Expansion Valve: Description and Operation

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.

If too little refrigerant enters the evaporator, poor cooling results. If too much refrigerant enters, it might not completely boil away and liquid refrigerant might return to the compressor, causing damage to the system.

A block-valve design of expansion valve is used on current BMW A/C systems. In the illustration on the opposing page, refrigerant enters at the upper right inlet. At the top 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.







Refrigerant from the outlet of the evaporator passes through the bottom left opening of the block valve. When the pressure at the evaporator outlet is high, this increases the pressure needed by the capillary tube, to open the valve Less refrigerant is provided to the evaporator (to prevent the evaporator from being flooded). When pressure at the outlet end of the evaporator is lower, less pressure is exerted on the bottom of the diaphragm. The diaphragm pushes down on the pushrod, allowing more refrigerant into the evaporator.