The return pipe and compressor of cold storage are frosted. Why？

The return pipe and compressor of cold storage are frosted. Why？

Yesterday, the reason for the frosting of the air-conditioning compressor and the compressor was released. Someone left us a message. I hope we can analyze the reasons for the frosting of the return pipe of the cold storage and the compressor.

Frosting at the return port of cold storage compressors is a very common phenomenon in refrigeration systems. Under normal circumstances, system problems will not immediately occur, and subtle frosting is generally not handled. If the frosting phenomenon is more serious, then you need to find out the cause of the frosting first.

1. Frosting at the return port of the compressor

Frosting at the air return 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?

If the volume and pressure of the refrigerant of the same quality are changed, the temperature will behave differently. If the compressor return air temperature is low, the return air pressure is generally low and the refrigerant volume of the same volume is high. 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 amount of heat required for the temperature value.

There are two causes of this problem:

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

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

2. The compressor return air is frosted due to less fluorine

1. Because the flow of refrigerant is extremely small. Too little refrigerant expansion will not use all of the evaporator area, but will only form a low temperature in the evaporator. In some areas, due to the small amount of refrigerant and rapid expansion, the local temperature will 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 low heat exchange in this area, the expansion of the refrigerant will transfer to other areas, and frost or icing of the entire evaporator will gradually appear, and the entire evaporator will form heat insulation. As a result, the expansion will spread to the compressor return pipe, causing the compressor return air to frost.

2. Due to the small amount of refrigerant. The low evaporation pressure of the evaporator leads to a low evaporation temperature, which will gradually cause condensation in the evaporator to form an insulating layer, and transfer the expansion point to the return air of the compressor to cause frosting of the return air of the compressor.

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

In fact, in most cases, for the frosting phenomenon, just adjust the hot gas bypass valve. Specific method: Open the rear end cover of the hot gas bypass valve, and then use a No. 8 Allen key to turn the adjustment nut inside clockwise. The adjustment process should not be too fast. Generally, you should pause after a half-turn to let the system run for a while After seeing the frosting situation, decide whether to continue the adjustment. After the operation is stable and the frosting of the compressor disappears, tighten the end cover.

Third, the cylinder head is frosted (the crankcase is frosted in severe cases)

Frost on the cylinder head is caused by a large amount of wet steam or refrigerant sucked into the compressor. The main reasons for this situation are:

1. The opening of the thermal expansion valve is adjusted too large, the temperature sensing bulb is installed incorrectly or the fixing is loose, so that the temperature felt is too high and the valve core opens abnormally.

The thermal expansion valve uses the superheat at the exit of the evaporator as the feedback signal, and compares it with the given superheat value to generate a deviation signal to adjust the refrigerant flow into the evaporator. It is a direct-acting proportional regulator. Transmitter, regulator and actuator are integrated.

According to different balance methods, thermal expansion valves can be divided into two types: internally balanced thermal expansion valves and externally balanced thermal expansion valves.

The opening of the thermal expansion valve is adjusted too large, the temperature sensing bulb is installed incorrectly or the fixing is loose, so that the temperature felt is too high and the valve core opens abnormally, causing a large amount of wet steam to be sucked into the compressor, causing frost on the cylinder head.

If the superheat at the outlet of the evaporator is too large, the overheating section at the rear of the evaporator is too long, and the cooling capacity will be significantly reduced; if the superheat at the outlet is too small, it may cause the compressor to strike or even frost the cylinder head. It is generally believed that the expansion valve should be adjusted to the evaporator outlet with a superheat of 3℃～8℃.

2. The liquid supply solenoid valve is leaking or the expansion valve is not closed tightly when it is shut down. A large amount of refrigerant liquid has accumulated in the evaporator before starting. This situation is also easy to cause compressor liquid shock!

3. Too much refrigerant in the system. The liquid level in the condenser is higher, the condensation heat exchange area is reduced, and the condensing pressure increases, that is, the pressure before the expansion valve increases, and the amount of refrigerant flowing into the evaporator increases. The liquid refrigerant cannot be completely evaporated in the evaporator. Therefore, the compressor sucks in wet steam, the cylinder hair is cold or even frosted, which may cause "liquid hammer", and the evaporation pressure will be high.