Summary of Air Filter Knowledge

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1. Main parameters of air filters 1.1 Dimensions of filters used in air conditioning and ventilation systems ⑴ The most commonly used filters in air conditioning and ventilation systems; whether they are frame-type, bag-type, or W-type, their nominal dimensions are usually 610mm X 610mm, which corresponds to the 24″X 24″ standard. The dimensions of the outer frames vary depending on the manufacturer, ranging from 592mm to 597mm per side. ⑵ The high-efficiency filters used at the end of purification systems are, in developed countries, generally 610 mm (24″) in size; their alternative dimensions are 203 mm, 305 mm, 762 mm, 915 mm, 1219 mm, 1524 mm, and 1829 mm (8″, 12″, 30″, 36″, 48″, 60″, 72″). ⑶ The common sizes of partitionless high-efficiency filters used in China are basically the same as those abroad. The common sizes for high-efficiency filters with partitions are 484mm X 484mm X 220mm (GB-01 type) and 630mm X 630mm X 220mm (GB-03 type). Here, the term GB has nothing to do with the so-called \"national standards\"; G stands for filter, while B stands for glass fiber. 1.2 Rated air volume of the filter ⑴ The rated air volume of a filter is the maximum amount of air that the filter can handle. It depends on the area of the filtering material (not the area of the filter itself; the area of the filtering material is often dozens of times larger than the area exposed to the airflow). If the flow velocity of the air through the filtering material remains the same, a larger area of filtering material will allow more air to pass through. Currently, the rated air volume of filters with the same structure depends on their size. ⑵ For filters with the same structure and filter media, once the final resistance is determined, increasing the filtration area by 50% will extend the filter’s service life by 70%-80%; when the filtration area is doubled, the filter’s service life will be approximately three times that of the original. 1.3 Initial and final resistance of the filter ⑴ The filter creates resistance to air flow; the amount of dust accumulated on the filter increases as it is used for longer periods. When the resistance of the filter reaches a certain specified value, the filter becomes unusable. ⑵ The resistance of a new filter is referred to as the \"initial resistance\", while the resistance value when the filter reaches the end of its useful life is called the \"final resistance\". Some filter models come with a parameter indicating the final resistance, and air conditioning engineers can also adjust the originally specified final resistance value based on the actual conditions on site. In most cases, the final resistance of the filter in use is 2 to 4 times the initial resistance. ⑶ The table below shows the recommended final resistance values for various filtration efficiency specifications. Recommended final resistance values, filtration efficiency specifications (%), and recommended final resistance (Pa): G3 (coarse efficiency): 100 – 200; G4: 150 – 250; F5 – F6 (medium efficiency): 250 – 300; F7 – F8 (high medium efficiency): 300 – 400; F9 – H11 (sub-high efficiency): 400 – 450. ⑷ Low-efficiency filters typically use coarse fiber filter media with a diameter of ≥10μm. Due to the large gaps between the fibers, excessive resistance may cause the dust accumulated on the filter to be blown off; in such cases, the resistance no longer increases, but the filtration efficiency is zero. Therefore, the terminal resistance values of filters below G4 must be strictly limited. ⑸ To ensure the effective use of filters at all levels, it is recommended to install resistance monitoring devices in each filtering section; the cheapest such device is a U-tube manometer. The inclined-tube differential pressure gauge is more accurate than the U-tube differential pressure gauge, and it also has a more attractive design. Pointer-type differential pressure gauges are of higher quality and cost more. ⑹ Control systems for regulating pressure differences generally use differential pressure transmitters, which can convert the resistance value into current or voltage signals that are sent to the control system. By adding a differential pressure switch, an alarm device for detecting excessive resistance can be created. 1.4 Filtration Efficiency ⑴ The “filtration efficiency” of an air filter refers to the ratio of the amount of dust captured by the filter to the amount of dust present in the original air: Image ⑵ The determination of filtration efficiency is inseparable from the testing methods used; different testing methods applied to the same filter will yield different efficiency values. Therefore, without a testing method, filter efficiency cannot be discussed. ⑶ Test methods used by manufacturers with different efficiencies, different **, and different brands vary. If you really need to know the specific efficiency figures, please don’t forget to specify the exact testing methods and the ways to calculate efficiency. 