How to Deal with the Malfunction of the Refrigeration Unit!

Most of the faults in refrigeration units are related to the compressor. By analyzing different problems of refrigeration unit faults, we can decide how to reasonably solve these faults. The following details the content of refrigeration equipment fault problems.
I. Fault Analysis and Treatment of the Cold Storage Temperature Not Dropping
The temperature in the cold storage was too high. After inspection, it was found that the temperature in the two cold storages was only between -4°C and 0°C, and the liquid supply solenoid valves in both storages were in the open state. The compressor started frequently, and the situation did not improve when switching to another compressor. However, there was thick frost on the suction pipe. When entering these two cold storages for inspection, it was found that there was thick frost on the evaporation coils. After defrosting, the situation improved. At this time, the number of compressor starts decreased, and the cold storage temperature also dropped, but it was still not ideal. Later, when checking the upper and lower limits of the low - pressure controller's action, it was found that they had been mistakenly adjusted to 0.11 - 0.15 MPa (gauge pressure, the same below). That is, the compressor stopped when the pressure was 0.11 MPa and started when the pressure was 0.15 MPa. The corresponding evaporation temperature range was approximately -20°C to -18°C. Obviously, this setting was too high, and the differential value was too small. Therefore, the upper and lower limits of the low - pressure controller were readjusted. After adjustment, the values were 0.05 - 0.12 MPa, and the corresponding evaporation temperature range was approximately -20°C to -18°C. After that, the system was restarted, and the operation returned to normal.
II. Several Reasons for Frequent Starting of the Refrigeration Compressor
The start and stop of the running compressor are controlled by high - and low - pressure relays. However, after most high - pressure relays trip, the compressor must be manually reset to restart. Therefore, the frequent start and stop of the compressor are generally not caused by the high - pressure relay but mainly by the low - pressure relay:
1. The differential value of the low - pressure relay is too small, or in the system where the cold storage temperature is controlled by a combination of a temperature relay and a low - pressure relay, the differential value of the temperature relay is too small.
2. The suction and discharge valves or safety valves of the compressor are leaking. So after shutdown, the high - pressure gas will leak into the low - pressure system, and the pressure will quickly rise, starting the compressor. After starting, the pressure in the low - pressure system is quickly pumped down, and the low - pressure relay acts to stop the compressor.
3. The automatic oil return valve of the oil separator is leaking.
4.

The expansion valve is ice - blocked.
III. Excessive Compressor Operating Time
The fundamental reason for the excessive operating time of the compressor lies in the insufficient refrigeration capacity of the device or the excessive heat load of the cold storage. Specifically, there are mainly the following reasons:
1. The evaporator is frosted too thickly or has too much oil stored.
2. The circulation volume of the 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 compressor's inability to increase the load, the actual gas delivery volume of the compressor is significantly reduced.
4. Due to the damage of the cold storage insulation, the loose closing of the cold storage door, or the placement of a large amount of hot goods, the heat load of the cold storage is too large.
5.

Control elements such as the temperature relay, low - pressure relay, or liquid supply solenoid valve malfunction, so that although the cold storage temperature has reached the lower limit, the compressor cannot stop in time.
After the compressor stops, the high and low pressures quickly balance. This is mainly caused by the 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, the high - pressure gas quickly enters the suction chamber.
IV. The Compressor Cannot Increase or Decrease the Load Normally
For the energy adjustment system controlled by oil pressure, the main reasons are as follows: The lubricating oil pressure is too low (usually 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 circuit is blocked; the cylinder and the piston or other mechanisms are jammed; the solenoid valve does not act properly, or the iron core has residual magnetism.
Faults and Analysis of the Refrigeration System
1. Frost on the Evaporator Coils
The frost on the evaporator coils should generally not exceed 3 mm. If the frost is too thick, the thermal resistance increases, resulting in a certain heat transfer temperature difference between the evaporator and the cold storage. The refrigerant in the evaporator cannot absorb enough heat to vaporize, and a large amount of refrigerant absorbs heat and vaporizes on the suction pipeline, increasing the frost on the suction 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 does not close, and there is still a certain heat load in the cold storage. After the pressure in the evaporator rises, the compressor starts again, causing frequent starts. 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 was too thick, reaching 1 - 2 cm, which seriously affected heat transfer, and the cold storage temperature could not drop.

After defrosting and restarting the system, the temperature in the two low - temperature cold storages could drop to 6 - 5°C.
2. Incorrect Setting Values of the High - and Low - Pressure Controllers
The refrigeration equipment uses R22 as the refrigerant. The high - pressure cut - off pressure (upper limit) is often selected as a gauge pressure of 1.7 - 1.9 MPa. The low - pressure relay pressure (lower limit) can be the refrigerant saturation pressure corresponding to the designed evaporation temperature minus 5°C (heat transfer temperature difference), but generally not lower than 0.01 MPa gauge pressure. The adjustment differential of the low - pressure switch is generally 0.1 - 0.2 MPa. 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 during commissioning.

