How to select the thermal expansion valve？
1. The role and classification of expansion valves
Expansion valve is an important part of refrigeration system, generally installed between liquid storage cylinder and evaporator. The expansion valve causes the medium temperature and high pressure liquid refrigerant to become low temperature and low pressure wet vapor through its throttling, and then the refrigerant absorbs heat in the evaporator to achieve the cooling effect. The expansion valve controls the valve flow rate through the change of the superheat at the end of the evaporator to prevent occurrence Insufficient utilization of evaporator area and cylinder knocking.
Commonly used expansion valves in refrigeration systems include thermal expansion valves and electronic expansion valves. Thermal expansion valve is divided into: internal balance thermal expansion valve and external balance thermal expansion valve, as shown in the figure:
(Internal balance thermal expansion valve above picture)
(External balance thermal expansion valve above picture)
2. Selection conditions of expansion valve
Four necessary conditions for selecting an expansion valve: refrigerant; evaporating temperature; condensing temperature; evaporating load.
2.1
Refrigerants:
R22: X (such as TEX2, TEX5, TGEX) (Danfoss)
R404a /R507: S (eg TES2, TES5, TGES) (Danfoss)
R134a: N R407C: Z (Danfoss)
2.2
Evaporation temperature:
Ordinary cold storage: -40℃～+10℃N series;
Quick-freezing cold storage: -60℃～-25℃, B series;
Cold storage: The evaporation temperature is 10℃ lower than the storage temperature, and the quick-freezing storage is 8℃;
Shell and tube dry evaporator: The evaporation temperature is 8°C lower than the outlet water temperature.
2.3
Condensing temperature:
Evaporative condenser: 36 ℃;
Water-cooled condenser: 40 ℃;
Air-cooled condenser: 45 ℃.
Economizer: The degree of subcooling can be ignored.
2.4
Evaporation load:
The evaporation load between the expansion valve and the bulb
Case 1: 1 expansion valve with 1 air cooler or 2 to 4 rows of pipes.
Situation 2: 1 expansion valve with 2 or more air coolers, 1 large air cooler with 2 or more expansion valves.
3. Selection of expansion valve
Example 1: R22 refrigerant, 0℃ storage, air-cooled condenser, ice jade air cooler DD-28.0/140.
Air cooler: evaporative load = heat exchange under design conditions.
1. Refrigerant: R22; ℃
2. Evaporation temperature: -10℃;
3. Condensing temperature: 45 ℃;
4. Evaporation load: 30.8KW expansion valve TEX5-4, TGEX-11 (12.5).
The expansion valve throttle flow is greater than the evaporative load.
Example 2: R22, -18℃ storage, water-cooled condenser, ice jade air cooler DD-28.0/140.
Air cooler: evaporative load = heat exchange under design conditions.
1. Refrigerant: R22;
2. Evaporation temperature: -28℃;
3. Condensing temperature: 40℃;
4. Evaporation load: 28KW expansion valve TEX5-5, TGEX-20.
The expansion valve throttle flow is greater than the evaporative load.
Example 3: R22, -18°C warehouse, evaporative cooling, 120m*3 sets of aluminum pipes.
Aluminum row tube: Evaporation load = heat transfer under design conditions
1. Refrigerant: R22;
2. Evaporation temperature: -28℃;
3. Condensing temperature: 36 ℃;
4. Evaporation load: 13.3KW (37W/m*120m*3) expansion valve TEX5-2, TGEX-8.
The expansion valve throttle flow is greater than the evaporative load.
Evaporation load = heat transfer under the design conditions of the evaporator. Select the expansion valve according to the evaporative load, and check with the cooling capacity of the unit. The expansion valve cannot be selected directly by the cooling capacity of the unit.
If the expansion valve is selected too small, the designed refrigeration effect cannot be achieved. If the expansion valve is selected too large, it will cause liquid return or compressor overload.
4. Installation of expansion valve
When the diameter of the return air pipe is less than 25mm, the temperature sensing bag can be tied on the top of the return air pipe; when the diameter is greater than 25mm, it can be tied at 45° of the lower side of the return air pipe to prevent oil accumulation at the bottom of the pipe from affecting the temperature sensing bag The right sense of temperature.
The temperature sensing bulb must be installed on the horizontal pipe at the outlet of the evaporator. According to the diameter of the return steam pipe, it must be installed in the correct position according to the instructions. It must not be installed on the vertical pipe or return bend. When installing the evaporator, we must consider leaving a horizontal return steam pipe installation position for the temperature sensor bulb.
5. Adjustment of expansion valve
1. 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 sensor). Connect the pressure gauge to the tee of the compressor low pressure valve. As shown below:
2.Let the compressor run for more than 15 minutes to enter a stable operation state, so that the pressure indicator and temperature display reach stable values. As shown below:
3.Use a thermometer to measure the difference between the temperature of the return pipe and the evaporation temperature (ie the actual superheat) and check the standard superheat (between 5-8°C) to determine whether the adjustment is appropriate. Using the suction pressure of the compressor as the saturation pressure in the evaporator, look up the table to get the approximate evaporation temperature. Use a thermometer to measure the temperature of the return air pipe and the evaporation temperature to see if the difference is within the normal range of 5-8.
If you feel that the degree of superheat is too small, that is, the flow is too large, you can turn the adjusting screw half a turn clockwise or (that is, increase the spring force and reduce the opening of the expansion valve) to reduce the flow. Conversely, if you feel that the degree of overheating 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 rate.
Precautions:
1. Before adjusting the thermal expansion valve, confirm that the abnormal refrigeration is caused by the deviation of the thermal expansion valve from the optimal operating point, not because of improper refrigerant charge in the system, blockage of the filter dryer and expansion valve, and failure of the solenoid valve , Caused by other reasons such as poor heat dissipation of the condensing fan.
2. The correctness of the sampling signal of the temperature sensor must be ensured. The installation position of the temperature sensor must be correct. It must not be installed directly under the pipe to prevent oil accumulation at the bottom of the pipe from affecting the correct temperature of the temperature sensor. Generally, new products have been debugged and proofread before they leave the factory.
3. The number of turns of the adjusting screw should not exceed half a turn at a time. There must be more than 15 minutes between adjusting the expansion valve.
