a vapor compression system includes an evaporator , a compressor , and a condenser interconnected in a closed - loop system . in one embodiment , a multifunctional valve is configured to receive a liquefied heat transfer fluid from the condenser and a hot vapor from the compressor . a saturated vapor line connects the outlet of the multifunctional valve to the inlet of the evaporator and is sized so as to substantially convert the heat transfer fluid exiting the multifunctional valve into a saturated vapor prior to delivery to the evaporator . the multifunctional valve regulates the flow of heat transfer fluid through the valve by monitoring the temperature of the heat transfer fluid returning to the compressor through a suction line coupling the outlet of the evaporator to the inlet of the compressor . separate gated passageways within the multifunctional valve permit the refrigeration system to be operated in defrost mode by flowing hot vapor through the saturated vapor line and the evaporator in a forward - flow process thereby reducing the amount of time necessary to defrost the system and improving the overall system performance . in one preferred embodiment of the invention , a heat source is applied to the heat transfer fluid after the heat transfer fluid passes through the expansion valve and before the heat transfer fluid enters the evaporator . the heat source converts the heat transfer fluid from a low quality liquid vapor mixture to a high quality liquid vapor mixture , or a saturated vapor .