Refrigeration Technology Sharing - Commissioning of Refrigeration Systems

2024-12-19

For newly installed and overhauled refrigeration systems, operational debugging must be carried out to identify and improve the quality and performance of the entire system.

The main contents of debugging include:

Adjusting the evaporation temperature;
Setting a reasonable condensation temperature;
Checking the suction temperature;
Checking the discharge temperature and subcooling temperature;
Verifying the set values of automatic protection elements, etc.

These temperature parameters directly affect the performance of the refrigerator (such as refrigeration capacity and power consumption, etc.), so they must be adjusted to stabilize them within a reasonable range.

Next, I will share with you the specific methods for debugging the refrigeration system.

I. Adjusting the Evaporation Temperature

Determining the evaporation temperature according to the cold room temperature
For refrigeration equipment, the cold room temperature refers to the refrigeration temperature of food; for air-conditioning equipment, the cold room temperature refers to the room temperature. The ultimate goal of the operation of the refrigeration unit is to reach the cold room temperature required by users. Under normal circumstances, the cold room temperature is mainly controlled by the evaporation temperature. The evaporation temperature (the boiling point of the refrigerant) directly affects the temperature of the cooled medium (such as the secondary refrigerant, chilled water and air), and the temperature of the cooled medium determines the temperature of the cold room. From the perspective of heat transfer, the greater the difference between the evaporation temperature and the cold room temperature, the better the heat transfer effect. However, if the temperature difference is too large, it means that the evaporation temperature is too low. According to the refrigeration principle, when the condensation pressure remains unchanged, the lower the evaporation temperature, the smaller the flow rate of the refrigerant and the unit refrigeration capacity, and the lower the refrigeration coefficient. Therefore, the process of adjusting the evaporation temperature is the process of selecting a reasonable heat transfer temperature difference. Theory and practice have proved that when the evaporator uses air as the heat transfer medium, if the air is in natural convection, the heat transfer temperature difference is generally 8 - 12 °C; if the air is in forced convection, the heat transfer temperature difference is generally 5 - 8 °C. When the evaporator uses chilled water or secondary refrigerant as the heat transfer medium, the heat transfer temperature difference is generally 4 - 6 °C.
Adjusting the evaporation temperature mainly relies on adjusting the evaporation pressure
On the premise of ensuring the maximum refrigeration capacity, the adjustment of the evaporation pressure is generally achieved by adjusting the opening degree of the expansion valve. The smaller the opening degree of the expansion valve, the lower the circulation amount of the refrigerant, and the relatively reduced amount of the refrigerant in the evaporator, so that the boiling amount of the refrigerant is less than the suction amount of the compressor, and the pressure in the evaporator will decrease. Conversely, the larger the opening degree of the expansion valve, the higher the evaporation pressure.
During the debugging process, the suction pressure of the compressor is usually approximately regarded as the evaporation pressure of the refrigerant in the evaporator, and the saturation temperature corresponding to this pressure is the evaporation temperature. By comparing the difference between the evaporation temperature and the cold room temperature with the above reasonable temperature difference, it can be known whether the adjustment of the evaporation pressure is appropriate.
For example, in a wall-tube refrigerated warehouse with an R22 compressor, from the saturated thermodynamic property table, it can be found that the evaporation temperature corresponding to the current suction pressure of R22 is about -25 °C. In a direct cooling system, it is usually required that the evaporation temperature is 5 - 10 °C lower than the cold room temperature. Then, at an evaporation temperature of about -25 °C, the requirement that the cold room temperature remains at about -15 - 20 °C can be met.
