The thermostatic expansion valve is one of the important components in the refrigeration unit, which reduces the flow of the condensed saturated liquid to the evaporation pressure while regulating the flow of refrigerant into the evaporator. According to the different balance methods, the thermostatic expansion valve can be divided into: external balance and internal balance.
The internally balanced equilibrium pressure is taken at the inlet of the evaporator and is used in systems where the pressure loss of the refrigerant in the evaporator is small. The equilibrium pressure of the external equilibrium type is taken at the outlet of the evaporator and is used in systems where the pressure loss of the refrigerant in the evaporator is large. The diagram below illustrates the difference between an internally balanced and externally balanced expansion valve.
As the transmitter of the thermostatic expansion valve, the location selection and actual installation of the thermostatic expansion valve are reasonable, which will directly affect the adjustment accuracy and working stability of the thermostatic expansion valve.
1. Installation and precautions of expansion valve
The body of the thermostatic expansion valve is installed as far as possible on a horizontal liquid pipe close to the inlet of the evaporator to reduce the pressure and temperature loss after the expansion valve is throttled. The valve body should be installed vertically and cannot be tilted or upside down. The valve body is generally installed at the outlet of the filter drier to prevent "dirty blockage" or "ice blockage" in the valve core. The temperature sensing bag is usually placed on the return pipe of the evaporator, and is tightly bandaged close to the wall of the tube, and the contact area should be cleaned;
The correct installation of the thermostatic expansion valve will directly affect the operating conditions of the refrigeration equipment, so it must be installed reasonably:
1. It must be installed above the suction pipe, and at the same time, it is necessary to ensure that the temperature sensing bag is close to the suction pipe, so as to improve the heat transfer effect of the refrigerant in the temperature sensing bag and the suction pipe, and reduce the time lag.
2. The temperature sensing bag should be installed on the horizontal suction pipe near the outlet of the evaporator as far as possible, far away from the suction port of the compressor, and should not be installed vertically. If the bag must be mounted on a vertical suction tube, the capillary of the bag should enter from the upper end of the pack, with the head of the pack facing down:
3. In order to protect the temperature bag from the air or liquid being cooled, a layer of insulation material should be wrapped. Special insulation is available or plastic insulation tape is available to ensure that the temperature of the suction tube is accurately detected by the sensor.
4. The temperature sensing bag needs to be fixed on the suction tube with copper sheets, stainless screws and nuts. Before installation, the contact of the suction tube and the temperature bag must be cleaned with steel wool to remove the oxide scale on the surface and expose the natural color of the metal, so as to avoid reducing the sensitivity of the sensor due to poor contact.
5. The liquid working fluid in the thermodynamic head is always kept in the temperature sensing bag, so the temperature sensing package is fixed on the suction pipe of the corrugated diaphragm cavity lower than the top of the valve body, and if the installation position of the temperature sensing bag must be higher than the corrugated diaphragm cavity on the top of the valve body, the capillary tube at the connection of the temperature sensing bag should be bent upward into an inverted "U" shape; It prevents the liquid from entering the corrugated diaphragm cavity and reduces the sensitivity and accuracy of the thermodynamic expansion valve.
6. The temperature sensor bag should not be installed in the place where there is liquid accumulation, oil accumulation, pipe joints, valves or other large metal parts in the suction pipe, so as not to cause the reaction lag of the temperature sensor bag and affect the stable operation of the thermal expansion valve.
7. According to the different diameter of the suction pipe, the installation angle of the temperature sensor bag on the horizontal suction pipe is different, when the diameter of the return pipe is less than 25mm, the temperature sensor bag can be tied to the top of the return pipe; When the diameter is greater than 25mm, it can be pierced at 45° on the underside of the return pipe.
The diameter of 12 ~ 16 mm is installed in the "1 o'clock" position of the suction tube;
The 18 ~ 22 mm diameter tube is installed in the "2 o'clock" position of the suction tube;
The 25 ~ 28 mm diameter is installed in the "3 o'clock" position of the suction tube;
The 32 ~ 35 mm diameter is installed in the "4-and-a-half" position of the suction tube.
This is done because the suction tube is mainly filled with evaporated refrigerant, but there is also a small amount of refrigerant liquid and oil. The diagram below shows refrigerant vapor and some liquid beads flowing through a larger diameter suction tube. Due to the large pipe diameter, the refrigerant vapor flow rate is often low, and refrigerant beads and oil are deposited at the bottom of the pipeline.
The temperature sensing bag of the larger diameter suction pipe should be located near the lower part of the tube, not at the upper end of the suction pipe, because the refrigerant liquid beads have a tendency to separate from the flowing vapor, and at the same time, the oil layer covered inside the suction pipe has a thermal insulation effect, and the temperature difference between the upper and lower sides of the horizontal pipe may be large, so the temperature sensing bag is usually installed between 45° below the horizontal axis of the large-diameter suction pipe, let's look at the physical picture.
2. Debugging of thermodynamic expansion valve
If the opening degree of the thermostatic 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 been evaporated on the way to flow in the evaporation tube, and there is no liquid refrigerant for evaporation in the subsequent evaporator tube, only the steam is superheated. As a result, the evaporation rate slows down, the refrigeration capacity per unit volume (time) decreases, and the refrigeration efficiency decreases.
The thermostatic expansion valve is opened too large: the flow rate of the refrigerant through is more, that is, the liquid supply of the thermostatic expansion valve to the evaporator is greater than the load of the evaporator, so that the liquid refrigerant evaporates excessively, which will cause some liquid refrigerants to evaporate in the evaporator and be sucked into the compressor together with the gaseous refrigerant, causing the wet stroke (liquid shock) of the compressor, making the compressor unable to work normally, causing a series of bad working conditions, and even the accident of damaging the compressor. At the same time, too large opening of the thermostatic expansion valve will also cause a decrease in cooling capacity, increase the power consumption of the compressor, and increase the power consumption.
Debugging method: If the flow rate is too large, the adjusting screw can be rotated in a clockwise direction (that is, the spring force can be increased and the opening degree of the thermal expansion valve can be reduced) to reduce the flow rate. As shown in the figure below:
Conversely, if the liquid supply is insufficient, the adjusting screw can be rotated in the opposite direction (counterclockwise) to increase the flow rate.
Due to the thermal inertia of the thermodynamic expansion valve temperature sensing system in actual work, the signal transmission lag is formed, and the next adjustment can only be made after the operation is basically stable. Regulating the expansion valve must be carried out carefully and patiently, and it generally takes 15-30 minutes to stabilize the adjustment pressure of the expansion valve on the suction pressure gauge.