Don't Worry When the Refrigeration Unit Malfunctions. Look Here!

2025-04-11

Many of the faults of the refrigeration unit are problems with the compressor. Analyze different problems of the refrigeration unit's faults to determine how to reasonably solve the faults of the refrigeration unit. The following is a detailed introduction to the content of the faults of the refrigeration equipment.
I. Fault Analysis and Treatment of the Inability to Lower the Temperature in the Cold Storage
The temperature in the cold storage is too high. After inspection, it is found that the temperatures of the two storage rooms are only -4℃ ～ 0℃, and the liquid supply solenoid valves of the two storage rooms are in the open state. The compressor starts relatively frequently. When another compressor is switched to work, the situation does not improve, and there is a thick frost on the return air pipe.
When entering these two storage rooms for inspection, it is found that there is a thick layer of frost on the evaporator coils. The situation improves after defrosting. At this time, the starting times of the compressor are reduced, and the temperature in the storage room also decreases, but it is still not ideal. Later, when checking the upper and lower limit values of the action of the low-pressure controller, it is found that they have been mistakenly adjusted to 0.11 — 0.15NPa (gauge pressure, the same below), that is, the compressor stops when the pressure is 0.11MPa, and the compressor starts when the pressure is 0.

Don't Worry When the Refrigeration Unit Malfunctions. Look Here!1744344601615

15Pa. The corresponding evaporation temperature range is about -20℃ — 18℃. Obviously, this setting is too high, and the amplitude difference value is also too small.
Therefore, readjust the upper and lower limits of the low-pressure controller. After adjustment, the values are: 0.05 — 0.12MPa, and the corresponding evaporation temperature range is about -20℃ — 18℃. After that, restart the system, and the operation returns to normal.
II. Several Reasons for the Frequent Starting of the Refrigeration Compressor
The compressor in operation is controlled to start and stop by the high and low-pressure relays. However, after most high-pressure relays trip, the compressor can only be restarted by manual reset. Therefore, the frequent starting and stopping of the compressor is generally not caused by the high-pressure relay but mainly by the low-pressure relay:

The amplitude difference value of the low-pressure relay is too small, or in the system where the temperature in the storage room is controlled jointly by the temperature relay and the low-pressure relay, the amplitude difference value of the temperature relay is too small;
The suction and discharge valves or safety valves of the compressor are leaking. So, after the compressor stops, the high-pressure gas will leak into the low-pressure system, and the pressure will rise quickly, causing the compressor to start. After starting, the pressure of the low-pressure system is quickly pumped down, and the low-pressure relay acts again to stop the machine;
The automatic oil return valve of the oil separator is leaking;
The expansion valve is ice-blocked.
III. The Compressor Runs for Too Long
The fundamental reason for the compressor running for too long is that the refrigeration capacity of the device is insufficient or the heat load of the cold storage is too large. Specifically, it mainly includes:
The evaporator is too thickly frosted or there is too much oil stored;
The circulation volume of the refrigerant in the system is insufficient, or the liquid refrigerant pipeline is not smooth enough;
Due to the leakage of the suction and discharge valve plates, the 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;
Due to the damage of the thermal insulation of the cold storage, the door of the storage room not being closed tightly, or a large amount of hot goods being put in, the heat load of the cold storage is too large;
Control components such as the temperature relay, low-pressure relay, or liquid supply solenoid valve fail, so that the temperature in the storage room has reached the lower limit, but the compressor cannot stop in time.
IV. The High and Low Pressures Balance Rapidly After the Compressor Stops
This is mainly caused by the serious leakage or fracture 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 the compressor stops, the high-pressure gas quickly enters the suction chamber.
V. The Compressor Cannot Increase or Unload the Load Normally
For the energy regulation system controlled by the oil pressure, the main reasons are as follows: The oil pressure is too low. (Generally caused by too large a bearing clearance and too large a 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
Frost on the Evaporator Coils
The frost on the evaporator coils generally should not exceed 3mm. If the frost is too thick, it will lead to an increase in the thermal resistance, resulting in a certain heat transfer temperature difference between the evaporator and the cold storage. The refrigerant cannot absorb sufficient heat to vaporize in the evaporator. A large amount of refrigerant absorbs heat and vaporizes on 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 will close down or close, and the compressor will stop due to low pressure soon.

However, the solenoid valve has not closed, and there is still a certain heat load in the cold storage. After the pressure in the evaporator rises, the compressor starts again, resulting in frequent starting. The thicker the frost on the evaporator is, the more serious this situation 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 has seriously affected the heat transfer, and the temperature in the storage room cannot be lowered. After defrosting and then running the system again, the temperatures in the two low-temperature storage rooms can drop to 6~5℃.
Incorrect Setting Values of the 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 saturated pressure of the refrigerant corresponding to the design evaporation temperature minus 5℃ (heat transfer temperature difference), but it is generally not lower than a gauge pressure of 0.01MPa. The adjustment amplitude difference of the low-pressure 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 subject to the value measured 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 disconnection pressure. Confirm that the pressure gauge is reliable before the test; the discharge valve should not be fully closed for safety.
Insufficient Refrigerant in the System
In the device with a liquid receiver, due to the regulating effect of the liquid receiver, unless the refrigerant is seriously insufficient, which causes the liquid supplied from the liquid receiver to the liquid pipe to be discontinuous, thus affecting the normal operation of the device, the general "insufficient refrigerant", that is, the low liquid level, will not have an obvious impact on the operation of the system.
However, in the device without a liquid receiver, since the amount of refrigerant in the system directly determines the level of the refrigerant liquid in the condenser, and thus affects the operation of the condenser and the subcooling degree of the liquid refrigerant, when the amount of refrigerant in the system is insufficient, the following changes in the operating conditions of the device will inevitably occur:
(1) The compressor keeps running, but the temperature in the storage room cannot be lowered;
(2) The discharge pressure of the compressor drops;
(3) The suction pressure of the compressor is relatively 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 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 degree of the thermal expansion valve is adjusted too small, phenomena such as a decrease in the suction pressure, the melting of part of the frost on the evaporator, and the melting of the frost on the suction pipe will also occur. Therefore, in the case where the refrigerant liquid level cannot be accurately observed.
In order to determine whether the amount of refrigerant in the system is insufficient, the following method can be adopted: Stop using the thermal expansion valve, open and properly 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 means that the thermal expansion valve is improperly adjusted; otherwise, it means that the refrigerant in the system is insufficient.
If the refrigerant in the system is insufficient, if it is not due to insufficient charging, it is always caused by leakage. Therefore, after determining that the refrigerant in the system is insufficient, the leakage should be detected first, and the refrigerant should be added after the leakage is eliminated.