Thermal expansion valve failure phenomenon_difference between internal and external balance of thermal expansion valve

Thermal expansion valve failure phenomenon

Fault phenomenon 1: The refrigerant flow provided by the thermal expansion valve is insufficient (small opening).

Fault phenomenon 2: The refrigerant flow provided by the expansion valve is too large (large opening).

Fault phenomenon three: The expansion valve sometimes supplies insufficient liquid and sometimes supplies too much liquid (fluctuation).

Thermal Expansion Valve Troubleshooting

Reason 1: Ice blockage

[Cause]: If the refrigerant contains a lot of water, when the evaporation temperature drops below 0℃, it may freeze at the throttle of the thermal expansion valve, causing "ice blockage". Possible causes of ice blockage are: 1) The system vacuum time is not enough to completely remove the moisture in the pipeline. 2) The water content of the refrigerant or lubricant added to the system exceeds the standard.

[Occurrence location]: Ice blockage usually occurs at the throttling hole of the expansion valve, because this is the place with the lowest temperature and the smallest hole diameter in the entire system. Observe the color of the test paper in the sight glass to determine whether there is too much water in the refrigerant.

[Solution]: Replace the filter drier to remove moisture from the system pipes; if necessary, vacuum the system and refill the refrigerant.

Reason 2: Dirty blockage

【Cause】: Impurities exist in the system.

The sources of impurities in the refrigeration system include: oxide film that was not cleaned during copper tube welding, metal shavings produced by compressor operation and wear, substances produced by chemical reactions of refrigeration oil, etc.

[Occurrence location]: Generally, dirty blockage occurs at the filter drier, where impurities in the system are intercepted by the filter, causing dirty blockage; it may also occur at the throttling hole of the expansion valve. After dirty blockage occurs, the evaporation pressure of the system becomes lower, the return air temperature increases, the superheat increases, and the system oil return is difficult.

[Solution]: For blockage at the filter drier, the filter drier needs to be replaced. If the expansion valve outlet is dirty and blocked, the outlet assembly of the split expansion valve can be removed and carefully cleaned; the integral expansion valve needs to be replaced.

Reason 3: Leakage of the filling medium of the temperature sensing package

[Cause]: If the filling medium in the temperature-sensing package leaks, the diaphragm will lose the power to open the expansion valve, causing the expansion valve to be partially or completely closed, and the system low pressure is very low. Common causes include:

1) The capillary tube of the temperature sensing package is broken.

The capillary tube breaks and the filling medium in the temperature-sensing package leaks, resulting in the failure to transmit the correct signal to the actuator of the thermal expansion valve. The possible locations of the break include the capillary tube, the connection between the temperature-sensing package and the capillary tube, and the welding between the capillary tube and the upper cover of the valve body.

2) Corrosion of temperature sensing bulb and capillary.

If there are a lot of highly corrosive substances in the application environment, they can destroy the passivation layer on the surface of stainless steel, thus accelerating the corrosion of the metal. At the same time, there are a lot of solid particles in the atmosphere, which may also cause pitting on the metal surface under the solid particles when they interact with sulfur dioxide and water. In addition, the corrosion rate is also related to relative humidity and temperature.

3) The capillary of the temperature sensing package is damaged.

Sharp objects may scratch, cut, squeeze or burn the capillary tube, causing the filling medium in the temperature sensing package to leak out, or block the capillary tube, affecting the transmission of the superheat signal to the upper cavity of the diaphragm, thus causing the expansion valve to fail to work properly.

【Solution】

In this case, the expansion valve needs to be replaced. Therefore, it is particularly required to avoid contact between the capillary and sharp objects during installation and use to avoid damage and burns, and to use protective materials to protect the capillary. For pitting corrosion, protective layers and corrosion inhibitors can be used to control the concentration of corrosive gases and solid particles in the atmosphere.

Reason 4: The temperature sensor is not installed in the correct position.