1.5 Dust Holding Capacity ⑴ The dust holding capacity of a filter refers to the weight of specific test dust that the filter can hold under specified test conditions. Here, “specific” refers to: A. standard test wind tunnels, as well as the related testing and measurement equipment ； B. Standard “road dust” that is much larger than actual atmospheric dust particles”* ； * The test dust specified by European and American standards is commonly known as ASHRAE dust; it consists of AC fine ash mixed with a specified proportion of fine carbon black and short fibers. The so-called AC fine ash refers to the airborne dust from a specific location in the desert region of Arizona, USA (Arizona Road Dust). Japan stipulated the use of its own \"Kantō clay,\" while China once specified the use of dust from the Loess Plateau. C. As agreed upon by the client and the testing party, or the testing and calculation methods specified by the standards ； D. Conditions for terminating the trial agreed upon by the client and the testing party. ⑵ The dust capacity is not the weight of atmospheric dust that the filter can hold at the time it becomes unusable. ⑶ The dust holding capacity does not have a direct correlation with the actual weight of dust that the filter can hold; isolated dust holding capacity data is of no use to users. The service life of filters can only be compared when the test conditions and the test dust are the same. ⑷ When measuring the dust holding capacity, a destructive dust generation test must be conducted on the filter. 2. Filter classification 2.1 Efficiency grading in China ⑴ There are two **standards for filters used in general ventilation, and both of these standards classify filters based on their efficiency according to the counting method for new filters. ⑵ Standard GB12218-89 is divided into five grades; the specific requirements are shown in the table below ； ⑶ The GB/T14295-93 standard is divided into four grades; the specific requirements are shown in the table below ； ⑷ The main difference between China’s current standard counting method and foreign counting methods is ； A. In domestic settings, only the efficiency of new filters is measured, while abroad, the efficiency of filters throughout the dust generation test is assessed ； B. Domestic measurements assess the filtration efficiency of all particles larger than a certain particle size, while foreign measurements evaluate the efficiency of particles within a specific particle size range ； C. Standard dust is used for counting measurements abroad, while atmospheric dust is used domestically. ⑸ The **standard GB13354-92 for the classification of high-efficiency filters stipulates that: A. Filters whose efficiency is ≥99.9% as determined by the sodium flame test specified in GB6165 are considered high-efficiency filters. B. Filters with a filtration efficiency of ≥99.999% for particles with a particle size of ≥0.1μm are called ultra-high-efficiency filters (also known as “very high-efficiency” filters). 2.2 European Efficiency Classification ⑴ The current European classification for filter efficiency is shown in the table below: * When the final resistance at the end of the test is 450 Pa, the average counting efficiency value at 0.4 μm corresponds to the efficiency value determined by the colorimetric method. Since it is a dust emission test, the average counting efficiency value is higher than the initial efficiency determined by China’s current methods. The new counting method standard from the European Committee for Standardization will replace the colorimetric method specified in the original EN779. 2.3 Comparison of Filter Efficiency Specifications To facilitate the comparison of the various efficiency specifications that may be encountered, Dr. Cai Jie, a specialist in filter research in China, specifically designed a chart for comparing efficiencies; Dr. Cai stated ； This comparison chart is for reference only; for accuracy, one should refer to the definitions of various testing methods and efficiency specifications. 3. Testing methods for filter filtration efficiency 3.1 Weight-based method – Arrestance ⑴ The weight-based method is generally used to measure low-efficiency filters that serve as pre-filters in central air conditioning systems. ⑵ The filter is installed in a standard test wind tunnel, with dust being generated continuously at the upstream end. At regular intervals, the weight of the dust that passes through the filter (or the amount of dust accumulated on the filter) is measured, thereby determining the filtration efficiency of the filter at that stage, expressed in terms of dust weight. The final weighing efficiency is the weighted average of the efficiencies at each testing stage based on the dust generation amount. ⑶ The dust source used for testing is standard dust with large particle sizes and high concentrations, and the dust used varies from country to country. ⑷ The termination conditions for the weight-based testing are: the final resistance value agreed upon with the user, or a final resistance value set by the tester themselves. Different final resistance values result in different weighing efficiencies. ⑸ The weight-based testing is a destructive test and cannot be used as a performance inspection in product manufacturing. ⑹ Relevant standards for the gravimetric test: American standard: ANSI/ASHRAE 52.1 – 1992; British standard: EN 779 – 1993; Chinese standard: GB 12218 – 1989. 