Commissioning of the Refrigeration System after Installation and Major Overhaul: Very Detailed!

For the refrigeration system newly installed or after major overhaul, it is necessary to conduct operation commissioning to appraise and perfect the quality and performance of the entire system.
The main contents of the commissioning include:

These temperature parameters directly affect the performance of the refrigerator (such as cooling capacity and power consumption, etc.), so it is necessary to adjust them to keep them stable within a reasonable range.

Adjusting the Evaporation Temperature
Determine the evaporation temperature according to the temperature of the cold storage. For refrigeration equipment, the temperature of the cold storage refers to the refrigeration temperature of the food; the ultimate goal of the operation of the refrigeration device is to reach the temperature of the cold storage required by the user. Under normal circumstances, the temperature of the cold storage 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 coolant, chilled water and air), and the temperature of the cooled medium in turn determines the temperature of the cold storage. From the refrigeration principle, it is known that when the condensing pressure remains unchanged, the lower the evaporation temperature is, the smaller the flow rate of the refrigerant and the unit cooling capacity will be, and the lower the refrigeration coefficient will be. 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 coolant as the heat transfer medium, the heat transfer temperature difference is generally 4-6°C.

Adjusting the evaporation temperature mainly depends on adjusting the evaporation pressure. On the premise of ensuring the maximum cooling 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 is, the lower the circulation volume of the refrigerant will be, and the amount of refrigerant in the evaporator will be relatively reduced, making the boiling amount of the refrigerant 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 is, the higher the evaporation pressure will be.

During the commissioning 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 temperature of the cold storage with the above-mentioned reasonable temperature difference, it can be known whether the adjustment of the evaporation pressure is appropriate.

In the air-conditioning chilled water unit, if the suction pressure of the R22 compressor is 0.45MPa, 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 air in the cold storage needs to be maintained at 5-10°C, it can meet the requirement that the supply air temperature of the fan coil unit is maintained at 13-18°C.

Commissioning Method of the Expansion Valve
Under stable working conditions, the adjustment of the evaporation temperature and evaporation pressure of the refrigeration system mainly involves the adjustment of the thermal expansion valve. For example, a cold storage uses a water-cooled unit, and the refrigerant is R22. It is required to maintain the temperature of the cold storage at about -10°C.

The commissioning process during the first trial operation of the unit is as follows:
At the beginning of the commissioning, due to the relatively high temperature of the cold storage, adjust the opening degree of the expansion valve to the state where the outlet of the evaporator starts to frost, and then open it a little more, and 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, as it is easy to cause "liquid slugging", but it should not be adjusted too small either, because an overly small opening degree will result in a too small cooling capacity and a too slow cooling rate. After the refrigeration operation is relatively stable, adjust the expansion valve again to make the frost layer reach the end of the return air pipe (that is, the suction port of the compressor), but frost is not allowed to form on the compressor cylinder, otherwise it is easy to cause "liquid slugging".
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 each adjustment, let it run for about 20 minutes. After repeated adjustments for many times, when the temperature of the cold storage drops to -10°C (that is, the evaporation temperature is -20°C).
While adjusting the expansion valve, attention should be paid to the changes of other operating parameters. According to the relationship between the condensing temperature and the cooling water temperature, the reasonable condensing temperature should be 5-9°C higher than 30°C. At the same time, the frost on the evaporator should be continuous and uniform, and the suction temperature should be between -5°C and 0°C (if there is a gas-liquid subcooler, it is advisable to maintain a superheat of 15°C). If there is no suction thermometer, it can be observed that the frost just reaches the suction port of the compressor, and at this time, the commissioning basically meets the design requirements.

Adjustment of the Evaporation Pressure
Regarding the adjustment of the evaporation pressure, for the compressor with an energy regulation device, the evaporation pressure can be adjusted by adjusting the gas delivery volume of the compressor. When changing the gas delivery volume of the compressor, for example, changing the compressor from 4-cylinder operation to 2-cylinder operation, the suction volume of the compressor is reduced by half, and the evaporation pressure will definitely increase. When there are multiple evaporators operating in parallel, changing the number of operating evaporators can also achieve the purpose of adjusting the evaporation pressure. For example, when the number of operating evaporators is reduced, it essentially reduces the evaporation area, which can make the evaporation pressure drop. However, the main purpose of these two types of adjustments is to adjust the cooling capacity, not the temperature of the cold storage.

02 Determine a Reasonable Condensing Temperature
The condensing temperature refers to the saturation temperature at which the refrigerant gas in the condenser condenses under a certain pressure. The condensing temperature is not the lower the better. This is because there is a certain heat transfer temperature difference between the condensing temperature and the cooling medium. From the perspective of heat transfer, the larger this temperature difference is, the greater the heat release amount will be. And the greater the heat release amount is, the more the liquefaction amount of the gaseous refrigerant will be, and the larger the circulation volume of the refrigerant will be. From this, it can be seen that the condensing temperature cannot be too low.
To adjust the condensing temperature appropriately, the most effective way is to lower the temperature of the cooling medium, or increase the flow rate and speed of the cooling medium. In this way, not only can the heat release amount of the condenser be increased, but also the condensing pressure and condensing temperature can be appropriately reduced. The level of the condensing pressure can be reflected by the pressure gauge installed on the compressor exhaust port.

01 Inspect the Suction Temperature
The suction temperature of the compressor should not be too low, and the suction temperature should be higher than the evaporation temperature by a certain value. Generally, for the Freon refrigeration device without a heat exchanger, the superheat is about 5°C.

02 Inspect 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 condensing temperature.

03 Inspect the Subcooling Temperature
To prevent the generation of flash gas 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 subcooling degree is, the larger the actual circulation volume of the refrigerant will be, and the higher the cooling capacity will be.

04 Commissioning of the Pressure Controller
Test method of the high-pressure controller: Close the water valve of the cooling water slightly or turn off the condensing fan to gradually increase the exhaust pressure, and check whether the exhaust pressure value when the high-pressure controller acts is consistent with the required protection pressure value.

05 Commissioning of the Pressure Difference Controller
Test method of the pressure difference controller: After the compressor is running normally and the pressure is stable, slowly rotate the oil pressure adjustment rod on the outside of the crankcase to gradually decrease 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. If not, the controller should be adjusted until it meets the requirements.