Thermostatic expansion valves (TXV) are responsible for metering the proper amount of refrigerant into an evaporator coil. When they fail, they either cause the evaporator to be flooded (overfed) or starved (underfed) for refrigerant. This can be observed by measuring the superheat value of the refrigerant as it leaves the evaporator.

A TXV that overfeeds an evaporator can be observed by measuring a lower than normal refrigerant superheat value at the outlet of the evaporator.

A TXV that underfeeds an evaporator can be observed by measuring a higher than normal refrigerant superheat value at the outlet of the evaporator.

Although a defective TXV can result in either a higher or lower refrigerant superheat value, it is definitely not the only possible cause. Before condemning a TXV, all other possible causes should be investigated and ruled out before looking at the TXV.

A defective TXV that results in a starved evaporator can be the result of a lost refrigerant charge within the sensing bulb of the valve. This is due to the fact that the refrigerant charge in its sensing bulb is the opening force of the TXV. When a loss of charge exists, there is not a sufficient force to throttle the valve open to its correct position.

PROCEDURES

The following procedure can be used to see if loss of charge in the bulb of a TXV is the cause of a higher than normal evaporator superheat at the outlet of the evaporator:

• First, if the valve has an external superheat adjustment, turn the adjustment nut counter- clockwise several turns. Allow the system to operate for 10 to 15 minutes and monitor the evaporator’s superheat value. If the evaporator’s superheat has lowered, the valve may just need to be adjusted. Try to further adjust the valve to obtain the correct evaporator superheat value.

• Next, remove the bulb from the suction line and hold it in your hand for several minutes to warm it up. Observe the suction pressure. If the valve has a refrigerant charge, it should open and you should see an increase in the suction pressure.

• If no change in the pressure occurs, it is reasonable to conclude that the TXV is defective and a likely cause is its sensing bulb has lost its refrigerant charge.

• Some TXVs have a power assembly that can be removed and replaced instead of changing the entire TXV. Once the power assembly has been removed, the bulb charge can also be checked. Try depressing the diaphragm with your thumb. If you are able to do this, then the assembly has lost its charge.

If a replacement TXV or power assembly cannot be immediately replaced and the system needs to be operational, one trick of the trade to use as a temporary fix is to install a dime or similar sized solid disk in between the power assembly and the body of the TXV. This will depress the push rods within the TXV and cause the valve opening to go to a more open position. The TXV will now meter more refrigerant in the evaporator but the valve will continue to feed at the same rate.  It is possible now to cause too much refrigerant to enter the evaporator, leading to liquid refrigerant flooding back to the compressor, resulting in compressor damage. When using this trick be sure to understand the pros and the cons of its application. You may fix the immediate problem but cause a more serious issue for you and your customer.

Publication date: 08/02/2010