I. Fault Analysis and Treatment of Inability to Lower the Cold Storage Temperature

II. Several Reasons for Frequent Starting of the Refrigeration Compressor

1. The amplitude difference of the low - pressure relay is too small. Or in a system where the cold storage temperature is jointly controlled by a temperature relay and a low - pressure relay, the amplitude difference of the temperature relay is too small.
2. The suction and discharge valves or the safety valve of the compressor leak. So after shutdown, high - pressure gas will leak into the low - pressure system, and the pressure will rise quickly, starting the compressor. After starting, the pressure of the low - pressure system is quickly pumped down, and the low - pressure relay acts to stop the machine.
3. The automatic oil - return valve of the oil separator leaks.
4. The expansion valve is blocked by ice.

III. The Compressor Runs for Too Long

1. The evaporator has too thick frost or too much oil deposit.
2. The circulating amount of refrigerant in the system is insufficient, or the liquid refrigerant pipeline is not smooth enough.
3. Due to the leakage of the suction and discharge valve plates, serious leakage of the piston rings, or the inability of the compressor to increase the load, the actual gas delivery volume of the compressor is significantly reduced.
4. Due to the damage of the heat insulation of the cold storage, the cold storage door not closing tightly, or a large amount of hot goods being put in, the heat load of the cold storage is too large.
5. Control components such as the temperature relay, low - pressure relay, or liquid - supply solenoid valve fail, so that the cold storage temperature has reached the lower limit, but the compressor cannot stop in time.
   After the compressor stops, the high and low pressures balance quickly. This is mainly caused by serious leakage or breakage of the suction and discharge valve plates, as well as the breakdown of the gasket between the high and low pressures of the cylinder. After shutdown, high - pressure gas quickly enters the suction chamber.

IV. The Compressor Cannot Increase or Unload the Load Normally

Refrigeration System Malfunctions and Analysis

1. Frost Formation on Evaporator Coils
   Normally, the frost on the evaporator coils should not exceed 3mm. If the frost is too thick, the thermal resistance increases, creating a certain heat - transfer temperature difference between the evaporator and the cold storage. As a result, the refrigerant cannot absorb sufficient heat for vaporization in the evaporator. A large amount of refrigerant absorbs heat and vaporizes in the return air pipeline, increasing the frost on the return air pipeline. In addition, since the superheat sensed by the expansion valve is too small or even zero, the expansion valve closes or shuts down, and the compressor quickly stops due to low pressure. However, the solenoid valve remains open, and there is still a certain heat load in the cold storage. After the evaporator pressure rises, the compressor starts again, resulting in frequent startups. The thicker the frost on the evaporator, the more serious this situation becomes. 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 has seriously affected heat transfer, and the cold storage temperature cannot drop. After defrosting and running the system again, the temperature of the two low - temperature cold storages can drop to - 6~ - 5℃.
2. Incorrect Set Values of High - and Low - Pressure Controllers
   The refrigerant used in the refrigeration equipment is R22. The high - pressure power - off pressure (upper limit) is mostly selected as a gauge pressure of 1.7 - 1.9MPa. The pressure of the low - pressure relay (lower limit) can be the saturation pressure of the refrigerant corresponding to the designed evaporation temperature minus 5℃ (heat - transfer temperature difference), but it is generally not lower than a gauge pressure of 0.01MPa. The adjustment range of the low - pressure switch is generally 0.1 - 0.2MPa. Sometimes the scale of the pressure control set value is not very accurate, and the actual action value should be based on the measured value during debugging. When testing the low - pressure controller, slowly close the suction stop valve of the compressor, pay attention to the indication value of the suction pressure gauge. The indication values when the compressor stops and restarts 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, which is the high - pressure cut - off pressure. Before the test, confirm that the pressure gauge is reliable; the discharge valve should not be fully closed for safety.
3. Insufficient Refrigerant in the System
   In a device with a liquid receiver, due to the regulating function of the liquid receiver, unless the refrigerant is severely insufficient, causing the liquid supplied from the liquid receiver to the liquid pipe to be discontinuous and thus affecting the normal operation of the device, a general "insufficient refrigerant" situation, that is, a low liquid level, will not significantly affect the operation of the system. However, in a device without a liquid receiver, the amount of refrigerant in the system directly determines the height of the refrigerant liquid level in the condenser, and thus affects the operation of the condenser and the sub - cooling degree of the liquid refrigerant. Therefore, when the amount of refrigerant in the system is insufficient, the following changes in the device operating conditions will inevitably occur:
   (1) The compressor runs continuously, but the cold storage temperature does not drop.
   (2) The discharge pressure of the compressor drops.
   (3) The suction pressure of the compressor is low, the suction superheat increases, the frost at the rear of the evaporator melts, and the cylinder head of the compressor gets hot.
   (4) A large number of bubbles can be seen in the liquid flow in the liquid - supply liquid indicator.
   (5) The liquid level in the condenser is significantly low. It should be noted that when the opening of the thermostatic expansion valve is adjusted too small, phenomena such as a decrease in suction pressure, partial melting of the frost on the evaporator, and melting of the frost on the suction pipe will also occur. Therefore, when the refrigerant liquid level cannot be accurately observed.
   To determine whether the amount of refrigerant in the system is insufficient, the following method can be used: Stop using the thermostatic expansion valve, open and appropriately adjust the manual expansion valve, observe the operation of the system to see if it can return to normal. If it can return to normal, it is due to improper adjustment of the thermostatic expansion valve; otherwise, it is due to insufficient refrigerant in the system. If the insufficient refrigerant in the system is not due to insufficient charging, it is always caused by leakage. Therefore, after determining that the refrigerant in the system is insufficient, leak detection should be carried out first, and then refrigerant should be added after eliminating the leakage.