3.2 Colorimetric method – Dust-spot: (1) The colorimetric method is used to measure filters intended for general ventilation, which offer high efficiency; most filters in central air conditioning systems fall into this category. ⑵ The test bench and test dust are the same as those used in the gravimetric method. ⑶ Sampling is taken before and after the filter using a sampling head equipped with high-efficiency filter paper. After each dust-generation test, the light transmittance of the high-efficiency filter paper on the sampling heads at the sampling points before and after the filter in a dust-free condition is measured. By comparing the differences in light transmittance of the filter paper, the so-called \"filtration efficiency\" is determined using a specified calculation method. The final colorimetric efficiency is the weighted average of the efficiencies at various test stages based on the amount of dust generated. ⑷ The conditions for terminating the test are similar to those used in the gravimetric method: either the final resistance value agreed upon with the user, or a final resistance value set by the tester themselves. A different final resistance value results in a different colorimetric efficiency. ⑸ The colorimetric test is a destructive test and cannot be used as a performance check in product manufacturing. ⑹ Relevant standards for the weight-based testing method: American standard: ANSI/ASHRAE 52.1 – 1992; British standard: EN 779 – 1993. China has never used the colorimetric method, nor are there any colorimetric testing rigs available in the country. ⑺ Colorimetry was once the commonly used testing method abroad, but it is gradually being replaced by counting methods. 3.3 Atmospheric dust counting method ⑴ China’s classification of the efficiency of ventilation filters is based on the atmospheric dust counting method. China’s standards for this counting process were established earlier than those in Europe and the United States; however, since they were developed using domestic counters and corresponding measurement techniques from the 1980s, the method is relatively crude. ⑵ The dust source is “atmospheric dust” in the atmosphere. ⑶ The instruments used to measure the number of dust particles are ordinary optical or laser particle counters. ⑷ The efficiency value of the atmospheric dust counting method only represents the initial efficiency of a new filter. ⑸ Standard: GB 12218 - 1989. 3.4 Counting method – Particle Efficiency ⑴ The test bench and the high-concentration test dust used for dust generation are similar to those used in the gravimetric method and colorimetric method. ⑵ The “amount” of dust refers to the number of particles in the fine particle size range, and the instrument used to measure the number of dust particles is a laser particle counter. ⑶ During the testing process, counting measurements were taken before and after each dust generation test, and the filtration efficiency for particles of various sizes was calculated. The test is stopped when the conditions for terminating it are met; the typical efficiency value of the filter is the weighted average of the instantaneous efficiencies at various stages within the specified particle size range, based on the amount of dust generated. ⑷ The counting efficiency is no longer a single value, but rather a filtration efficiency curve across different particle sizes. Tests in Europe have shown that when the ultimate resistance of the test is 450 Pa, the counting efficiency at 0.4 μm is close to that of the conventional colorimetric method. ⑸ European standards specify the use of specific polydisperse droplets for counting measurements, such as the DENS spray emitted by a Laskin nozzle, or polystyrene latex spheres. * *Polystyrene latex balls (Latex) are often used as standard particles for calibrating particle counters. ⑹ American standards specify the use of bleaching powder for counting measurements. When measuring the efficiency values for different particle size ranges using filters of various grades, the ultimate resistance encountered during testing also varies depending on the efficiency grade. ⑺ A full counting efficiency test is a destructive test and cannot be used for routine inspection of products. Manufacturers may omit the particle generation process and merely measure the initial counting efficiency of the filter. ⑻ Relevant standards for the counting method test: American standard: ASHRAE 52.2 – 1999; European standard: PREN 779 (CEN draft, 1999; this standard will replace the colorimetric method specified in EN 779:1993). ⑼ Colorimetry was once the commonly used testing method abroad, but it is gradually being replaced by counting methods. 3.5 Oil Mist Method
⑴ The oil mist method was commonly used in the former Soviet Union, West Germany, and China. Currently, it is no longer used abroad; in China, only some filter material manufacturers still employ this method. ⑵ The dust source is oil mist; Germany requires the use of paraffin oil, with oil mist particle sizes ranging from 0.3μm to 0.5μm. The Chinese standards do not specify the types of oil; they only stipulate that the average diameter of oil mist particles is 0.28μm – 0.34μm. The “quantity” refers to the number of particles within this small particle size range. A laser particle counter is the instrument used to measure the number of dust particles. ⑶ During the test, the “quantity” measured is the turbidity of air containing oil mist. The testing instrument used is a turbidimeter; the filtration efficiency of the filter (or filtering material) against oil mist particles is determined based on the difference in turbidity of the air samples. ⑷ Relevant standards: Chinese standard: GB 6165 – 85; German standard: DIN 24184 – 1990. 