Thermostatic expansion valve capacity is dependent on vapor free liquid entering the valve. If there is flash gas in the entering liquid, valve capacity is reduced substantially because• Refrigerant mass flow passing through the valve is significantlydiminished because the two-phase flow has a lower density• Flow of the compressible vapor fraction chokes at pressure ratiosthat typically exist across expansion valves and further restrictsliquid-phase flow rate• Vapor passing through the valve provides no refrigerating effect. Flashing of liquid refrigerant may be caused by pressure drop in the liquid line, filter-drier, vertical lift, or a combination of these. If refrigerant subcooling at the valve inlet is not adequate to prevent flash gas from forming, additional subcooling means must be provided.