What Are the Reasons Why the Temperature in the Quick-freezing Cold Storage Cannot Be Lowered?

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°C to 0°C, 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 layer of 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 0.15MPa. The corresponding evaporation temperature range is about -20°C to 18°C. 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°C to 18°C. After that, restart the system, and the operation returns to normal.
II. Faults 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 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 5~6°C.
III. 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.