The constant pressure valve is the forerunner of the thermostatic expansion valve. It is called an automatic expansion valve due to the fact that it opens and closes automatically without the aid of any external mechanical device. These expansion valves are basically pressure regulating devices. These valves maintain a constant pressure at outlet. They sense and keep the evaporated pressure at a constant value by controlling the liquid-refrigerant flow into the evaporator, based on the suction pressure. Refrigerant flows at a rate that exactly matches compressor capacity. Their applications are limited because of the constant cooling load.