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1 Introduction
More projects in the central air conditioning, air handling units in use a very short time would need to replace the filter, greatly increasing the user running costs, also causing the waste of raw materials. Therefore need a designer, the engineering contractor for the filter performance for a reasonable choice for the filter to become the development trend.
2, the filter
2.1 The filtering mechanism
Hit → stick
Dust particles in the air, or move with the flow inertial motion, or no regular exercise, or by the role of a certain field strength. When the movement of the particles hit the obstacle, the gravitational force between the surface of the particles and obstacles to make it stick to the obstacle.
Fiber filter material
The filter material should be able to: both effectively block dust particles, not too much resistance to airflow. Non-woven fiber materials and special paper that meets this requirement. Messy interwoven fibers formed numerous barrier to particles, the wide space between the fibers allow air flow through.
The principle of inertia
Inertial motion of large particles in the gas stream. Airflow Obstacle bypass the particles deviate from the direction of air flow due to inertia and hit the obstacle. Particles, the greater the inertia force, the greater the likelihood of the impact of obstacles, so the better the filtering effect.
Diffusion theory
Small particles as irregular movement. Used for mathematical processing of the irregular movement of the \"diffusion and mass transfer theory, diffusion theory one said. The smaller the particles, the random motion is more violent, more opportunities for the impact of obstacles, so the better the filtering effect.
Efficiency varies with the particles size
The filter traps dust, the amount of unfiltered the amount of dust in the air filtration efficiency. Less than 0.1μm (micron) particles, mainly for the proliferation of sports, the smaller the particle, the higher the efficiency; particles larger than 0.5μm for the inertial motion, the greater the particle, the higher the efficiency. Between 0.1μm and 0.5μm, the efficiency of a lowest point.
Resistance
Fiber so that the airflow bypass, resulting in a small resistance. Numerous fiber resistance and is the filter resistance.
The filter resistance increased with increasing air flow resistance by increasing the area of ​​the filter material, the relative wind speed can be reduced through the filter to reduce the filter.
    Filter formed on airflow resistance. Filter fouling, resistance increases, when the resistance increases to a specified value, the filter scrap. The resistance of the filter referred to as \"initial resistance\"; resistance when the corresponding filters scrap value \"end resistance\". The final choice of resistance is directly related to the filter service life, the air flow range, the system energy consumption. In most cases, the final resistance of 2 to 4 times the initial resistance.
    Filter is dirty, the faster growth and resistance. The high end resistance value does not mean that the life of the filter will be significantly extended, but it will make the air conditioning system, dropped. Therefore, there is no need to end resistance value is set too high.
    Low-efficiency filters commonly used diameter ≥ 10mm coarse fiber filter. The large gap between the fiber, too much resistance there may be dust on the filter to disperse, at this time, the resistance is no longer increased, but the filtration efficiency is reduced to zero. Therefore, we must strictly limit the final resistance value of the G4 filter.
    Each filter section should be installed resistance monitoring device. Resistance will depend on the instrument to judge, not only by the operator.
    Dynamic performance
    Dust capture additional resistance of air flow, so the use of filters, gradually increasing resistance. Captured dust to form a new obstacle, therefore, the filtration efficiency is slightly improved.
    Capture the dust gathered in most of the windward side of the filter material, filter the larger the area, which can hold more dust, the longer filter life.
    Filter scrap
    More on the dust filter, the greater the resistance. When the resistance to achieve the design does not allow the degree of filter life terminated. Sometimes, too much resistance causes the filter to capture dust flying, this is the case, the filters need to be scrapped.
 3, clean design specifications
    3.1 clean grade
    In 1963, the U.S. cleanroom standards of FED-STD-209 per cubic feet of ≥ 0.5mm number of maximum allowable concentration of dust, clean room is divided into several grades, such as 100, 10,000, 100,000. Many countries in the world to follow suit.
    In 1999, the International Organization for Standardization ISO issued the first part of an international standard ISO 14644-1-1 clean room and controlled clean environment: air cleanliness. Adopted a new classification standard.
    In 2001, the new clean room design standards using the ISO classification.
    Electronics industry and the pharmaceutical industry is the closest relationship with the clean room, the two industries. The ISO standard is an electronics industry immediately switch to the ISO standard defines the level of clean room and pharmaceutical industry is still used in the old clean-level requirements. The 1998 version of GMP increased by 300,000 than the previous version.
    3.2 purification
    Layout and installation methods for the selection of clean grade requirements of the air conditioning unit air filter should meet the following requirements:
    1, the early effect of the air filter should not use oil-soaked filter.
    2, the efficiency of air filters should be centrally set in the positive pressure to clean up the air conditioning system segment.
    3, high efficiency air filters or sub-efficiency air filters should be set at the end of the purification of air-conditioning systems.
    4, efficiency, sub-efficient, high efficiency air filters should be selected according to the rated air flow.
