Correct adjustment method of opening degree of thermal expansion valve

Correct adjustment method of opening degree of thermal expansion valve

The expansion valve is one of the four major components of the refrigeration system. It is an important device for regulating and controlling the flow and pressure of the refrigerant entering the evaporator, and it is also the "boundary line" between the high and low pressure sides. Its adjustment is not only related to the normal operation of the entire refrigeration system, but also an important indicator to measure the level of operator skills.

　　 Today we will tell you about the troubleshooting of the thermal expansion valve, and how to adjust the opening of the thermal expansion valve, hot dry goods!

　　 One, the fault phenomenon of improper adjustment of the thermal expansion valve

　　 1. The opening is too small

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. The refrigerant has evaporated during the flow of the evaporator tube, and there is no liquid in the evaporator tube after this. The refrigerant is available for evaporation, only the steam is superheated.

　　 At the same time, the outlet pressure of the expansion valve is too low, and the corresponding evaporation pressure and temperature are also too low. As a result, the evaporation rate slows down, the cooling capacity per unit volume (time) decreases, and the cooling efficiency decreases.

　　 Therefore, a considerable part of the evaporator fails to give full play to its performance, resulting in insufficient cooling capacity and reducing the cooling effect of the air conditioner. Most of the compressors of special air conditioners in computer rooms 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 discharge temperature of the compressor will increase. The lubricating oil becomes thinner, the lubrication quality is reduced, and the working environment of the compressor deteriorates, which will seriously affect the working life of the compressor.

　　2, the opening is too large

If the thermal expansion valve is opened too large, the flow of refrigerant will be too much, that is, the amount of liquid supplied by the thermal expansion valve to the evaporator is greater than the load of the evaporator, and the liquid refrigerant will evaporate too much, which will cause part of the liquid refrigerant to evaporate in the evaporator , Together with the gaseous refrigerant, is sucked into the compressor, causing the wet stroke (liquid hammer) of the compressor, making the compressor unable to work normally, causing a series of severe working conditions and even damage to the compressor by the cylinder.

　　 At the same time, the thermal expansion valve is opened too large, so that the corresponding evaporation pressure and temperature of the refrigerant entering the evaporator are also too high, the cooling capacity decreases, the compressor power consumption increases, and the power consumption increases.

Since the refrigeration power of the new compressor is not equipped with the system, the adjustment pressure of the expansion valve is always lower than normal, indicating that the compressor power is too large, resulting in large suction volume, too small evaporation area, contradictory operating conditions, and then open it again. The expansion valve sucks in over-humid gas because the liquid cannot boil and evaporate, causing the compressor to frost.

　　 2. The adjustment premise of the thermal expansion valve

1. Before adjusting the thermal expansion valve, it must be confirmed that the abnormal refrigeration is caused by the deviation of the thermal expansion valve from the best working point of z*, not because of the improper refrigerant charge of the system, blockage of the filter dryer and expansion valve, and electromagnetic It is caused by other reasons such as valve failure, poor heat dissipation of the condensing fan, refrigeration power of the newly replaced compressor and unequipped system.

　　 At the same time, the correctness of the sampling signal of the temperature sensor must be ensured. The installation position of the temperature sensor must be correct, and it must not be installed directly under the pipe, to prevent factors such as oil accumulation at the bottom of the pipe from affecting the correct temperature of the temperature sensor. In addition, if it is a thermal expansion valve that has lost its adjustment function, when a new expansion valve is used, the new product has been debugged and checked before leaving the factory, so it is not necessary to adjust the expansion valve and install it directly.

　　2, stop. Insert the probe of the digital thermometer into the insulation layer at the return port of the evaporator (corresponding to the position of the temperature sensing bulb). Connect the pressure gauge to the tee of the compressor low pressure valve.

　　3. Turn on the compressor and let the compressor run for more than 15 minutes to enter a stable operation state, so that the pressure indicator and temperature display reach stable values.

　　 Three, the adjustment steps of the thermal expansion valve

When adjusting, first unscrew the bonnet under the thermal expansion valve, and turn the valve stem clockwise to close the pinhole opening of the valve body, that is, the liquid supply flow is reduced (in short, the needle is turned down); otherwise, the needle The opening of the hole is large, that is, the flow of the liquid supply is increased (in short, the reverse rotation is large).

　　 is the same as adjusting the water valve to control the water flow. The flow adjustment should be carried out during the normal operation of the refrigeration system, and should be operated slowly and adjusted gradually. The judgment basis for adjusting the liquid supply flow rate is as follows:

　　(1) Determine whether the adjustment size is appropriate by the shape change of the frosting of the thermal expansion valve.

