Frosting in the Cold Storage? A Detailed Explanation of the 9 Reasons for Frosting of Air Coolers and 4 Defrosting Methods~~

2025-03-11

As an important component of the cold storage refrigeration system, when the air cooler operates at a temperature lower than 0°C and lower than the air dew point, frost begins to form on the surface of the evaporator. As the operating time increases, the frost layer will become thicker and thicker.
A thicker frost layer will lead to two main problems:
I. The heat transfer resistance increases, and the cooling capacity inside the evaporator coil cannot effectively pass through the pipe wall and the frost layer to be transferred into the cold storage.
II. Another problem:
The thicker frost layer forms a greater air resistance for the fan motor, resulting in a decrease in the air volume of the air cooler, which also reduces the heat transfer efficiency of the air cooler.
Some simple analyses have been made on the reasons for the frosting of the air cooler (evaporator) as follows:

Insufficient air volume supply, including blockage of the air supply and return ducts, blockage of the filter screen, blockage of the fin gaps, the fan not rotating or the rotation speed decreasing, etc., resulting in insufficient heat exchange, a decrease in the evaporation pressure, and a decrease in the evaporation temperature.
Problems with the heat exchanger itself. With common use of the heat exchanger, its heat exchange performance deteriorates, causing the evaporation pressure to decrease.
The external temperature is too low. Generally, the temperature in civil refrigeration will not be lower than 20°C. Refrigeration in a low-temperature environment will result in insufficient heat exchange and a low evaporation pressure.
The expansion valve is blocked or the pulse motor system for controlling the opening degree is damaged. In a system that has been operating for a long time, some sundries will block the expansion valve port, making it unable to work properly, reducing the refrigerant flow rate, and causing the evaporation pressure to decrease. Malfunction of the opening degree control will also lead to a decrease in flow rate and pressure.
Secondary throttling. There are pipe bends or blockages by sundries inside the evaporator, causing secondary throttling, resulting in a decrease in pressure and temperature in the part after the secondary throttling.
Poor system matching. To be more accurate, the evaporator is relatively small or the operating conditions of the compressor are too high. In this case, even if the performance of the evaporator is fully utilized, due to the high operating conditions of the compressor, the suction pressure will be low and the evaporation temperature will drop.
Lack of refrigerant, resulting in a low evaporation pressure and a low evaporation temperature.
The relative humidity inside the cold storage is high, or the installation position of the evaporator is incorrect, or the cold storage door is opened and closed frequently.
Incomplete defrosting. Due to insufficient defrosting time and the unreasonable position of the defrosting reset probe, the air cooler starts to operate when the evaporator is not completely defrosted. After multiple cycles, the local frost layer on the evaporator freezes and accumulates and becomes larger.
Why does the evaporator in the cold storage frost? Since frost forms on the surface of the evaporator in the cold storage, it hinders the conduction and dissipation of the cooling capacity of the refrigeration evaporator (pipes), affecting the refrigeration effect. When the thickness of the frost layer (ice layer) on the surface of the evaporator reaches a certain extent, the refrigeration efficiency even drops to less than 30%, resulting in a large waste of electrical energy and shortening the service life of the refrigeration system. Therefore, it is necessary to carry out the defrosting operation of the cold storage within an appropriate cycle.
Defrosting methods for the cold storage:
Hot gas defrosting (hot fluoride defrosting, hot ammonia defrosting), water spraying defrosting, electrical defrosting, mechanical (manual) defrosting, etc.
Hot Gas Defrosting
——Suitable for defrosting the pipe rows in large, medium and small cold storages:
Directly introduce the hot high-temperature gaseous condenser into the evaporator without throttling. The temperature of the evaporator rises, prompting the melting or subsequent peeling of the junction between the frost layer and the pipe rows. Hot gas defrosting is economical and reliable, convenient for maintenance and management, and its investment and construction difficulty are not great.
Water Spraying Defrosting
——Mostly applied to defrosting large and medium-sized air coolers:
Regularly spray the evaporator with normal temperature water to melt the frost layer. Although water spraying defrosting has a good defrosting effect, it is more suitable for air coolers. For evaporator coils, it is difficult to operate. A solution with a relatively high freezing point, such as a 5% to 8% concentrated brine, can also be sprayed on the evaporator to prevent the formation of frost.
Electrical Defrosting
——Electric heating tubes are mostly used for medium and small air coolers:
Electric heating wires are mostly used for the electrical heating defrosting of aluminum pipe rows in medium and small cold storages. It is simple and easy to use for air coolers. However, in the case of cold storages with aluminum pipe rows, the construction difficulty of installing electric heating wires on the aluminum fins is not small, and the failure rate in the future is relatively high, the maintenance and management difficulty is large, the economy is poor, and the safety coefficient is relatively low.
Mechanical and Manual Defrosting
——Suitable for defrosting the pipe rows in small cold storages:
Manual defrosting of the pipe rows in the cold storage is relatively economical and is a primitive defrosting method. Manual defrosting is not practical for large cold storages. The operation of looking up is difficult, physical strength is consumed too quickly, staying in the cold storage for too long is harmful to health, defrosting is not easy to be thorough, and it may cause the deformation of the evaporator, and may even damage the evaporator, leading to the occurrence of refrigerant leakage accidents.
The purposes of defrosting the cold storage:
Improve the refrigeration efficiency of the system.
Ensure the quality of the frozen products in the cold storage.
Save electrical energy.
Extend the service life of the cold storage system.