Thermostatic expansion valves (TXVs) come in many different shapes and sizes. Although they all are designed to control the superheat value of the refrigerant leaving an evaporator, there are some differences among TXVs that either enhance their operation or slightly change how they operate. One of these differences is in the design of the valve’s power element. There are different types of power assemblies that can be used on the body of an expansion valve. One of these types is the maximum operating pressure (MOP) power element.

An expansion valve with an MOP-type power element is designed to limit the evaporator pressure from rising above a predetermined value. Above its MOP setting, the valve will essentially close, preventing the evaporator pressure from rising higher. This prevents a compressor’s motor from overloading if the suction pressure reaches a value beyond its designed operating parameters. This type of valve tends to serve the same basic function as a crankcase pressure regulator (CPR). Because of this, it is not normally recommended that an MOP-type expansion valve and a CPR be used on the same system as they may fight each other for control of the suction pressure.

The limiting characteristic of an MOP expansion valve is obtained from the refrigerant charge within its sensing bulb. The sensing bulb will have a limited quantity of liquid refrigerant. At a predetermined temperature, all of the liquid refrigerant will have evaporated and any further increase in the sensing bulb’s temperature will have a negligible effect on opening the valve’s port. Since the bulb pressure is the opening force of a TXV, the port within the valve will essentially close, preventing any further increase in the evaporator pressure.

Although this type of valve does offer the advantage of protecting a compressor from overloading, there is a disadvantage with the use of an MOP TXV. If the body of the valve becomes colder than the sensing element, the bulb’s charge can migrate to the valve body and the sensing bulb will lose its ability to properly control the operation of the valve. For this reason, it is recommended that the sensing bulb be insulated and the valve body be installed in a warmer location than the sensing bulb. So remember, if during a repair you remove the insulation around the bulb of the TXV, you should always replace it before leaving the job. If the power element is an MOP type, you can affect the operation of the system and create an additional problem. 

Also, when adjusting the superheat spring tension of this type of valve, you will slightly change the MOP setting of the valve. An increase in the superheat spring pressure (turn the valve stem clockwise) will slightly decrease the MOP setting. A decrease in the superheat spring pressure (turn the valve stem counter-clockwise) will slightly increase the MOP setting.

Understanding how a TXV with an MOP power element operates differently from other expansion valves will definitely help a technician better understand the operation of a system and troubleshoot the system more efficiently. 

Publication date: 8/7/2017

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