It is recommended to install the temperature sensor package horizontally on the return air pipe near the evaporator outlet. During installation, the temperature sensor package needs to be fixed with a mounting belt, and the surface of the return air pipe needs to be derusted. If it is a steel pipe, the surface should be painted with silver paint after derusting to ensure good contact between the temperature sensor package and the return air pipe.

Reason 5: The temperature sensor fails to sense the accurate temperature.

The temperature at the temperature sensor is affected by other factors. The temperature sensing is delayed and dirt causes difficulty in heat transfer.

Reason 6: Migration of filling medium (with MOP function)

For expansion valves with MOP function, the amount of filling medium filled in the temperature-sensing package and valve head is relatively small. If the temperature of the valve head is lower than that of the temperature-sensing package, the filling medium will also migrate from the temperature-sensing package to the valve head, causing the expansion valve to fail to work properly. The process of filling medium migration is related to temperature difference and time.

【Solution】: During transportation, storage, installation, commissioning and use, the temperature of the temperature-sensing package should always be kept lower than that of the temperature-sensing element. If the filling medium has migrated, the filling medium can be returned to the temperature-sensing package by cooling the temperature-sensing package (recommended) or heating the valve head. This process is affected by temperature difference and time.

Reason 7: The valve core is stuck

[Cause]: The system refrigerant contains too much water, or the refrigerant contains corrosive substances, which causes serious corrosion inside the expansion valve, making the valve core unable to move and the expansion valve unable to be adjusted.

[Solution]: Replace the expansion valve and dry filter to remove moisture and other impurities from the system.

Reason 8: The capacity of the selected expansion valve is too small, the superheat adjustment is improper, etc.

Fault phenomenon 2: The refrigerant flow provided by the expansion valve is too large (large opening)

[Fault phenomenon]: Liquid backflow; low superheat; suction temperature and pressure fluctuations.

【Cause】：

1) The valve core of the thermal expansion valve is stuck.

2) The external balance pipe is blocked. The sensed pressure cannot be quickly transmitted to the bottom of the diaphragm, resulting in a small valve closing force and a large valve opening.

3) Problems with the installation of the temperature sensor package. Incorrect installation position and method, or poor contact and insulation may cause excessive liquid supply.

4) The cooling capacity of the selected expansion valve is too large.

5) Improper adjustment of superheat of thermal expansion valve.

Fault phenomenon 3: The expansion valve sometimes supplies insufficient fluid, and sometimes supplies too much fluid (fluctuation)

【Cause】：

1) The cooling capacity of the selected expansion valve is too large, causing fluctuations.

2) The temperature sensor is installed in an inappropriate position.

3) The dispensing head does not dispense liquid evenly.

The difference between internal and external balance of thermal expansion valve

Thermal expansion valves can be divided into internally balanced and externally balanced according to the different ways in which the evaporation pressure is transmitted to the valve diaphragm. The differences are as follows:

Internally balanced - transfers the evaporator pressure internally to the diaphragm.

External balance - the evaporator pressure is transferred from the evaporator outlet to the diaphragm through an external connection.

Note: delta t is 10 k (medium temperature – evaporation temperature). If the pressure drop across the evaporator is high, an expansion valve with external balancing should be used. This means that systems with a split head must use an expansion valve with external balancing.

Now that we know the difference between internally balanced and externally balanced expansion valves, what are the differences in their usage scenarios?

Due to the pressure drop in the evaporator, the pressure value collected by the internal balanced expansion valve is larger than that collected by the external balanced pressure. Since the expansion valve itself is only a mechanical component, it will not know this deviation and will not make corrections, which will cause the internal balanced expansion valve to make wrong judgments.

Therefore, for situations where the evaporator pressure drop is large, it is recommended to use an externally balanced thermal expansion valve.

The specific usage occasions are as follows:

External balance:

1. Where the pressure drop is large, such as when a plate evaporator is used

2. Units with large heat exchange capacity, such as screw compressors and shell and tube evaporators

3. Refrigeration system using a liquid distributor (the pressure drop of the liquid distributor is relatively large)