Trouble shooting and correct selection method for thermal expansion valve debugging

Trouble shooting and correct selection method for thermal expansion valve debugging

As we all know, the thermal expansion valve is one of the four major components in the refrigeration system. It is responsible for reducing the refrigerant from the condensing pressure to the evaporating pressure in the system, and proportionally controlling the flow of the refrigerant. The quality of the thermal expansion valve in a system will directly affect the operating performance of the entire system. Therefore, timely troubleshooting of the thermal expansion valve and proper selection are of great significance to the operating life, cooling effect, and operating cost of the air conditioning system. .

Trouble shooting and correct selection method for thermal expansion valve debugging

　　 One, the working principle of the thermal expansion valve

Trouble shooting and correct selection method for thermal expansion valve debugging

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 the different balancing methods, the thermal expansion valve is divided into external balance and internal balance, and it is mostly used in central air conditioning systems External balance type. It is composed of induction mechanism, actuator, adjustment mechanism and valve body. When working, the temperature sensing bulb fixed on the evaporator outlet pipe senses the overheating temperature of the evaporator outlet, so that pressure is generated in the temperature sensing bulb, and it is transmitted to the space above the diaphragm by the capillary tube, and the diaphragm is elastic under the action of pressure. The deformation method transmits the signal to the thimble (actuator), thereby adjusting the opening of the valves and controlling the flow of refrigerant.

　　 2. Analysis of several failures in the operation of thermal expansion valve

　　 1. Blockage failure of thermal expansion valve

　　1.1 Causes of blockage

　　 Thermal expansion valve blockage faults in refrigeration systems often occur, including "dirty blockage" and "ice blockage". The main cause of dirty blockage is the presence of impurities in the system, such as welding slag, copper filings, iron filings, fibers, etc. The cause of ice blockage is that the system contains too much water (humidity), and the ways to generate humidity are:

　　1) During installation, the vacuuming time of the system was not enough, and the moisture in the pipeline could not be exhausted; the welding process of the pipeline connection was not good, and there were leaks.

　　2) When charging the system with refrigerant, the air in the connecting hose was not blown out of the hose.

　　3) When replenishing lubricating oil for the system, air enters.

　　1.2 Location of blockage

　　 Generally, dirty clogging occurs on the filter drier, and the impurities in the system are intercepted by the filter, causing dirty clogging. When it happens, the system first manifests as the return air temperature rises and the superheat rises. After the fault is serious, the system stops running. If the impurities in the system are not removed, the system cannot be restarted. Ice blockage generally occurs at the orifice of the expansion valve, because this is the place with the lowest temperature and smallest aperture in the entire system. Because the system is not cooling, the overall temperature of the system rises. As the temperature increases, the ice block will gradually melt, and then the system resumes its refrigeration capacity. As the overall temperature of the system drops again, the ice block will appear again. Therefore, ice jam is a repetitive process.

　　1.3 How to eliminate blockage

　　 So how to eliminate the clogging fault? For the dirty clogging, if it is not very serious, just change a filter dryer. If it is very serious, it is necessary to clean up the impurities in the system piping, vacuum, and refill the refrigerant. For minor ice blockage, you can apply a hot towel to the ice blockage. If the degree of ice blockage is serious and has affected the normal operation of the system, you must replace the filter dryer, remove the water in the system pipeline again, and vacuum, Recharge refrigerant.

　　2, the temperature sensor is faulty

　　2.1 Common causes of faults in the temperature sensing package

　　 When the expansion valve supplies liquid frequently and sometimes less or the expansion valve is not closed, the degree of superheat, the degree of subcooling is incorrect, etc. in the system, the reason may be that the temperature sensor is out of order. include:

　　1) The capillary tube of the temperature sensing bag is broken, causing the filling material in the temperature sensing bag to leak out, resulting in failure to transmit the correct signal to the actuator of the thermal expansion valve.

　　2) The wrapping position of the temperature sensor is incorrect.

　　2.2 How to deal with the fault of the temperature sensor

　　Generally, the temperature sensor should be installed on the return pipe of the horizontal section of the evaporator outlet as far as possible. It should be far from the compressor suction port and nearest to the evaporator, and it is not suitable for vertical installation. When the diameter of the horizontal return pipe is less than 7/8" (22mm), the temperature sensing bulb should be installed on the top end of the return pipe, that is, "one o'clock" of the suction pipe. When the diameter of the horizontal return pipe is greater than 7/8", the sensor The temperature bulb should be installed below the axis of the return pipe at about 45 degrees to the horizontal axis, that is, the "3 o'clock" position of the suction pipe. Because installing the temperature sensor bulb on the upper part of the suction pipe will reduce the sensitivity of the reaction and may cause too much refrigerant in the evaporator, installing the temperature sensor bulb on the bottom of the suction pipe will cause turbulence in the liquid supply, because there is always a small amount The liquid refrigerant flows to the location where the bulb is installed, causing the temperature of the bulb to change rapidly.

