How a thermal expansion valve works
What's inside a Thermal Expansion Valve and how the Thermostatic Expansion Valve or TXV works in an HVAC refrigeration system. Learn the basic operating principles that allow the thermal expansion valve to meter the flow of refrigerant to the evaporator of chillers and air conditioning units.
Scroll down to see the YouTube video
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where do you find the regulator
Expansion valves are located between the condenser and the evaporator in a refrigeration cycle.
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| Where to find a thermal expansion valve |
In this model that is a T2 thermal expansion valve. We have the main body which is made of brass. In the body we have the refrigerant inlet at the bottom of the valve. Then the coolant outlet on one side and on the other side we have a plug that can be removed. Below is a screw that is used to manually adjust superheat, we'll see how that works later.
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| Parts of a TXV |
At the top we have this big head called a power head, a very thin coil of tubing known as a capillary tube and there's a large bulb at the end known as a sensor bulb, they're all stainless steel.
The coil is stretched and the bulb sits on the evaporator outlet to detect superheating.
We have already covered in detail how thermal expansion valves and electronic expansion valves work as well as the different types of expansion valves used in chillers.
What are thermal expansion valves used for?
Expansion valves control the flow of refrigerant through the evaporator in response to the cooling load. It measures superheat at the outlet and responds by increasing or decreasing the amount of refrigerant flowing through the evaporator to try to maintain a constant superheat. It also ensures that the refrigerant boils off the evaporator as slightly superheated vapor and prevents liquid refrigerant from entering the compressor. Liquids cannot be compressed, so if they get into compressors they can cause serious damage and even destroy them.
The bulb is filled with a refrigerant that is completely separate from the refrigerant in the rest of the system, these two refrigerants never meet or mix, they are always separate.
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| inside a thermal expansion valve |
Overheating causes the refrigerant inside the bulb to boil, and as it boils, it builds pressure. The pressure moves along the hollow capillary tube and into the power head. The motorized head controls the flow of coolant and then we'll look inside.
We put a removable cartridge inside the inlet of the regulator. This has an orifice that works with the valve to control the flow of coolant. They come in different sizes depending on the cooling capacity and the refrigerant used.
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| TXV Removable Orifice |
I'm just going to put the valve in a vise to hold it steady while I open it up.
Due to the delicate parts inside, I used a hacksaw to open this one up. It takes a long time, but the angle grinder could break the internal components and I want to be able to show you those parts.
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| What is inside a thermal expansion valve? |
We have the main body that holds everything together.
We have the refrigerant inlet coming out of the lower part of the main body, in this vertical tube. Next we have the refrigerant outlet in the horizontal tube. Thus, the refrigerant leaves the condenser and enters the valve body through the inlet as a saturated liquid at high pressure and medium temperature. It then goes through the valve body and when it exits, it exits the valve through the outlet and then it will be a low pressure, low temperature liquid/vapor mixture.
So what causes the change of pressure, temperature, phase and also controls the flow of refrigerant. Well, we can see this little pin here.
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| thermal expansion valve pin |
It is connected to the diaphragm in the power head. The diaphragm is a thin sheet of flexible metal. As the diaphragm moves up and down, the pin moves up and down. Underneath the diaphragm we have a spring that pushes against the diaphragm, we can use that to adjust the superheat and we'll get to that a bit later in the video.
Above the diaphragm, we have this empty chamber that is connected to the capillary tube and then to the sensing bulb. The chamber, capillary tube, and bulb are all hollow.
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| Inside a thermal expansion valve sensing bulb |
I'm just going to cut the sensor bulb with an angle grinder to show you the inside, as you can see it's just an empty cylinder that is usually filled with coolant. The refrigerant in the bulb and capillary is completely separate from the main refrigerant circulating in the system, this isolated refrigerant only moves between the bulb, capillary tube and the top of the diaphragm.
The sensor bulb is located at the outlet of the evaporator. As the cooling load on the evaporator increases, the superheat at the evaporator outlet increases. Since the sensing bulb is in direct contact with the tubing at the evaporator outlet, heat energy is transferred and causes the refrigerant within the sensing bulb to expand and boil. Because the sensor bulb, capillary tube and chamber are all hollow and form a sealed system. As the refrigerant expands and boils, the internal pressure increases. This pressure travels along the capillary tube and into the chamber above the diaphragm. As the pressure builds, it pushes down on the diaphragm and the diaphragm pushes down on the pin. The pin controls how much coolant can flow, but to do that we need an extra part.
Inside the valve inlet we place an orifice assembly. These are replaceable and available in different sizes to suit the cooling capacity and refrigerant used. Inside is a small strainer that protects the valve from clogging, then there is a small orifice, or hole, which is blocked by a spring-loaded plug. Our pin, in the main valve, presses on this plug to open the valve. The further down the cap is pushed, the more coolant can flow.
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| Thermal expansion valve port |
As the refrigeration load on the evaporator increases, the superheat at the outlet increases. The sensing bulb at the outlet detects this and the refrigerant inside boils, causing the pressure to rise along the capillary tube.
This pressure pushes the diaphragm down and this pushes down on the pin, which opens the valve and allows more refrigerant to flow. As the refrigerant circulates, the superheat decreases and therefore the pressure in the sensing bulb and capillary tube decreases, meaning there is less pressure pushing the diaphragm down. The spring then pushes the diaphragm up, causing the pin to move up, and as the pin moves up, the spring-loaded cap begins to close the port, reducing the amount of refrigerant that it can flow. This is constantly repeated and stabilizes the valve to ensure the correct amount of refrigerant flows.
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| How to adjust the superheat in the expansion valve |
We mentioned setting the superheat control earlier. Well this cap on the side is internally threaded and if we turn it, it will move the slider up or down depending on how we turn it. As the slider is moved up and down, the force applied by the spring to the bottom of the diaphragm changes, which changes the sensitivity of the device and allows you to adjust the expander and adjust the superheat.
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