Expansion Valve - How it Works

THERMAL EXPANSION VALVE

A thermal expansion valve (often abbreviated as TEV, TXV, or TX valve) is a component in refrigeration and air conditioning systems that controls the amount of refrigerant flow into the coil thereby controlling the superheating at the outlet of the evaporator. Thermal expansion valves are often referred to generically as "metering devices".

Flow control, or metering, of the refrigerant is accomplished by use of a temperature sensing bulb, filled with a similar gas as in the system. The sensing bulb mounts on the refrigerant line leaving the coil.  As the temperature on the bulb increases, the refrigerant expands, pressing down to open the needle valve, allowing more refrigerant flow to the coil. As the suction line temperature decreases, so does the pressure in the bulb and therefore on the spring causing the need valve to close.

COMMON EXPANSION VALVE FAILURES

The most common failure with an expansion valve is loss of charge in the sensing bulb. With no refrigerant pressure to expand and open the needle valve, the expansion valve remains in the closed position, not allowing refrigerant flow into the coil, resulting in very low suction pressure or pressures in a vacuum.

The most common cause of refrigerant pressure loss is a broken or worn-through sensing bulb line.

• Look for areas where the sensing bulb line has come into contact with other copper lines or metal cabinetry.
• Look and feel to see if the line has been rubbed and compromised.

It is good maintenance practice to make sure the sensing bulb line is not touching itself or other copper lines.

TESTING AN EXPANSION VALVE

1. Temperature Response Test
   • The best way to test an expansion valve is to see how the suction pressure responds to a change in the bulb temperature. Remove the sensing bulb from the suction line and change its temperature.
     • As temperature increases, the suction pressure should climb.
     • As temperature decreases, the suction pressure should drop.
   • If the pressure does not respond to a temperature change, the valve is likely bad.
2. Superheat Test
   • At proper charge, an expansion valve should be set to maintain a superheat between 8 and 18 deg.
     • Check system subcooling to make sure system has adequate charge. 
     • Check the system superheat.
     • If the superheat is high (25 deg or higher), the valve may be bad.