. • The geometrical parameters such as contact line length per lobe, blow hole area and discharge port position etc .. The increase of the quantity of liquid refrigerant injected always results in a slight decrease of the coefficient of performance but the use of the injection of liquid refrigerant eliminates the need for separate oil cooling. If the discharge temperature is lower than that permitted, the quantity of liquid refrigerant injected should be as small as possible. This quantity can he predicted by the computer program of the mathematical model for the working process. Increases of rotational speed and quantity of oil injected always result in a decrease of leakage through any leakage path the consequence of which is an increase in the volumet,ric efficiency. But on the other hand these increases will result in increases of fluid-dynamic losses, fluid and mechanical friction losses etc., so as to increase the energy consumption. For the rotational speed and quantity of oil injected there exist optimum values, between which there is a relationship. So far these optimum values have been ol>taiued from tests or experience only, since there are no reliable theories of fluid-dynamics and fridion yet, which can he used to develop a mathematical model to predict the increase of energy cons urn ption. The purpose of the procedure for the optimization of the geometrical parameters is to combine the smallest blow hole area, with the shortest contact line length per lobe and the correct discharge port position suitable for a given discharge pressure. This can be achived by the use of test results and two computer programs: • The geometrical calculation program. • The program simulating the working process. For a give~ wrap angle and screw pitch, the blow hole area and contact line length per lobe are totally decided by the end profiles of the male and female rotors. The purposes of optimizing the