When the filter of air conditioning unit (also known as air conditioning box) is dusty, the air resistance will increase and the fan efficiency will become poor; and once the heat exchanger is dusty, scaling phenomenon will lead to the increase of heat transfer resistance. When the air conditioner is corroded by dirt for a long time, the pressure resistance of the pipeline will be poor, and even the leakage of the system will occur. However, after the scaling of the water spray system, the nozzle will often be blocked, which seriously affects the heat exchange efficiency of the water spray room.
I. common filter forms of air conditioner
1 primary filter
The primary effect filter is usually made of metal mesh, ceramic ring, glass fiber, rough hole polyurethane foam or all kinds of man-made fibers as filter layer, plus network and fixed frame filter. In recent years, non-woven fabrics are being developed and utilized. All of the above filters need to be cleaned and replaced manually. In order to reduce the workload of cleaning the filter and improve the operation efficiency, the drive winding filter can be used. The filter is mainly composed of metal mesh (fiber cloth) filter material and filter material roll. The filter screen rotates slowly driven by the transmission mechanism, and the sticky dust cleans itself in the oil tank, and works continuously.
2-way filter
Generally, the filter is made of glass fiber of about 10p.m diameter, polyethylene foam plastic with medium porosity, or filter made of acrylic, polypropylene and polyester. Different filtering effects will be obtained under different filter thickness and filtering speed. In order to improve the efficiency of filtration and increase the air volume of treatment, drawer type or bag type filtration elements are often made. At present, the medium efficiency filter has gradually adopted the chemical fiber filter. This kind of filter material is made of felt, and the fiber density is different along the direction of filter material thickness, from coarse to fine. Under the same conditions, the chemical fiber filter has the advantages of high efficiency, small resistance and large volume of dust, compared with foam plastics or glass fiber filter media.
3 high efficiency filter

The high efficiency filter (www.rfilter. Com) is mainly used in the air-conditioning system with super clean requirements. As the final filter, it should be equipped with primary and intermediate prefilters. The filter material of the filter is superfine cotton fiber, superfine glass wool, etc. The fiber diameter of these filter materials is generally less than 1 m, and the filter materials are made into paper pieces. In order to reduce the flow resistance and increase the diffusion effect on the micro dust, low filtration rate (such as 2.0-2.5cm / s) is adopted. Fold the paper multiple times for this purpose. The filtering area is 50-60 times of the ventilation area. After the filter paper is folded, the middle channel is separated by corrugated separator. The high efficiency filter can be used in the non-toxic biological clean room to effectively catch the bacteria in the air dust particles.

II. Causes and hazards of dust and scale formation
In addition to the super clean air conditioning system, the general comfort air conditioning unit often uses the coarse effect filter. In the process of use, the dust particles in the air are adsorbed on the filter net, which inevitably causes the filter blockage, increases the resistance of air flow, and affects the efficiency of the fan. In serious cases, the pressure head of the fan is not enough to meet the air supply requirements, so the filter screen should be cleaned regularly to meet the operation conditions of the air conditioning unit.
But in the actual use process. Some construction units ignore the problem of maintenance convenience, while operators only consider how to facilitate and extend the single operation time of air conditioning unit when maintaining and managing air conditioning unit. Sometimes even remove the coarse effect filter. In this way, the dust will be absorbed between the fins of the heat exchanger of the air conditioning unit after the unit operates for a certain period of time, resulting in the decrease of the heat exchange efficiency and the sharp decrease of the air supply volume. In addition, due to the dust sticking on the fins, the dew can't develop into bead like condensation and can't drop to the water pan smoothly, which also increases the heat transfer resistance.
Surface heat exchanger for both heat and cold. If hot water or steam is used as heat medium, it will inevitably form scale (mainly scale) in the pipeline. Because the heat conductivity of the steel wall of the heat exchanger is large (λ≌ 50W / m · K), and the heat conductivity of the scale is small (λ = 1.2 ~ 1.4W / m · K), only 2.4 ~ 2.8% of the former. In this way, the heat transfer resistance of heat exchanger is increased, and the effect of air conditioning unit is obviously reduced. For the water spray room, dust and scale also have a serious impact. We can analyze it from the following factors that affect the heat exchange efficiency of the spray chamber:
(1) mass velocity of air u p (U is air velocity, M / S; P is air density, kg / M). When the fins of the filter or heat exchanger are blocked, the air mass flow through the air conditioning unit decreases and the air flow rate decreases. The results show that the heat exchange efficiency of the spray chamber becomes worse because of the decrease of the mass flow rate and the air velocity.