3.6 Sodium Flame Method ⑴ The sodium flame method originated in the UK and was widely used in parts of Europe during the 1970s to 1990s. With the spread of scanning methods, this technique is no longer used internationally; however, a considerable number of manufacturers of high-efficiency filters in China still employ the sodium flame method. ⑵ The dust source is a monodisperse phase of sodium chloride (NaCl) salt spray. The “quantity” being tested is the brightness of the hydrogen flame in the presence of the salt spray; the main instrument used is a photometer. ⑶ The particle size of the aerosol generated upon nebulization of sodium chloride solution ranges from 0.2 μm to 2.0 μm, with a median particle size of approximately 0.6 μm; the measured value for existing devices in China is 0.50 μm. ⑷ During the test, brine splashes due to agitation by compressed air; the resulting fine test salt particles, formed after drying, enter the air duct. Samples are taken both before and after the filter. The air sample containing these salt particles causes the hydrogen flame to turn blue and become brighter. The concentration of salt particles in the air is determined based on the brightness of the flame, which in turn is used to ascertain the filter’s efficiency in removing salt particles from the air. ⑸ Relevant standards: Chinese standard: GB 6165–85; British standard: BS 3928–1969; European standard: Eurovent S 4/4. 3.7 DOP method – Dioctyl Phthalate ⑴ The Chinese translation for DOP is “dioctyl phthalate”. It is a commonly used plasticizer in the plastics industry, as well as a common cleaning agent. The method of using 0.3μm DOP droplets as a test aerosol to determine the filtration efficiency of high-efficiency filters is known as the DOP method; the resulting filtration efficiency is referred to as the DOP efficiency. This testing method originated in the United States and is widely used internationally; however, it has never been implemented in China. ⑵ The DOP liquid is heated to produce vapor, which then condenses into tiny droplets under specific conditions. After removing the droplets that are too large or too small, droplets of 0.3μm in size remain as dust sources; this method is also known as the “thermal DOP method”. *The use of 0.3μm particles is specified because it was believed in the past that filters find it most difficult to filter out dust particles of 0.3μm size. ⑶ In the “cold DOP method”, compressed air is used to bubble DOP liquid; this causes it to be atomized via a Laskin nozzle, thereby generating artificial dust in the form of a mist. The cold DOP method produces polydisperse DOP dust particles with sizes ranging from 0.1 μm to 1.0 μm; particles larger than or equal to 0.35 μm account for over 90% of them. This method is frequently employed in tests on ventilation filters as well as in scan tests of filters. ⑷ The filter efficiency measured using polydisperse DOP is higher than that using monodisperse DOP, and no conversion relationship between the two exists at present. ⑸ Fog-like DOP 0.3μm microdroplets are introduced into the air duct; by measuring the turbidity of the air samples before and after the filter, the filtration efficiency of the filter against 0.3μm dust can be determined. ⑹ DOP has been used in the testing of high-efficiency filters for almost 40 years. In recent years, there have been concerns that the cyclophenols it contains are carcinogenic; as a result, monodisperse DOS DEHS is now being used instead. These substances are harmful to the production of ICs and disk drives, which is why monodisperse polystyrene latex spheres (SPLS) with particle sizes ranging from 0.1μm to 1.0μm are commonly used nowadays. ⑺Relevant standard: U.S. military standard: MIL-STD-282. 3.8 Counting Scan Method (MPPS method) – Most Penetratiable Particulate Size ⑴ The current mainstream testing method for high-efficiency filters internationally. ⑵ A counter is used to conduct continuous scanning inspections of the entire outlet surface of the filter; it provides the number and particle size of dust at each point. This method not only allows for the measurement of the filter’s average efficiency but also enables the comparison of the local efficiency at various points. ⑶ As the name implies, the MPPS method is used to determine the filtration efficiency for the particle size of dust that is most difficult to filter out. European experience shows that the particle size of dust that is most difficult to filter lies somewhere between 0.1μm and 0.25μm, while American standards specify that measurements should be taken only within the range of 0.1μm to 0.2μm. ⑷ The dust source used in the experiments was the polydisperse phase DOP droplets generated by Laskin nozzles, or solid dust with a defined particle size. ⑸ If a nucleation counter is used in the test, a monodisperse test dust with a known particle size must be employed. ⑹ The MPPS method is the most stringent approach for testing high-efficiency filters, and it is an inevitable trend to replace various other traditional testing methods with this one. ⑺ Relevant standards: American standard: IES – RP – CC007.1 – 1992; European standards: EN 1882.1 – 1882.5 – 1998 – 2000. 