Air conditioning unit filter and heat exchange equipment dust and scale treatment method
Summary:
Dust and fouling occur on filters and heat exchangers, which are not economical for the operation of air conditioning units and cannot guarantee normal use requirements. This article puts forward the hazards of dust and fouling of air-conditioning units, and proposes solutions to the problems.
1 Introduction
After the air-conditioning unit (also known as the air-conditioning box) dust formation occurs, the air resistance will increase and the fan efficiency will deteriorate; and once the heat exchanger has dust formation and fouling, it will cause heat transfer and thermal resistance. Increase. In addition, long-term corrosion by dirt can easily cause the pressure resistance of the pipeline to deteriorate, and even the phenomenon of system leakage; after the water spray system is fouled, the nozzles are often blocked, which seriously affects the heat exchange efficiency of the water spray chamber. In order to improve the efficiency of the air-conditioning system and extend the service life of the equipment, it is entirely possible and necessary to design a set of simple and easy maintenance devices to maintain the air-conditioning equipment regularly and conveniently. To add this device, we only need to add an additional solution tank or replace the ordinary water spray pump with an acid-resistant pump, so it is extremely economical in the actual maintenance process.
2 Commonly used filter forms of air conditioners
2.1 Primary filter
The primary filter generally uses metal mesh, porcelain ring, glass filament, coarse-pored polyurethane foam or various man-made fibers as the filter layer, plus a network and a fixed frame to form the filter. In recent years, non-woven fabrics are being developed and utilized. The above-mentioned filters need to be cleaned and replaced manually. In order to reduce the workload of cleaning the filter and improve the operation efficiency, a drive-wound filter can be used. This kind of filter is mainly composed of metal mesh (fiber cloth) filter material and filter material roll. The filter screen rotates slowly under the drive of the transmission mechanism, and the sticky dust is cleaned by itself in the oil tank and works continuously.
2.2 Medium-efficiency filter
This kind of filter is generally selected with a diameter of about 10p. m around glass fiber, medium-fine-pore polyethylene foam or filter made of acrylic, polypropylene, polyester, etc. as filter materials. Under different filter layer thickness and filtration rate, different filtration effects will be obtained. In order to improve the filtration efficiency and increase the processing air volume, drawer-type or bag-type filter elements are often made. At present, medium-efficiency filters have gradually adopted chemical fiber filters. This filter material is made into a felt shape, and the fiber density is different along the thickness of the filter material, from coarse to fine. Under the same conditions, chemical fiber filters have higher efficiency and lower resistance than foam or glass fiber filters. Advantages such as large dust holding capacity.
2.3 High efficiency filter
HEPA filters are mainly used in air conditioning systems that require ultra-cleanliness. As the final filtration, preliminary and intermediate pre-filters should be installed before it. The filter material of this filter is ultra-fine cotton fiber, ultra-fine glass wool and so on. The fiber diameter of these filter materials is generally less than 1 m, and the filter materials are made into paper sheets. In order to reduce the flow resistance and increase the diffusion effect of fine dust, a low filtration rate (such as 2.0 to 2.5 cm/s) is used. To do this, fold the paper several times. The filtering area is 50-60 times the ventilation area. After the filter paper is folded, the middle channel is separated by corrugated separators. High-efficiency filters can be used in biological clean rooms with non-toxic requirements to achieve effective trapping of bacteria in air dust particles.
3 Causes and hazards of dust and fouling
In addition to air-conditioning systems that require ultra-cleanliness, general comfort air-conditioning units often use coarse-efficiency filters. During use, the dust particles in the air are adsorbed on the filter, which inevitably causes the filter to be blocked, increases the resistance of the airflow, and affects the efficiency of the fan. In severe cases, the pressure head of the fan is not enough to meet the air supply requirements, so The filter should be cleaned regularly to meet the operating conditions of the air conditioning unit. But in actual use. Some construction units neglected the issue of convenience for maintenance, and operators only consider how to facilitate and extend the single operation time of the air-conditioning unit when maintaining and managing the air-conditioning unit. Sometimes even the coarse filter is removed. In this way, after the unit has been running for a certain period of time, dust will be adsorbed between the fins of the heat exchanger of the air conditioning unit, which will cause the heat exchange efficiency to drop and the air supply volume to drop sharply. In addition, by. Because the dust sticks to the fins, the condensed water cannot develop into a bead-like condensation during work and cannot smoothly drip to the water tray, which also increases the heat transfer resistance.
