Working Principle, Installation and Commissioning of Thermostatic Expansion Valve

Thermal Expansion Valve (TXV/TEV) is a key core component of refrigeration, air conditioning and cold chain systems. Improper selection, installation or commissioning can easily lead to a 15%-30% drop in refrigeration efficiency, and even lead to compressor burnout.

This article analyzes its working principle, installation and commissioning points, to help the relevant personnel to avoid misunderstandings, to ensure stable and efficient operation of the system.

What Is a Thermostatic Expansion Valve?

Thermal expansion valve (TXV/TEV) is a key throttling device in refrigeration system, the core function of high-pressure liquid refrigerant pressure reduction and throttling, according to the evaporator heat load precision liquid supply, to ensure full heat transfer and avoid liquid shock.

Compared with other expansion devices, capillary load adaptation is narrow, electronic expansion valve (EEV) cost is high, float valve is suitable for large container level control; thermal expansion valve by virtue of the advantages of mechanical self-regulation, widely used in the supermarket cold chain, industrial chiller, central air conditioning and other scenarios, accounting for more than 60% of the medium and large refrigeration system.

In all kinds of refrigeration scenarios that require stable temperature control, the thermal expansion valve is the key connecting part between the condenser and the evaporator, and its performance directly determines the energy consumption and service life of the system.

Main Components of a Thermostatic Expansion Valve

The precise work of thermal expansion valve depends on the cooperation of various components, the core components and functions are as follows:

• Power element (temperature sensing package + diaphragm): as the valve “sensing organ”, the temperature sensing package is close to the suction pipe, filled with the same type of refrigerant, which can sense the suction temperature change in real time, and drive the diaphragm to drive the valve to open and close through pressure transfer;
• Valve body and seat: refrigerant “channel gate”, valve seat precision design determines the throttling effect, high-quality brass valve body corrosion resistance, good sealing, suitable for high-pressure long-term working conditions;
• Superheat adjusting spring: set the base control pressure, change the valve opening sensitivity by adjusting the tightness, suitable for different refrigeration system requirements;
• Balance tube: there are two types of balance, internal balance type simple structure, suitable for evaporator resistance system, external balance type can offset the pressure drop of the evaporator, suitable for large or long pipeline system;
• Inlet screen: filter refrigerant impurities, to avoid valve seat clogging, most of the valve failure stems from the screen is not cleaned in time.

Working Principle of a Thermostatic Expansion Valve

Basic Working Logic

The core of thermal expansion valve is “supplying liquid on demand”, automatically adjusting the refrigerant flow rate with the heat load of the evaporator: when the heat load increases, the pressure of the sensing package rises to push the diaphragm to open the valve to increase the supply of liquid; the valve closes down to reduce the supply of liquid when the heat load decreases to ensure the best heat exchange of the evaporator.

The self-regulation does not require additional power, relying on thermal and pressure balance, because of the reliable structure, low maintenance costs, widely favored by the industrial scene.

Valve on the “Tripartite Force Balance”

valve opening and closing by three groups of force dynamic balance decision, is the key to understanding the principle of operation:

1. temperature sensing package generated by the opening force (the higher the temperature the greater the pressure), the evaporator within the closing resistance, manually set the spring adjusting force, the three together to determine the valve initial opening threshold.
2. Three-way force balance when the valve opening stability, any change in the force to break the balance, the valve that is adjusted to match the opening of the refrigerant flow and heat load.

Superheat Control

Superheat refers to the refrigerant vapor temperature in the suction pipe is higher than the evaporation temperature of the value of the core indicators of thermal expansion valve control.

Too little superheat is easy to burn the compressor liquid strike, too large is not enough liquid supply to the evaporator, refrigeration efficiency, data show that every deviation from the optimal value of 1 ℃, the system energy consumption increased by 3% -5%.

High-quality thermal expansion valve can control the degree of superheat in the set range, taking into account the safety of the compressor and evaporator heat transfer efficiency, so the degree of superheat adjustment is the key to debugging.

Common Types of Thermal Expansion Valves

According to the structure and application scenarios, thermal expansion valves are mainly divided into the following categories, which need to be accurately matched with the system requirements when choosing:

Categorized According to the Balancing Method

• Internal balancing type: no external balancing pipe is required, suitable for small systems with evaporator pressure drop ≤ 0.05MPa, easy to install;
• External balancing type: external balancing tube needs to be installed, suitable for large/long pipeline cold chain system with large pressure drop of evaporator, which can avoid abnormal superheat caused by pressure drop.

Classification According to Valve Port Structure

• Balance port type: strong anti-dirty plugging ability, in the poor water quality, refrigerant purity scenario, the failure rate can be reduced by more than 40%;
• Conventional type: conventional structure, suitable for scenes with clean medium and stable working conditions.

Classification by Spool Replaceability

• Replaceable spool type: high flexibility, no need to replace the whole valve for later maintenance, only change the spool can be adapted to different refrigerants/conditions, suitable for industrial refrigeration systems with changing conditions;
• Fixed spool type: the spool is not replaceable, suitable for fixed scenes with stable working conditions.

