Flash Gas in the Liquid Line

Flash gas at the inlet of the TEV will obstruct the flow of liquid refrigerant, and therefore, starve the evaporator.

Note: If the pressure on a liquid line refrigerant decreases while its temperature remains the same, or the temperature increases while the pressure stays the same, some of the refrigerant will flash into gas. If this occurs and flash gas enters the TEV:
a) The valve will operate inefficiently.
b) The system loses some of its capacity.
c) Superheat increases.

Probable Cause: Remedy:
Insufficient refrigerant charge. Add refrigerant to the system- this raises head pressure. There are several methods to follow for charging a system:
Excessive pressure drop in liquid line due to fluid friction or excessive vertical list of liquid line. If there is no sight galss in the liquid line at the TEV inlet:
a.) Flash gas can sometimes be detected by a characteristic steady whistling sound at the TEV.
b.) Flash gas can also be detected by installing a pressure tap at or near the TEV inlet. Take a pressure and temperature reading at this location. If the temperature measured is above its saturation temperature (taken from a pressure-temperature chart), then flash gas is present
a) On a TEV non-receiver system, by subcooling the liquid refrigerant by 10°F (at full load, if possible).
b) On a TEV receiver system, charge per the usual manner until no bubbles are seen thru the sight glass.
c) On a system which has a condenser, receiver, and pressure adjustable valves, the minimum pressure diff. should be 10 psi between condenser flooding valve (IPR) and receiver pressure valve (OPR).
d) Or, you can sufficiently subcool the liquid to ensure solid refrigerant at the TEV inlet.
Lack of positive head pressure control will cause erratic head pressures. Review or install positive head pressure control.