First, frosting on the compressor return air shows that the temperature of the compressor return air is too low, so what will cause the temperature of the compressor return air to be too low?
Everyone knows that if the volume and pressure of the refrigerant of the same quality are changed, the temperature will have different performance, that is, if the liquid refrigerant absorbs more heat, the refrigerant of the same quality will have higher pressure, temperature and volume, and if it absorbs less heat, the pressure, temperature and volume will be lower.
In other words, if the return air temperature of the compressor is low, the return air pressure will generally be low and the refrigerant volume with the same volume will be high. The root cause of this situation is that the refrigerant flowing through the evaporator cannot fully absorb the heat required to expand to the predetermined pressure temperature value, resulting in low temperature, pressure and volume values of the return air.
There are two causes of this problem:
1. The liquid refrigerant supply of throttle valve is normal, but the evaporator cannot absorb heat normally to supply refrigerant for expansion.
2. The evaporator absorbs heat normally, but the throttle valve supplies too much refrigerant, that is, the refrigerant flow is too large. We usually understand that there is too much fluorine, that is, too much fluorine will also cause low pressure.
Second, the compressor return air is frosted due to less fluorine
1. Due to the extremely small flow rate of refrigerant, the refrigerant will start to expand in the first expandable space after flowing out of the rear end of the throttle valve. Most of us see that frost formation at the liquid separation head at the rear end of the expansion valve is often caused by lack of fluorine or insufficient flow rate of the expansion valve. Too little refrigerant expansion will not utilize the entire evaporator area, and will only form low temperature in the evaporator area. The rapid expansion in some areas due to the small amount of refrigerant will cause the local temperature to be too low, resulting in frost formation on the evaporator.
After local frost formation, due to the formation of a heat insulation layer on the evaporator surface and low heat exchange in this area, refrigerant expansion will be transferred to other areas, gradually causing frosting or icing of the entire evaporator. The entire evaporator forms a heat insulation layer, and then expansion will spread to the compressor return air pipe, resulting in frosting of the compressor return air.
2. Due to the small amount of refrigerant, the low evaporation pressure of the evaporator leads to the low evaporation temperature, which will also gradually lead to condensation of the evaporator to form a heat insulation layer and transfer the expansion point to the return air of the compressor, resulting in frosting of the return air of the compressor. Both of the above two points will show frost on the evaporator before the return air of the compressor is frosted.
In fact, for frosting in most cases, it is only necessary to adjust the hot gas bypass valve. The specific method is to open the end cover at the rear of the hot gas bypass valve, and then turn the adjusting nut inside clockwise with the No.8 Allen wrench. The adjustment process should not be too fast. Generally, the adjustment process should be suspended after half a turn or so, so that the system can be operated for a period of time before deciding whether to continue the adjustment according to frosting conditions. When the operation is stable and the frost on the compressor disappears, tighten the end cover.
For models below 15 cubic meters, since there is no hot gas bypass valve, if frost is serious, the take-off pressure of the condensing fan pressure switch can be appropriately increased. The specific method is to find the pressure switch, remove the adjusting nut of the pressure switch to fix the small piece, and then rotate clockwise with a Phillips screwdriver. The whole adjustment also needs to be carried out slowly. Adjust for half a turn to see if the adjustment is needed.
Three, the cylinder head frosting (serious crankcase frosting), always due to a large number of wet steam or refrigerant suction compressor. The main reasons for this situation are:
1. The opening of the thermal expansion valve is adjusted too large, and the temperature sensing bulb is installed incorrectly or fixed loosely, so that the sensed temperature is too high and the valve core opens abnormally.
The thermostatic expansion valve is a direct-acting proportional regulator that uses the superheat degree at the outlet of the evaporator as a feedback signal to generate a deviation signal after comparing it with a given superheat degree value to regulate the refrigerant flow entering the evaporator. It integrates a transmitter, a regulator and an actuator.
When the measured parameter of the transmitter deviates from the given value, the physical quantity of the transmitter changes and generates enough energy to directly push the actuator to act, and the position change of the actuator is proportional to the adjusted parameter.
Thermal expansion valves can be divided into internal balance thermal expansion valve and external balance thermal expansion valve according to different balance modes. The liquid refrigerant evaporates in the evaporator and absorbs heat. When it flows to the outlet of the evaporator, it has completely vaporized and has a certain degree of superheat. The temperature sensing tube of the thermal expansion valve is tightly attached to the evaporator outlet pipeline and senses the temperature at the evaporator outlet. If the liquid filled in the temperature bag is the same as the refrigerant, the liquid pressure above the diaphragm of the thermal expansion valve is greater than the liquid pressure below the diaphragm, and the higher the temperature at the outlet of the evaporator, i.e. the higher the superheat degree, the greater the liquid pressure above the diaphragm.
This pressure difference is balanced by the tension of the push rod and the adjusting spring under the diaphragm. If the tension of the adjusting spring is changed, the jacking force of the ejector rod can be changed, thus changing the opening degree of the needle valve. Obviously, the degree of superheat of evaporator will also lead to the change of needle valve opening. When the adjusting spring is adjusted at a certain position, the expansion valve will automatically change the opening of the needle valve according to the temperature of the evaporator outlet to keep the superheat degree of the evaporator outlet at a certain value.
The opening of the thermal expansion valve is adjusted too large, the temperature sensing bulb is installed incorrectly or is fixed loosely, so that the sensed temperature is too high and the valve core opens abnormally, causing a large amount of wet steam to be sucked into the compressor and causing frosting on the cylinder head. The thermostatic expansion valve is used to adjust the superheat degree when the evaporator works.
If the superheat degree at the evaporator outlet is too high, the superheat section at the rear of the evaporator will be too long and the refrigerating capacity will be significantly reduced. The outlet superheat degree is too low, which may cause the compressor liquid to blow and even frost on the cylinder head. It is generally believed that the expansion valve should be adjusted to a superheat of 3℃ ~ 8℃ at the evaporator outlet.
2. When the liquid supply solenoid valve leaks or stops, the expansion valve is not tightly closed, causing a large amount of refrigerant liquid to accumulate in the evaporator before starting. The temperature relay and electromagnetic valve are used together to control the reservoir temperature.
The temperature sensing bulb of the temperature relay is placed in the cold storage, when the temperature of the cold storage is higher than the upper limit of the starting setting value, the contact of the temperature relay is connected, the solenoid valve coil is energized, the valve is opened, and the refrigerant enters the evaporator to cool down; When the reservoir temperature is lower than the lower limit of its set value, the contact of the temperature relay opens, cutting off the coil current of the solenoid valve, closing the solenoid valve, and stopping the refrigerant from entering the evaporator, thus the reservoir temperature can be controlled within the required range.
3, start the compressor suction cut-off valve open too big or open too early.
4. When there is too much refrigerant in the system, the liquid level in the condenser is relatively high, the condensation heat exchange area is reduced, and the condensation pressure is increased, i.e. the pressure in front of the expansion valve is increased, the amount of refrigerant flowing into the evaporator is increased, and the liquid refrigerant cannot be completely evaporated in the evaporator. Therefore, the compressor sucks wet steam, the cylinder head is cold and even frosted, which may cause "liquid strike", and the evaporation pressure is also relatively high.