Shutdown process simulation of a refrigeration plant having a twin screw compressor

Non-return valves in refrigeration systems are sources of inefficiency due to flow pressure drop, and of unreliability due to the accumulation of flow debris. Hence the attractiveness of schemes to remove them. In twin screw compressor plants of a certain configuration the suction non-return valve was removed and a reverse rotation brake was fitted to the free end of the motor shaft to prevent backflow of the (oil-laden) refrigerant and reverse rotation of the compressor rotors immediately following plant shutdown. Brake failures occurred on site. This paper describes one part of the subsequent investigation, namely the adaptation of a program created to calculate normal running torque to the process calculating the maximum static torque to occur following shutdown. Shutdown torques up to 60% greater than the running torque are predicted but are thought to be underestimates due to the distribution of oil following shutdown being different from that during normal operation. The authors believe that, following shutdown, the oil seals the exit leakage path across the contact line but clears away from the inlet path through the blow hole allowing unexpectedly high cavity pressures and torques to develop. In existing plants fitted with brakes chosen on the assumption that the shutdown torque will be no greater than the running torque, a safe shutdown may be obtained by opening the slide valve prior to shutdown.