Analysis of the causes of low evaporation temperature and poor refrigeration effect in cold storage

During the operation of cold storage, a common misunderstanding is that the lower the evaporation temperature, the better the cooling effect. However, in actual operation, when the evaporation temperature of the cold storage is too low, the cooling effect will often become worse. This article briefly analyzes the common reasons for low evaporation temperature and poor refrigeration effect in cold storage for your reference.

1. Why does the cooling effect deteriorate if the evaporation temperature is too low?

To understand this phenomenon, we need to understand the basic principles of cold storage refrigeration. The cold storage refrigeration system achieves the cooling effect by evaporating the refrigerant in the evaporator and absorbing heat. If the evaporation temperature is too low, it may appear that the cooling capacity is greater, but in fact it will lead to a reduction in cooling capacity. In the actual application process, the common reasons why the evaporation temperature of the cold storage is too low and the refrigeration effect is poor are as follows.

1. The evaporator is too frosted and the heat transfer effect is seriously reduced.

The surface temperature of the evaporator in the cold storage is usually below 0°C, and the humidity in the warehouse is high. The moisture in the air can easily form frost or even ice on the evaporator surface. When the evaporation temperature is too low, frost will form faster and thicker.

Impact mechanism: An excessively thick frost layer will hinder heat transfer, significantly reducing the heat transfer coefficient of the evaporator and increasing the thermal resistance. As a result, the refrigerant cannot fully absorb heat in the evaporator, and the cooling effect is greatly reduced.

Solution: Perform manual defrost or electric defrost regularly. Methods such as "stop defrost" (stop the operation of the compressor, open the warehouse door to allow the warehouse temperature to rise, and restart after the frost layer automatically melts) or "frost flush" (move the goods out of the warehouse and flush the surface of the evaporator drain pipe with higher temperature tap water).

2. Lubricating oil accumulates in the evaporator, forming an oil film that hinders heat transfer.

During the operation of the cold storage, part of the refrigeration oil will enter the evaporator along with the refrigerant. When the evaporation temperature is too low, lubricating oil is more likely to form an oil film on the wall of the evaporation coil.

Influence mechanism: The oil film will significantly reduce the heat transfer efficiency of the evaporator, causing the heat transfer coefficient to become smaller and the thermal resistance to increase. The amount of refrigerant evaporated decreases, resulting in a decrease in system evaporation pressure, further deteriorating the refrigeration effect.

Solution: Drain the oil from the system regularly, and use hot ammonia to flush out the lubricating oil in the evaporator. It is recommended to perform a system oil drain check every 3-6 months.

3. The expansion valve is opened too small and the system has insufficient liquid supply.

The expansion valve is a key component in controlling refrigerant flow. When the expansion valve is opened too small, the liquid supply to the system is insufficient.

Impact mechanism: Insufficient liquid supply leads to a decrease in evaporation pressure, which in turn reduces the evaporation temperature. But at the same time, the evaporation of the refrigerant decreases, the cooling capacity of the system decreases, and the cooling effect becomes worse.

Solution: Check and adjust the opening of the expansion valve to ensure that the liquid supply meets the system demand. Adjust the opening of the expansion valve in a timely manner according to changes in warehouse load.

4. The cooling capacity of the compressor is too large, "a big horse and a small cart"

Cold storage compressors are usually matched according to the maximum load of the refrigeration system, while the maximum load of fruit and vegetable cold storage mainly occurs during the storage stage of goods, and the load is less than 50% at other times.

Impact mechanism: When the temperature at the end of storage drops to the appropriate storage temperature, the system load is greatly reduced, but the compressor still operates at high load, forming a "big horse and small cart" phenomenon. This results in too low evaporation temperature, reduced cooling efficiency, and increased energy consumption.

Solution: Reduce the number of compressors turned on in time according to changes in warehouse load, or use energy adjustment devices to reduce the number of working cylinders. It is recommended to install an automatic load adjustment system to automatically adjust the compressor operating status according to the actual load.

Shanghai KUB Refrigeration Equipment Co., Ltd.

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