3.9 Photometer scanning ⑴ There are no corresponding standards for the method of leak detection using photometer scanning. ⑵ A photometer is used to scan the entire outlet surface of the filter in order to detect leaks; this scanning method allows for the rapid and accurate identification of leak points in the filter. However, since dust sources are generally multi-dispersed phases, and the photometer itself cannot determine the particle size of the dust, the \"filtration efficiency\" indicated by this scanning method has little practical significance. ⑶ The photometer scanning method is very effective for quality control during the production process, and the testing equipment required is relatively simple. Some manufacturers believe that as long as the quality and specifications of the filter media are strictly controlled, the efficiency of the filter is determined; therefore, performing photometer scans solely for leak detection is sufficient to ensure filter quality. However, this approach is not easily accepted by users. 3.10 Fluorescence method for uranium ⑴: This method is used only in France, and at present it is applied solely to the testing of certain filters used in the nuclear industry. In fact, French filter manufacturers used to prefer the DOP method over the fluorescence method they had established themselves; now, the counting method specified by the European Standardization Organization has been adopted as the standard, resulting in less use of the fluorescence method. ⑵ The test dust source for the fluorescence method is sodium fluorescein dust generated by a sprayer; according to French standards, the average count value of the dust particle size produced by the dust-generating device is 0.08 μm, and the average volume value of the particle size is 0.15 μm. ⑶ During the test, samples are taken before and after the filter; then sodium fluorescein on the sampling filter paper is dissolved in water, and the fluorescence intensity of the sodium fluorescein solution under specific conditions is measured. This intensity indirectly reflects the weight of the dust, and the efficiency of the filter is determined by the difference in fluorescence intensity between the samples taken before and after the filter. ⑷ Relevant standard: French standard: NF X44 - 011 - 1972. 3.11 Other inspection methods: (1) Variable air volume leak detection – if the filter’s efficiency decreases when the air volume is reduced, there is definitely a leak. This method can only determine whether there is a leak in the filter, but it cannot locate the exact site of the leak. ⑵ Smoke testing for leaks: In a dark room, smoke is generated upstream of the filter, and a strong light beam is directed at the outlet side of the filter. When there is a leak in the filter, a wisp of smoke can be clearly seen at the location of the leak, allowing for accurate identification of the leak site. ⑶ In terms of contamination testing, some users worry that the dust used in tests might contaminate the filters. They often request that filter manufacturers use solid-particle dust that they consider to be safe ； Some pharmaceutical factories require the direct use of outdoor atmospheric dust. 4. Applications of filters 4.1 Determining the efficiency of each filter stage appropriately ⑴ Generally, the final filter determines the degree of air purification. ⑵ The filters at various stages upstream serve only a protective purpose and are collectively referred to as “pre-filters”. ⑶ The efficiency of filters at each stage should be properly configured; if the efficiency specifications of adjacent filters differ too much, the preceding filter will not be able to protect the one following it ； If the difference between the two stages is small, then the burden on the latter stage is too light. ⑷ A reasonable configuration is to install a primary filter every 2–4 stages. According to the current European standards for filter efficiency, if an H13 high-efficiency filter is used at the final stage, a three-stage protection system consisting of F5 – F8 – H10 filters can be employed in the preceding stages; the H13 high-efficiency filter itself can last up to eight years. ⑸ The service life of high-efficiency filters at the end of clean rooms should be 5–15 years. The main factors affecting this service life are the quality of the pre-filters themselves and whether they are properly configured. ⑹ A filter with an efficiency of at least F8 must be used for protection in front of the high-efficiency filter in the clean room. ⑺ In urban central air conditioning systems, G3 – F6 are common primary filters. ⑻ Key points: The performance of the final filter must be reliable, the efficiency and configuration of the pre-filter should be appropriate, and the maintenance of the primary filter must be convenient. 