The thermostatic expansion valve, commonly known as the TEV or TXV, is a device mainly used in air conditioning and refrigeration units to control the refrigerant that flows into the evaporator. The controlled flow is essential in achieving the evaporator’s optimal efficiency and eliminating the excess refrigerant from flooding back into the compressor chamber.

Understanding Parts of the TEV

The thermostatic expansion valve consists of the valve body, which encloses the other functional components. The diaphragm is made of a thin, flexible, metallic material designed to move and apply force on the pin. The pin, also referred to as the needle, moves up and down to regulate the opening of the orifice, which determines how the refrigerant flows. The spring is essential in counteracting the force of the pin and usually operates in the opposite direction. Within the valve body, there is the sensing bulb and capillary line. The bulb is used to measure the temperature levels at the exit point in the evaporator. Based on the bulb’s temperature reading, it influences the opening or closing of the valve.

Understanding How the TEV Controls the System

The TEV has a design feature to separate high pressure and low-pressure areas within the air conditioning system. Typically, the TEV is responsible for limiting the amount of liquid refrigerant that flows into the evaporator. Doing so reduces the refrigerant’s pressure as it flows through the fluid line past the orifice opening.

By regulating how the refrigerant flows into the evaporator, the TEV ensures a steady supply of the refrigerant while at the same time matching its flow rate against the speed at which the refrigerant boils off within the evaporator coil.

For the evaporator to match this flow, it reacts to the variables of the temperature of the vapor of the refrigerant as it’s exiting the evaporator, and the pressure within the evaporator itself, by using the movable pin against the spring pressure to regulate the refrigerant flow into the evaporator with high precision.

Understanding How the TEV Transfers Energy

The TEV restricts the flow of the refrigerant, causing a pressure drop of the refrigerant. As a result of the decreased pressure, some of the liquid refrigerants are converted to gas.

The flash gas produced represents a high magnitude of energy transfer, since the sensible heat of the liquid refrigerant is converted to latent heat. The vapor and low-pressure liquid combination is then directed into the evaporator, where it evaporates, absorbing heat from the system load.

The pressure drop within the TEV is essential in the operation of the refrigerant. For optimal efficiency, the TEV must precisely regulate the refrigerant flow in response to system conditions.

Understanding the Pressures Acting Inside the TEV

Typically, the TEV has three special forces that work in synchronization to ensure the efficiency of the valve is achieved. The first pressure usually operates on top of the TEV while exerting force on the inside of the power element above the diaphragm. This pressure causes the diaphragm to shift downwards, resulting in a similar downward movement of the pin, and the valve opens. The rise in the evaporator temperature leads to the expansion of the gas in the feeler bulb and a surge in gas pressure within the power element. Consequently, the pin shifts downwards, resulting in the opening of the valve.

The second pressure acts below the diaphragm and is due to the refrigerant pressure within the evaporator and is responsible for the upward movement of the diaphragm, thus closing the valve.

Generally, the two pressures act against the first pressure. Therefore, for the valve to open, pressure one needs to be more than the total force of the two counteracting pressures.

The Basic Operation of The TEV

The TEV keeps the high-pressure liquid refrigerant from gaining entry into the evaporator, and therefore, it regulates its flow significantly. The temperature of the refrigerant is drastically reduced when it enters the TEV, allowing it to boil at reduced temperatures. Boiling is crucial because it will enable the refrigerant to take in heat and redirect it to the compressor.

Therefore, being at higher pressure, the refrigerant is directed through the orifice opening and significantly experiences a reduction in force, resulting in some of the liquid refrigerants vaporizing and the rest maintaining a liquid state.

Next, the resultant combination of vapor and liquid is directed into the evaporator, where it gains heat by
gathering heat from the available air. Due to the extreme temperatures within the evaporator, the refrigerant is subjected to more thermal energy causing it to change phase into a saturated vapor. The latent heat maintains a stable temperature, and as a result, no notable temperature change is experienced.

The refrigerant continually gains more heat, its temperature significantly increases when the phase change is complete, and the vapor produced is then superheated. The bulb keeps the temperature variations to regulate the refrigerant contained in the valve.

What Is the Effect Of an Increase in the Cooling Load

The increase in the cooling load results in more evaporation of the refrigerant in the evaporator. As a result, both the superheat and refrigerant temperatures rise at the exit of the evaporator. It’s essential to maintain the superheat within the designed limits, and this is made possible by directing a higher amount of refrigerant into the evaporator. Therefore, the pin is forced downwards to compress the spring, allowing the refrigerant to flow through the orifice opening.

The sensing bulb usually contains a small amount of refrigerant that is confined in a closed system. When the bulb detects the rise of the temperature, the resultant thermal energy leads to the evaporation of the refrigerant in the bulb, resulting in the pressure increasing, and moves through the capillary tube and gets in the evaporator. Eventually, the diaphragm is forced downwards, pushing down the pin. The spring is compressed, and more refrigerant makes its way into the evaporator.

The TEV adjusts to the correct position and ensures that the force exerted on the diaphragm is equal to or greater than the counteracting force of the spring. As a result, the refrigerant flows through, decreasing its superheat temperature.

What Happens When the Cooling Load Decreases

The reduction in the cooling load results in a decrease in the superheat temperature. The temperature reduction is detected, and the sensing bulb minimizes the refrigerant flow into the evaporator. The refrigerant in the capillary tube makes its way back into the bulb, and the main valve closes. At the same time, the superheat temperature begins to rise as this happens. Eventually, the TEV equalizes, allowing the appropriate levels of refrigerant to get into the evaporator.

If the TEV fails to react to the superheat, it can potentially allow the liquid refrigerant to flow into the compressor. This can be catastrophic to the compressor operation since liquids aren’t easily compressible, resulting in severe damage to the unit.

The installation of HVAC systems should be performed by a trained technician. At Shines Energy, our technicians specialize in AC repair, maintenance, and installation in Dartmouth and the surrounding areas. Furthermore, we provide indoor air quality and water heater services.

Contact Shines Energy today for more information.

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