The thermostatic expansion valve (TEV or TXV) is a device that is essential in managing refrigerant flow. For this reason, the TXV is useful in refrigeration and air conditioning units and is found next to the evaporator.
Functional Parts of the TXV
The TXV comprises several functional components that work harmoniously to achieve the needed efficiency. The valve body houses all these components; an opening called an orifice controls the refrigerant flow. Additionally, the valve body houses the diaphragm, a flexible material made of metal, and it flexes to apply the needed pressure to the pin.
The needle is another crucial functional component that moves rhythmically up and down to change the opening size in the orifice, thus regulating the refrigerant flow. There is also the spring located within the valve body, and its primary purpose is to counter the pressure exerted by the pin and works in the opposite direction. Furthermore, the valve body has a sensing bulb responsible for detecting the temperature level at the exit point of the evaporator. The reactions of the sensing bulb dictate whether the valve opens or closes.
Pressures Acting Inside the TXV
Typically, three pressures act on the inside of the TEV. The first pressure is exerted at the top area of the TXV and is usually within the power element. As a result of this pressure, the diaphragm moves down, and similarly, the needle also moves down, causing the valve to open.
An increase in the temperature within the evaporator leads to the gas expansion in the feeler bulb, causing the gas pressure in the power element to increase. As a result, the needle moves down, causing the valve to open.
Another pressure operates on the lower side of the diaphragm, resulting from the pressure inside the evaporator. This pressure usually moves the diaphragm upward, closing the valve opening. The third pressure is the spring pressure, which tends to be constant and works to close the valve opening.
The two latter pressures typically work to counteract the pressure at the top area of the diaphragm. Overall, the first pressure aims to open the valve, while the second and third pressure combination tends to close the TXV. As a result, if the valve has to open at a point, the first pressure must be greater than the combined force of the second and third forces.
How the Thermostatic Expansion Valve Works
Typically, the valve works to hold the liquid refrigerant, which has high pressure from making its way into the condenser and regulating the refrigerant flow. The thermostatic expansion valve causes a significant decrease in refrigerant pressure, thus allowing boiling at low temperatures. The boiling is essential as it helps the refrigerant to acquire heat and direct it into the compressor.
Next, the liquid refrigerant, which is under high pressure, is forced through the orifice opening, which significantly causes a reduction in its pressure when it flows past. As the pressure reduces, it causes some of the refrigerant to vaporize while some remains in a liquid state. This process has a similar operation to how a spray bottle nozzle works. When the trigger is pulled, the high-pressurized water inside is ejected through the small opening into a low-pressure external environment. This makes the water comes out as part liquid and part vapor.
The combination of the gas and liquid is injected into the evaporator and gains heat from the pipe. As the refrigerant flows, it is exposed to extreme thermal energy and changes state into a complex gas in the evaporator coil. As a result, the temperature remains constant because of the latent heat.
The refrigerant continues gathering thermal energy, and after the phase change, its temperature begins to increase, resulting in the superheating of the refrigerant vapor. The sensing bulb is alert to these temperature variances to regulate the refrigerant in the main valve body.
Effect of a Rise in the Cooling Load
When there is a significant rise in the cooling load, it results in more refrigerant evaporating within the evaporator. At the exit point of the evaporator, both the superheat and the temperature start to rise.
Since it’s desirable to keep the superheat temperature within correct limits, it’s essential to reduce its high temperature, which is made possible by letting more refrigerant flow. As a result, the pin is pushed downwards and exerts a force on the spring, which adjusts to facilitate the refrigerant to flow past the orifice in the valve body.
The sensing bulb notes the temperature change, and as a result, the refrigerant, usually held in a constrained space within the bulb, evaporates. Because of the limited area where this refrigerant is held, pressure builds up, moving through the capillary tube into the top of the TEV. The diaphragm is pushed down, exerting a downward force on the pin, which in turn exerts pressure on the spring, and it moves to enable the refrigerant to flow in the evaporator.
Next, the valve readjusts to ensure the pressure directed on the diaphragm is more than the pressure produced by the spring. This readjustment allows the desired refrigerant level to make its way through; consequently, the superheat lowers.
Lowering of the Cooling Load
If the cooling load faces a significant decrease, it leads to a decline in the superheat temperature. The sensing bulb detects this change, reducing the flow rate of the refrigerant. The refrigerant that is held in the capillary tube begins to settle back into the bulb, and the valve opening is closed. Eventually, the TXV equalizes, and the desired amount of refrigerant flows through and equalizes with the superheat settings.
Suppose the valve doesn’t respond to the superheat, in that case, it can allow the liquid to pass through into the compressor. The result can be fatal for the compressor since fluids aren’t easily compressible and lead to severe internal compressor damages.
The TEV is crucial in keeping the evaporator active by allowing refrigerant to flow per the cooling load. The valve allows the evaporator to operate as per the requirements and eliminates any possible wastage of evaporator capacity. The valve enables the enclosure air conditioner to work at optimal capacity to deal with the heat loads. For instance, it can function at a greater capacity when the heat load rises.
The TXV, therefore, is a significant part that works with precision to regulate how the refrigerant flows as a response to systematic conditions. For this reason, the TXV has gained popularity in the manufacturing of HVAC systems. This is all you need to be aware of in regards to the primary function of the TEV.
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