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Analysis of the reason for cold storage not cooling

Update:10-06-2020
Summary:

1. There is more air or freezing oil in the evaporator, […]

1. There is more air or freezing oil in the evaporator, the heat transfer effect is reduced
Once more freezing oil is attached to the inner surface of the heat transfer tube of the evaporator, its heat transfer coefficient will be reduced. Similarly, if there is more air in the heat transfer tube, the heat transfer area of ​​the evaporator is reduced, and its heat transfer The efficiency will also decrease significantly, and the temperature of the warehouse will slow down. Therefore, in daily operation and maintenance, care should be taken to remove the oil on the inner surface of the heat transfer tube of the evaporator and exhaust the air in the evaporator in time to improve the heat transfer efficiency of the evaporator.
Second, the cold storage due to poor thermal insulation or sealing performance, resulting in large loss of cooling capacity
The poor insulation performance of cold storage is due to the insufficient thickness of the insulation layer of pipes, warehouse insulation walls, etc., and poor insulation and insulation effects. It is mainly caused by the selection of the insulation layer thickness during design or the poor quality of insulation materials during construction. In addition, in the process of construction and use, the heat and moisture resistance of the heat insulation material may be destroyed, causing the heat insulation layer to be damp, deformed, or even eroded. Its heat insulation ability is reduced, the loss of cold storage capacity of the warehouse is increased, and the temperature of the warehouse is significantly slowed down. . Another important reason for the large cold loss is the poor sealing performance of the warehouse, and more hot air invades the warehouse from the air leakage. Generally, if condensation occurs on the sealing strip of the warehouse door or the insulation of the cold storage insulation wall, it means that the seal is not tight. In addition, frequent opening and closing of warehouse doors or more people entering the warehouse together will also increase the cooling capacity loss of the warehouse. Try to avoid opening the warehouse door to prevent a lot of hot air from entering the warehouse. Of course, when the storage in the cold storage warehouse is frequent or the purchase volume is too large, the heat load increases sharply, and it usually takes a long time to cool down to the specified temperature.
3. The compressor efficiency is low, and the cooling capacity cannot meet the warehouse load requirements
Due to the long-term operation of the compressor, due to the serious wear of the cylinder liner and piston ring, and the increase of the cooperation clearance, the sealing performance will decrease accordingly, the air transmission coefficient of the cold storage compressor will also decrease, and the cooling capacity will decrease. When the cooling capacity is less than the warehouse thermal load, it will cause the warehouse temperature to drop slowly. The cooling capacity of the compressor can be roughly judged by observing the suction and discharge pressure of the compressor. If the cooling capacity of the compressor is reduced, the common method is to replace the cylinder liner and piston ring of the compressor. If it still fails to work after replacement, other factors should be considered, even disassembly and overhaul to eliminate the failure factors.


4. The amount of refrigerant in the system is insufficient and the cooling capacity is insufficient
There are two main reasons for insufficient refrigerant circulation. One is insufficient refrigerant charge. At this time, only a sufficient amount of refrigerant needs to be added. Another reason is that there is a lot of refrigerant leakage in the system. In this case, you should first find the leak point, focus on checking the connection of each pipe and valve, find out the leaked part is repaired, and then charge a sufficient amount of refrigerant.
Fifth, the frost on the surface of the evaporator is too thick or too much dust, the heat transfer effect is reduced
Another important reason for the slow temperature drop in the cold storage is the low heat transfer efficiency of the evaporator, which is mainly caused by the excessively thick frost layer or excessive dust accumulation on the surface of the evaporator. Because the surface temperature of the cold storage evaporator is mostly below 0 ℃, and the warehouse humidity is relatively high, the moisture in the air is prone to frost or even freeze on the surface of the evaporator, which affects the heat transfer effect of the evaporator. In order to prevent the frost layer on the surface of the evaporator from being too thick, it needs to be defrosted regularly. Two simpler defrosting methods are introduced below: ① Shut down and defrost. That is, stop the operation of the compressor, open the warehouse door, and let the warehouse temperature rise. After the frost layer automatically melts, restart the compressor. ② Frost. After the goods in the warehouse are moved out, the surface of the evaporator drain pipe is directly washed with higher temperature tap water to dissolve or fall off the frost layer. In addition to excessively thick frost, the heat transfer effect of the evaporator will be poor, and the surface of the evaporator will not be cleaned for a long time and the dust accumulation is too thick, and its heat transfer efficiency will be significantly reduced.
Sixth, the throttle valve is improperly adjusted or blocked, the refrigerant flow rate is too large or too small
Improper adjustment or blockage of the throttle valve will directly affect the refrigerant flow into the evaporator. When the opening of the throttle is too large, the refrigerant flow is too large, and the evaporation pressure and evaporation temperature will also increase, and the temperature of the warehouse will decrease slowly. At the same time, when the opening of the throttle is too small or blocked, the refrigerant The flow rate is also reduced, the cooling capacity of the system is also reduced, and the temperature of the warehouse will also slow down. Generally, it can be judged whether the refrigerant flow rate of the throttle valve is proper by observing the evaporation pressure, the evaporation temperature and the frosting condition of the suction pipe. Throttle blockage is an important factor that affects the refrigerant flow rate. The main causes of throttle blockage are ice blockage and dirty blockage. Ice blockage is due to the poor drying effect of the dryer. The refrigerant contains moisture. When it flows through the throttle valve, the temperature drops below 0°C. The moisture in the refrigerant forms ice and blocks the throttle valve hole; dirty plugging is Due to the accumulation of more dirt on the inlet filter of the throttle valve, the refrigerant flow is not smooth, resulting in blockage.

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