Categorized by temperature scenarios, low-temperature type (-40℃-10℃) is suitable for freezers, medium-temperature type (-10℃10℃) is used for supermarket refrigerated cabinets, and high-temperature type (10℃~30℃) is suitable for comfort refrigeration scenarios such as central air-conditioning, and choosing the wrong type will directly lead to the system can not work properly.

Installation of Thermostatic Expansion Valve

Installation is a key link in determining the performance of the thermal expansion valve, many failures are in fact improperly installed buried hidden dangers, the following steps must be strictly enforced.

Pre-Installation Checks

First of all, we should confirm whether the valve selection is correct: the valve capacity needs to be matched with the system cooling capacity, the deviation can not be more than 10%, otherwise there will be insufficient or excessive liquid supply; the refrigerant type should be corresponded to, for example, R22 refrigerant valve can not be used in the R410A system, otherwise the seals will be damaged.

It is also important to check whether the valve is broken in appearance, whether the screen is clean, and whether the diaphragm has any leakage – I have come across a project where the impurities clogged the valve seat because I did not check the screen before installation, and the system was running for 3 days before the refrigeration failure occurred.

In addition, to confirm the suitability of the evaporator and the valve, such as external balancing valve requires the evaporator to reserve balance pipe interface, check in advance to avoid rework.

Correct Mounting Position

The valve should be installed on the liquid pipe between the condenser outlet and the evaporator inlet, and the distance from the evaporator should not exceed 3 meters as far as possible, so as to reduce the impact of pipeline pressure drop.

The installation direction must be vertical, the valve stem facing up, the deviation can not exceed 15 °, otherwise it will lead to valve spool stagnation, affecting the accuracy of flow regulation.

If there is liquid refrigerant accumulation in the liquid pipe, the liquid storage bend should be installed in front of the valve to avoid the refrigerant with liquid into the valve, resulting in valve seat wear.

Installation Specification of Temperature Sensing Package

The temperature sensing package must be tightly attached to the horizontal section of the suction pipe, and the position is selected at 10-15 cm after the evaporator outlet, away from the compressor suction port and valve to avoid temperature interference.

When installing the suction pipe to remove the surface of the oil and oxidation layer, the temperature sensing package with a bandage fixed firmly to ensure that the full fit – many technicians want to save trouble, not tightly wrapped, resulting in the temperature sensing package sensing temperature deviation, the valve inaccurate operation.

If the suction pipe is installed vertically, the temperature sensing package should be installed on the side of the pipe; horizontal pipe is installed on the top or side of the pipe at 45 ° position, to avoid the accumulation of liquid affect the temperature detection.

After fixing, the temperature sensing package should be insulated, and the thickness of the insulation layer should not be less than 20mm, and it should not be connected with the insulation layer of the pipeline, so as to prevent the condensation from affecting the detection accuracy.

Installation Requirements of External Balance Pipe

Only the external balance type valve needs to install the external balance pipe, the interface should be selected on the suction pipe between the evaporator outlet and the temperature sensing package, at least 5 cm away from the temperature sensing package to avoid mutual interference.

The connection of the balance pipe should be sealed tightly, there can be no leakage, otherwise it will lead to inaccurate pressure detection of the evaporator; the pipeline should be as short and straight as possible, the length of which should not be more than 5 meters, so as to reduce the pressure loss.

Note that the balance pipe can not be installed in the suction pipe at the accumulation of liquid or elbow, the pressure fluctuations in these locations will affect the stability of valve regulation.

Commissioning of Thermostatic Expansion Valve

After installation, debugging is the last step to make the system reach the best state, especially the superheat adjustment, which is directly related to the system efficiency and equipment safety.

System Preparation before Commissioning

First of all, the system should be vacuumed, the vacuum degree needs to reach -0.1MPa, keep it for more than 2 hours to ensure that there is no moisture and air in the system – the moisture will lead to the valve ice blockage, and the air will affect the refrigeration efficiency.

Then charge the refrigerant according to the system requirements, the charge should refer to the equipment manual, not too much or too little, run for 10 minutes after charging, let the system stabilize.

Also check the system voltage, compressor current is normal, fans, pumps and other auxiliary equipment is running well, to create a stable working condition for debugging.

Initial Start-Up Procedure

After starting the system, do not rush to adjust the valve, first let the system run for 30 minutes to achieve stable working conditions.

During this period, observe the suction pressure, evaporation temperature and compressor operation status, and record the initial data – for example, the suction pressure of the medium-temperature refrigeration system should be 0.2-0.3MPa, if the deviation is too large, it may be a problem of the installation process, which needs to be investigated before debugging.

If abnormal noise or vibration occurs in the system, it should be stopped immediately for checking and eliminating problems such as valve stagnation or pipeline blockage.

