What Is a Thermostatic Expansion Valve?

The thermal expansion valve, also known as TXV or TEV, is a common feature in most refrigeration or air conditioning systems, and it’s usually located just before the evaporator. The thermal static expansion valves work with precision to control the refrigerant’s flow rate into the evaporator chamber, which is essential in improving the efficiency of the evaporator while at the same time holding back the excellent liquid refrigerant from flooding back to the compressor section.

TXV has a design feature that allows it to create and maintain a separation between the high- and low-pressure sides of an air conditioning system. Usually, liquid refrigerant is at high pressure as it enters the TXV valve via the fluid line. Its pressure is significantly reduced by the TXV when it regulates the amount of refrigerant flowing into the evaporator.

Main Components of the TXV

• The valve body
• The pin
• The sensing Bulb
• The spring
• The diaphragm

The Working Principle of the TXV

The thermostatic expansion valve is designed to precisely hold the liquid refrigerant under high pressure from entering the compressor and control the evaporator’s amount.

The TXV reduces the pressure of the refrigerant, allowing it to boil at lower temperatures. Boiling is essential in enabling the refrigerant to absorb heat from the air and directing it to the compressor. Having acquired a high pressure, the refrigerant is directed through the orifice opening, significantly reducing its pressure. Some of the refrigerants evaporate, while some remain in a liquid state.

The vapor refrigerant is directed into the evaporator, absorbing heat in the air within the pipe. While the refrigerant moves through the evaporator, it undergoes a phase change into a saturated vapor at the evaporator coil. After the phase change, the refrigerant gathers more thermal energy, and this superheats the vapor produces. The sensing bulb maintains the temperature to regulate the refrigerant within the valve body.

Increase in Cooling Load

An increase in the cooling load results in the evaporation of more refrigerant within the evaporator. Consequently, the superheat increases causing an upsurge in the refrigerator temperature at the evaporator exit.

Next, to maintain stable and desired superheat temperature, the refrigerant’s flow into the evaporator is increased by forcing the pin downwards, thus compressing the spring and allowing more refrigerant to flow past the orifice.

The sensing bulb contains some refrigerant held in a closed system to adjust the volume of the bulb powerhead and within the capillary tube. As thermal energy moves through the pipe walls into the bulb, it evaporates the refrigerant contained within the bulb. Since the refrigerant is enclosed in limited space, it increases pressure, which pushes the capillary tube to the top of the expansion valve. As a result, the diaphragm is forced downwards, exerting pressure on the pin, compressing the spring, and allowing the liquid refrigerant into the evaporator chamber.

Finally, the valve readjusts to ensure that the force acting on the diaphragm is equal to the force exerted by the spring in the opposite direction, thus allowing the required amount of refrigerant to flow, reducing the refrigerant superheat; this is detected by the sensing bulb and readjusted until an equilibrium is achieved.

Decrease in Cooling Load

A decrease in the cooling load causes a reduction in superheat temperature. This is detected by the sensing bulb, which regulates the refrigerant’s flow into the evaporator. The refrigerant within the capillary flows back to the bulb while the main valve starts to close. Similarly, the temperature of the superheat begins to rise during this process.

Finally, when equilibrium within the valve is realized, the required amount of refrigerant is allowed to pass through.

Functions of the Thermal Expansion Valve

The TXV has several essential functions that are highlighted below.

Reducing Pressure of the Refrigerant

The primary function of the thermostatic expansion valve is to regulate and reduce the pressure of the liquid refrigerant from compressor pressure to match the evaporator pressure. This is facilitated by the orifice opening, which is located within the TXV. It significantly reduces the pressure of the refrigerant as it passes through to match the evaporator temperature. As a result, there is a reduction in the refrigerant’s temperature, which brings about a cooling effect within the evaporator.

Regulating Flow of the Refrigerant as Per the Requirements

Additionally, the TXV guides the flow rate of the refrigerant into the evaporator in response to the load level within it, which is essential in preventing the flood back of the refrigerant into the compressor, ultimately increasing the efficiency of the compressor and evaporator.

Maintaining the Evaporator Active

The TXV regulates the flow of the refrigerant depending on the cooling load within it. When the loads are at a higher value, the flow increases and is reduced if it’s at a lower value. Similarly, since the valve allows the flow of the refrigerant based on the requirements, it ensures that there is no waste in the capacity of the evaporator.

Advantages of the TXV

The thermostatic expansion valve plays a significant role in the HVAC industry, making it commonly used in manufacturing for the following advantages.

Higher Efficiency

The valve allows air conditioners to operate optimally, allowing them to handle the heat load as required. This allows the enclosure ac to operate at higher capacities for increased heat loads.

Eliminates Risk of Compressor Breakdown

The TXV ensures that the vaporization of the liquid refrigerant within the evaporator is complete, thus eliminating the possibility of the liquid slugging or the particles of the liquid refrigerant reaching the compressor chamber, which would be catastrophic for its operation.

Enables Better Temperature Control

The TXV’s ability to control the flow rate of the refrigerant assists in regulating the temperature of the evaporator. This is facilitated through the capillary tubes, which allow a constant flow of the refrigerant, offering immediate responses to the changing heat loads.

Handling the Variations in the Refrigerant Charge

The TXV offers greater tolerance to the variations of the refrigerant charge because of its ability to adapt the flow of the refrigerant to match the heat load.

Adaptable Refrigerant Flow

Last but not least, the thermal expansion valve regulates the flow rate of the refrigerant in response to the heat load in the enclosure, thus allowing immediate response to either increase or decrease the amount of heat being produced by the equipment.

Application of the TXV

The TXV is commonly used to manufacture high capacity systems such as high capacity split air conditioners, industrial refrigerators, and packaged air conditioners. These and more are the many advantages of the thermal expansion valve and its application in various industries.

At Air Max HVAC, we provide a full spectrum of HVAC services including furnace repair and air conditioner repair, AC installation and AC maintenance services in the Burbank, CA. area. Additionally, we also offer humidifier and dehumidifier services, indoor air quality audit, and IAQ products. Contact us today to learn more about the services we offer or to get a quote for our services.