The air conditioning system is a refrigeration circulation system composed of compressors, condensers, capillaries, evaporators and other components, among which the pipeline connecting the indoor unit and the outdoor unit is also called the connecting pipe. There are two types of connecting pipes: return pipes (large pipes) and liquid supply pipes (small pipes), which play a vital role in air conditioning systems. However, in actual operation, frost may occur in both large and small pipes, which seriously affects the cooling effect and operation safety of the air conditioner. This article will focus on the causes, hazards, and solutions of frosting in large and small air conditioning pipes.
1. The role of large and small tubules
1. Large pipe (return pipe)
The large pipe, also known as the return pipe or low pressure pipe, is a copper pipe that connects the outlet of the evaporator of the indoor unit to the suction port of the compressor of the outdoor unit. Its function is to transport the refrigerant vapor that absorbs heat from the evaporator into low temperature and low pressure to the compressor to provide source power for the refrigeration cycle. The diameter of the large pipe is thicker, generally 9.52mm or 12.7mm, to reduce the refrigerant flow resistance and improve the return air efficiency.
2. Small tube (liquid supply tube)
A small tube, also known as a liquid supply or high-pressure pipe, is a copper tube that connects the outlet of the outdoor unit's condenser to the inlet of the indoor unit's capillary. Its function is to transport the high-temperature and high-pressure liquid refrigerant after condensation and exothermy of the condenser to the capillary, and then enter the evaporator through throttling and depressurization. The small tube diameter is small, generally 6.35mm, to maintain a high pressure and temperature and prevent the refrigerant from vaporizing prematurely.
2. Causes, effects and treatment of large tube frosting
1. The cause of frosting in the large tube
(1) The evaporation temperature is too low. When the evaporation temperature is lower than the ambient air dew point, the water vapor in the air will condense into frost on the surface of the large tube.
(2) Poor insulation of large pipes. If the insulation layer of the large pipe is damaged or detached, the cold pipe wall is directly exposed to the air, which is more likely to cause frost.
(3) Insufficient heat return to the air. The suction superheat of the compressor is too low, so that there is too much liquid refrigerant in the tube, and the cryogenic liquid and the tube wall exchange heat and frost.
(4) The pressure on the low-pressure side of the system is too low. Due to refrigerant leakage, throttling element blockage and other reasons, the low pressure of the system is seriously low, which aggravates frosting.
2. The effect of frosting in large tubes
(1) Reduce the evaporation pressure and evaporation temperature, and the refrigeration capacity decreases. Frosting of the large tube forms additional thermal resistance, which hinders the efficient heat absorption of the evaporator and reduces the overall cooling effect.
(2) Increase the suction resistance of the compressor, causing surge and even burning. Frosting of the large pipe blocks the return air channel, making it difficult for the compressor to suction, increasing the load, and even damage the liquid hammer.
(3) Accelerate the deterioration of compressor oil and affect lubrication. Frosty, low-temperature return air carries a large amount of moisture into the compressor, contaminating and emulsifying the lubricating oil.
(4) Trigger the "flash evaporation" effect and reduce the heat exchange capacity of the evaporator. After the liquid refrigerant enters the large tube, it flashes due to a sudden pressure drop, reducing the effective evaporation area.
3. Treatment method of large tube frosting
(1) Increase the evaporation temperature. Reasonably set the superheat of the expansion valve or the length of the capillary tube to increase the evaporation pressure and evaporation temperature, and reduce frosting.
(2) Strengthen the insulation of large pipes. High-quality rubber and plastic sponge and other thermal insulation materials are used to continuously and reliably insulate the large pipe and isolate air moisture.
(3) Increase the suction superheat of the compressor. Fine-tune the opening of the expansion valve or replace the capillary tube to ensure that the compressor sucks in superheated vapor and prevents liquid shock.
(4) Check and repair refrigerant leaks. Regularly check the tightness of pipelines and valves, and repair or replace leaks in time to ensure that the system pressure is normal.
3. Causes, effects and treatment of tubular frosting
1. The cause of tubular frosting
(1) The condensing temperature is too low. When the condensation temperature is too low, the temperature of the high-pressure liquid in the small tube also decreases, and it is easy to coagulate on the tube wall.
(2) The flow rate of the tubule is too low. Due to poor heat transfer in the condenser or improper selection of the tube diameter, the refrigerant flow rate in the tube is reduced, and frost is more likely to form at low temperatures.
(3) The condensing pressure is too high. When the condensation pressure of the system exceeds the normal value, the temperature of the refrigerant in the tube decreases, and it is more prone to frost formation at low ambient temperatures, such as at night.
2. The effect of tubular frosting
(1) Block the small tube, obstruct the flow of refrigerant, and reduce the liquid supply of the evaporator. The frosting of the small tube increases the flow resistance, reduces the amount of liquid entering the evaporator, and reduces the refrigeration effect.
(2) Cause throttling element failure. Frosting in the tubules can entrap solid ice crystals in the refrigerant, clog the capillary or damage the expansion valve, causing the system to spiral out of control.
(3) Exacerbate the abnormal operation of the compressor. Insufficient liquid supply reduces the evaporation pressure, and the compressor starts and stops frequently or vibrates at low frequency, shortening the service life.
3. Treatment of tubular frosting
(1) Increase the condensing temperature. Clean the condenser, maintain good ventilation and heat dissipation, and if necessary, shade the sun in summer to increase the condensation temperature.
(2) Accelerate the flow rate of the tubule. Appropriately reduce the diameter of the small tube, reduce the length of the condenser tube, and ensure the rapid passage of high temperature and high pressure liquid refrigerant.
(3) Control the condensing pressure. Adjust or replace the fan motor in time, clean the fins, reduce the condensation pressure, and avoid low temperature of the small tube.
Fourth, the causes, effects and treatment of capillary frosting
1. Causes of capillary frosting
(1) The small tube contains cream. As mentioned earlier, tubular frosting carries ice crystals into the capillaries, causing them to "freeze and plug".
(2) The superheater has poor effect. The indoor fan and the electronic expansion valve are not properly matched, resulting in the return temperature of the superheater being too low and the front end of the capillary condensation.
(3) The compressor discharge pressure is high. The compressor discharge pressure is too high, causing the condensate temperature at the front of the capillary to drop sharply and frost at night when the ambient temperature is low.
2. The effect of capillary frosting
(1) The capillary tube is completely blocked, the refrigerant cannot flow through, and the compressor idles without load.
(2) Reduce the capillary passing capacity, and the refrigeration capacity of the evaporator decreases sharply. Frosting reduces the inner diameter of the capillary, increases the flow resistance, and the liquid supply is insufficient.
(3) A small part of the ice crystals precipitate into the evaporator with the liquid flow, blocking the capillary network of the evaporator and reducing the heat transfer efficiency.
(4) Repeated "freezing plugging" makes the capillary fatigue, local deformation and rupture, and the danger occurs.
3. Treatment method of capillary frosting
(1) Timely treatment of upstream tubular frosting. Refer to the previous method to eliminate the frosting of the tubule and cut off the source of capillary frost.
(2) Improve the heat exchange effect of the superheater. Check whether the indoor fan is stalled, and replace the motor if necessary; Adjust or replace the EEV valve body to match the return air temperature of the superheater.
(3) Optimize the condensing pressure and compressor discharge temperature. Clean the condenser, correct the amount of refrigerant, and replace the compressor if necessary to avoid subcooling at the front of the capillary.
(4) Purge the capillary regularly. During system maintenance, the capillary tube is removed and repeatedly purged with high-pressure nitrogen to expel ice crystals and impurities.