A thermal expansion valve in refrigeration or air conditioning systems uses a temperature sensing bulb to open a valve, allowing liquefied refrigerant to move from the high pressure side of the system to the low pressure side before entering the evaporator. As the evaporator temperature decreases, pressure on the bulb decreases, allowing a spring to close the valve, according to Reference.com.
Liquid refrigerant on the high side of the system is warm. The expansion valve removes pressure from liquid refrigerant so that is able to expand and convert from a liquid to a vapor as it enters the evaporator. As refrigerant leaves the evaporator valve, it is significantly colder. The orifice inside the valve removes pressure but not heat. However, as the molecules of liquid refrigerant spread, heat disperses due to the phase change. In the refrigeration cycle, refrigerant is coldest as it leaves the expansion valve.
Inside the evaporator coil, the cold gas absorbs heat from inside the home to provide cooling. This warm gas travels through the system to the compressor. Increasing the pressure of the refrigerant causes an increase in its boiling point, concentrating the heat absorbed from the home to the temperature of the outside air. As this hot gas passes through the condenser coils, heat transfers to the cooler outside air, causing the liquid to condense into a liquid.