Engineers summarize: 7 key points for debugging refrigeration systems

I. Adjust the evaporation temperature

1. Determine the evaporation temperature based on 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 device is to achieve the cold room temperature required by the user. 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 coolant, refrigerant 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. From the principle of refrigeration, when the condensing pressure remains unchanged, the lower the evaporation temperature, the smaller the flow rate and unit cooling capacity of the refrigerant, and the lower the refrigeration coefficient. Therefore, the adjustment process of 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, the air is natural convection, and the heat transfer temperature difference is generally 8-12℃; when the air is forced convection, the heat transfer temperature difference is generally 5-8℃. When the evaporator uses refrigerant water or refrigerant as the heat transfer medium, the heat transfer temperature difference is generally 4-6℃.

2. Adjusting the evaporation temperature mainly depends on adjusting the evaporation pressure. Under 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, the lower the circulation volume of the refrigerant, and the refrigerant in the evaporator is relatively reduced, so that the boiling volume of the refrigerant is less than the suction volume of the compressor, and the pressure in the evaporator will decrease. Conversely, the greater the opening degree of the expansion valve, the higher the evaporation pressure. During the debugging process, the suction pressure of the compressor is usually regarded as the evaporation pressure of the refrigerant in the evaporator, and the saturation temperature corresponding to this pressure is the evaporation temperature. 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-type cold storage, when the R22 compressor, from the saturated thermal properties table, it can be found that the evaporation temperature corresponding to the current suction pressure of R22 is about -25℃. In the direct cooling system, the evaporation temperature is usually required to be 5~10℃ lower than the cold room temperature. Then at the evaporation temperature of -25℃, the requirement of maintaining the cold room temperature at -15~20℃ can be met. For example, in an air-conditioning chiller, if the suction pressure of the R22 compressor is 0.45MPa, the corresponding evaporation temperature is about 3℃. Considering that the temperature difference between the refrigerant water temperature and the evaporation temperature needs to be maintained at 4~5℃, and the temperature difference between the refrigerant water and the cold room air needs to be maintained at 5~10℃, the fan coil can maintain a supply air temperature of 13~18c.

3. Debugging method of expansion valve. Under stable working conditions, the adjustment of the evaporation temperature and evaporation pressure of the refrigeration system is mainly the adjustment of the thermal expansion valve.

For example, a cold storage uses a water-cooled unit with R22 as the refrigerant. It is required to keep the temperature of the cold room at about -10℃. 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 commissioning operation. 3.2 At the beginning of the commissioning, due to the relatively high temperature of the cold room, adjust the opening of the expansion valve to the state where the evaporator outlet begins to frost, and then open it a little wider, and let the system run for a while. It should be pointed out that the opening of the expansion valve should not be too large, which is easy to cause "liquid hammer", but the opening should not be adjusted too small, because too small will cause too small cooling capacity and too slow cooling speed. 3.3 After the refrigeration operation is relatively stable, adjust the expansion valve again so that the frost layer forms at the end of the return air pipe (i.e. the suction port of the compressor), but frost is not allowed on the compressor cylinder, otherwise it is easy to cause "liquid hammer". 3.4 In the process of adjusting the expansion valve, the adjustment amount each time should not be too large. Generally, it is adjusted by 1/2~1/4 turn each time, and after adjusting once, let it run for about 20 minutes. After repeated adjustments, the cold room temperature drops to -10℃ (that is, the evaporation temperature is -20℃). 3.5 While adjusting the expansion valve, attention should be paid to changes in other operating parameters. According to the relationship between the condensing temperature and the cooling water temperature, the reasonable condensing temperature should be 5~9℃ higher than 30℃. At the same time, the frost of the evaporator is continuous and uniform, and the suction temperature is -5~0℃ (if there is a gas-liquid subcooler, it is advisable to keep the superheat at 15℃). If there is no suction thermometer, it can be seen that the frost just forms at the suction port of the compressor. The debugging has basically met the design requirements.

4. Adjustment of evaporation pressure Regarding the adjustment of evaporation pressure, for compressors with energy regulation devices, the evaporation pressure can be adjusted by adjusting the air delivery of the compressor. When the air delivery of the compressor is changed, for example, the compressor is changed from 4-cylinder operation to 2-cylinder operation, the air delivery of the compressor is reduced by half, and the evaporation pressure will inevitably increase. When multiple evaporators are working in parallel, changing the number of evaporators can also achieve the purpose of adjusting the evaporation pressure. For example, when the number of workstations is reduced, the evaporation area is actually reduced, which can reduce the evaporation pressure. However, the main purpose of these two adjustments is to adjust the cooling capacity, not the cold room temperature.

