First, the failure analysis and treatment of the cold storage temperature does not drop
The temperature of the refrigerator is too high. After inspection, the temperature of the two warehouses was only -4°C to 0°C, and the liquid supply solenoid valves of the two warehouses were opened. The compressor started frequently, but the situation did not improve when switching to another compressor, but there was thick frost on the return air pipe. After entering the two warehouses, it was found that thick frost had formed on the evaporating coils, and the situation improved after defrosting. At this time, the start-up time and storage temperature of the compressor are reduced, but not ideal. Then check the upper and lower limits of the low-pressure controller action, and found that the misadjustment is 0.11-0.15npa, that is, stop the compressor when the pressure is 0.11mpa, and start the compressor when the pressure is 0.15pa. The corresponding evaporation temperature range is about -20°C to 18°C. Obviously, this setting is too high and the amplitude difference is too small. Therefore, readjust the upper and lower limits of the low pressure controller. The adjusted value is 0.05-0.12mpa, and the corresponding evaporation temperature range is about -20°C-18°C. Afterwards, reboot the system and resume normal operation.
2. Several reasons for frequent start-up of refrigeration compressors
Running compressors are started and stopped by the high and low voltage relays, but after tripping most high voltage relays, a manual reset must be made to restart the compressor. Therefore, the frequent start and stop of the compressor is generally not caused by the high-voltage relay, but mainly by the low-voltage relay:
1. The temperature difference between the relay amplitude and the low-voltage relay is too small, or the temperature difference between the relay amplitude and the low-voltage relay is too small;
2. The suction and exhaust valve or safety valve of the compressor leaks, so after the shutdown, the high-pressure gas will leak into the low-pressure system, and the pressure will rise rapidly to start the compressor. After starting, the pressure of the low-voltage system drops rapidly, the low-voltage relay operates, and the compressor stops;
3. The automatic oil return valve of the lubricating oil separator leaks;
4. Expansion valve ice plug.
3. The compressor runs for too long
The root cause of the long running time of the compressor is the insufficient cooling capacity of the unit or the excessive heat load of the cold storage, mainly including:
1. The evaporator has too much frost or too much oil storage;
2. The refrigerant circulation in the system is insufficient, or the liquid refrigerant pipeline is not smooth enough;
3. Due to the leakage of the intake and exhaust valve plates, the serious leakage of the piston ring or the failure of the compressor to increase the load, the actual gas delivery of the compressor is significantly reduced;
4. The heat insulation layer of the cold storage is damaged, the door is not closed tightly or a large number of hot items are released, resulting in excessive thermal load of the cold storage;
5. The temperature relay, low voltage relay or liquid supply solenoid valve and other control components are faulty, causing the storage temperature to reach the lower limit. But the compressor cannot stop in time.
4. After the compressor stops, the high and low pressures are quickly balanced
This is mainly due to the serious leakage or fracture of the suction and exhaust valve plates, the rupture of the gasket between the high pressure and low pressure of the cylinder, and the rapid entry of high pressure gas into the suction chamber after the shutdown.
5. The compressor cannot be loaded or unloaded normally
For the energy regulation system controlled by oil pressure, the main reason is: the lubricating oil pressure is too low. (Generally caused by excessive bearing clearance and pump clearance), it can be solved by tightening the oil pressure regulating valve; the unloading cylinder piston leaks oil seriously, and the oil circuit is blocked; the oil cylinder is stuck on the piston or other mechanisms; the solenoid valve does not work normally, Or the iron core has residual magnetism.
6. Refrigeration system failure
1. Frosting on the evaporator coil: the frosting on the evaporator coil should not exceed 3mm. If the frosting is too thick, the thermal resistance will increase, resulting in a certain heat transfer temperature difference between the evaporator and the cold storage. The refrigerant cannot absorb enough heat to evaporate in the evaporator. A large amount of refrigerant absorbs heat on the return pipe and evaporates, which increases the frosting of the return pipe; in addition, the superheat sensed by the expansion valve is too small or even zero, causing it to close or close, and the compressor will stop at low pressure soon. However, the solenoid valve is not closed, and there is still a certain heat load in the cold storage. After the evaporator pressure rises, the compressor starts up again, causing frequent starts. The thicker the frost on the evaporator, the worse this condition will be. In fact, the frost on the evaporator coils of the two low-temperature cold storages in this system is too thick, reaching 1-2cm, which seriously affects the heat transfer and cannot reduce the storage temperature. After defrosting, run the system again, and the temperature of the two low-temperature warehouses can drop to 6-5°C.
2. The setting value of the high and low pressure controller is incorrect: the refrigerant used in the refrigeration equipment is R22, and the high voltage cut-off pressure (upper limit) is mostly selected as the gauge pressure of 1.7-1.9mpa. The pressure (lower limit) of the low-voltage relay can be the refrigerant saturation pressure corresponding to the design evaporation temperature -5°C (heat transfer temperature difference), but generally not lower than the gauge pressure of 0.01 MPa. The adjustment range difference of the low-voltage switch is generally 0.1-0.2MPa. Sometimes the scale of the pressure control setting value is not accurate, and the actual action value is subject to the value measured during debugging. When testing the low-pressure controller, slowly close the suction shut-off valve of the compressor, and pay attention to the indication value of the suction pressure gauge. The indication values when the compressor is stopped and restarted are the upper and lower limits of the low pressure controller. To test the high-pressure controller, slowly close the discharge stop valve of the compressor, and read the reading of the discharge pressure gauge when the compressor stops, that is, the high-pressure cut-off pressure. Verify the reliability of the pressure gauge before the test; to ensure safety, the discharge valve should not be completely closed.
3. Insufficient refrigerant in the system: In a device with a liquid storage tank, due to the adjustment function of the liquid storage tank, unless due to a serious shortage of refrigerant, the liquid supplied by the liquid storage tank cannot be continuous, which will affect the normal operation of the device. “Low Refrigerant”, i.e. a low fluid level, will not have a significant impact on the operation of the system. However, in a device without a liquid storage tank, since the amount of refrigerant in the system directly determines the liquid level of the refrigerant in the condenser, thereby affecting the operation of the condenser and the subcooling degree of the liquid refrigerant, when the amount of refrigerant in the system When it is insufficient, it will inevitably lead to the following changes in the working conditions of the equipment:
(1) The compressor keeps running, but the storage temperature cannot be lowered;
(2) The exhaust pressure of the compressor is reduced;
(3) The suction pressure of the compressor is low, the suction superheat increases, the frost on the back of the evaporator melts, and the compressor cylinder head heats up;
(4) A large number of bubbles can be seen in the liquid flow center of the liquid supply indicator;
(5) The liquid level of the condenser is obviously low.
When the opening of the thermal expansion valve is adjusted too small, the suction pressure will decrease, the evaporator will be frosted and melted, and the suction pipe will be frosted and melted. Therefore, when the refrigerant level cannot be accurately observed. In order to judge whether the amount of refrigerant in the system is insufficient, the following methods can be used:
Stop using the thermal expansion valve, open and adjust the manual expansion valve appropriately, and observe the system operation to see if it can return to normal. If it can return to normal, it means that the thermal expansion valve is not adjusted properly, otherwise there is a lack of refrigerant in the system. Insufficient refrigerant in the system (if not an insufficient charge) is the cause of the leak. Therefore, after it is determined that the system refrigerant is insufficient, the leak should be detected first, and the refrigerant should be added after the leak is eliminated.
Post time: Mar-17-2023