Why does the air cooler frost and what are the methods for defrosting?

December 5, 2023

As an important component of the refrigeration system in a cold storage, the air cooler starts to frost on the surface of the evaporator when it operates at temperatures below 0 ℃ or below the air dew point. As the operating time increases, the frost layer will become thicker. A thicker frost layer can lead to two main problems: one is an increase in heat transfer resistance, making it difficult for the cooling capacity inside the evaporator coil to effectively pass through the tube wall and the frost layer to transfer to the cold storage; Another issue: The thicker frost layer creates significant wind resistance for the fan motor, leading to a decrease in the air volume of the air cooler and also reducing the heat transfer efficiency of the air cooler. Some simple analyses have been conducted on the causes of frost formation in air coolers (evaporators):


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1. Insufficient air supply, including blockage of the outlet and return air ducts, filter screens, fin gaps, fan failure or reduced speed, resulting in insufficient heat exchange, decreased evaporation pressure, and decreased evaporation temperature;


2. The problem with heat exchangers themselves is that they are commonly used, resulting in a decrease in heat transfer performance and a decrease in evaporation pressure;


3. The external temperature is too low, and civilian refrigeration generally does not fall below 20 ℃. Refrigeration in low-temperature environments can cause insufficient heat exchange and low evaporation pressure;


4. The expansion valve is blocked by gambling or the pulse motor system that controls the opening is damaged. In a system that operates for a long time, some debris may block the expansion valve opening, making it unable to work properly, reducing the refrigerant flow rate and evaporating pressure. Improper opening control can also cause a decrease in flow rate and pressure;


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5. Secondary throttling, caused by pipe bending or debris blockage inside the evaporator, results in secondary throttling, causing a decrease in pressure and temperature in the part after secondary throttling;


6. Poor system matching, more precisely, refers to a small evaporator or high compressor operating conditions. In this case, even if the evaporator performance is fully utilized, high compressor operating conditions can cause low suction pressure and a decrease in evaporation temperature;


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



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8. The relative humidity inside the warehouse is relatively high, or the installation position of the evaporator is incorrect or the cold storage door is frequently opened and closed;


9. Defrosting is not clean. Due to insufficient defrosting time and unreasonable position of the defrosting reset probe, the evaporator will start running when defrosting is not clean. After multiple cycles, the local frost layer of the evaporator will form ice and accumulate larger.


Why does the evaporator in the cold storage frost? Due to frost on the surface of the evaporator in the cold storage, it hinders the conduction and emission of cold energy in the refrigeration evaporator (pipeline), affecting the refrigeration 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 to below 30%, resulting in significant waste of electrical energy and shortening the service life of the refrigeration system. Therefore, it is necessary to perform defrosting operations in the cold storage at appropriate intervals

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The defrosting methods for cold storage include hot air defrosting (hot fluorine defrosting, hot ammonia defrosting), water spray defrosting, electrical defrosting, mechanical (manual) defrosting, etc.


1. Hot air defrosting - suitable for defrosting the exhaust pipes of large, medium, and small cold storage units:

Directly pour the hot and high-temperature gaseous condensate into the evaporator without intercepting the flow, causing the temperature of the evaporator to rise and promoting the melting or subsequent peeling of the frost layer at the junction with the exhaust pipe. Hot air defrosting is economically reliable, easy to maintain and manage, and its investment and construction difficulty are also not high.


2. Spray defrosting - commonly used for defrosting large and medium-sized air conditioners:

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 difficult to operate for evaporative coils. Alternatively, a solution with a higher freezing temperature, such as 5% to 8% concentrated salt water, can be sprayed onto the evaporator to prevent frost formation.


3. Electric defrosting - electric heating tubes are commonly used for small and medium-sized air coolers:

Electric heating wire is commonly used for electric heating and defrosting of aluminum exhaust pipes in small and medium-sized cold storage. It is simple and convenient to use for air coolers; But for the situation of aluminum tube cold storage, the construction difficulty of installing electric heating wires on aluminum fins is not small, and the failure rate in the future is also relatively high, maintenance and management are difficult, the economy is poor, and the safety factor is relatively low.


4. Mechanical manual defrosting - suitable for defrosting small cold storage pipes:

The manual defrosting of cold storage pipes is relatively economical, using the original defrosting method. It is not realistic to use manual defrosting for larger cold storage facilities. It is difficult to operate with the head up, and physical energy consumption is too fast. Staying in the warehouse for too long is harmful to physical health, and defrosting is not easy to complete. It may cause deformation of the evaporator and even damage it, leading to refrigerant leakage accidents.


Purpose of defrosting in cold storage:

1. Improve the refrigeration efficiency of the system;

2. Ensure the quality of frozen products in the warehouse;

3. Save electricity;

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



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