What to do if the refrigeration unit fails!

Update:20 Aug 2021
Summary:

Many of the failures of the refrigeration unit are comp […]

Many of the failures of the refrigeration unit are compressor problems. According to the different problems of the refrigeration unit failures, we analyze and decide how to solve the failures of the refrigeration unit reasonably. As Industrial refrigeration Manufacturers ,Shanghai Zhaoxue Refrigeration Equipment Co., Ltd.will introduce in detail the failure problems that are easy to occur in the refrigeration equipment.

 

   I.Failure analysis and treatment of the cold storage temperature cannot drop
   The temperature of the cold storage is too high. After inspection, it was found that the temperature of the two storages was only -4℃~0℃, and the liquid supply solenoid valves of the two storages were in an open state. The compressor starts frequently, and the situation does not get better when switching to another compressor, but there is thick frost on the return pipe. After entering the two warehouses, it was found that there was thick frost on the evaporator coils. After defrosting, the situation improved. At this time, the number of compressor starts was reduced and the temperature of the warehouse was also lowered, but it was not ideal. . Later, the upper and lower limits of the action of the low pressure controller were checked, and it was found to be misadjusted to 0.11—0.15NPa (gauge pressure, the same below), that is, the compressor is stopped when the pressure is 0.11MPa, and the compressor is started when the pressure is 0.15Pa. The evaporation temperature range is about -20℃-18℃. Obviously, this setting is too high and the amplitude difference is too small. Therefore, re-adjust the upper and lower limits of the low-pressure controller. After adjustment, the value is: 0.05—0.12MPa, and the corresponding evaporation temperature range is about -20°C to 18°C. After that, restart the system and work back to normal.

   II.several reasons that cause the refrigeration compressor to start frequently
The compressor in operation is controlled by the high and low pressure relays to start and stop, but after most of the high voltage relays are tripped, they must be reset manually to restart the compressor. Therefore, the frequent start and stop of the compressor is generally not caused by the high voltage relay. Mainly caused by low voltage relay:
  1. The amplitude difference of the low-voltage relay is too small, or the temperature relay is too small in the system where the warehouse temperature is controlled by the temperature relay and the low-voltage relay;
        2. The compressor's suction and discharge valve or safety valve leaks, so the high-pressure gas will leak to the low-pressure system after shutdown. The pressure will quickly rise and the compressor will be started. After startup, the pressure of the low-pressure system is quickly pumped down. The relay operates and stops again;
  3, the automatic oil return valve of the lubricating oil separator is leaking;
  4, the expansion valve ice plug.

  III. Compressor running time is too long
  The root cause of the long running time of the compressor is that the cooling capacity of the device is insufficient or the heat load of the cold storage is too large. The specific main reasons are:
   1. The evaporator has too thick frost or too much oil;
  2, the circulating amount of refrigerant in the system is insufficient, or the liquid refrigerant pipeline is not unobstructed;
  3. Due to the leakage of the suction and exhaust valve, the severe leakage of the piston ring or the inability of the compressor to increase the load, the actual air delivery volume of the compressor is significantly reduced;
  4. Due to the thermal insulation damage of the cold storage, the door is not closed tightly or a large number of hot goods are released, the heat load of the cold storage is too large;
  5. Control elements such as temperature relay, low voltage relay or liquid supply solenoid valve malfunction, so that the storage temperature has reached the lower limit. However, the compressor cannot be shut down in time.

  IV. After the compressor stops, the high and low pressure quickly balance
   This is mainly caused by serious leakage or fracture of the suction and exhaust valve, as well as the breakdown of the gasket between the high and low pressure of the cylinder, and the high pressure gas quickly enters the suction chamber after shutdown.

   V. The compressor cannot be increased or unloaded normally
   For the oil pressure control energy regulation system, the main reasons are: the oil pressure is too low. (Generally caused by excessive bearing clearance and excessive pump clearance), it can be solved by tightening the oil pressure regulating valve; the piston of the unloading cylinder has serious oil leakage and the oil circuit is blocked; the oil cylinder and the piston or other mechanisms are jammed; the solenoid valve does not operate Spirit, or the iron core has remanent magnetism.

