What are the reasons for compressor return air frosting?

Frosting at the air return port of the cold storage compressor is a very common phenomenon in the refrigeration system. Under normal circumstances, it will not cause system problems immediately, and minor frosting generally does not need to be dealt with. If the frosting phenomenon is serious, it is first necessary to find out the cause of the frosting.

1. Frosting at the air return port of the compressor

Frosting at the return air port indicates that the return air temperature of the compressor is too low, so what will cause the return air temperature of the compressor to be too low?

For the same quality refrigerant, if the volume and pressure are changed, the temperature will behave differently. If the return air temperature of the compressor is low, it will generally show low return air pressure and high refrigerant volume of the same volume at the same time. The root cause of this situation is that the refrigerant flowing through the evaporator cannot fully absorb itself and expand to the predetermined pressure. The heat required for the temperature value.

There are two things that cause this problem:

1. The liquid refrigerant supply of the throttle valve is normal, but the evaporator cannot absorb heat normally;

2. The heat absorption of the evaporator works normally, but the supply of refrigerant to the throttle valve is too much, that is, the flow of refrigerant is too much. We usually understand that there is too much refrigerant.

Second, due to the lack of fluorine, the return air of the compressor is frosted

1. Due to the low flow of refrigerant

Too little refrigerant expansion will not utilize the entire evaporator area, and will only form a low temperature locally in the evaporator. Due to the small amount of refrigerant, the rapid expansion of some areas will cause the local temperature to be too low, and the evaporator will frost.

After partial frosting, due to the formation of a heat insulation layer on the surface of the evaporator and the heat transfer in this area is low, the expansion of the refrigerant will be transferred to other areas, and the phenomenon of frosting or freezing of the entire evaporator will gradually appear, and the entire evaporator will form heat insulation Layer, so the expansion spreads to the compressor return air pipe, causing the compressor return air to frost.

2. Due to the low amount of refrigerant

The low evaporating pressure of the evaporator leads to low evaporating temperature, which will also gradually lead to the condensation of the evaporator to form a heat insulation layer, and the transfer of the expansion point to the return air of the compressor will cause the return air of the compressor to frost.

Both of the above two points will show the frosting of the evaporator before the return air of the compressor is frosted.

In fact, in most cases, for the phenomenon of frost contact, it is only necessary to adjust the hot gas bypass valve. The specific method: open the rear end cover of the hot gas bypass valve, and then use a No. 8 inner hexagonal wrench to turn the adjusting nut clockwise. The adjustment process should not be too fast. Generally, it will stop for a while after turning about half a circle, and let the system run for a while. Check the frosting situation later to decide whether to continue to adjust. Wait until the operation is stable and the frosting of the compressor disappears before tightening the end cover.

3. Cylinder head frosting (in severe cases, crankcase frosting)

Cylinder head frosting is caused by a large amount of wet steam or refrigerant being sucked into the compressor. The main reasons for this are: 

1. The opening of the thermal expansion valve is adjusted too large, and the temperature sensing package is installed incorrectly or fixed loosely, so that the temperature felt is too high and the valve core is opened abnormally.

The thermal expansion valve is a direct acting proportional regulator that uses the superheat at the outlet of the evaporator as a feedback signal to compare it with a given superheat value to generate a deviation signal to adjust the refrigerant flow into the evaporator. Transmitter, regulator and actuator in one.

According to different balance methods, thermal expansion valves can be divided into:

• Internally balanced thermal expansion valve;

• Externally balanced thermostatic expansion valve.

The opening of the thermal expansion valve is adjusted too large, and the temperature sensing package is installed incorrectly or loosely fixed, so that the temperature sensed is too high and the valve core is opened abnormally, causing a large amount of wet steam to be sucked into the compressor, resulting in frosting on the cylinder head.

If the superheat degree at the outlet of the evaporator is too high, the superheat section at the rear of the evaporator will be too long, and the cooling capacity will be significantly reduced; if the superheat degree at the outlet is too small, it may cause liquid shock of the compressor or even frost on the cylinder head. It is generally believed that it is advisable for the expansion valve to be adjusted to the outlet of the evaporator with a working superheat of 3°C to 8°C. 

2. The solenoid valve for liquid supply leaks or the expansion valve does not close tightly when shutting down

As a result, a large amount of refrigerant liquid has accumulated in the evaporator before starting. This situation is also likely to cause compressor liquid shock!

3. Too much refrigerant in the system

The liquid level in the condenser is high, the condensation heat transfer area is reduced, and the condensation pressure is increased, that is, the pressure in front of the expansion valve increases, and the amount of refrigerant flowing into the evaporator increases, and the liquid refrigerant cannot be completely evaporated in the evaporator. Therefore, the compressor sucks in wet steam, the head of the cylinder is cold or even frosted, which may cause "liquid shock", and the evaporation pressure will be high at the same time.

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