What is a Thermostatic Expansion Valve?

Figure 1: A sample of a Parker thermostatic expansion valve.

The thermostatic expansion valve (TXV or TEV), also called the thermal expansion valve, controls the openness degree through the control of the superheat of the gaseous refrigerant at the outlet of the evaporator. It is a throttling device, an important component of the refrigeration device and one of the four basic components of a refrigeration system (the other three: compressor, evaporator, condenser). The TXV can be divided into internally equalized and externally equalized ones.

Functions of the Thermostatic Expansion Valve

The TXV can achieve the throttling from the condensing pressure to the evaporating pressure and control the flow rate of the refrigerant. Small as it is, it matters a lot. It directly determines how well the refrigeration system works. It is intended to provide fluid to the evaporator in the best manner, so as to ensure a constant superheat of the refrigerant vapor at the outlet of the evaporator.

Here are the roles of the TXV in the refrigeration system:
1) Throttling and pressure reduction: it throttles and depressurizes the high-temperature and high-pressure liquid refrigerant condensed by the condenser into a low-temperature and low-pressure vapor-liquid mixture that is easy to evaporate.

2) Adjusting the flow rate: according to the temperature signal obtained by the sensing bulb, the expansion valve can automatically adjust the flow rate of the refrigerant entering the evaporator to adapt to the changing cooling load.

3) Maintain a certain superheat, prevent fluid pound and abnormal superheat: it helps the evaporator to keep a certain superheat by controlling the flow rate, which guarantees the effective use of the total volume of the evaporator and prevents the liquid refrigerant from entering the compressor and causing fluid pounds. It can control the degree of superheat within a certain range to prevent the occurrence of abnormal superheat.

Figure 2: Working principles of a refrigeration system adopting a thermostatic expansion valve.

Comparisons between the Thermostatic Expansion Valve and the Electronic Expansion Valve

Among the various types of throttling devices, capillary tubes are cheaper and electronic expansion valves (EEVs) offer greater superheat control across a large load range. To know the TXV better, we can compare it with a similar device: the electronic expansion valve.

While the EEV performs better in terms of providing constant superheat, it costs more. And usually, the better performance cannot compensate for the higher price. In contrast, the TXV can significantly improve the efficiency but only costs a little more in comparison with its cheaper counterparts such as the capillary tube.

What’s more, the TXV is more self-contained, while the EEV needs additional parts to work properly, such as a controller and a sensor, which may cause extra fees.
Summarization of the comparisons is shown in the table below.

Figure 3: Comparisons between the thermostatic expansion valve and the electronic expansion valve.

The EEV has overcome some disadvantages of the TXV, but when it comes to relatively low accuracy requirement and limited budget, the TXV is a better choice.

Related Info