II. Determine 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 condensation temperature is not the lower 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, and the greater the heat release, the more gaseous refrigerant is liquefied, and the greater the circulation 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. From the principle of refrigeration, it is known that if the condensation temperature is too high, the cooling capacity will decrease and the power consumption will increase. Therefore, the condensation temperature cannot be too high or too low, and should be reasonably adjusted according to the specific temperature changes of the cooling medium.

The reasonable temperature difference between the condensation temperature and the cooling medium is as follows: 1) When air is used as the cooling medium, the condensation temperature is required to be 8 to 12 ° C higher than the air temperature. 2) When water is used as the cooling medium, the temperature difference required for the condensation temperature to be higher than the inlet water temperature is: t= t- tw= t+ tz Where: tk condensation temperature, tw cooling water inlet temperature t: cooling water temperature rise in the condenser (i.e. inlet and outlet water temperature), generally t= 2~～4C; t——the difference between the condensation temperature and the cooling water outlet water temperature, generally t= 5~9℃. There is a one-to-one correspondence between the condensation temperature and the condensation pressure. Theoretically, the condensation temperature can be adjusted by adjusting the condensation pressure. But in fact, the condensation pressure cannot be adjusted at will like the evaporation pressure. To adjust the condensation temperature appropriately, the most effective way is to lower the temperature of the cooling medium, or to increase the flow rate and speed of the cooling medium. This can not only increase the heat release 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. Check the suction temperature, exhaust temperature and supercooling temperature

1. Intake temperature In order to ensure the safe operation of the compressor and prevent wet steam from entering the cylinder to produce "liquid hammer" phenomenon, the suction temperature of the compressor cannot be too low. The suction temperature should be higher than the evaporation temperature by a certain value and have a reasonable suction superheat (the difference between the suction temperature of the compressor and the evaporation temperature at the end of the evaporator). Under normal circumstances, the superheat of the Freon refrigeration device without a heat exchanger is about 5°C. 2. Exhaust temperature The exhaust temperature is related to the suction temperature, compression ratio, refrigerant adiabatic index and other factors. Since the exhaust of the compressor is in an overheated state, the exhaust 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 exhaust temperature. Excessive exhaust temperature will cause the temperature of the refrigeration oil to rise, reduce viscosity, affect the lubrication effect, easily cause wear of the operating parts, and even cause carbonization, so that the valve plate is not closed tightly, which directly affects the reliability and economy of the compressor operation. 3. 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 chart that the greater the degree of subcooling, the greater the actual circulation volume of the refrigerant and the higher the refrigeration capacity.

IV. Debugging pressure controller and differential pressure controller

1. Debugging of pressure controller Test method of high-pressure controller: close the water valve of cooling water or turn off the condensing fan to gradually increase the exhaust pressure, and check whether the exhaust pressure value when the high-pressure controller is activated is consistent with the required protection pressure value.

Test method of low-pressure controller: after the compressor is running, slowly screw in the valve stem of the suction valve, gradually approach the shut-off position, so that the suction pressure gradually decreases, and check whether the low-pressure controller is in accordance with the required pressure value when it is activated. 2. Debugging of differential pressure controller Test method of differential pressure controller: after the compressor is running normally and the pressure is stable, slowly rotate the oil pressure regulating 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. If it does not match, the controller should be adjusted until it meets the requirements.

The pressure displayed on the oil pressure gauge does not represent the quality of the compressor lubrication conditions. The high-pressure oil output by the oil pump is fed to the oil pressure gauge, lubricating components, and unloading and energy regulating 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 (i.e. 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 surface and the accurate response of the energy regulating mechanism be achieved. The debugging of the differential pressure controller is to check whether the oil pressure differential controller can work when the difference between the oil pump pressure and the crankcase pressure is lower than the specified value.

V. System flow and balance adjustment

The balance of refrigerant and water flow is crucial to the overall efficiency and performance of the system. Uneven flow may lead to uneven temperature in some areas or poor cooling effect. By adjusting the flow meter or balancing valve, it can be ensured that each part obtains the right amount of refrigerant and water flow. Especially in multi-circuit systems, ensuring the flow balance of each circuit is the key to ensuring the overall performance of the system.

VI. Inspection and adjustment of electrical control systems

The commissioning of electrical control systems includes ensuring that all sensors, switches and controllers are properly connected and calibrated. The readings of temperature and pressure sensors should be checked to be accurate and the control loops should respond according to the predetermined logic. In addition, the user interface and alarm system should be checked to see if they are working properly to ensure that operators can effectively monitor and control the system.

VII. Noise and vibration control

The noise and vibration during system operation not only affect comfort, but may also be an early warning sign of system problems. Check whether the fan and compressor are installed firmly and whether the pipes are properly supported and soundproofed. If necessary, shock absorbers or sound insulation materials can be installed to reduce noise and vibration.