   VI. Refrigeration system failure
   1. Frosting on the evaporator coil
The frost on the evaporator coil should not exceed 3mm in one strand. If the frost is too thick, the thermal resistance will increase, and there will be a certain heat transfer temperature difference between the evaporator and the cold storage, and the refrigerant will not be absorbed in the evaporator. Enough heat to vaporize, a large amount of refrigerant absorbs heat and vaporizes on the return line, which increases the frosting of the return line; in addition, the superheat experienced by the expansion valve is too small or even zero, causing it to be closed or closed, and the compressor is very Quickly stop at low pressure. 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 again, causing frequent starts. The thicker the evaporator frost, the more serious the situation. In fact, the frost on the two low-temperature cold storage evaporator coils of this system is too thick, reaching 1-2cm, which has seriously affected the heat transfer, and the storage temperature cannot be lowered. After defrosting, and then running the system, the storage temperature of the two low-temperature storages can already drop to 6~5℃.
  2, the setting value of the high and low voltage controller is wrong
  The refrigerant used in the refrigeration equipment is R22, and the high-voltage breaking voltage (upper limit) is usually 1.7-1.9MPa gauge pressure. The low pressure relay pressure (lower limit) can be the refrigerant saturation pressure corresponding to the design evaporation temperature minus 5°C (heat transfer temperature difference), but it is generally not lower than the gauge pressure of 0.01 MPa. The adjustment amplitude difference of the low-voltage switch is generally 0.1~0.2MPa. Sometimes the scale of the pressure control setting value is not very accurate, and the actual action value should be based on the value measured in debugging. When testing the low-pressure controller, slowly close the compressor suction shut-off valve. Pay attention to the indication value of the suction pressure gauge. The indication value when the compressor is stopped and restarted is the upper and lower limits of the low-pressure controller. The test high-pressure controller can slowly close the compressor discharge shut-off valve, and read the reading of the discharge pressure gauge when the compressor stops, which is the high-pressure cut-off pressure. Before the test, the pressure gauge must be confirmed to be reliable; the discharge valve should not be fully closed to ensure safety.
   3. Insufficient refrigerant in the system
In a device with a liquid reservoir, due to the adjustment function of the liquid reservoir, unless the refrigerant is severely insufficient, the liquid supplied from the liquid reservoir into the liquid pipe cannot be continuous, which affects the normal operation of the device, except for the general "refrigeration "Insufficient agent" means that the liquid level is low, which will not have a significant impact on the operation of the system. However, in a device without a liquid receiver, because the amount of refrigerant in the system directly determines the level of refrigerant in the condenser, and therefore affects the operation of the condenser and the degree of subcooling of the liquid refrigerant, so When the amount of refrigerant in the system is insufficient, it will inevitably cause the following changes in the operating conditions of the device:
  (1) The compressor is running continuously, but the temperature of the storage can't drop;
  (2) The discharge pressure of the compressor drops;
  (3) The suction pressure of the compressor is low, the suction superheat is increased, the frost at the back of the evaporator melts, and the compressor cylinder hair is hot;
   (4) A large number of bubbles can be seen in the center of the liquid flow in the liquid supply indicator;
  (5) The liquid level in the condenser is obviously low.
   It should be noted that when the opening of the thermal expansion valve is adjusted too small, the suction pressure will drop, the evaporator part will be frosted and melted, and the suction pipe will be frosted and melted. Therefore, in the case where 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, observe the system's work to see if it can return to normal, if it can return to normal, it is thermal Improper adjustment of the expansion valve, otherwise the refrigerant in the system is insufficient. Insufficient refrigerant in the system, if the non-charged quantity is not enough, it is always caused by leakage. Therefore, after the system is identified as insufficient refrigerant, leak detection should be performed first, and the refrigerant should be added after the leakage is eliminated.