　　A. If the expansion valve body is completely frosted, it indicates that the flow rate is too small and large, and it should be increased;

　　 B. If the frosting shape does not change when it is enlarged, the expansion valve orifice may be partially blocked and should be cleaned;

　　 C. If only the outlet side of the expansion valve body is frosted, it indicates that the flow rate is too large and should be adjusted down;

　　D. If the outlet side and lower part of the expansion valve body are frosted obliquely at 45°C, the inlet side should not be frosted, indicating that the adjustment is accurate and suitable;

　　E. If only the inlet side of the expansion valve body is frosted, it indicates that the filter screen at the inlet of the valve body is blocked and should be cleaned;

F. If the expansion valve body is completely frost-free, it indicates that there is no flow. The refrigerant may have leaked out or the stop valve in the pipeline has not been opened or the expansion valve temperature probe capillary has leaked or the expansion valve orifice is blocked or the valve body inlet is filtered The net part is blocked and should be cleaned.

　　(2) It is also possible to judge whether the adjustment size is appropriate by the change in the shape of the frost at the compressor suction pipe.

　　A. If the hoarfrost forms on the suction stop valve, it indicates that the flow rate is too large and should be reduced;

　　 B. If the hoarfrost does not form the suction pipe, it indicates that the flow rate is too small and should be increased.

　　(3) In addition, judge whether the adjustment is appropriate by the pressure value of the low pressure side.

　　(4) The uniform and complete condition of the evaporator coil frosting is used to judge whether the adjustment size is appropriate.

　　(5) Under normal circumstances, the expansion valve is very quiet when it is working. If there is a clear "silk" sound, it means that the refrigerant in the system is insufficient.

　　(6) Use a thermometer to measure the difference between the temperature of the return pipe and the evaporation temperature (ie the actual superheat) and check the standard superheat (between 5-8°C) to determine whether the adjustment is appropriate.

　　Using the suction pressure of the compressor as the saturation pressure in the evaporator, look up the table to get the approximate evaporation temperature.

　　 Use a thermometer to measure the temperature of the return air pipe and check whether it is within the normal range of 5-8°C compared with the evaporation temperature. The suction pressure and the return pipe temperature must be read at the same time, otherwise the calculated actual superheat will be inaccurate.

　　In adjustment,

　　A. 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 of the thermal expansion valve) to reduce the flow;

　　 B. Conversely, if you feel that the superheat is too high, that is, the liquid supply is insufficient, you can turn the adjusting screw 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, the signal transmission lags, and the next adjustment can be made after the operation is basically stable.

　　 Do not use rapid adjustment with large fluctuations during adjustment, so that the refrigeration system operates unstable and the effect of adjustment cannot be grasped.

Rough adjustment should be started first (the thermal expansion valve of the special air conditioner in the computer room generally adopts the pressure rod type and the loose gear type. The loose gear type uses a small gear to drive a large gear. The number of adjustment turns is relatively large. Generally, it can be adjusted. 2~4 turns; the number of adjustable turns of the pressure rod type is relatively small, each adjustment is 1/4 turn; the thermal expansion valve of the air conditioner adopts a loose gear type), observe the change of the pressure value of the pressure gauge on the low pressure side, and wait for the pressure value of the gauge to stabilize After that, make fine adjustments. Adjust 1/2, 1/3, 1/4 circle each time.

　　Adjusting the expansion valve must be carried out carefully and patiently. The pressure must be adjusted through the heat exchange between the evaporator and the warehouse temperature, boil (evaporate), and then enter the compressor suction chamber through the pipeline to reflect on the pressure gauge. It takes a time process. Every time the expansion valve is adjusted, it usually takes 15-30 minutes to stabilize the adjustment pressure of the expansion valve on the suction pressure gauge. In addition to observing the changes in the low pressure value, it is also necessary to observe the frosting at the expansion valve body, evaporator coil and compressor suction pipe to determine whether the adjustment is appropriate.

　　 Note:

　　1. Before the expansion valve is adjusted, the system should not be injected from the high-pressure side for leak detection and refrigerant filling. Otherwise, it is difficult to inject, and the pressure at the high and low pressure ends cannot be balanced. Because the expansion valve is a divider connecting the high and low pressure of the system, if the evaporator outlet pipe at the expansion valve temperature probe is not hot or superheated when the refrigeration system is not in normal operation, the expansion valve outlet valve port is closed and cannot be opened. The pipeline is cut off and the high and low pressure sides are blocked, so the pressure at the high and low pressure ends cannot be balanced.

　　2. The refrigeration system consisting of a thermal expansion valve and a single-phase hermetic compressor will have difficulty starting the compressor. The reason is the same as above.