Surface heat exchanger for both cold and heat. If the heating medium uses hot water or steam, it will inevitably form scale (mainly scale) in the pipeline. Because the thermal conductivity λ of the steel wall of the heat exchanger is large (λ≌ 50w/m·k), the thermal conductivity λ of scale is very small (λ = 1.2～1.4W/m·k), only 2 of the former ．4～2.8%. This leads to an increase in the heat transfer resistance of the heat exchanger, which significantly reduces the effect of the air conditioning unit. For the spraying room, dust and fouling also have a serious impact. We can analyze it from the following factors that affect the heat exchange efficiency of the spray chamber:
(1) The mass flow velocity of air up (u is the air velocity, m/s; p is the air density, kg/m). When the fins of the filter or heat exchanger are blocked, the mass flow of air passing through the air conditioning unit decreases, and the air velocity drops. Experiments have proved that due to the decrease of air mass flow rate and the decrease of air velocity, the heat exchange efficiency of the spray chamber becomes worse.
(2)Water spray coefficient μ. The spray coefficient refers to the amount of water used per kg of air. If the air volume passing through the spray chamber is G (kg/la) and the total spray volume is W (kg/h), the spray coefficient μ is expressed as:
When the system pipes become fouled and become thinner, the nozzles are blocked, which causes a decrease in the amount of water sprayed and a decrease in the spray coefficient. Practice has proved that the reduction of the spray coefficient within a certain range will cause the heat exchange efficiency of the spray chamber to decrease.
(3) The structural characteristics of the spray chamber. The structural characteristics of the spray chamber mainly refer to the nozzle form, aperture, nozzle arrangement density, number of rows, row spacing, spray direction, and cross-sectional size of the spray chamber. Experiments have proved that when other conditions are the same, the nozzle diameter is small, the sprayed water droplets are small, which increases the contact area with the air, so the heat exchange efficiency is good. However, the smaller the aperture of the nozzle, the easier it is to block the nozzle. Once the nozzle is blocked due to fouling, the design requirements cannot be guaranteed and the advantages cannot be reflected.
4 Treatment methods for dust and fouling
4.1 Treatment of the filter
Since the filters of air conditioning units are basically detachable, it is convenient to clean or replace. Normally, flush with water. However, if there is oily dust in the use environment, washing with clean water is often not effective, and then it needs to be treated with dust (a washing liquid). The specific method is: remove the dusty filter, soak it with clean water, and dry it. Then evenly pour (or spray) an appropriate amount of liquid medicine on the filter, and allow the liquid to penetrate by itself for 3 to 5 minutes. Finally, rinse with clean water until all the dirt is removed, and normal operation can be restored.
4.2 Treatment of spray pipes and nozzles
We can set an inlet valve and a drain valve at the highest point and the lowest point of the spray chamber system respectively (see Figure 1). A solution tank is installed on the top of the air conditioning unit. During operation, first close the outlet valve before the spray pump, open the drain valve to drain the water in the spray pipe, and then close the nozzle. Fill the pipeline with the prepared scale remover (such as scale) from the solution tank through the inlet valve. When the predetermined time (generally 3 to 4 hours) is reached, open the drain valve to drain the liquid, and finally rinse with water for 2 to 3 times. If the spray pump used is an acid-resistant pump (an additional acid-resistant pump can also be installed), we can set a pipe behind the pump to connect to the solution preparation tank (see Figure 2) to form a cycle. When using, drain the water in the water pan through the drain valve, then pour the prepared scale remover into the circulating water tank, start the spray pump for forced circulation for 1 to 2 hours, and finally circulate the water for 2 to 3 times.
4.3 Treatment of heat exchanger
In order to make the heat exchanger surface fins fully contact with the chemical liquid, it is advisable to use circulating spraying on the heat exchanger surface and forced circulation inside. Both can use the same water tank to form a cycle, and the flow can be adjusted through their respective valves (see Figure 3). The number of liquid chemical nozzles can be determined according to the windward area of the heat exchanger and the spray area of a single nozzle. Other parameters such as pump lift, flow, nozzle spray pressure, pipe diameter, etc. can be easily checked from the nozzle spray performance chart Got. In the specific operation, first spray the surface fins of the heat exchanger with clean water. After the surface of the fins are fully wetted with dust and no dripping water, start spraying the liquid medicine. After the liquid medicine is in full contact with the fins for 10-15 minutes, use clean water. Thoroughly flush it.
5 Conclusion
5.1 The equipment and pipes used for descaling and cleaning should be pre-considered when the equipment is installed. If it is not considered during installation, it can be temporarily installed during specific operations. After the operation, the relevant equipment and pipes do not need to be removed. Use it during operation.
5.2 After descaling and cleaning operations, be sure to rinse the chemical liquid to prevent the residual chemical liquid from corroding the equipment.
5.3 The maintenance of air-conditioning units is generally carried out in spring and autumn. Regular economic and effective maintenance through the above methods can ensure the heat and moisture exchange efficiency of the unit and the fan pressure head and purification requirements, while extending the service life of the unit and improving the economic benefits of the enterprise.
