Why is the air cooler frosted? What are the methods for defrosting?

June 2, 2021

As an important component of the cold storage refrigeration system, the air cooler will start to form frost on the surface of the evaporator when the temperature of the air cooler is lower than 0°C. As the operating time increases, the frost layer will become thicker and thicker. The thicker frost layer will cause two main problems: one is that the heat transfer efficiency is reduced, and the cold in the evaporator pipe cannot be effectively transmitted through the tube wall and the frost layer to the cold storage; the other problem: the thicker frost layer For the fan motor, a large wind resistance is formed, which causes the air volume of the air cooler to decrease, and also reduces the heat transfer efficiency of the air cooler. Some simple analyses have been made about the cause of frosting on the air cooler (evaporator):

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1. Insufficient air supply, including the blockage of the outlet and return air duct, the filter screen, the fin gap, the fan does not rotate or the rotation speed, etc., resulting in insufficient heat exchange, lower evaporating pressure, and lower evaporating temperature;

2. The problem of the heat exchanger itself, the heat exchanger is commonly used, the heat exchange performance decreases, and the evaporation pressure is reduced;

3. If the external temperature is too low, civil refrigeration will generally not be lower than 20°C, and refrigeration in a low temperature environment will cause insufficient heat exchange and low evaporation pressure;

4. The expansion valve is damaged by the gambling plug or the pulse motor system that controls the opening. Due to the long-term operation of the system, some debris will be blocked in the expansion valve port and make it unable to work normally, which reduces the refrigerant flow rate and the evaporation pressure. In addition, the abnormal opening control will also cause the flow rate to decrease and the pressure to decrease;

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5. Secondary throttling, pipe bends or clogging of debris inside the evaporator, causing secondary throttling, reducing the pressure and temperature of the part after the secondary throttling;

6. Poor system matching. To be precise, the evaporator is small or the compressor working condition is too high. In this case, even if the evaporator performance is fully utilized, the compressor working condition will cause low suction pressure and evaporate. Temperature drop;

7. Lack of refrigerant, low evaporation pressure and low evaporation temperature;

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8. The relative humidity in the warehouse is relatively high, or the evaporator is installed in the wrong position or the cold storage door is frequently opened and closed;

9. The defrosting is not clean. Due to the insufficient defrosting time and the unreasonable position of the defrosting reset probe, the evaporator will be turned on when the defrosting is not clean. The local frost layer of the evaporator will become icy after multiple cycles. Accumulation becomes bigger.

Why does the cold storage evaporator form frost? Because the surface of the evaporator in the cold storage is frosted, it hinders the conduction and dissipation of the cold energy of the refrigeration evaporator (pipe) and affects the cooling effect. When the thickness of the frost layer (ice layer) on the surface of the evaporator reaches a certain level, the refrigeration efficiency even drops below 30%, resulting in a large waste of electricity and shortening the service life of the refrigeration system. Therefore, it is necessary to perform cold storage defrosting operations within an appropriate period.

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Cold storage defrosting methods: hot gas defrosting (hot fluorine defrosting, hot ammonia defrosting), water spray defrosting, electrical defrosting, mechanical (manual) defrosting, etc.

1. Hot gas defrosting——Applicable to large, medium and small cold storage pipe defrosting: directly enter the hot high-temperature gaseous condensate into the evaporator without interception, and the temperature of the evaporator rises, prompting the frosting layer and the pipe joint to melt or Then peel off. Hot gas defrost is economical and reliable, maintenance and management are convenient, and its investment and construction are not difficult.

2. Water spray defrosting-mostly used in large and medium-sized air coolers for defrosting: Regularly spray and cool the evaporator with room temperature water to melt the frost layer. Although water spray defrosting has a good defrosting effect, it is more suitable for air coolers, and it is difficult to operate for evaporating coils. It is also possible to spray the evaporator with a solution with a higher freezing temperature, such as 5% to 8% concentrated brine, to prevent the formation of frost.

3. Electric heating defrosting-electric heating tubes are mostly used for medium and small air coolers: electric heating wires are mostly used for electric heating and defrosting of aluminum pipes in medium and small cold storage. It is simple and easy to use for air coolers and convenient to use; but for aluminum pipe cold storage. In fact, the construction difficulty of installing the heating wire on the aluminum fin is not small, and the failure rate in the future is also relatively high, the maintenance and management are difficult, the economy is poor, and the safety factor is relatively low.

4. Mechanical manual defrosting——Applicable to defrosting of small cold storage pipes: manual defrosting of cold storage pipes is more economical and the most primitive defrosting method. It is unrealistic to use manual defrosting for large cold storages. It is difficult to operate with the head up, and the physical energy is consumed too quickly. It is harmful to your health if you stay in the storage for too long. The defrost is not easy to complete, and it may cause the evaporator to deform or even break. The evaporator caused a refrigerant leakage accident.

The purpose of cold storage defrosting:

1. Improve system refrigeration efficiency;

2. Guarantee the quality of frozen in the warehouse;

3. Save electricity;

4. Extend the service life of the cold storage system.