Why Does the Air Conditioner Evaporator Frost?

Frosting on the air conditioner evaporator is a relatively common phenomenon in the refrigeration industry and also a technical issue that receives significant attention. Today, let's take a look at why the air conditioner evaporator frosts and how to solve this frosting problem.

However, for these eight reasons, we cannot simply generalize. We need to look at the actual situation specifically:
Both low evaporation temperature and insufficient heat exchange are indispensable. So, ① - ③ can individually cause the evaporator to frost, while ④ - ⑧ only cause a low evaporation temperature. If the external conditions for heat exchange are good enough, it is difficult for frosting to occur. Taking ⑦ as an example, if the lack of refrigerant causes the evaporation temperature to drop to -3°C, but if my refrigeration operating condition is 27/19°C in summer, with sufficient air volume and good heat exchanger performance, there will be no frosting phenomenon.

It should be added that: The evaporation temperature mentioned earlier refers to the evaporation temperature when there is liquid gasification. When the refrigerant completely turns into a gas state, with the progress of heat exchange, the gaseous refrigerant will become superheated.

So, generally, the evaporator will have partial frosting, especially in cases of ④⑤⑦. In the case of good air volume, external temperature, and heat exchanger performance, ④ and ⑦ will only cause frosting at the outlet of the expansion valve and the front section of the evaporator (there are also cases where the frost layer deteriorates the heat exchange performance and causes frosting in the rear section as well). ⑤ will cause the evaporator after the secondary throttling to frost.

Based on this, personally, I think the likelihood of the reasons for the heat exchanger frosting is ①②③ > ⑥⑧ > ④⑤⑦. Of course, the frosting of the evaporator is mostly the result of the combined effect of the above factors.

Let's discuss the frosting of the return air pipe: The difference between the return air pipe and the heat exchanger is that it does not have forced convective heat exchange with air. So, it is very easy to frost when its temperature is lower than zero degrees. Therefore, except for ④ and ⑦, all others may cause the return air pipe to frost, among which the possibility of secondary throttling is relatively high. Personally, I think the order of importance is ⑤ > ①②③ > ⑥⑧.
2. Different Frosting Directions, Different Judgments of the Reasons
Basically, if the evaporator frosts, it basically starts from the following two directions.

Of course, in addition to looking at the frosting direction, if the surface of the evaporator fins is frosted, a more detailed analysis is needed...
Further Refinement of the Reasons for Evaporator Frosting
The surface of the evaporator fins is frosted, the temperature of the frosted part of the evaporator is lower than 0°C, and the frosting speed is related to the ambient temperature and humidity. There are two phenomena of evaporator frosting:
(1) One phenomenon is that the frost starts from the outlet of the evaporator.
As time goes by, the entire surface of the evaporator is covered with frost. This phenomenon indicates an excessive liquid supply for the evaporator (the capillary system reacts obviously). For the entire refrigeration system of a household air conditioner, it may be that there are problems with the return air system and the control system:
a. Reasons such as dust blockage of the return air filter, dust blockage on the surface of the evaporator fins, a damaged fan capacitor or a decrease in capacitance, the fan motor not rotating or with a reduced rotation speed, etc., result in a decrease in the heat exchange load of the evaporator.
b. Refrigeration operation in a low ambient temperature or long-term refrigeration operation when the resistance value of the return air temperature sensor changes. In a household air conditioner, if the control system has an anti-freezing protection function, after the refrigeration system has been running for a period of time and the control system detects that the temperature of the evaporator coil is lower than 0°C, the controller will automatically turn off the compressor and the outdoor fan for protection, and the indoor fan will operate at a high wind speed. During the protection period, when the temperature of the indoor evaporator coil is greater than 10°C, the protection will be cancelled and the refrigeration work will be resumed. If it is found that the surface of the evaporator has been frosted for a long time without melting, it may be that the evaporator coil sensor and the return air temperature sensor of the control system are faulty or the control system does not have an anti-freezing protection function.
c. Some air conditioners have a continuous operation function.

When the continuous operation function is turned on, the operation of the refrigeration system is not controlled by the temperature of the temperature sensor. After running for a long time, the surface of the evaporator will also frost. Just turn off the continuous operation function. When repairing, first check the evaporator fins, the fan system, the return air temperature and the coil temperature sensor of the controller, and the ambient temperature at the time of operation. Only after confirming that there are no problems with the checked components can it be determined whether the refrigerant charge in the system is excessive. An excessive refrigerant charge is generally a phenomenon that may occur after the system has been refilled with refrigerant. It should be noted here that when the refrigerant charge in the system is excessive, the surface of the evaporator may not frost when operating under high ambient temperature conditions, but it may frost when operating under low ambient temperature conditions.
(2) The other phenomenon is that the frost starts after the capillary tube throttling.
As time goes by, part of the surface at the inlet of the evaporator is severely frosted, and most of the surface of the evaporator has no condensed water.

This phenomenon indicates an insufficient liquid supply for the evaporator of the refrigeration system.
For the entire refrigeration system of a household air conditioner, it may be that there is a leak in the system or a semi-blockage phenomenon in the throttling system.
Distinguish between a leak in the system or a semi-blockage in the throttling system:
a. The air outlet temperature of the condenser is high, the current is large, and there is partial frosting at the inlet of the evaporator without refrigeration. This is mostly a semi-blockage phenomenon in the throttling system. The faulty parts are the dirty blockage of the filter in front of the capillary tube inlet or the indoor and outdoor connecting liquid pipes (thin pipes) being flattened to form secondary throttling.
b. The air outlet temperature of the condenser is not high, the current is small, and there is partial frosting at the inlet of the evaporator without refrigeration. This is mostly a leak point in the pipeline of the refrigeration system. The faulty parts are the pipeline of the system, the welding points, the interface nuts, the valve cores, the valve caps, especially the parts with oil stains in the pipeline of the system. In the control system of a household air conditioner, there should be a system capacity protection function. In the refrigeration working mode, after the compressor has been running continuously for 30 minutes, if the temperature of the indoor evaporator coil is ≥ 25°C (some manufacturers use the condition that the indoor ambient temperature - the indoor coil temperature ≤ 4°C), the compressor and the outdoor fan will be stopped, and the indoor fan will operate at the set wind speed. To exit the protection state, it generally requires power off. Some control systems do not have a capacity protection function. When the compressor runs continuously, the compressor may have an overheating protection.