Why Do Unit Air Coolers Frost? What Are The Methods of Defrosting?

Why do unit air coolers frost? What are the methods of defrosting?

As an important component of the cold storage refrigeration system, when the unit air cooler operates at temperatures below 0°C and below the dew point of the air, frost starts to form on the surface of the evaporator, and the frost layer becomes thicker and thicker with the increase in operating time. The thicker frost layer will lead to two main problems: one is the increase of heat transfer thermal resistance, the evaporator coil inside the cold can not effectively through the tube wall and the frost layer to the cold storage; the other problem: the thicker frost layer on the fan motor to form a larger wind resistance, resulting in a reduction in the air volume of the unit air cooler, and likewise reduces the heat transfer efficiency of the unit air cooler.

Unit air cooler (evaporator) frost causes:

1, insufficient air supply, including out and back air duct blockage, filter blockage, fin gap blockage, fan does not rotate or speed reduction, resulting in insufficient heat transfer, evaporative pressure reduction, evaporative temperature reduction;

2, the heat exchanger itself, the heat exchanger is commonly used, the heat transfer performance decreases, so that the evaporating pressure decreases;

3, the external temperature is too low, civil refrigeration is generally not less than 20 ℃, refrigeration in a low-temperature environment will cause insufficient heat transfer, low evaporative pressure;

4, the expansion valve is blocked or control the opening of the pulse motor system damage, long-term operation of the system, some debris will be blocked in the expansion valve mouth so that it can not work properly, reducing the refrigerant flow, so that the evaporative pressure is lowered, the opening of the control is out of order will also result in a reduction in the flow rate, the pressure is reduced;

5, secondary throttling, evaporator internal pipe bending or debris blockage, resulting in secondary throttling, so that the part after the secondary throttling pressure reduction, temperature reduction;

6, the system is poorly matched, to be precise, the evaporator is smaller or the compressor condition is too high, in this case, even if the evaporator performance is fully played, due to the compressor condition is too high will cause the suction pressure is low, the evaporating temperature drops;

7, the lack of refrigerant, evaporative pressure is low, evaporative temperature is low.

8, the relative humidity in the library is large, or the evaporator installation location is not right or frequent switch cold storage door;

9, defrost is not clean, due to defrost time is not enough, plus defrost reset probe location is not reasonable, resulting in the evaporator defrost is not clean when the power on the operation, the evaporator local frost layer after more than one cycle after the ice and the accumulation of large.

Cold storage defrosting methods: hot gas defrosting (hot fluorine defrosting, hot ammonia defrosting), water spray defrosting, electrical defrosting, mechanical (artificial) defrosting and so on.

Hot gas defrosting - applicable to large, medium and small cold storage pipe defrosting:

Directly to the hot high-temperature gaseous condensate without intercepting the flow into the evaporator, the evaporator temperature rises, prompting the frost layer and the combination of tube melting or peeling off. Hot gas defrosting economy is reliable, easy to maintain and manage, and its investment and construction is not difficult.

Water spray defrosting - mostly used in large and medium-sized unit air cooler defrosting:

Regularly using room temperature water spray cooling evaporator, to melt the frost layer. Water spray defrosting although defrosting effect is very good, but it is more suitable for the air cooler, for the evaporator coil, difficult to operate. You can also use the freezing point of a higher temperature solution such as 5% -8% concentrated brine spray evaporator, to prevent the formation of frost.

Electrothermal defrost - electric heaters are mostly used in small and medium-sized unit air coolers:

Electric heaters are mostly used in small and medium-sized cold storage aluminum tube heating defrost, for the unit air cooler is simple and easy to use, convenient; but for the case of aluminum tube cold storage, aluminum fins to install electric heaters construction difficulty is not small, and later failure rate is also relatively high, maintenance and management is difficult, the economy is also poor, the safety factor is relatively low.

Mechanical artificial defrost - small cold storage row tube defrost applicable:

Cold storage row tube artificial defrost is more economical, the original defrost method. Larger cold storage with artificial defrost is not realistic, tilting the head of the operation is difficult, physical energy consumption is too fast, stay in the library for too long is harmful to health, defrost is not easy to be thorough, may cause the distortion of the evaporator, and may even smash the evaporator guide to the cold media leakage accident occurs.

Cold storage defrost purpose:

Improve the cooling efficiency of the system;

Ensure the quality of frozen products in the warehouse;

Saving electricity;

Extend the service life of cold storage system.