(2) spray coefficient μ. The spray coefficient is the amount of water used to treat each kg of air. When the scale of the system pipe becomes thinner and the nozzle is blocked, the water spray quantity will be reduced and the water spray coefficient will be reduced. It has been proved that the decrease of spray coefficient in a certain range will result in the decrease of heat exchange efficiency.
(3) structural characteristics of water spray chamber. The structural characteristics of the spray chamber mainly refer to the nozzle form, aperture, nozzle layout density, number of rows, distance between rows, spray direction and section size of the spray chamber. The experiment shows that under the same conditions, the small nozzle aperture will result in the small water droplets and increase the contact area with air, so the heat exchange efficiency is good. However, the smaller the aperture of the nozzle, the more likely it is to cause nozzle blockage. Once the nozzle is blocked due to scaling, it can not meet the design requirements, and the advantages can not be reflected.
III. treatment of dust and scale
1 treatment of filter
As the filter of air conditioning unit is basically detachable, it is convenient to clean or replace. Generally, it can be washed with clean water. However, if there is oil-bearing dust in the use environment, it is often ineffective to wash it with clean water. At this time, it needs to be treated with polyester dust (a kind of washing liquid). The specific method is: remove the filter full of dust and drench it with clear water to dry the water. Then pour (or spray) a proper amount of liquid medicine on the filter evenly, and let the liquid medicine penetrate for 3-5 minutes by itself. Finally, wash with clean water until all dust is removed, and normal operation can be resumed.
2 treatment of spray pipe and nozzle
We can set a water inlet valve and a drain valve at the highest point and the lowest point of the sprinkler system. A solution water tank is arranged on the top of the air conditioning unit. During operation, first close the water outlet valve in front of the water spray pump, open the drain valve to drain the water in the water spray pipe, and then close the nozzle. Fill the pipeline with the prepared scale remover (such as crud) from the solution water tank through the water inlet valve. When the scheduled time (generally 3-4 hours) is reached, open the drain valve to remove the liquid medicine, and then rinse with clean water for 2-3 times. If the water spray pump is acid resistant (or add another acid resistant pump), we can set a branch pipe in the water inlet pipe behind the pump to connect with the solution preparation water tank to form a cycle. During use, the water in the water pan shall be discharged through the drain valve, then the prepared scale remover shall be poured into the circulating water tank, the water spray pump shall be started for forced circulation for 1-2 hours, and finally the clean water shall be used for circulation for 2-3 times.
3 treatment of heat exchanger
In order to make the fins on the surface of the heat exchanger fully contact with the liquid medicine, the surface of the heat exchanger should be circularly sprayed, while the internal part should be circularly forced. Both of them can use the same water tank to form a cycle, and the flow can be adjusted by respective valves. The number of liquid medicine nozzle rows can be determined according to the size of the windward area of the heat exchanger and the spray area of a single nozzle. Other parameters such as pump head, flow, nozzle spray pressure, pipe diameter, etc. can be easily found from the spray performance chart of the nozzle. In the specific operation, spray the fins on the surface of the heat exchanger with clear water first. When the dust on the surface of the fins is fully wet and there is no water dripping, spray the liquid medicine in the sun. After the liquid medicine fully contacts the fins for 10-15 minutes, rinse them thoroughly with clear water.
IV. precautions
1. Equipment and pipes used for descaling and cleaning shall be considered in advance during equipment installation. If they are not considered during installation, they can be temporarily added during specific operation. After operation, relevant equipment and pipes may not need to be removed and will be used again in the next operation.
2. After the descaling and cleaning operation, the liquid medicine must be washed to prevent the residual liquid medicine from corroding the equipment.
3. The maintenance of air conditioning unit is generally carried out in spring and autumn. By means of the above methods, the heat and moisture exchange efficiency of the unit, the pressure head of the fan and the purification requirements can be ensured, the service life of the unit can be prolonged, and the economic benefits of the enterprise can be improved.
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