Working principle of the hottest thermal expansion

2022-07-25
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Working principle of thermal expansion valve

1 overview

thermal expansion valve is an important component of refrigeration device and one of the four basic equipment in refrigeration system. It realizes the pressure drop from condensation pressure to evaporation pressure, and controls the flow of refrigerant at the same time; Although its volume is small, it plays a huge role. Its work directly determines the operation performance of the whole system. However, in practical work, the operation of the thermal expansion valve is often ignored, making the thermal expansion valve a dead angle in equipment maintenance. The regular inspection and adjustment of 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

2.1 working principle of the thermal expansion valve

the thermal expansion valve controls the refrigerant flow into the evaporator by feeling the overheating degree 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, external balance thermal expansion valve is generally used. The thermal expansion valve is composed of an induction mechanism, an actuator, an adjusting mechanism and a valve body. The induction mechanism is filled with Freon working medium, and the temperature sensing package 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 whole induction system is at the corresponding saturation pressure Pb. This pressure will be transmitted to the ejector rod through the diaphragm to the valve core. In the diaphragm at the upper part of the pressure chamber, only the main chain contains CPB, and below the diaphragm is the spring force Pt and evaporation pressure P0 of the adjusting spring. When the three are in balance, there is pb=pt+po. When the heat load of the evaporator increases, the outlet superheat is high, Pb increases, pb>pt+po, and the resultant force causes the ejector rod and valve core to move down, the opening of the thermal expansion valve increases, and the refrigerant flow increases proportionally. On the contrary, the opening of the thermal expansion valve becomes smaller, and the refrigerant flow decreases proportionally. Therefore, the refrigeration equipment realizes the self-regulation of the refrigeration system by controlling the degree of superheat through the thermal expansion valve

2.2 determine the correct degree of superheat

to ensure that the thermal expansion valve works at the best matching point, it must ensure that the thermal expansion valve has an appropriate degree of superheat. The superheat of the thermal expansion valve is composed of static assembly superheat and effective superheat. The superheat required to make the valve start to open is called opening superheat, also known as static assembly superheat. Generally, the static assembly superheat is about 3 ℃. The increment of superheat 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 value is related to the stiffness of the spring and the stroke of the valve element. Generally, the effective superheat is about 2~5 ℃. Generally, the sum of the static assembly superheat and the effective superheat of the thermal expansion valve is called the working superheat, which is usually called the superheat. Therefore, only when we ensure that the superheat is within an appropriate range can the refrigeration system reach the maximum cooling capacity without causing wet stroke. The superheat degree of industrial oil cooler is required to be between 5~8 ℃. If it is found that the superheat is not within this range, it shall be adjusted

3 check the necessity of adjusting the thermal expansion valve

the refrigeration equipment has just been put into operation, and the thermal expansion valve does not need to be adjusted. However, after the equipment has been used for several years, the opening of the thermal expansion valve is affected due to the wear of the valve needle, impurities in the system, blockage of the valve hole and weakening of the spring force, so that the components of the thermal structure can be adjusted and the expansion valve deviates from its working point, It shows that the opening of the thermal expansion valve is 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 evaporation pipe. In the following period, there is no liquid refrigerant to evaporate in the evaporator pipe, only the steam is overheated. Therefore, a considerable number of evaporators fail to give full play to their efficiency, resulting in insufficient cooling capacity and reducing the cooling effect of the equipment. Most compressors of industrial oil coolers use the steam from the evaporator to cool the compressor. If the thermal expansion valve is not opened enough, the steam will overheat too much, the cooling effect on the compressor will be reduced, the exhaust temperature of the compressor will increase, the lubricating oil will become thinner, the lubrication quality will be reduced, and the working environment of the compressor will deteriorate, which 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 reduced, the running time of the compressor is increased and the power consumption is increased

on the contrary, if the thermal expansion valve is opened too much, that is, the liquid supply from the thermal expansion valve to the evaporator is greater than the evaporator load, some refrigerant will not be able to evaporate in the evaporator and enter the compressor together with gaseous refrigerant, causing wet stroke, even cylinder flushing accident, and damaging the compressor. At the same time, the thermal expansion valve is opened too large, which increases the evaporation temperature, reduces the refrigeration 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 make it work at the best matching point

4 adjustment process of the thermal expansion valve

4.1 inspection before the adjustment of the thermal expansion valve

before the adjustment of the thermal expansion valve, it must be confirmed that the abnormal refrigeration of the equipment is caused by the deviation of the thermal expansion valve from the optimal operating point, rather than due to less freon, clogged drying filter, filter, fan and other reasons. 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 to the lower side of the air return pipe, and must not be installed directly below the pipe, so as to prevent the accumulation of oil at the bottom of the pipe and other factors from affecting the correct temperature sensing of the temperature sensing package. It can not be installed on the riser. Check the control mode of condenser fan, and try to use speed control to ensure constant condensing pressure

4.2 precautions for adjustment of the thermal expansion valve

the adjustment of the thermal expansion valve must be carried out under the normal operation of the refrigeration unit. Since the thermometer cannot be placed on the evaporator surface, 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 looking up the table. Measure the temperature of the air return pipe with a thermometer and check the superheat by comparing it with the evaporation temperature. During adjustment, if the superheat is too small, turn the adjusting screw clockwise (that is, increase the spring force and reduce the opening of the thermal expansion valve) to reduce the flow; On the contrary, if the superheat 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. Due to certain thermal inertia in the temperature sensing system of the thermal expansion valve in actual work, different experimental rates with different signals will result in different experimental result transmission lag. After the operation is basically stable, the lubricating agent can be added between the contact surfaces of various friction pairs with relative activity for the next adjustment. Therefore, the whole adjustment process must be patient and meticulous, and the number of turns of the adjusting screw should not be too much or too fast at a time (the adjusting screw of the straight rod thermal expansion valve rotates for one turn, and the superheat changes by about 1~2 ℃)

4.3. Measuring method for superheat of thermal expansion valve

the steps are as follows:

1) shutdown. Insert the probe of the digital thermometer into the insulation layer at the air return port of the evaporator (corresponding to the position of the temperature sensing package). Connect the pressure gauge to the tee of the compressor low pressure valve

2) start the machine, let the compressor run for more than 15 minutes, enter the stable operation state, and make the pressure indication and temperature display reach a stable value

3) read out the temperature T1 of the digital thermometer and the temperature T2 corresponding to the pressure measured by the pressure gauge, and the superheat is the difference t1- T2 between the two readings. Note that both readings must be read at the same time

the superheat of the thermal expansion valve should be between 5~8 ℃. If not, make appropriate adjustment. It can be seen that the temperature of the compressor casing after adjustment will change significantly compared with that before adjustment

5. Maintenance cycle of the thermal expansion valve

after long-term statistics on the operation of the industrial oil cooler, it is found that the deviation of the thermal expansion valve from the working point usually occurs in the middle and late stages of its service life. Therefore, it is decided that the inspection and adjustment of the thermal expansion valve should focus on the middle and late stages of the equipment life. The following is the inspection cycle of the thermal expansion valve established according to the actual statistics

check and adjustment cycle of thermal expansion valve

after the 9th year in the first 4 years and 5~8 years before use

3 times/year 2 times/year 3 times or more

6 Conclusion

although it is troublesome to regularly check and adjust the thermal expansion valve, it has important practical significance for improving the cooling effect of the air conditioner, prolonging the service life of the air conditioner, saving energy and ensuring equipment safety

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