V. The Compressor Cannot Increase or Unload the Load Normally

• The oil pressure is too low (generally caused by excessive bearing clearance and excessive pump clearance), which can be solved by tightening the oil - pressure regulating valve.
• The piston of the unloading cylinder leaks oil seriously, and the oil passage is blocked.
• The cylinder and the piston or other mechanisms are jammed.
• The solenoid valve does not operate properly, or the iron core has residual magnetism.

Refrigeration System Malfunctions and Analysis

1. Frost Formation on Evaporator Coils
   Normally, the frost on the evaporator coils should not exceed 3mm. If the frost is too thick, it will increase the thermal resistance, creating a certain heat - transfer temperature difference between the evaporator and the cold storage. As a result, the refrigerant cannot absorb sufficient heat for vaporization in the evaporator. A large amount of refrigerant absorbs heat and vaporizes in the return air pipeline, increasing the frost on the return air pipeline. Additionally, because the superheat sensed by the expansion valve is too small or even zero, the expansion valve closes or shuts down, causing the compressor to stop due to low pressure soon. However, the solenoid valve remains open, and there is still a certain heat load in the cold storage. After the evaporator pressure rises, the compressor starts again, resulting in frequent startups. The thicker the frost on the evaporator, the more serious this situation becomes. In fact, the frost on the evaporator coils of the two low - temperature cold storages in this system is too thick, reaching 1 - 2cm. This has seriously affected heat transfer, and the cold - storage temperature cannot drop. After defrosting and running the system again, the temperature of the two low - temperature cold storages can drop to - 6~ - 5℃.
2. Incorrect Set Values of High - and Low - Pressure Controllers
   The refrigerant used in the refrigeration equipment is R22. The high - pressure power - off pressure (upper limit) is mostly selected as a gauge pressure of 1.7 - 1.

   

   9MPa. The pressure of the low - pressure relay (lower limit) can be the saturation pressure of the refrigerant corresponding to the designed evaporation temperature minus 5℃ (heat - transfer temperature difference), but it is generally not lower than a gauge pressure of 0.01MPa. The adjustment range of the low - pressure switch is generally 0.1 - 0.2MPa. Sometimes the scale of the pressure control set value is not very accurate, and the actual action value should be based on the measured value during debugging. When testing the low - pressure controller, slowly close the suction stop valve of the compressor, and pay attention to the indication value of the suction pressure gauge. The indication values when the compressor stops and restarts 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, which is the high - pressure cut - off pressure. Before the test, make sure the pressure gauge is reliable; the discharge valve should not be fully closed to ensure safety.
3. Insufficient Refrigerant in the System
   In a device with a liquid receiver, due to the regulating function of the liquid receiver, unless the refrigerant is severely insufficient, which makes the liquid supplied from the liquid receiver to the liquid pipe discontinuous and thus affects the normal operation of the device, a general "insufficient refrigerant" situation, that is, a low liquid level, will not significantly affect the operation of the system. However, in a device without a liquid receiver, the amount of refrigerant in the system directly determines the height of the refrigerant liquid level in the condenser, and thus affects the operation of the condenser and the sub - cooling degree of the liquid refrigerant. Therefore, when the amount of refrigerant in the system is insufficient, the following changes in the device's operating conditions will inevitably occur:
   • (1) The compressor runs continuously, but the cold - storage temperature does not drop.
   • (2) The discharge pressure of the compressor drops.
   • (3) The suction pressure of the compressor is low, the suction superheat increases, the frost at the rear of the evaporator melts, and the cylinder head of the compressor gets hot.
   • (4) A large number of bubbles can be seen in the middle of the liquid flow in the liquid - supply liquid indicator.
   • (5) The liquid level in the condenser is significantly low.