When testing the low - pressure controller, slowly close the compressor suction stop valve 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 compressor discharge stop valve and read the reading on the discharge pressure gauge when the compressor stops, which is the high - pressure cut - off pressure. Before the test, ensure that the pressure gauge is reliable; the discharge valve should not be fully closed for safety reasons.
3. Insufficient Refrigerant in the System
In a device with a liquid receiver, due to the regulating effect of the liquid receiver, unless the refrigerant is seriously 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, the general "insufficient refrigerant" (i.e., low liquid level) will not have a significant impact on the system operation. However, in a device without a liquid receiver, since the amount of refrigerant in the system directly determines the refrigerant liquid level in the condenser and thus affects the operation of the condenser and the sub - cooling degree of the liquid refrigerant, 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 decreases.
(3) The suction pressure of the compressor is low, the suction superheat increases, the frost at the rear of the evaporator melts, and the compressor cylinder head heats up.
(4) A large number of bubbles can be seen in the middle of the liquid flow in the liquid supply indicator.
(5) The liquid level in the condenser is significantly low.
It should be noted that when the opening of the thermal expansion valve is adjusted too small, phenomena such as a decrease in suction pressure, partial melting of frost on the evaporator, and melting of frost on the suction pipe will also occur. Therefore, when the refrigerant liquid level cannot be accurately observed, the following method can be used to determine whether the amount of refrigerant in the system is insufficient: Stop using the thermal expansion valve, open and appropriately adjust the manual expansion valve, and observe the operation of the system to see if it can return to normal. If it can, it is due to improper adjustment of the thermal expansion valve; otherwise, it is due to insufficient refrigerant in the system. Insufficient refrigerant in the system, if not due to insufficient charging, is always caused by leakage. Therefore, after determining that the refrigerant in the system is insufficient, leakage detection should be carried out first, and refrigerant should be added after eliminating the leakage.

This is mainly caused by the 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, the high - pressure gas quickly enters the suction chamber.
V. The Compressor Cannot Increase or Decrease the Load Normally
For the energy adjustment system controlled by oil pressure, the main reasons are as follows: The lubricating oil pressure is too low (usually 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 circuit is blocked; the cylinder and the piston or other mechanisms are jammed; the solenoid valve does not act properly, or the iron core has residual magnetism.
Faults and Analysis of the Refrigeration System
1. Frost on the Evaporator Coils
The frost on the evaporator coils should generally not exceed 3 mm. If the frost is too thick, the thermal resistance increases, resulting in a certain heat transfer temperature difference between the evaporator and the cold storage. The refrigerant in the evaporator cannot absorb enough heat to vaporize, and a large amount of refrigerant absorbs heat and vaporizes on the suction pipeline, increasing the frost on the suction 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 does not close, and there is still a certain heat load in the cold storage. After the pressure in the evaporator rises, the compressor starts again, causing frequent starts. 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 was too thick, reaching 1 - 2 cm, which seriously affected heat transfer, and the cold storage temperature could not drop. After defrosting and restarting the system, the temperature in the two low - temperature cold storages could drop to 6 - 5°C.
2. Incorrect Setting Values of the High - and Low - Pressure Controllers
The refrigeration equipment uses R22 as the refrigerant. The high - pressure cut - off pressure (upper limit) is often selected as a gauge pressure of 1.7 - 1.9 MPa. The low - pressure relay pressure (lower limit) can be the refrigerant saturation pressure corresponding to the designed evaporation temperature minus 5°C (heat transfer temperature difference), but generally not lower than 0.01 MPa gauge pressure. The adjustment differential of the low - pressure switch is generally 0.1 - 0.2 MPa. 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 during commissioning. When testing the low - pressure controller, slowly close the compressor suction stop valve 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 compressor discharge stop valve and read the reading on the discharge pressure gauge when the compressor stops, which is the high - pressure cut - off pressure. Before the test, ensure that the pressure gauge is reliable; the discharge valve should not be fully closed for safety reasons.
3. Insufficient Refrigerant in the System
In a device with a liquid receiver, due to the regulating effect of the liquid receiver, unless the refrigerant is seriously 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, the general "insufficient refrigerant" (i.e., low liquid level) will not have a significant impact on the system operation. However, in a device without a liquid receiver, since the amount of refrigerant in the system directly determines the refrigerant liquid level in the condenser and thus affects the operation of the condenser and the sub - cooling degree of the liquid refrigerant, 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 decreases.
(3) The suction pressure of the compressor is low, the suction superheat increases, the frost at the rear of the evaporator melts, and the compressor cylinder head heats up.
(4) A large number of bubbles can be seen in the middle of the liquid flow in the liquid supply indicator.
(5) The liquid level in the condenser is significantly low.
It should be noted that when the opening of the thermal expansion valve is adjusted too small, phenomena such as a decrease in suction pressure, partial melting of frost on the evaporator, and melting of frost on the suction pipe will also occur. Therefore, when the refrigerant liquid level cannot be accurately observed, the following method can be used to determine whether the amount of refrigerant in the system is insufficient: Stop using the thermal expansion valve, open and appropriately adjust the manual expansion valve, and observe the operation of the system to see if it can return to normal. If it can, it is due to improper adjustment of the thermal expansion valve; otherwise, it is due to insufficient refrigerant in the system. Insufficient refrigerant in the system, if not due to insufficient charging, is always caused by leakage. Therefore, after determining that the refrigerant in the system is insufficient, leakage detection should be carried out first, and refrigerant should be added after eliminating the leakage.
There are various faults in the refrigeration unit, and most of them are related to the compressor. Through the above analysis of different faults of the refrigeration unit, we can better understand the causes of these faults and take corresponding solutions. When dealing with the faults of the refrigeration unit, we should pay attention to details, carefully check each component, and make accurate judgments and treatments according to the specific situation. Only in this way can we ensure the normal operation of the refrigeration unit, improve work efficiency, and reduce economic losses.