Another example is in an air-conditioning chiller unit. If the suction pressure of the R22 compressor is 0.45 MPa and the corresponding evaporation temperature is about 3 °C, considering that the temperature difference between the chilled water temperature and the evaporation temperature needs to be maintained at 4 - 5 °C and the temperature difference between the chilled water and the cold room air needs to be maintained at 5 - 10 °C, the requirement for the supply air temperature of the fan coil unit to remain at 13 - 18 °C can be met.
Debugging methods for the expansion valve
Under the condition of stable working conditions, the adjustment of the evaporation temperature and evaporation pressure of the refrigeration system is mainly the adjustment of the thermostatic expansion valve. For example, a cold storage uses a water-cooled unit with R22 as the refrigerant and is required to maintain the cold room temperature at about -10 °C. During the first trial operation of the unit, the debugging process is as follows:
3.1 Start the compressor to put the refrigeration unit into the debugging operation.
3.2 At the beginning of debugging, since the cold room temperature is relatively high, adjust the opening degree of the expansion valve until the outlet of the evaporator starts to frost, and then open it a little larger. Let the system run for a period of time. It should be pointed out that the opening degree of the expansion valve should not be too large. If it is too large, "liquid slugging" is likely to occur, but it should not be adjusted too small either, because if it is too small, the refrigeration capacity will be too small and the cooling speed will be too slow.
3.3 After the refrigeration operation is relatively stable, adjust the expansion valve again to make the frost layer form to the end of the return pipe (that is, the suction port of the compressor), but frost is not allowed to form on the compressor cylinder, otherwise "liquid slugging" is likely to be caused.
3.4 During the operation process of adjusting the expansion valve, the adjustment amount each time should not be too large. Generally, adjust 1/2 - 1/4 turn each time, and after adjusting once, let it run for about 20 minutes. After repeated adjustments for many times, when the cold room temperature drops to about -10 °C (that is, the evaporation temperature is about -20 °C).
3.5 While adjusting the expansion valve, attention should be paid to the changes of other operating parameters. According to the relationship between the condensation temperature and the cooling water temperature, a reasonable condensation temperature should be 5 - 9 °C higher than 30 °C. At the same time, the frosting of the evaporator should be continuous and uniform, and the suction temperature should be in the range of about -5 - 0 °C (if there is a gas-liquid subcooler, it is advisable to maintain a superheat degree of 15 °C). If there is no suction thermometer, it can be seen that the frost just forms at the suction port of the compressor. At this point, the debugging basically meets the design requirements.
Adjustment of the evaporation pressure
Regarding the adjustment of the evaporation pressure, for compressors with energy regulation devices, the evaporation pressure can be adjusted by adjusting the displacement of the compressor. When the displacement of the compressor is changed, for example, when the compressor changes from operating with 4 cylinders to operating with 2 cylinders, the suction amount of the compressor is reduced by half, and the evaporation pressure will surely increase. When multiple evaporators work in parallel, changing the number of working evaporators can also achieve the purpose of adjusting the evaporation pressure. For example, when the number of working evaporators is reduced, the evaporation area is actually reduced, which can make the evaporation pressure drop. However, the main purpose of these two adjustments is to adjust the refrigeration capacity rather than the cold room temperature.

