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What are the reasons for fault judgment and analysis of thermal expansion valve with small opening?

January 12,2024

Reason 1: Ice blockage

[Causes]
If the refrigerant contains a high amount of water, when the evaporation temperature drops below 0°C, it is possible for ice to form at the thermal expansion valve outlet, causing a phenomenon known as "ice blockage."
Ways moisture can enter the system:

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During installation, if the vacuuming time is insufficient, moisture inside the pipes may not be completely removed; poor welding at pipe connections leading to leaks.

When charging the refrigerant into the system, air inside the connecting hose is not purged.

When adding lubricating oil to the system, air enters. [Location of Occurrence] Ice blockages generally occur at the throttling orifice of the expansion valve because this is the coldest and smallest area in the entire system. As the system stops cooling, the overall temperature of the system rises. With the temperature increase, the ice blockage gradually melts, and then the system resumes its cooling capacity. As the system's temperature decreases again, ice blockages may reoccur. Therefore, ice blockages are a recurring process. [Treatment] For minor ice blockages, a hot towel can be applied to the blockage. If the ice blockage is severe and has affected the normal operation of the system, the filter drier needs to be replaced. After that, the moisture in the system piping needs to be removed, and then the system should be evacuated, recharged with refrigerant. The color of the test paper in the Sight Glass can also be observed to determine if there is an excessive amount of water in the refrigerant. If confirmed, the filter drier or refrigerant needs to be replaced.

Reason 2: Dirt blockage

[Causes]
Impurities exist in the system. Sources of impurities in the refrigeration system include: copper pipe joints with uncleared oxide film, metal fines generated from compressor wear, and substances produced from chemical reactions of refrigeration machine oil.

[Location of Occurrence]
Generally, dirt blockages occur at the filter drier, where impurities in the system are trapped, causing the blockage phenomenon. Alternatively, dirt blockages can occur at the throttling orifice of the expansion valve. The system initially shows an increase in suction temperature and superheat. If the fault becomes severe, the system will stop operating. If the impurities in the system are not removed, the system cannot be restarted.

[Treatment]
If the blockage is not severe, replacing the filter drier is sufficient. If it is severe, the impurities in the system piping need to be cleaned out, followed by evacuation and recharging of the refrigerant. For impurities clogging the expansion valve flow port, for expandable valves with replaceable valve cores, the flow component can be disassembled and carefully cleaned; other types of expansion valves need to be replaced.

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HONGSEN Thermal Expansion Valve For Refrigeration System

Reason 3: Leakage of fill medium in the temperature sensing bulb

[Causes]
If the fixed quantity of fill medium inside the sensing bulb leaks, the bellows membrane loses the power to open the expansion valve, causing the partial or complete closure of the expansion valve and very low system pressure. Common reasons include:

Breakage of the capillary tube in the sensing bulb, causing the fill medium to leak out, resulting in an incorrect signal transmission to the actuator of the thermal expansion valve. The breakage can occur at the capillary tube, the connection between the sensing bulb and the capillary tube, or the welding point where the capillary tube enters the valve body cover.

Corrosion of the capillary tube in the sensing bulb. If the application environment contains a large amount of highly corrosive substances, they can destroy the passivation layer on the stainless steel surface, accelerating corrosion of the metal. Additionally, if there are numerous solid particles in the atmosphere, when various dust and sulfur dioxide in the air interact, corrosion is accelerated, causing pitting on the metal surface under the solid particles. Furthermore, the corrosion rate is related to relative humidity and temperature.

Damage to the capillary tube in the sensing bulb. Sharp objects can scrape, cut, crush, or burn the capillary tube, causing the fill medium inside the sensing bulb to leak or block the capillary tube, affecting the transmission of the superheat signal to the upper chamber of the membrane, thereby preventing the expansion valve from working properly.

