Expansion valve principle and failure analysis and adjustment
In large and medium-sized air conditioners and heat pump hot water systems, thermal expansion valves are widely used as throttling components of refrigeration systems to control the amount of refrigerant supplied due to their advantages of stable regulation and reliable quality. The following aspects will introduce the thermal expansion valve, an important refrigeration component.
1. Structural composition of thermal expansion valve
The air conditioner thermal expansion valve is composed of a temperature sensing bulb, a capillary tube, a throttle valve needle, an ejector rod, a fixed value spring and an adjusting screw.
2, the working principle of thermal expansion valve
The thermal expansion valve controls the refrigerant flow into the evaporator by sensing the superheat of the gaseous refrigerant at the outlet of the evaporator. According to the different balance methods, the thermal expansion valve is divided into external balance and internal balance, while in the central air conditioning system, the external balance is mostly used, which is composed of an induction mechanism, an actuator, an adjustment mechanism and a valve body. When working, the temperature sensing package fixed on the outlet pipe of the evaporator senses the superheated temperature of the evaporator outlet, so that pressure is generated in the temperature sensing package, and is transmitted to the space above the diaphragm by the capillary tube. The deformation method transmits the signal to the thimble (actuator), thereby adjusting the opening of the valve and controlling the flow of the refrigerant.
The expansion valve is affected by three forces to adjust its opening degree. The top is the pressure of the temperature sensor; the left is the adjustment spring pressure, and the right is the evaporation pressure. The pressure of the temperature sensing bulb provides the valve opening force, and the regulating spring pressure and evaporation pressure provide the valve closing force.
Through the comparison of the above two figures, the difference is that the sampling points of evaporation pressure are different. The inner balance collection point is the outlet position of the expansion valve, and the outer balance evaporation pressure collection point is the outlet position of the evaporator. As we all know, the function of the thermal expansion valve is to control the superheat degree of the evaporator outlet, that is to say, the response of the external balance thermal expansion valve is correct under any conditions.
3. Analysis of several failures in the work of thermal expansion valve
3.1 Blocking fault
3.1.1 Causes of blockage
The blockage of the thermal expansion valve in the refrigeration system is a frequent occurrence, including "dirty blockage" and "ice blockage". 1) The main reason for dirty blockage is the presence of impurities in the system, such as welding slag, copper filings, iron filings, fibers, etc. 2) The reason for ice blockage is that the system contains too much moisture (moisture), and the ways of generating moisture are: during installation, the vacuuming time of the system is not enough, and the moisture in the pipeline cannot be exhausted; the pipeline connection The welding process at the place is not good, and there are air leakage points. Air in the connecting hose was not blown out of the hose when charging the system with refrigerant. Enter air when re-lubricating the system.
3.1.2 Location of blockage
In general, the dirty blockage occurs on the dry filter, and the impurities in the system are intercepted by the filter, resulting in dirty blockage. When it occurs, the system first manifests as the return air temperature rises and the superheat degree rises. After the failure is serious, the system stops running. If the impurities in the system are not removed, the system cannot be turned on again. Ice blockage generally occurs at the throttle hole of the expansion valve, for example, because this is the place with the lowest temperature and the smallest hole diameter in the entire system. Since the system is no longer refrigerated, the overall temperature of the system rises. As the temperature increases, the ice block will gradually melt, and then the system will restore the cooling capacity. As the overall temperature of the system decreases again, ice block will occur again. Therefore, ice blockage is an iterative process.
3.1.3 Elimination method of blockage
So how to troubleshoot the blockage? For dirty blockage, if it is not serious, just replace the filter drier. If it is very serious, it is necessary to re-clean the impurities in the system pipeline, vacuumize, and recharge the refrigerant. For slight ice blockage, a hot towel can be applied to the blockage area. If the degree of ice blockage is serious and has affected the normal operation of the system, the filter dryer should be replaced, the water in the system pipeline should be removed again, and the vacuum should be applied. Recharge refrigerant.
3.2 Temperature sensor failure
3.2.1 Common causes of temperature sensor failure
When the liquid supply of the expansion valve is too long or too little or the opening of the expansion valve is not too small, and the superheat and subcooling are incorrect, the reason may be that the temperature sensor is faulty. Including: the capillary tube of the temperature sensing package is broken, so that the filling material in the temperature sensing package is leaked, resulting in the inability to transmit the correct signal to the actuator of the thermal expansion valve; the wrapping position of the temperature sensing package is incorrect.
