Expansion Valve
The expansion valve meters the flow of refrigerant into the evaporator to match the refrigerant flow with the heat of the air passing through the evaporator.
The expansion valve is attached to the inlet and outlet ports of the evaporator. The valve consists of an aluminum housing containing inlet and outlet. The expansion valve controls the amount of refrigerant released into the evaporator. It is fitted to the evaporator inlet/outlet pipes. The valve separates the high-pressure side of the system from the low-pressure side. A small passage, or "orifice," allows only a small amount of liquid into the evaporator. The amount of refrigerant that it allows through depends on the evaporator temperature and pressure, and the temperature of the air passing through the evaporator.
A block-valve design of expansion valve is used on MINI A/C systems. The refrigerant enters at the upper right inlet. At the left of the valve there is a capillary tube filled with an inert gas, that senses the temperature of the air coming into the housing from the plenum. When the air temperature in the plenum rises, the pressure in the capillary tube increases. This pushes down on a diaphragm and pushrod assembly, which increases the size of the orifice opening, allowing more refrigerant into the evaporator and providing more cooling. When plenum temperature falls, the pressure in the capillary tube falls. The spring pushes up on the pushrod, making the orifice opening smaller; less refrigerant is allowed into the evaporator, allowing less cooling.