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Fault Analysis and Troubleshooting of Thermal Expansion Valve in Water Chiller
The thermal expansion valve, one of the four major components of a water chiller, is divided into an external balance type and an internal balance type according to the different balancing methods. However, in central air-conditioning systems, the external balance type is mostly used, consisting of a sensing mechanism, an actuator, an adjustment mechanism, and a valve body. During operation, the temperature sensing bag fixed on the evaporator outlet pipe senses the overheating temperature at the evaporator outlet, causing pressure to be generated in the temperature sensing bag and transmitted to the space above the membrane by the capillary tube. Under the action of pressure, the membrane transmits the signal to the thimble (actuator) in an elastic deformation manner, thereby adjusting the opening of the valve and controlling the flow of refrigerant.
Common faults and troubleshooting analysis during operation of thermal expansion valve:
First: Improper adjustment of thermal expansion valve
(1) The superheat of the expansion valve: When the thermal expansion valve is at a certain opening degree, the corresponding superheat is called the working superheat, which is the superheat of the thermal expansion valve. This includes static superheat (SS) and open superheat (OS).
(2) Static superheat: When the thermal expansion valve is in the open position, the spring force is minimal, and the superheat controlled by the thermal expansion valve is minimal, known as static superheat SS.
(3) Dynamic superheat degree: After the expansion valve hole is opened, the valve hole opening degree increases with the increase of the outlet steam superheat degree. From the valve hole opening to full opening, the value of its superheat degree increase is called dynamic superheat degree OS.
Correct adjustment method for thermal expansion valve:
(1) Before adjusting the thermal expansion valve, it must be confirmed that the abnormal cooling of the air conditioner is caused by the deviation of the thermal expansion valve from the optimal working point, rather than due to low Freon, clogged drying filters, filter screens, fans, belts, and other reasons. At the same time, it is necessary to ensure the correctness of the sampling signal of the temperature sensor. The temperature sensor must be installed at the correct location, and must not be installed directly below the pipe to prevent factors such as oil accumulation at the bottom of the pipe from affecting the correct temperature sensing of the temperature sensor.
(2) Precautions for adjusting the thermal expansion valve
The adjustment of the thermal expansion valve must be carried out under the normal operating state of the refrigeration device. Due to the inability to place a thermometer on the surface of the evaporator, the suction pressure of the compressor can be used as the saturation pressure within the evaporator to obtain an approximate evaporation temperature by looking up the table. Use a thermometer to measure the temperature of the return pipe and compare it with the evaporation temperature to check the degree of superheat. During adjustment, if you feel that the degree of superheat 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; On the contrary, 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. Due to certain thermal inertia in the actual work of the thermal expansion valve temperature sensing system, signal transmission lags behind, and the next adjustment can only be made after the operation is basically stable. Therefore, the entire adjustment process must be patient and meticulous, and the number of turns of the adjusting screw should not be too many or too fast at one time.
(3) Specific adjustment steps for the thermal expansion valve
1) Shutdown. Insert the probe of the digital thermometer into the insulation layer at the evaporator return port (corresponding to the temperature sensing bulb position). Connect the pressure gauge to the tee of the compressor low pressure valve.
2) Start the machine and let the compressor run for more than 15 minutes to enter a stable operation state, so that the pressure indication 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 degree of superheat is the difference between the two readings T1 and T2.
Second: blockage fault of thermal expansion valve
Blocking failures of thermal expansion valves in refrigeration systems often occur, including "dirty blocking" and "ice blocking.". The main reason for fouling is the presence of impurities in the refrigeration system, such as welding slag, copper filings, iron filings, fibers, etc. The reason for ice blockage is that the system contains too much water (moisture), which can be generated through the following ways: 1) During installation, the system did not have enough vacuum pumping time to completely extract the moisture in the pipeline, or the welding process at the pipeline connection was poor, resulting in air leakage points. 2) When filling the system with refrigerant, the air in the connecting hose was not blown out of the hose. 3) When replenishing the system with lubricating oil, air enters. Generally, dirty blockage occurs on the drying filter, and impurities in the system are blocked by the filter, resulting in dirty blockage. When it occurs, the system first exhibits an increase in return air temperature and superheat. After a serious fault occurs, the system stops operating. If impurities in the system are not removed, the system cannot be restarted. Ice blockage generally occurs at the orifice of the expansion valve, as this is the lowest temperature and smallest pore size area in the entire system. As the refrigeration system is not cooling, the overall temperature of the system rises. As the temperature increases, the ice blockage will gradually melt, and then the system will restore its cooling capacity. As the overall temperature of the system decreases again, ice blockage will occur. Therefore, ice blockage is a recurrent process.
Troubleshooting for clogging: If the clogging is not very serious, replace the industrial chiller with a drying filter. If it is very serious, it is necessary to re clean the impurities in the refrigeration system pipeline, vacuum, and refill the refrigerant. For slight ice blockage, a hot towel can be applied to the ice blockage. If the ice blockage is severe and has affected the normal operation of the system, the filter dryer should be replaced, the moisture in the system pipeline should be removed, vacuum pumped, and refrigerant refilled.
Third: Temperature sensing bulb fault
When there are phenomena in the system such as excessive or insufficient supply of liquid by the expansion valve, or the expansion valve is not turned down, or the degree of superheat or undercooling is incorrect, the cause may be a malfunction of the temperature sensing bag. Including: 1) The capillary tube of the temperature sensitive bulb is broken, causing the filling material in the temperature sensitive bulb to leak, resulting in the failure to transmit the correct signal to the actuator of the thermal expansion valve. 2) The temperature sensing wrapping position is incorrect.
Troubleshooting method for temperature sensitive bulb: Generally, the temperature sensitive bulb should be installed on the return pipe at the horizontal section of the evaporator outlet as far as possible, away from the compressor suction port, and should not be installed vertically. When the diameter of the horizontal return pipe is less than 7/8 "(22mm), the temperature sensor should be installed at the top end of the return pipe, that is, the" one o'clock "position of the suction pipe. When the diameter of the horizontal return pipe is greater than 7/8", the temperature sensor should be installed below the axis of the return pipe at about 45 degrees to the horizontal axis, that is, the "three o'clock" position of the suction pipe. Because installing a temperature sensitive bulb on the top of the suction tube can reduce the sensitivity of the reaction, which may cause excessive refrigerant in the evaporator. Installing a temperature sensitive bulb on the bottom of the suction tube can cause a disturbance in the supply of liquid, as there is always a small amount of liquid refrigerant flowing to the location where the temperature sensitive bulb is installed, resulting in a rapid change in the temperature of the temperature sensitive bulb.
The thermal expansion valve is one of the four major components in a refrigeration system, which is responsible for reducing the refrigerant from the condensation pressure to the evaporation pressure in the system, and controlling the refrigerant flow proportionally. The quality of the thermal expansion valve in a system directly affects the operational performance of the entire system. Therefore, timely troubleshooting and appropriate and correct selection of the thermal expansion valve are of great significance to the operating life, cooling effect, and operating cost of the chiller.
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