Thermal Expansion Valve (TXV): Everything You Need To Know

Figure 1: Thermal expansion valve.

The thermal expansion valve (TXV) is an important part of the refrigeration device and one of the four basic components in the refrigeration system. It uses the sensing bulb installed at the outlet of the evaporator to sense the superheat of the refrigerant gas (the superheat refers to the temperature difference between the evaporation temperature and the outlet temperature of the evaporator in the system) to control the refrigerant flow into the evaporator.

This article introduces the components, types, working principle, functions, fault phenomenon (bad thermal expansion valve symptoms), etc. of the thermal expansion valve, and appropriate figures are available to help you understand the relevant knowledge of the thermal expansion valve.

1. What is a Thermal Expansion Valve

The thermal expansion valve (thermostatic expansion valve) is an important self-control component of the refrigeration system, for example HVAC thermal expansion valve, and is generally installed between the liquid receiver and the evaporator. It uses the temperature change of the sensing bulb as a signal to adjust the opening of the valve and change the refrigerant flow rate, so that the medium-temperature and high-pressure refrigerant becomes low-temperature and low-pressure wet vapor through its throttling, and then the refrigerant absorbs heat in the evaporator to achieve the cooling effect.

In addition to the thermal expansion valve as the throttling element, the refrigeration system can also use capillary tubes and electronic expansion valves.

1.1 Thermal Expansion Valve Components

Figure 2: Components of a thermal expansion valve.

Valve body: made of aluminum or brass, it provides refrigerant circulation channels, connection port with the system, and installation position for parts in the valve;
Diaphragm: It transmits the force of the power head to the transmission plate with the smallest resistance, and generally made of stainless steel or copper alloy, its characteristics are determined by material, state, thickness, corrugation shape, etc.;

Power head cover: it together with the power head seat, diaphragm, sensing bulb and capillary tube, forms power head element (some products do not have a sensing bulb and capillary tube), and the material is stainless steel or brass;
Sensing bulb: it feels the external temperature, and transmits the pressure converted from temperature to power head, and the material is copper;

Capillary tube: it transmits the pressure of the sensing bulb, and the material is copper;
Transmission plate: it transmits the force of the diaphragm to the push rod, and the material is aluminum or brass;
Power head seat: It connects the valve body, accommodates the transmission plate, and forms the power head together with the power head cover, diaphragm, sensing bulb and capillary tube, and the material is stainless steel;

Push rod: it transmits the force of the diaphragm to the valve core;
O-ring: it makes a seal between the transmission rod and the valve body without excessive frictional resistance;
Valve core: Under the action of the power head and spring force, it controls the flow area of the valve port, and the material is stainless steel or brass;

Adjusting spring: It provides a certain pre-tightening force according to the superheat requirement of static assembly, and the material is stainless steel or spring steel;
Adjusting screw: it adjusts the spring pre-tightening force.

1.2 Thermal Expansion Valve Types

Expansion valves commonly used in refrigeration systems mainly include thermal expansion valves and electronic expansion valves. Thermal expansion valves are divided into internally equalized type and externally equalized type according to different balance methods.

1) Structure of internally equalized expansion valve: the temperature sensing bulb is filled with refrigerant and placed on the outlet pipe of the evaporator. The sensing bulb and the upper part of the diaphragm are connected through a capillary tube to sense the temperature of the refrigerant at the outlet of the evaporator, and what the lower part of the diaphragm feels is the evaporator inlet pressure.

Figure 3: Internally equalized thermostatic expansion valve.

2) Structure of the externally equalized expansion valve: the structural principle of the externally equalized expansion valve is basically the same as that of the internally equalized expansion valve. The difference is: the internally equalized expansion valve diaphragm feels the evaporator inlet pressure; while the externally equalized expansion valve diaphragm feels the evaporator outlet pressure.

Figure 4: Externally equalized thermostatic expansion valve.

2. How a Thermal Expansion Valve Works

The thermal expansion valve consists of an induction mechanism, an actuator, an adjustment mechanism and a valve body. When the valve is working, the sensing bulb fixed on the evaporator outlet pipe senses the superheated temperature, which makes the pressure generate in the sensing bulb. And this pressure is transmitted to the upper space of the diaphragm by the capillary tube. Under the action of pressure, the diaphragm transmits the signal to the push rod (actuator) in the form of elastic deformation, thereby adjusting the opening of the valve and controlling the flow of refrigerant.

