The evaporator is a device which cools and dehumidifies the air before the air enters the vehicle. High-pressure refrigerant flows through the expansion tube and becomes low-pressure refrigerant before entering the evaporator. The heat in the air passing through the evaporator core is lost to the cooler surface of the core. This cools the air. As the process of heat loss from the air to the evaporator core surface is taking place, any moisture (humidity) in the air condenses on the outside of the evaporator core. This moisture is drained off as water.