Air conditioning evaporator frosting is a common phenomenon and a technical problem that is highly valued. Today we will take a look at why the air conditioning evaporator frosts and how to solve this problem.
1. 8 Reasons for Evaporator Frosting
The reasons for evaporator frosting are generally: low evaporation temperature and insufficient heat exchange of the evaporator (liquid low-temperature and low-pressure refrigerant cannot be effectively evaporated in the evaporator). We can analyze it from the following 8 reasons:
1. Insufficient Air Supply. The detailed reasons include blockage of the air outlet and return ducts, blockage of the filter, blockage of the fin gap, fan failure or fan speed reduced, etc., resulting in insufficient heat exchange, reduced evaporation pressure, and reduced evaporation temperature.
2. Problems with the Evaporator Itself. The evaporator is often used, and its heat exchange performance decreases, causing the evaporation pressure to decrease.
3. Too Low External Temperature. Civil refrigeration equipment generally cannot be used in an environment below 20°C. Refrigeration work in a low-temperature environment will cause insufficient heat exchange and low evaporation pressure.
4. Expansion Valve Blocked or the Pulse Motor System that Controls the Opening Damaged. In a long-term running system, some debris may block the expansion valve port, which will make it unable to work normally, reducing the refrigerant flow and the evaporation pressure.
5. Secondary Throttling. Pipe bending or blockage inside the evaporator results in secondary throttling, causing the pressure and temperature of the part through the secondary throttling to decrease.
6. Poor System Matching. To be precise, the evaporator is small or the compressor size is too big. In this case, even if the evaporator performance is fully utilized, the big compressor size will cause low suction pressure and low evaporation temperature.
7. Lack of Refrigerant. It causes low evaporation pressure and low evaporation temperature.
8. Too Much Refrigerant. Many people think that if there is too much refrigerant, the evaporation pressure will rise and there will be no frost. However, when there is too much refrigerant, the excess refrigerant is basically in liquid form in the pipeline between the rear section of the condenser and the front of the expansion valve. At this time, the system circulation slows down, the liquid supercooling increases, the expansion valve opening decreases, the evaporation temperature decreases, and it may also cause the return air pipe temperature to be negative.
However, we cannot talk about these 8 reasons in general. The specific situation depends on the actual situation:
Low evaporation temperature and insufficient heat exchange are both indispensable, so 1-3 can only cause evaporator frost, while 4-8 only cause low evaporativity. If the external conditions are conducive enough to heat exchange, it is difficult to frost.
Take 7 as an example. When the lack of refrigerant causes the evaporation temperature to drop to -3℃, but if my cooling condition is 27/19℃ in summer, the air volume is sufficient, and the heat exchanger performance is good, there will be no frost.
2. Analyze the Causes of Evaporator Frosting in Detail
The surface of the evaporator fins is frosted, and the evaporator temperature at the frosted part is lower than 0℃. The frosting speed is related to the ambient temperature and humidity.
2.1 Frosting at the Evaporator Outlet
As time goes by, frost covers the entire evaporator surface. This phenomenon is a manifestation of excessive liquid supply for the evaporator (the capillary tube system reacts obviously), and for the entire refrigeration system of the household air conditioning, it may be a problem with the return air system and control system:
a. Dust blockage of the return air filter, dust blockage on the evaporator fin surface, fan capacitor broken or with reduced capacitance, no rotation of the fan motor, slow speed, etc., resulting in a reduction in the heat exchange load of the evaporator.
b. Refrigeration runs under low ambient temperature, or runs for a long time under the condition that the resistance of the return air temperature sensor changes.
c. Some air conditioning systems have a continuous operation function. When the continuous operation function is turned on, the refrigeration system operation is not controlled by the temperature sensor, and frost will appear on the evaporator surface after a long operation time. Just turn off the continuous operation function to solve the problem.
2.2 Frosting after the Capillary Tube Throttling
As time goes by, parts of the surface of the evaporator are severely frosted starting from the evaporator inlet, and most of the evaporator surfaces have no condensation water. This phenomenon is a manifestation of insufficient liquid supply for the evaporator of the refrigeration system. For the entire refrigeration system of the household air conditioning, it may be that the system has a leak or the throttling system is partially blocked. Ways to distinguish whether the system is leaking or the throttling system is partially blocked:
a. The condenser air outlet temperature is high, the current is large, and there is partial frost at the evaporator inlet and no refrigeration. This is mostly caused by a partially-blocked throttling system. The filter before the capillary tube inlet is dirty and blocked, or the indoor and outdoor connecting liquid pipes (thin tubes) are flattened to cause secondary throttling.
b. The condenser air outlet temperature is not high, the current is small, and there is partial frost at the evaporator inlet and no refrigeration. This is mostly caused by a leak in the refrigeration system pipeline. The most likely places for leakage are system pipelines, welding points, interface pins, valve cores, valve caps, and especially the parts with oil stains on the system pipelines.
2.3 Frosting from the Compressor Return Gas Port and Gradually towards the Evaporator
(1) The throttle valve opening is too large, and the manifestation is that the condenser return liquid temperature and pressure are normal or slightly higher. Frosting starts from the compressor return gas port and gradually moves towards the evaporator.
(2) The evaporator cannot absorb heat normally, or the cold generated by the evaporator cannot be taken away. The manifestation is that the return liquid temperature and pressure are normal or slightly low, and frost gradually forms from the compressor return gas port to the evaporator. After a long time, the entire section from the compressor return gas port to the evaporator and the throttle valve to the evaporator is frozen, and finally low pressure appears.
The inspection method is:
● Check whether the air filter is clean;
● Check whether the fan speed is normal;
● Check whether the evaporator is clean;
● Check whether the air supply system is blocked.
2.4 Frosting from the Expansion Valve to the Evaporator
(1) The refrigerant is too little (leakage), the manifestation is that the condenser return liquid temperature is high but the pressure is not high, and frost gradually forms from the throttle valve to the evaporator.
(2) The expansion valve opening is too small, the manifestation is that the condenser return liquid temperature and pressure are normal or slightly low, and frost gradually forms from the throttle valve to the evaporator.
(3) Check the sight glass to see the condition of the refrigerant. If there is severe bubbling, there may be a system leak. If there is no bubbling or very little bubbling, there may be a small opening of the expansion valve or a blockage. Generally, there will be a large temperature difference before and after the blockage position.