Figure 5: Thermal expansion valve structure.

Let's take a look at the working principle of the thermal expansion valve through the force analysis.

Figure 6: Expansion valve three-force balance control principle.

P1 = P2 + F
P1: The pressure generated by the filling medium (refrigerant) in the sensing bulb, acting on the top of the diaphragm;
P2: Evaporating pressure (for internally equalized type, it is the evaporator inlet pressure, and for externally equalized type, it is the evaporator outlet pressure), acting under the diaphragm;
F: Spring force, acting below the diaphragm.

When the temperature of the sensing bulb decreases, P1 When the temperature of the sensing bulb rises, P1>P2+F, the opening of the valve port increases, and the refrigerant flow rate increases proportionally.

Note: other forces, such as diaphragm stiffness, frictional resistance of transmission parts, fluid force on the valve core, etc., are not considered in the analysis.

3. Functions of Thermal Expansion Valve

Thermal expansion valves are widely used in refrigerators, air conditioners, central air-conditioning chillers, and HVAC systems. In the refrigeration system, thermal expansion valve, for example AC thermal expansion valve (thermal expansion valve air conditioning), is installed on the evaporator inlet, which has three functions:

Figure 7: Throttling function of thermal expansion valve in refrigeration system.

① Throttling and depressurization
The high-temperature and high-pressure refrigerant condensed by the condenser is throttled and depressurized, and becomes a low-temperature and low-pressure gas-liquid mixture that is easy to evaporate. It enters the evaporator to evaporate, and absorbs external heat.

② Flow adjusting
According to the temperature signal obtained by the sensing bulb or the power head, the expansion valve can automatically adjust the refrigerant flow into the evaporator to meet the changing needs of the cooling load.

③ Maintaining a certain degree of superheat, preventing liquid slugging and abnormal overheating
The expansion valve make the evaporator have a certain degree of superheat by adjusting the flow rate, ensures the effective use of the total volume of the evaporator, and avoids liquid refrigerant entering the compressor to cause liquid slugging; at the same time, it can control the degree of superheat within a certain range to prevent abnormal overheating.

4. How Do You Know if a TXV Valve is Bad? (Bad Thermal Expansion Valve Symptoms)

Common failure phenomena of thermal expansion valves include the following three aspects.
·Failure symptom 1: The refrigerant flow rate provided by the thermal expansion valve is insufficient (small opening).
·Symptom 2: The refrigerant flow rate provided by the expansion valve is too large (large opening).
·Symptom 3: The expansion valve sometimes supplies insufficient refrigerant liquid, and sometimes supplies too much (fluctuation).

4.1 Symptom 1: Insufficient Refrigerant

【Reason】:
1): Ice blockage. If there is a lot of water in the refrigerant, when the evaporation temperature drops below 0°C, it may freeze at the throttling port of the thermal expansion valve, resulting in "ice blockage".

Figure 8: Frosting on the outside of the thermal expansion valve.

2) : Dirty blockage. There are impurities in the system. The sources of impurities in the refrigeration system include: the oxide film that was not removed during the welding of the copper tube, the metal fines produced by the wear and tear of the compressor operation, and the substances produced by the chemical reaction of the refrigeration oil.

3) : Filling medium leakage of the sensing bulb.
4) : Incorrect installation position of the sensing bulb.
5) : The sensing bulb fails to feel the accurate temperature.
Reason 6: Filling medium migration (with MOP function)

6) : The valve core is stuck.
7) : The capacity of the selected expansion valve is too small, and the superheat degree is not adjusted properly.

4.2 Symptom 2: Too Large Refrigerant Flow Rate

【Reason】:
1) The valve core of the thermal expansion valve is stuck.
2) The external equalizer line is blocked. The sensed pressure cannot be quickly transmitted to the diaphragm underneath, resulting in a small valve closing force and a large opening of the valve port.
3) There is a problem with the installation of the sensing bulb. If the installation location and method are wrong, or the contact is not good, and the heat preservation is not good, it may cause too much liquid supply.
4) The cooling capacity of the selected expansion valve is too large.
5) Improper adjustment of superheat degree of thermal expansion valve.

4.3 Symptom 3: Refrigerant Flow Rate Fluctuation

【Reason】:
1) The cooling capacity of the selected expansion valve is too large to cause fluctuations.
2) The installation position of the temperature sensing bulb is inappropriate.
3) Uneven dispensing of liquid from the dispensing head.

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