Screw compressor

Abstract

A screw compressor is for use of a screw chiller, and comprises a pair of screw rotors and a casing housing the screw rotors, a capacity control valve for varying a ratio of volume, a motor for driving the screw rotors and an inverter for varying the rotational speed of the motor. The screw compressor is controlled using rotational speed control means by the inverter and mechanical capacity control means by the capacity control valve independently or combined together according to loads. The maximum efficient point in a capacity control performed solely by the inverter is set to a rotational speed side lower than the rated operation point. In a region where the rotational speed is higher than the maximum efficient point, the inverter solely takes control from a rated rotational speed to a high rotational speed side.

1 . A screw compressor for a screw chiller, comprising:
 a pair of screw rotors;   a casing housing the screw rotors;   a capacity control valve for varying a ratio of volume;   a motor for driving the screw rotors; and   an inverter for varying a rotational speed of the motor,   wherein the screw compressor is controlled using rotational speed control means by the inverter and mechanical capacity control means by the capacity control valve either independently or combined together according to loads, and   wherein the maximum efficient point in the capacity control performed solely by the inverter is set to a rotational speed side lower than a rated operation point, and in a region where the rotational speed is higher than the maximum efficient point, the inverter solely takes control from a rated rotational speed to a high rotational speed side.   
   
   
       2 . The screw compressor according to  claim 1 , wherein when an operation at a rotational speed region at or below the maximum efficient point is demanded, the rotational speed control means by the inverter and the mechanical capacity control means by the capacity control valve are used concurrently according to capacities so that efficiency becomes the maximum. 
   
   
       3 . The screw compressor according to  claim 1 , wherein the capacity control valve varies a compression starting position provided in the casing. 
   
   
       4 . The screw compressor according to  claim 1 , wherein the maximum efficient point is set to around 80% of a rated refrigerating capacity when an independent capacity control by the inverter is performed. 
   
   
       5 . The screw compressor according to  claim 2 , wherein a position of the capacity control valve is controlled based on the rotational speed of the motor according to a suction pressure, a discharge pressure and a load of the pair of the screw rotors. 
   
   
       6 . The screw compressor according to  claim 1 , wherein when an anomaly occurs on the inverter and a continuous operation of the motor by the inverter is made impossible, the motor is directly connected to a commercial power in an emergency manner and the capacity controlled operation by the capacity control valve is continued same as before.