II. Determining a Reasonable Condensation Temperature
The condensation temperature refers to the saturation temperature when the refrigerant gas in the condenser condenses under a certain pressure. The lower the condensation temperature, the better. This is because there is a certain heat transfer temperature difference between the condensation temperature and the cooling medium. From the perspective of heat transfer, the greater the temperature difference, the greater the heat release amount, and the greater the heat release amount, the greater the liquefaction amount of the gaseous refrigerant and the greater the circulation amount of the refrigerant. It can be seen from this that the condensation temperature cannot be too low.
However, the condensation temperature cannot be too high either. According to the refrigeration principle, if the condensation temperature is too high, the refrigeration capacity will decrease and the power consumption will increase. Therefore, the condensation temperature should neither be too high nor too low, and reasonable adjustments should be made according to the specific temperature changes of the cooling medium.
The reasonable temperature differences between the condensation temperature and the cooling medium are as follows:

When using air as the cooling medium, it is required that the condensation temperature be 8 - 12 °C higher than the air temperature.
When using water as the cooling medium, the temperature difference required for the condensation temperature to be higher than the inlet water temperature is: t = tk - tw = tw + tz
Where: tk is the condensation temperature, tw is the inlet water temperature of the cooling water, t is the temperature rise of the cooling water in the condenser (that is, the difference between the inlet and outlet water temperatures), generally t = 2 - 4 °C; tz is the difference between the condensation temperature and the outlet water temperature of the cooling water, generally tz = 5 - 9 °C.
There is a one-to-one correspondence between the condensation temperature and the condensation pressure. Theoretically speaking, the condensation temperature can be adjusted by adjusting the condensation pressure. But in fact, the condensation pressure cannot be adjusted as freely as the evaporation pressure.
To adjust the condensation temperature appropriately, the most effective way is to reduce the temperature of the cooling medium or increase the flow rate and speed of the cooling medium. This can not only increase the heat release amount of the condenser but also appropriately reduce the condensation pressure and condensation temperature. The level of the condensation pressure can be reflected by the exhaust pressure gauge installed on the compressor.

III. Checking the Suction Temperature
To ensure the safe operation of the compressor and prevent wet steam from entering the cylinder and causing the "liquid slugging" phenomenon, the suction temperature of the compressor should not be too low. It should be higher than the evaporation temperature by a certain value, having a reasonable suction superheat degree (the difference between the compressor suction temperature and the evaporation temperature at the end of the evaporator). Under normal circumstances, for a Freon refrigeration unit without a heat exchanger, the superheat degree is about 5 °C.

IV. Checking the Discharge Temperature
The discharge temperature is related to factors such as the suction temperature, compression ratio, and adiabatic index of the refrigerant. Since the exhaust of the compressor is in a superheated state, the discharge temperature is much higher than the condensation temperature.
The higher the suction temperature, the greater the compression ratio, and the higher the adiabatic index of the refrigerant, the higher the discharge temperature. If the discharge temperature is too high, it will cause the temperature of the refrigeration oil to rise, reduce the viscosity, affect the lubrication effect, easily cause wear of the operating parts, and even cause carbon deposition, resulting in the valve plate not closing tightly, directly affecting the reliability and economy of the compressor operation.

V. Checking the Subcooling Temperature
To prevent flash gas from being generated in the liquid pipe before the expansion valve, the liquid before throttling should have a certain degree of subcooling. It can be seen from the pressure-enthalpy diagram that the greater the degree of subcooling, the greater the actual circulation amount of the refrigerant and the higher the refrigeration capacity.

VI. Debugging of the Pressure Controller
Test method for the high-pressure controller: Close the water valve of the cooling water or turn off the condenser fan to gradually increase the exhaust pressure, and check whether the exhaust pressure value when the high-pressure controller operates is consistent with the required protection pressure value.
Test method for the low-pressure controller: After the compressor starts running, slowly turn the valve stem of the suction valve to gradually approach the closed position to gradually reduce the suction pressure, and check whether the operation of the low-pressure controller is consistent with the required pressure value.

VII. Debugging of the Differential Pressure Controller
Test method for the differential pressure controller: After the compressor operates normally and the pressure is stable, slowly rotate the oil pressure adjusting rod on the outside of the crankcase to gradually reduce the oil pressure and reduce the oil pressure difference. Check whether the action value of the oil pressure difference is consistent with the required protection value.

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If they are not consistent, the controller should be adjusted until it is consistent with the requirements.
The level of the pressure shown on the oil pressure gauge does not represent the quality of the lubrication conditions of the compressor. The high-pressure oil output by the oil pump is divided into one path for the oil pressure gauge, one path for the lubricated parts, and one path for the unloading and energy regulation mechanisms. The high-pressure oil output by the oil pump must overcome the pressure of the refrigerant in the crankcase and the resistance in the oil pipe before it can enter the normal use state. The suction pressure (that is, the pressure in the crankcase) is always changing. Only by maintaining a certain difference between the oil pressure and the pressure in the crankcase can the normal lubrication of the friction surfaces and the accurate response of the energy regulation mechanism be achieved.
The debugging of the differential pressure controller is to check whether the oil pressure difference controller can operate when the difference between the oil pump pressure and the crankcase pressure is lower than the specified value.