Why Can't the Temperature of the Quick-Freeze Cold Storage Room Be Reduced?

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

The cold storage room temperature is too high. Inspection revealed that the temperatures of the two storage rooms were only -4°C to 0°C, and the liquid supply solenoid valves for both storage rooms were in the open position.

The compressor starts frequently. When the other compressor started, the situation did not improve, and a thick layer of frost formed on the return air duct. Inspection of the two storage rooms revealed a thick layer of frost on the evaporator coils.

After defrosting, the situation improved. At this point, the compressor start time shortened, and the storage room temperature also dropped, but it was still not ideal. Later, when checking the upper and lower limit values of the low-pressure controller, it was found that they had been incorrectly adjusted to 0.11–0.15 MPa (gauge pressure, same below), meaning the compressor stopped at 0.11 MPa and started at 0.15 MPa. The corresponding evaporation temperature range was approximately -20°C to 18°C. Clearly, this setting was too high, and the differential 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, with an evaporation temperature range of approximately -20°C to 18°C. After restarting the system, operation returned to normal.

II. Refrigeration System Fault - Frost on Evaporator Coils

Frost on the evaporator coils should generally not exceed 3 mm.

If the frost is too thick, it increases thermal resistance, causing a temperature difference in heat transfer between the evaporator and the cold storage room. The refrigerant cannot absorb enough heat to vaporize in the evaporator. A large amount of refrigerant will absorb heat and vaporize in the return air duct, increasing frost on the return air duct.

In addition, the superheat sensed by the expansion valve becomes too small or even zero, causing the expansion valve to close or nearly close, and the compressor quickly stops due to low pressure. However, the solenoid valve has not yet closed, and there is still a heat load in the cold storage room. After the evaporator pressure rises, the compressor restarts, leading to frequent starts. The thicker the frost on the evaporator, the more severe the situation. In fact, in this system, the frost on the evaporator coils of the two low-temperature cold storage rooms was excessively thick, reaching 1 to 2 cm, which severely affected heat transfer, and the storage room temperature could not be lowered. After defrosting and running the system again, the temperature of the two low-temperature storage rooms could be reduced to 5–6°C.

III.

System Refrigerant Shortage In systems with a liquid receiver, due to the regulating effect of the receiver, unless the refrigerant is severely insufficient, causing the liquid supply to the liquid line to be discontinuous and affecting normal operation, a general "refrigerant shortage" (i.e., low liquid level) will not have a significant impact on system operation. However, in systems without a liquid receiver, since the amount of refrigerant in the system directly determines the liquid level in the condenser, affecting condenser operation and the subcooling of the liquid refrigerant, when the system refrigerant charge is insufficient, the following changes in operating conditions are inevitable:

(1) The compressor continues to run, but the storage room temperature cannot be lowered;

(2) The compressor discharge pressure drops;

(3) The compressor suction pressure is relatively low, suction superheat increases, frost at the rear of the evaporator melts, and the compressor cylinder head becomes hot;

(4) In the liquid sight glass, a large number of bubbles can be seen in the center of the liquid flow;

(5) The liquid level in the condenser is noticeably lower.