What is an Expansion Valve?

Jeri Sullivan

An expansion valve is a piece of equipment that reduces pressure in a system. The most common form of these valves is a thermal expansion valve (TMV), which is used in heating, ventilation, and air conditioning (HVAC) systems. The two main types of air conditioner expansion valves are thermostatic expansion valves and capillary tubes.

An air conditioning unit has three main parts: the compressor, the condenser and the evaporator, which has the expansion valve.
An air conditioning unit has three main parts: the compressor, the condenser and the evaporator, which has the expansion valve.

A thermostatic expansion valve controls the flow of refrigerant and can function at various temperatures or pressures. The capillary tube is responsible for transmitting the pressure levels in the system to the sensing tube attached to the TMV and basically acts as a throttle. An air conditioning unit has three main parts: the compressor, condenser, and evaporator. This valve is part of the evaporator.

To cool a building, refrigerated gas, such as Freon®, runs through the compressor where it becomes hot and pressurized. The gas then runs through coils that cause the heat to dissipate, which condenses it back to a liquid. The condensed liquid then runs through an expansion valve where it evaporates and becomes a cold gas, which pulls heat from the surrounding air. The gas runs through another set of coils, and a blower blows the cold air inside the building and the heat that was generated during the process is vented to the outside. A refrigerator or freezer works in the same manner.

The entire process is performed under pressure, which requires the use of a thermostatic expansion valve. When the pressure inside the compressor reaches a preset level, the TMV opens, which allows the pressure to decrease. The Freon® then moves through the valve to the evaporator. This maintains both the flow and pressure within the air conditioning system.

The capillary tube contains a filter on one end made of a fine mesh, which prevents debris from getting inside the tube. It works in much the same way as a thermostatic expansion valve, but it cannot withstand significant changes in temperature or pressure. The capillary tube is designed for constant temperature and pressure, and operates as a throttle between the condenser and evaporator.

The tube is typically long and narrow to allow for some Freon® evaporation to occur as it travels through the tube. This is required in order to make the Freon® entering the evaporator similar in temperature to that already in the evaporator. Similar temperatures make the system run more efficiently and speed up the cooling process.

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Discussion Comments


Okay, the part where you're talking about freon says the cold liquid has to be heated then recooled. Why does it have to start out refrigerated if the point is to compress it to heat it into a gas?

I know that the expansion valve is a pressure relief valve that lets off the pressure once it's liquid to let it expand into gas again, this time cold. Maybe the reason it starts out cold is because a certain amount of pressure is required?

I'm still trying to figure this out because I want a way to a portable air conditioner that I can run on solar power. I think I have a long way to go but it would be awesome to be able to cool a car with a broken A/C using the power off the very sun that's giving you the overheating problem.

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