Abstract: In the conventional mono-component refrigeration with propylene，ethylene or methane and the latest multi-component mixed refrigeration of ethylene plants，the superheated extent of returning refrigerant vapor to be compressed varied from null or a big value to am appropriate degree are discussed. The restrictions between superheated extent and its implementing elements，such as the type of heat exchanger used，the temperature control of outlet vapor and the addition intermediate cooler of refrigerant compressor are presented. Its influences on the introduction of inter-stage suction drums，the optimized design of liquid quench line and liquid drain system are analyzed further.