Superheat Adjustment Procedure

1. The first step is to measure the temperature of the suction pipe temperature sensing package position with a thermometer, and then read the evaporation pressure through a pressure gauge to convert the evaporation temperature, and the difference between the two is the actual degree of superheat.
2. The second step, according to the application scenarios to adjust the degree of superheat: medium temperature refrigeration system (such as supermarket refrigeration) is recommended to be set at 5-8 ℃, low-temperature systems (such as freezers) is set at 3-5 ℃, the central air-conditioning is set at 6-10 ℃.
3. Adjustment, clockwise rotation of the adjusting screw, superheat increases; counterclockwise rotation, superheat decreases, each time the rotation angle is not more than 1/4 turn, after adjustment, wait 15 minutes, so that the system is stable and then measured, to avoid frequent adjustments lead to fluctuations in working conditions.

It should be noted here that the adjustment should not be rushed, some systems need 1-2 hours to fully stabilize, patience to get accurate results.

Performance Verification

After commissioning, you need to observe the operating conditions: evaporator frost uniformity and no abnormality, the compressor runs smoothly, the current is normal and there is no overheating.

During normal operation, the refrigerant should be completely evaporated at the outlet of the evaporator (no obvious frost on the suction tube, except for the low temperature system), and the cooling capacity of the system should reach the design requirements.

Abnormal treatment: uneven frost is mostly due to insufficient liquid supply, need to reduce the degree of superheat; frequent start-stop compressor may be a small degree of superheat (liquid strike risk), need to increase the degree of superheat.

Common Installation and Commissioning Mistakes

In many years of hands-on instruction, I have found that many technicians will fall into the following misconceptions, resulting in abnormal system operation, which must be avoided:

• Wrong installation position of the temperature sensing package: such as installed in the elbow of the suction pipe, close to the compressor suction port or not close to the pipeline, will lead to inaccurate temperature detection, valve misadjustment, and in serious cases, can lead to the loss of control of the degree of superheat, the compressor burned out;
• Improperly set superheat: empirical settings ignore the differences in working conditions, such as low-temperature system set to 8 ℃ will lead to insufficient liquid supply, refrigeration efficiency decreased by 25%, medium-temperature system set to 3 ℃ is easy to trigger liquid shock;
• Misuse of balanced valves: large systems with internal balanced instead of external balanced, when the evaporator pressure drop of more than 0.05MPa, there will be a large degree of superheat, insufficient liquid supply, long-term operation of the energy consumption soared;
• System cleaning is not in place: impurities or moisture into the valve is easy to cause valve seat clogging, ice blocking, 30% of the valve failure stems from system contamination, pre-installation cleaning and vacuuming can not be omitted;
• Improper valve selection: selection of large easy to appear “hunting” phenomenon (large fluctuations in the degree of superheat), selection of small is not enough liquid supply, evaporator heat transfer is not sufficient.

Troubleshooting Thermostatic Expansion Valve Issues

When the system has poor refrigeration effect, compressor abnormality and other problems, the following failure phenomena can be categorized according to the investigation of thermal expansion valve problems:

1. Unstable superheat: improper installation of temperature sensing package, valve selection, superheat is set too high; first check the temperature sensing package fit tightly, and then adjust the degree of superheat; if there is still no improvement, replace the valve with the appropriate capacity.
2. Compressor liquid strike: superheat is too small, the valve supply too much liquid; immediately shut down, increase the superheat setting, check whether the temperature sensing package is wrapped in condensate; troubleshooting restart observation.
3. Insufficient refrigeration: insufficient liquid supply, specifically including valve clogging, superheat setting is too high, balance tube leakage; first check the balance tube sealing, and then disassemble the valve to clean the screen, and finally adjust the degree of superheat.
4. Valve adjustment is still unable to normal working conditions: valve stem jamming, diaphragm leakage, indicating that the valve has been damaged; timely replacement of the valve, replacement needs to match the type of refrigerant and system capacity, not arbitrary replacement.

Best Practices for Long-Term Reliable Operation

To make the thermal expansion valve work reliably for a long time, routine maintenance and standardized operation are essential.

Regular inspection is very important, it is recommended to check once a month whether the insulation of the temperature-sensitive package is intact, whether there is any leakage of the valve, whether the screen is clogged or not, and to find out the problems and deal with them in time, so as to avoid the expansion of small faults.

Keep the system clean is the key, a comprehensive annual cleaning of the system, replace the filter cartridge, to avoid the accumulation of impurities leading to valve failure; charging refrigerant to ensure the purity, to avoid corrosion of poor quality refrigerant seals.

Do a good job of data recording, each time after the commissioning of the superheat setting value, operating pressure, current and other data to be recorded in detail, later maintenance can be used as a reference to facilitate rapid troubleshooting.

In addition, the operator should be familiar with the valve’s operating principle and operating specifications, to avoid blind adjustment – many failures are caused by improper human operation, regular professional training, can effectively reduce the failure rate.

Conclusion

thermal expansion valve is small, but the refrigeration system “heart valve”, understanding of its working principle, accurate installation and scientific debugging, directly determines the system efficiency, stability and service life. Relevant technicians, designers and students to master these core knowledge and skills, can be less detour in practice.

If you encounter problems in selection, installation or commissioning, you can check against the methods in this article; welcome to share practical experience. Standardized operation and regular maintenance can make the refrigeration system continue to operate stably and ensure the reliable supply of cold source.