Membrane leakage. Welding slag, metal fines, and other fine particles can enter the membrane inside the valve body, causing the membrane to rupture, resulting in the loss of fill medium inside the sensing bulb. If the customer's system contains a large number of impurity particles and is installed horizontally, the particles can easily accumulate on the outer circumference of the membrane. During operation, the membrane is subjected to alternating loads, causing significant local stress concentration and fatigue rupture of the membrane. [Treatment] In this situation, the expansion valve needs to be replaced. Meanwhile, customers should avoid contact damage to the capillary tube and use protective materials to protect the capillary tube from burns. For localized corrosion, temporary protective layers and corrosion inhibitors can be used, and the concentration of corrosive gases and solid particles in the atmosphere should be controlled. Keep the outer balance tube clean and prevent welding slag, metal fines, and other fine particles from entering.

Reason 4: Improper packaging of the sensing bulb

In general, the sensing bulb should be installed on the horizontal section of the outlet of the evaporator's return pipe, away from the compressor's suction port and close to the evaporator. Vertical installation is not suitable.

When the diameter of the horizontal return pipe is less than 5/8" (16mm), the sensing bulb should be installed at the top end of the return pipe, i.e., the "12 o'clock" position of the suction pipe. When the diameter of the horizontal return pipe is greater than 1", the sensing bulb should be installed below the axis of the return pipe at an angle of about 45 degrees to the horizontal axis, i.e., the "3 o'clock" position of the suction pipe. Installing the sensing bulb at the top of the suction pipe reduces the sensitivity of the reaction, possibly resulting in excessive refrigerant in the evaporator. 

Installing the sensing bulb at the bottom of the suction pipe causes disorder in the supply of liquid because a small amount of liquid refrigerant always flows to the location where the sensing bulb is installed, causing rapid changes in the temperature of the sensing bulb. During installation, the sensing bulb should be securely wrapped with a copper strip, and the surface of the return pipe should be rust-free. If it is a steel pipe, after the surface is rust-free, silver paint should be applied to ensure good contact between the sensing bulb and the return pipe. The sensing bulb must be lower than the upper chamber of the membrane, and the head of the sensing bulb should be placed horizontally. If the relative position is higher than the upper chamber of the membrane, the capillary tube should bend upward into a U-shape to prevent liquid from entering the upper chamber of the membrane. To prevent the liquid refrigerant or oil from accumulating in the horizontal pipe section where the sensing bulb is located and affecting the performance of the sensing bulb when the system suddenly shuts down, the pipe section behind the sensing bulb should have a certain slope to facilitate the flow of the refrigerant liquid or oil.

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Reason 5: Poor wrapping or insulation of the sensing bulb

Incorrect temperature perception
Delayed temperature perception
Difficulty in heat transfer due to dirt

Reason 6: Migration of fill medium in the temperature sensing element (with MOP function)

Under normal circumstances, the fill medium in the entire temperature sensing element should remain in the sensing bulb, but sometimes, due to improper installation of the sensing bulb, the fill medium migrates from the sensing bulb to other parts.
Another reason is that if the temperature of the valve or temperature sensing element is lower than that of the sensing bulb, the fill medium will also migrate to the upper chamber of the bellows membrane on the expansion valve body, causing the fill medium to flow back from the sensing bulb to the temperature sensing element, thereby causing the expansion valve to malfunction. Some expansion valves are insulated to prevent excessively low temperatures.

Reason 7: Blockage or closure of the outer balance tube due to compression or bending

The outer balance tube provides closing force. If a certain pressure is sealed in the tube, it may cause the valve to close, resulting in insufficient liquid supply.

Reason 8: Stuck valve core

[Causes]
The refrigerant in the system contains a high amount of water, or the refrigerant contains corrosive substances, resulting in severe corrosion inside the expansion valve, making the valve core unable to move, and the expansion valve unable to adjust.
[Treatment]
Observe the color of the test paper in the center of the sight glass to determine if there is an excessive amount of water in the refrigerant. If confirmed, the filter drier or refrigerant needs to be replaced.

Reason 9: Selection of an expansion valve with insufficient capacity, improper adjustment of superheat, etc.


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