　　 When installing, the temperature sensor package needs to be wrapped with a copper sheet, and the surface of the air return pipe should be derusted. If it is a steel pipe, the surface should be painted with silver paint after rust removal to ensure good contact between the temperature sensor package and the return air pipe. The temperature sensing bulb must be lower than the upper cavity of the valve top diaphragm, and the head of the temperature sensing bulb should be placed horizontally or downwards. When the relative position is higher than the upper cavity of the diaphragm, the capillary tube should be bent upwards into a U shape to prevent liquid from entering the membrane. On-chip cavity.

　　3.3 Improper adjustment

　　3.3.1 Regarding expansion valve adjustment related concepts

　　 Speaking of adjustment, we must first understand a few concepts

　　 (1) The degree of superheat of the expansion valve: the thermal expansion valve is at a certain degree of opening, and the corresponding degree of superheat is called the working superheat, which is the so-called superheat of the thermal expansion valve. Including static superheat (SS) and open superheat (OS).

　　 (2) Static superheat: When the thermal expansion valve is in the open position, the spring force is the smallest. At this time, the superheat controlled by the thermal expansion valve is the smallest, which is called the static superheat SS.

(3) Dynamic superheat: After the valve hole of the expansion valve is opened, the opening of the valve hole increases with the increase of the outlet steam superheat. From the opening of the valve hole to the full opening, the value of the increase of the superheat is called the dynamic superheat OS .

　　3.3.2 Correct adjustment method of expansion valve

(1) Before adjusting the thermal expansion valve, it is necessary to confirm that the abnormal air conditioner refrigeration is caused by the deviation of the thermal expansion valve from the optimal operating point, and not due to low freon, clogged filter, filter, fan, belt, etc. of. At the same time, the correctness of the sampling signal of the temperature sensing bulb must be ensured, and the temperature sensing installation position must be correct, and it must not be installed directly under the pipeline to prevent factors such as oil accumulation at the bottom of the tube from affecting the correct temperature sensing of the temperature sensing bulb.

　　 (2) Matters needing attention when 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. Since 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 looking up the table. Use a thermometer to measure the temperature of the return air 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 reduce the opening of the thermal expansion valve) to reduce the flow; on the contrary, if you feel that the degree of overheating is too high, That is, if the liquid supply is insufficient, the adjustment 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, the signal transmission lags, 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.

　　 (3) Specific adjustment steps of thermal expansion valve

　　 1) Shut down. 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.

　　2) Turn on the compressor, let the compressor run for more than 15 minutes, enter a stable operation state, and make the pressure indicator and temperature display reach stable values.

　　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 between the two readings T1-T2.

　　 Note that these two readings must be read at the same time. The superheat of the thermal expansion valve should be between 5-8℃, if not, make appropriate adjustments. The adjustment steps are: first remove the protective cover of the thermal expansion valve, then turn the adjustment screw 2-4 turns, wait for the system to run stably, re-read the reading, calculate the superheat, whether it is in the normal range, if not, repeat the previous operation until it meets Requirements, the adjustment process must be careful and careful.

　　 Three, the option of thermal expansion valve

　　 1. The purpose of correct selection of thermal expansion valve

　　The selection of thermal expansion valve plays an important role in the performance of the whole system. The correct selection of thermal expansion valve will make the evaporator use to the maximum and make the evaporator always match the heat load.

　　2, the phenomenon when the thermal expansion valve does not match the system

When there is a mismatch, the refrigerant flow of the system will be more and sometimes less, which will cause the cooling capacity of the thermal expansion valve to be large and small. Cause adverse effects. When the cooling capacity is too large, it will cause oscillations, intermittently causing excessive liquid supply to the evaporator, resulting in sharp fluctuations in the suction pressure of the compressor, and even liquid refrigerant entering the compressor, causing liquid hammer ( Wet stroke) phenomenon.

　　 3. Basis for selection

　　 Choose according to the type of refrigerant in the refrigeration system, the evaporating temperature range and the size of the evaporator overheating load.

　　 4.3.1 Selection method and general steps

　　The general steps are as follows:

　　1) Determine the refrigerant type of the system.

　　 2) Determine the evaporator's evaporating temperature, condensing temperature and cooling capacity.

　　3) The pressure difference between the inlet and outlet of the thermal expansion valve.