    Resistance, the efficiency of similar high efficiency air filters should be set in the same clean room.
    6, installation of high efficiency air filters should be simple and reliable, easy leak detection and replacement.
    4, the filter is a reasonable choice
    Under normal circumstances, the last one filter decides the degree of air purification, upstream of the filter at all levels play a protective role, which protects the downwind side filter to prolong its life, or the protection of the air conditioning system to ensure its normal work.
    Last a filter efficiency of air conditioning design, should be determined according to the cleanliness requirements of the user, and then select the protective effect of the filter, if the filter needs to protect the additional filter, and then in the upwind side. The protective effect of filters collectively, the \"pre-filter.
    Should properly match the filter efficiency at all levels. Adjacent to the two filter efficiency specifications that much difference, one not have a role of protect; if two or less, the latter a burden is too small.
    The service life of high efficiency filter on the end of the clean room should be 5 to 15 years, the most important factor affecting the life of the pros and cons of pre-filter.
    When the classification of the use of \"G, ~ F– H-~ U-efficiency specifications can be easily estimated that the filter efficiency required at all levels. In G2, ~ H12, every 2 to 4 stalls set up a filter. For example: G4, → F7 → H10, in which the level of cleanliness of the end of H10 (high) and filter decided to air the F7 protection H10, G4, protection of the F7.
    Before the end of the clean room efficiency (HEPA) filter should be efficient specifications not less than F8 filter to protect; ultra-efficient (ULPA) filters can be used before F9 ~ H11 filter. Central air-conditioning should be efficiency specifications not less than the F5\\ s filters to protect.
    No wind, low-pollution areas, before the F7 filter or not to set pre-filter; Central air-conditioning system, G3, ~ F6 is the primary filter.
    Whether it should set up a filter efficiency level of the pre-filter to protect, the need for designers and field engineers will use the environment, spare parts costs, energy consumption, maintenance costs and other factors to consider the decision.
    Filter life also depends on the choice of filter area and filter materials. The commencement of the filters in the filter area is often on the windward side of the filter several times, several times, sometimes a hundred times. Filter area, can accommodate dust, long filter life. Filter area, on the low speed of the airflow through the material and the resistance of the filter. Increase the filtration area is the most effective means to extend filter life.
    Experience has shown that for the same kind of structure, the same filter of the filter, when the final resistance to determine the filter area increased by 50%, filter life will be extended by 70% to 80%; area doubled the life of the filter about three times the original.
    Extended life also means that maintenance labor cost reduction and risk reduction. In addition, the filter area increases after the initial resistance will reduce the costs of energy consumption of air conditioning systems will be less. For end users, the choice of filter area filter must be cost-effective.
    Most filters for general ventilation and clean room with a one-time, or can not be cleaned, or from an economic point of view to consider not worth cleaning. High efficiency filters, use the occasion is very particular about the filter wash is not bad, but also best not to wash, unless sure to thoroughly clean after the cleaning performance does not change, and test means to prove it.
    Some cleaning on the traditional + method is water washed plus hand rubbing, washable filtering media firm, such as the manufacture of crude fiber G2, ~ G4, the efficiency of the filter material, of course, you have to determine whether anti filter auxiliary materials water. F6 more than the efficiency of ventilation filters, filter material, the fiber is generally between ∮ 0.5 ~ ∮ 5μm, it is not strong, stand up and rub the F6 above filters are disposable. In fact, you see the material, will be able to determine whether it can be cleaned.
    The user of the developed countries is very little to clean the filter, certain principles can be cleaned, cleaning filters, labor-intensive, labor costs are high, and the filter price is relatively low.
4, air conditioning manufacturers filter configuration
    Domestic air-conditioning manufacturers generally clean grade selection of the filter configuration:
    100 clean: panel G4 (F5) + bag F7 (F8) + bag the F9 + plate of H13 + (the end of the duct FFU filter unit);
    1000 clean: panel G4 (F5) + bag F7 (F8) + bag the F9 + plate the H11 + (the end of the duct FFU filter unit);
    10000 clean: panel G4 (F5) + bag F7 (F8) + bag the F9 + plate the H11;
    100 000 clean: panel G3, ~ F5 + bag F7 (F8) + bag the F9;
    300 000 clean: panel G3 to F5 + bag the F6 ~ the F8;
    Comfort Air Conditioning: plate G2, ~ F5.
    According to some process requirements, within the air conditioning unit to set the UV germicidal lamp, deodorizing activated carbon filter, ozone generator devices.
    Summary: master filters principle, clean specification, repeated replacement of the correct choice of filter, you can reduce run-time cost of filters, filter the rational allocation of the machine to reduce drag, thus reducing the motor power, control cost and operating costs are greatly reduced. Modern building energy efficiency requirements, applied to the filter selection is particularly important in the central air conditioning unit.