What is an Expansion Valve?
Expansion valves are expansion devices used to control the flow of refrigerant within a refrigeration system. Expansion valves work to facilitate the change of refrigerant from a higher-pressure liquid state in the condensing unit to a lower-pressure gaseous state in the evaporator. Expansion valves can be found throughout the buildings you occupy, in any machine with a refrigeration cycle, and even in the toilets you use. Understanding how expansion valves work can help you to better understand some potential solutions to problems that arise with these devices.
Types of Expansion Valves
The most commonly found types of expansion valves are automatic expansion valves, thermostatic expansion valves (also known as thermal expansion valves), and float valves.
Automatic Expansion Valves
Automatic Expansion Valves (AEV) regulate the flow of liquid refrigerant from the liquid line to the evaporator by using a pressure-actuated diaphragm. A crucial role of AEVs is to maintain a specific superheat in the evaporator to make sure heat absorption within the evaporator is efficient.
The high-pressure liquid refrigerant entering the expansion valve is quite warm. This may be verified by feeling the liquid line at its connection to the expansion valve. The liquid refrigerant leaving the expansion valve is quite cold. The orifice within the valve does not remove heat, but accomplishes heat transfer only by reducing presÂsure. Heat molecules contained in the liquid refrigerant are thus allowed to spread as the refrigerant moves out of the orifice. Under greatly reduced pressure, the liquid refrigerant is at its coldest as it leaves the expansion valve and enters the evaporator.
Pressures at the inlet and outlet of the expansion valve will closely approximate gauge pressures at the inlet and outlet of the compressor in most systems. The closeness of the components causes the similarity of pressures to each other. The slight variation in pressure readings of a very few pounds is due to resistance, causing a pressure drop in the lines and coils of the evaporator and condenser.
Two types of valves are used on machine air conditioning systems:
- Internally-equalized valve – most commonly used valve
- Externally-equalized valve – used in systems needing more special control
Thermostatic Expansion Valves
Thermostatic Expansion Valves (abbreviated TXV and TEV) use a valve mechanism to control the flow of liquid refrigerant into the evaporator coil. The pressure in the evaporator controls the flow. This type of metering device can operate well when the load fluctuates and hence is suitable for use in air conditioning systems. When the evaporator warms, the valve provides a higher flow rate, and when it cools, it reduces the flow rate.
A sensing bulb detects the temperature of the coil and is usually located at a higher temperature within the evaporator. Inside the sensing bulb is a small amount of refrigerant. This refrigerant is separated from the rest of the system, constrained to just the volume within the bulb and capillary tube. When the temperature of the evaporator increases due to the demand for cooling, the pressure in the bulb will also increase, hence pushing the spring to open the valve.
Similarly, when the temperature of the evaporator reduces due to a lack of demand for cooling, the pressure in the bulb will drop, hence causing the spring to close the valve.
Float Valve
Float Valve is actuated by a float that is immersed in the liquid refrigerant. The low-side float and high-side-float are used to control the flow of liquid refrigerant. The low-pressure side float helps to maintain a constant level of liquid refrigerant in the evaporator. It opens when there is no liquid in the evaporator and closes when there is liquid in the evaporator.
The high-side float is located at the high-pressure side of the system and maintains a constant level of refrigerant in the condenser. When the compressor operates, the condensed refrigerant flows to the float chamber and opens the valve. This causes the refrigerant to flow into the evaporator where it is stored. As the liquid level falls in the float chamber, the valve opening will close, hence preventing the liquid from flowing to the evaporator.
Common Uses for Expansion Valves
Expansion valves are an essential part of almost all refrigeration cycles. Thermostatic expansion valves are most commonly used in your HVAC system, while almost every toilet has a float valve. Your AC also might contain a float valve, to prevent the overflow of water and resulting damage to your home. Automatic expansion valves are used in a wide range of refrigeration systems, including air conditioning systems, heat pumps, and commercial refrigeration and air conditioning units.
FAQs About Expansion Valves
What is an expansion valve and what does it do in my HVAC system?
Expansion valves are a crucial component in any refrigeration cycle. They control the flow of refrigerant into the evaporator coil and regulate the pressure and temperature of the refrigerant. Thermostatic expansion valves (TXV) are the most common type found in residential homes, while electronic expansion valves (EEV) are more precise and found in newer systems.
How can I tell if my expansion valve is faulty or needs replacement?
Unfortunately, symptoms of a faulty expansion valve are similar to those indicating other AC issues. That said, if your expansion valve needs to be replaced, you may be seeing frost or ice on the evaporator coil, higher energy bills, and reduced system efficiency.
You may also notice your AC isn’t as cool as normal and runs in shorter cycles.
Can I replace the expansion valve in my AC?
Experts do not recommend that homeowners try to replace or repair expansion valves themselves. Beyond safety concerns surrounding working with a high-pressure system, working with hazardous chemicals, like refrigerant, requires a license.
Professional-grade tools are also necessary to make these repairs, which are not cheap and are not common in household tool boxes.
