Principle, installation, debugging, and case studies of expansion valves

1.The role of thermal expansion valves in refrigeration systems

The thermal expansion valve (throttling mechanism) is one of the four basic system components necessary to achieve a refrigeration cycle.

① Throttling and depressurization: throttle and depressurize the condensed liquid refrigerant at room temperature and high pressure in the condenser, regulate the refrigerant flow through the evaporator, and control the superheat at the outlet of the evaporator. Superheat=return air temperature - evaporation temperature.

② Adjusting flow rate: Based on the temperature signal obtained from the temperature sensor, the expansion valve can automatically adjust the refrigerant flow rate entering the evaporator to adapt to the constantly changing refrigeration load.

③ Maintain a certain degree of superheat, prevent wet compression and abnormal overheating: The expansion valve adjusts the flow rate to ensure that the evaporator has a certain degree of superheat, ensuring the effective utilization of the total volume of the evaporator, and avoiding liquid refrigerant from entering the compressor and causing liquid shock; At the same time, it can control the degree of overheating within a certain range to prevent the occurrence of abnormal overheating.

Thermal expansion valves are divided into two types: internal balance and external balance expansion valves.

    ①Internal balance expansion valve: Temperature sensing bulb pressure=spring pressure+evaporator inlet pressure;

    ②Internal balance expansion valve: Under the valve diaphragm, the inlet pressure of the evaporator is felt;

External balance expansion valve: Temperature sensing bulb pressure=spring pressure+evaporator outlet pressure.

   ①External balance expansion valve: feels the outlet pressure of the evaporator;

   ②External balance expansion valve: used in systems with high evaporator resistance; 

2. Installation of thermal expansion valve

If the diameter of the copper tube is greater than 35mm, the temperature sensing bulb should be installed on the horizontal return pipe at the outlet of the evaporator as much as possible, and generally should be at least 1.5m away from the compressor suction port. The temperature sensor should never be placed on pipelines with accumulated liquid. And tightly wrap it against the pipe wall, and the contact area should be cleaned of oxide skin to expose the true color of the metal.

When the diameter of the return airway is less than 25mm, the temperature sensing bulb can be tied to the top of the return airway; When the diameter is greater than 25mm, it can be tied at a 45 ° angle below the return pipe to prevent factors such as oil accumulation at the bottom of the pipe from affecting the correct temperature sensing of the temperature sensor

3. Adjustment of expansion valve

The adjustment of the thermal expansion valve must be carried out under normal refrigeration operation. If the degree of superheat is too small, that is, the flow rate is too high, the adjusting screw can be turned clockwise by half a turn or (increasing spring force, reducing the opening degree of the expansion valve) to reduce the flow rate. Conversely, if the degree of superheat is too large, that is, insufficient liquid supply, the adjusting screw can be turned in the opposite direction (counterclockwise) to increase the flow rate. Adjust the number of turns of the screw to not exceed half a turn at a time. The interval between adjusting the expansion valves must be at least 15 minutes.

The superheat of the expansion valve should be between 5-8 ℃. If not, adjust it.

Example of Thermodynamic Expansion Valve Adjustment

A constant temperature and humidity air conditioner operates with both compressors, with a return air temperature of 22.5 ℃ and a supply air temperature of 16.8 ℃. The refrigeration effect of the air conditioner is not significant. Observation revealed sufficient Freon, eliminating low fluoride and filter blockage.

A constant temperature and humidity air conditioner operates with both compressors, with a return air temperature of 22.5 ℃ and a supply air temperature of 16.8 ℃. The refrigeration effect of the air conditioner is not significant. Observation revealed sufficient Freon, eliminating low fluoride and filter blockage. Further inspection revealed that the return air of the two air conditioning compressors was overheated and the temperature at the outlet of the expansion valve was low. The digital thermometer measured the outlet temperature of the evaporator as 18 ℃, and the pressure gauge measured the return air pressure as 3.2kg/cm2, corresponding to a temperature of -5 ℃ and a superheat degree of 23 ℃, which deviated significantly from the normal superheat degree. Therefore, the diagnosis was that the opening degree of the expansion valve was not enough, and it was decided to adjust the opening degree of the expansion valve.

After correct adjustment, the outlet temperature of the evaporator is 12 ℃, and the return air pressure measured with a pressure gauge is 4.8kg/cm2, corresponding to a temperature of 4.5 ℃. After subtraction, the superheat is 7.5 ℃.

In this adjustment, the optimal superheat values are 7.6 ℃ and 7.9 ℃, respectively.

If an experienced refrigeration worker, they can observe the condensation situation in the compressor return pipe and measure the current to make a judgment. The rated current of this compressor is 14A. According to the ammeter, the adjusted current of the compressor is as follows.