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Detailed explanation of the six working principles of thermal expansion valves


This chapter explains the six working principles of the thermal expansion valve in detail for everyone. Understanding this knowledge is the basis for learning about the chiller system, so it is also very important!

1 Overview: The thermal expansion valve is an important component of the refrigeration device and is one of the four basic equipment in the refrigeration system. It realizes the pressure drop from the condensing pressure to the evaporating pressure, while controlling the flow of the refrigerant; although its volume is small, its role is huge, and its work is good or bad, which directly determines the operating performance of the entire system. However, in actual work, the operation of the thermal expansion valve is often neglected, making the thermal expansion valve a dead angle in equipment maintenance. Regularly checking and adjusting the thermal expansion valve is of great significance to the operating life of the refrigeration equipment, saving energy and reducing operating costs.
2 Analysis of the working process of the thermal expansion valve (1) Working principle of the thermal expansion valve: The thermal expansion valve controls the flow of refrigerant entering the evaporator by sensing the superheat of the gaseous refrigerant at the outlet of the evaporator. According to different balance methods, the thermal expansion valve is divided into external balance type and internal balance type. In industrial cooling equipment, externally balanced thermal expansion valves are generally used. Thermal expansion valve consists of induction mechanism, actuator, adjustment mechanism and valve body. The induction mechanism is filled with Freon working medium, and the temperature sensing bag is set at the evaporator outlet. There is a temperature difference between the outlet temperature and the evaporation temperature, which is usually called superheat. After the temperature sensing package senses the evaporator outlet temperature, the entire induction system is at a corresponding saturation pressure Pb. This pressure will be transmitted to the ejector rod through the diaphragm to the spool. The diaphragm in the upper part of the pressure chamber has only Pb, and there is a spring force Pt and an evaporation pressure P0 of the adjusting spring under the diaphragm. When the three are in equilibrium, Pb = Pt + Po. When the heat load of the evaporator increases, the superheat of the outlet is high, Pb increases, Pb> Pt + Po, the combined force moves the ejector rod and valve core downward, the opening of the thermal expansion valve increases, and the refrigerant flow rate increases proportionally. Conversely, the opening of the thermal expansion valve becomes smaller, and the refrigerant flow rate decreases proportionally. Therefore, the refrigeration equipment is self-adjusted by the thermal expansion valve by controlling the superheat degree. (2) Determine the correct superheat degree: To ensure that the thermal expansion valve works at the best matching point, it is necessary to ensure that the thermal expansion valve has a suitable superheat degree. The superheat degree of the thermal expansion valve is composed of the static assembly superheat degree and the effective superheat degree. The superheat required to make the valve start to open is called the open superheat, also known as the static assembly superheat. The general static assembly superheat is about 3 ° C. The superheat increase required from the opening of the thermal expansion valve to the rated opening is called the effective superheat or variable superheat of the thermal expansion valve. The magnitude of the value is related to the stiffness of the spring and the stroke of the valve core. Generally, the effective superheat is about 2 ~ 5 ° C. Generally, the sum of the static assembly superheat and the effective superheat of the thermal expansion valve is called the working superheat. The superheat. Therefore, only by ensuring that the superheat is within the appropriate range, the refrigeration system can reach the maximum cooling capacity without causing wet strokes. The superheat of industrial oil coolers is required to be between 5 ~ 8 ℃. If you find that the superheat is not in this range, adjust it.

