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What Is a Thermal Expansion Valve?

By Jordan Weagly
Updated May 17, 2024
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A thermal expansion valve usually is a mechanical device in an air-conditioning or refrigeration system that regulates refrigerant material pressures. Within these systems, thermal expansion valves might be considered metering devices that simultaneously monitor and control the amount of refrigerant being used by a cooling system at any time. As the temperature changes inside a space, so does the amount of refrigerant necessary to maintain a desired temperature. The thermal expansion valve controls the variable levels of refrigerant.

In many cooling systems, refrigerant is held as a liquid in one chamber and released as a gas in another. This action drives the cooling process by pulling heat out of that space to change the phase of the refrigerant from liquid to gas and back to a liquid again. A thermal expansion valve is normally the point in a cooling system at which liquid is released to become a gas and initiate the cooling process.

This part of the process usually depends on the temperature of internal components within the entire cooling system. A typical thermal expansion valve includes a component called a thermal bulb. This chamber is filled with a small amount of the refrigerant liquid that is insulated from air temperatures and connected directly to a system of coils through which refrigerant passes, usually called an evaporator. When temperature changes occur, the thermal bulb is affected by those changes through its connection to the thermal expansion valve and the rest of the cooling system.

As the temperature changes across the system, especially within the evaporator, this indicates a change in temperature for the space being cooled. The material in the thermal bulb must expand and contract alongside any temperature change because the thermal bulb maintains a consistent size despite temperature changes. Higher temperatures in the cooling system force the liquid and gas inside the thermal bulb to expand. This expansion forces open a valve, allowing more refrigerant to pass into the evaporator. Lower temperatures, on the other hand, make the liquid in the thermal bulb contract, which closes the valve and allows less refrigerant to pass.

Thermal expansion valve installation is usually considered an integral part of refrigeration and air-conditioning systems of virtually any size. Many cooling systems integrate at least one thermal expansion valve, because they are considered quite accurate and consistent in their functionality. Systems that do not utilize a thermal expansion valve are often considered inaccurate or inconsistent.

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