Hot gas defrost for evaporators

Hot gas defrosting consists of stopping the flow of refrigerant to the evaporator and introducing steam at high pressure. The flow chart of a hot gas defrost with dry expansion evaporator is shown in the figure. During the defrost, the solenoid valve closes shutting off the flow of liquid to the evaporator. The hot gas defrost valve opens by introducing steam from the compressor discharge which initially heats the evaporator tray and then the inside of the coil by defrosting it. A pressure control valve maintains a back pressure in the evaporator in order to increase the saturation temperature. A check valve must be installed between the inlet and outlet of the evaporator in order to prevent expansion valve fluid from migrating to the hot gas manifold during normal operation