3.2.2 Troubleshooting method of temperature sensor
In general, the temperature sensor package should be installed on the return pipe of the horizontal section of the evaporator outlet as far as possible. It should be away from the suction port of the compressor and close to the evaporator, and should not be installed vertically. Because installing the temperature sensor at the top of the suction pipe will reduce the sensitivity of the reaction, it may cause too much refrigerant in the evaporator, and installing the temperature sensor at the bottom of the suction pipe will cause disturbance of the liquid supply, because there is always a small amount of The liquid refrigerant flows to the location where the temperature sensor is installed, resulting in a rapid change in the temperature of the temperature sensor. During installation, the temperature sensing package should be wrapped with copper sheets, and the surface of the return air pipe should be derusted. The temperature sensor package must be lower than the upper chamber of the valve top diaphragm, and the head of the temperature sensor package should be placed horizontally or downward. When the relative position is higher than the upper chamber of the diaphragm, the capillary should be bent upward into a U shape to prevent liquid from entering the film. On-chip cavity.
4. Adjustment of thermal expansion valve
4.1 Regarding the adjustment of expansion valve, we must first understand several concepts
1) The degree of superheat of the expansion valve: when the thermal expansion valve is at a certain opening, the corresponding degree of superheat is called the degree of working superheat, that is, the degree of superheat of the thermal expansion valve. Includes static superheat (SS) and open superheat (OS).
2) Static superheat degree: When the thermal expansion valve is in the open position, the spring force is the smallest, and the superheat degree controlled by the thermal expansion valve is the smallest at this time, which is called the static superheat degree SS.
3) Dynamic superheat degree: After the valve hole of the expansion valve is opened, the valve hole opening degree increases with the increase of the outlet steam superheat degree. From the valve hole opening to the full opening, the value of the superheat degree increasing is called the dynamic superheat degree OS.
4.2 Correct adjustment method of expansion valve
Before adjusting the thermal expansion valve, it must be confirmed that the abnormal cooling is caused by the deviation of the thermal expansion valve from the optimal working point, rather than due to insufficient Freon, clogging of the drying filter, filter screen, fan, belt and other reasons. At the same time, it is necessary to ensure the correctness of the sampling signal of the temperature sensor. The installation position of the temperature sensor must be correct, and it must not be installed directly under the pipeline, so as to prevent factors such as oil accumulation at the bottom of the pipe from affecting the correct temperature sensing of the temperature sensor.
4.3 Matters needing attention when adjusting thermal expansion valve
The adjustment of the thermal expansion valve must be carried out under the normal operation of the refrigeration unit. Since a 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 looking up the table. Use a thermometer to measure the temperature of the return gas pipe and compare it with the evaporation temperature to check the superheat. During adjustment, if you feel that the superheat is too small, you can turn the adjusting screw clockwise to increase the spring force, reduce the opening degree of the thermal expansion valve, and reduce the flow; If the liquid is insufficient, turn the adjusting screw in the opposite direction (counterclockwise) to increase the flow. Due to the thermal inertia of the thermal expansion valve temperature sensing system in actual work, the signal transmission lag is formed, and the next adjustment can 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 adjustment screw should not be too fast or too fast.
4.4 Specific adjustment steps of thermal expansion valve
Shutdown: Insert the probe of the digital thermometer into the insulation layer at the return air port of the evaporator (corresponding to the position of the temperature sensor bulb). Connect the pressure gauge to the tee of the compressor low pressure valve. Power on: let the compressor run for more than 20 minutes, enter a stable operation state, and make the pressure indication and temperature display reach a stable value. 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 between the two readings T1-T2. Note that both readings must be taken at the same time. Thermal expansion valve superheat should be between 3-8 degree , if not, make appropriate adjustments. The adjustment steps are: first remove the protective cover of the thermal expansion valve, then turn the adjustment screw 2 to 4 turns, wait for the system to run stably, re-read, calculate the superheat, whether it is in the normal range, if not, repeat the previous operation until it meets the requirements, the adjustment process must be careful and careful.