3 Check the necessity of adjusting the thermal expansion valve: the refrigeration equipment has just been put into operation, the thermal expansion valve is not adjusted, but after several years of continuous use of the equipment, due to the wear of the valve needle, the system has impurities, the valve hole is blocked and the spring force Reasons such as weakening affect the opening degree of the thermal expansion valve, which makes the thermal expansion valve deviate from its operating point, which is manifested by the opening degree of the thermal expansion valve being too small or too large.
If the opening degree of the thermal expansion valve is too small, it will cause insufficient liquid supply, so that there is not enough Freon to evaporate in the evaporator, and the refrigerant has evaporated during the flow in the evaporator tube. In the following paragraph, the evaporator tube No liquid refrigerant is available for evaporation, only steam is overheated. Therefore, a considerable part of the evaporator fails to give full play to its efficiency, resulting in insufficient cooling capacity and reducing the cooling effect of the equipment. Most of the compressors of industrial oil coolers use the steam returned from the evaporator to cool the compressor. If the thermal expansion valve is not opened enough, the steam will be overheated, the cooling effect on the compressor will be reduced, and the compressor's exhaust temperature will increase. As the lubricant becomes thinner, the lubrication quality decreases, and the working environment of the compressor deteriorates, it will seriously affect the working life of the compressor and even burn the compressor. According to analysis, it is related to excessive overheating. In addition, because the temperature of the cooled medium cannot be lowered, the operating time of the compressor is increased, and the power consumption is also increased.
In contrast, if the thermal expansion valve is opened too large, that is, the thermal expansion valve supplies more liquid to the evaporator than the evaporator load, it will cause some refrigerants to have time to evaporate in the evaporator and enter the compressor together with the gaseous refrigerant, causing Wet strokes, or even cylinder accidents, damage the compressor. At the same time, the thermal expansion valve is opened too large, which increases the evaporation temperature, reduces the cooling capacity, increases the power consumption of the compressor, and increases the power consumption. Therefore, it is necessary to regularly check and adjust the thermal expansion valve to try to make the thermal expansion valve work at the best matching point.
4 Adjustment process of thermal expansion valve (1) Inspection before adjustment of expansion valve: Before adjusting the thermal expansion valve, it must be confirmed that the abnormal cooling of the equipment is caused by the thermal expansion valve deviating from the optimal operating point, not because of low Freon, dry filtration Caused by blockage of filters, filters, fans, etc. At the same time, the correctness of the sampling signal of the temperature sensing package must be ensured. The temperature sensing package must be installed horizontally at a position of 45 degrees below the return pipe. It must not be installed directly below the pipe to prevent factors such as oil accumulation at the bottom of the pipe from affecting the sense The temperature pack is correct for temperature. Not to be installed on the riser. Check the condenser fan control method, and try to use speed control to ensure that the condensing pressure is constant. (2) Precautions when adjusting the thermal expansion valve: The adjustment of the thermal expansion valve must be performed under the normal operation of the refrigeration device. Because the thermometer cannot be placed on the surface of the evaporator, the suction pressure of the compressor can be used as the saturation pressure in the evaporator, and the approximate evaporation temperature can be obtained by checking the table. Use a thermometer to measure the temperature of the return pipe and compare it with the evaporation temperature to check the superheat. During the adjustment, if you feel that the degree of overheating is too small, you can turn the adjusting screw clockwise (that is, increase the spring force and decrease the opening degree of the thermal expansion valve) to reduce the flow rate; otherwise, if you feel that the degree of overheating is too large, That is, the liquid supply is insufficient, the adjusting screw can be turned in the opposite direction (counterclockwise) to increase the flow rate. Due to the thermal inertia of the thermal expansion valve temperature sensing system in actual work, a signal transmission lag is formed, and the next adjustment can be performed after the operation is basically stable. Therefore, the entire adjustment process must be patient and meticulous. The number of turns of the adjustment screw should not be too fast at one time (the adjustment screw of the straight-rod thermal expansion valve rotates once, and the change in superheat degree changes about 1 ~ 2 ° C). (3) Method for measuring superheat of thermal expansion valve.
Proceed as follows:
1) Stop. Insert the probe of the digital thermometer into the heat insulation layer of the evaporator air outlet (corresponding to the position of the temperature sensor). Connect the pressure gauge to the tee of the compressor low-pressure valve.
2) Turn on the machine and let the compressor run for more than 15 minutes to enter a stable operating state, so that the pressure indicator and temperature display reach a stable value.
3) Read the temperature T1 of the digital thermometer and the temperature T2 corresponding to the pressure measured by the pressure gauge. The superheat is the difference T1-T2 of the two readings. Note that both readings must be read at the same time.
The superheat degree of the thermal expansion valve should be between 5 ~ 8 ℃. If it is not, adjust it appropriately. It can be seen that the temperature of the compressor casing after adjustment will be significantly changed from that before adjustment.

5. Thermal expansion valve maintenance cycle: After long-term statistics on the operation of industrial oil coolers, it is found that the deviation of the thermal expansion valve from the operating point usually occurs in the middle and late life of the service life. Therefore, it is decided to focus on the inspection and adjustment of the thermal expansion valve. In the middle and later stages of equipment life, the following is the thermal expansion valve inspection cycle established based on actual statistics. Thermal expansion valve inspection and adjustment cycle 4 years before use 5 to 8 years 3 times after the 9th year / 2 times per year / 3 times or more

6. Concluding remarks: Although regular inspection and adjustment of the thermal expansion valve is somewhat troublesome, it has important practical significance for improving the cooling effect of the air conditioner, extending the life of the air conditioner, saving energy, and ensuring the safety of the equipment.



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