The evaporator is tasked with removing heat and humidity from the passenger’s compartment. It’s located in a plastic housing, typically a heating/cooling unit. It’s a heat exchanger consisting of aluminum tubes and fins. A metering device controls the temperature of the evaporator by controlling the flow of liquid refrigerant into the core. A decrease in flow and pressure from the metering device causes a decrease in temperature in the evaporator core.
Refrigerant enters the evaporator from the bottom inlet tube as low pressure liquid. As it passes through the core and toward the top, it starts absorbing heat. Warm air from the passenger’s compartment cools as it passes through the evaporator’s fins and tubes.
Refrigerant has a very low boiling point. As it absorbs heat from the cabin, it changes its state from a liquid to a vapor. This heat exchange causes the refrigerant to boil and expand. This expansion phase is why the outlet tube is larger than the inlet.