Pilot Operated Absolute Suction Throttling Valve
Most modern STVs are of the pilot operated absolute (POA) type. A POA STV is really nothing more than a spring-loaded valve, controlled by an evacuated bellows and needle valve assembly inside a housing. The valve operates independently of atmospheric pressure, and is unaffected by changes in altitude. By providing an opposing force to evaporator pressure, the valve can maintain the evaporator pressure with a high degree of accuracy (within a 1 psi range).
As long as the evaporator pressure remains above a predetermined level, the POA STV remains open to allow refrigerant to flow freely out of the evaporator. When the pressure drops below the predetermined level, the valve closes, and the refrigerant flow from the evaporator is restricted. The pressure in the evaporator then increases, which raises the temperature and prevents icing on the outside of the evaporator. The opening and closing cycle of the valve continues as long as the compressor is running.