4.2 Selection of high-efficiency filters ⑴ Generally, for filters made of the same material, those with higher efficiency have greater resistance and are also more expensive. ⑵ For cleanrooms with high cleanliness requirements, more efficient HEPA or ULPA filters can be used, while for cleanrooms with lower cleanliness requirements, less efficient HEPA filters can be employed. ⑶ The change in filter efficiency at high dust levels has little impact on the cleanliness of a clean room; therefore, clean rooms with low cleanliness requirements should not use high-efficiency filters with high performance. ⑷ At low dust generation levels, high-efficiency filters with higher efficiency provide significant benefits for cleanliness at low air velocities. Therefore, for cleanrooms that require high cleanliness levels, it is necessary to use filters with higher efficiency while simultaneously reducing the face velocity of the air flow. 4.3 The impact of wind speed on filters ⑴ In the vast majority of cases, the lower the wind speed, the better the performance of the filter. ⑵ For high-efficiency filters, if the wind speed is reduced by half, the dust penetration rate decreases by one order of magnitude (the efficiency value increases by 9 points); if the wind speed is doubled, the penetration rate increases by one order of magnitude (the efficiency value decreases by 9 points). ⑶ For high-efficiency filters, the flow rate of air through the filter media is generally between 0.01 and 0.04 m/s; within this range, the resistance of the filter is directly proportional to the volume of air being filtered. For a filter with a rated airflow of 1000 m³/h and an initial resistance of 250 Pa, when the actual airflow during use is only 500 m³/h, its initial resistance can be reduced to 125 Pa. ⑷ For general ventilation filters, the air velocity passing through the filter medium ranges from 0.13 to 1.0 m/s. In this range, the relationship between resistance and airflow is no longer linear; instead, it follows an upward-curving trend. When the airflow increases by 30%, the resistance may increase by as much as 50%. ⑸ Filter resistance is a very important parameter; do not forget to ask the filter supplier for the airflow-resistance curve. 4.4 Use filters with a large filtering area. ⑴ The filtering area referred to here is the area of the filtering material in the filter; the filtering area of a single filter is often several times, dozens of times, or even hundreds of times the area exposed to the wind. ⑵ With a large filtering area, the airflow velocity through the filter material is reduced, resulting in lower resistance for the filter; at the same time, it can handle more dust. Therefore, increasing the filtration area is the most effective way to extend the filter’s service life. ⑶ Experience shows that for filters with the same structure and filter media, once the final resistance is determined, an increase of 50% in the filtration area results in a 70–80% increase in the filter’s service life; when the filtration area is doubled, the filter’s service life becomes three times that of the original. ⑷ The size of the filtration area has little impact on filtration efficiency. ⑸ For end-users, it is definitely cost-effective to choose filters with a large filtration area. 4.5 High-efficiency filters must be tested individually. ⑴ Different filter manufacturers may use various testing methods, but the bottom line is that each high-efficiency filter must undergo routine testing. ⑵ Leaks in the filter are a fatal flaw; it is not possible to detect such leaks visually. A high-efficiency filter with leaks can cause the entire project to fail in environments requiring high cleanliness levels. By choosing high-efficiency filters that have not been tested individually, one assumes the risk of project failure. ⑶ It should be noted that the third-party test reports and product certification documents provided by many filter manufacturers only reflect the performance of the samples sent for testing, and they cannot guarantee that your batch of filters is of qualified quality. 4.6 Selection of Filters for Central Air Conditioning Systems ⑴ Central air conditioning systems require high-quality filters for protection; if low-efficiency filters are used, the following problems will occur in such systems: ● Airway blockages and fouling of the fans, resulting in a reduced airflow volume ； ● Black stains may appear near the air outlet; ● The efficiency of the heat exchange components decreases ； ● Failure of measurement and control components for temperature, humidity, etc ； ● Failure of the dynamic terminal air supply device ； ● Total heat exchange unit failed ； ● The dust accumulated in pipes, with its moderate temperature and humidity, provides an ideal environment for microorganisms to thrive. ⑵ Experience in developed countries shows that with F5 filters, central air conditioning systems need to be cleaned every 5–8 years, whereas with such filters, no cleaning is required over 30 years. ⑶ For a good air conditioning system, the filter efficiency rating should be F6 – F7. ⑷ In developed countries, the cost of cleaning an air conditioning system is 20 times the difference in price between good and poor-quality filters. ⑸ Key point: Central air conditioning systems require good filters for protection; inefficient filters will result in high costs for both users and contractors. An F7 efficiency filter can protect the air conditioning system for 30 years. 4.7 Air filters used in automotive painting (spraying) lines ⑴ If dust particles of 5μm size get mixed into the paint layer, these particles can be seen with the naked eye as defects. To ensure the quality of the paint on car surfaces, automobile manufacturers must use a large number of air filters to remove dust from the air in the spraying and baking processes. ⑵ The painting line is a long tunnel through which the vehicle bodies move. The ceiling of this tunnel is covered with a thick layer of non-woven fabric, which serves both to dampen and regulate airflow as well as to filter air. The efficiency level of this damping material is roughly equivalent to F5; however, filtration efficiency is not that important here. What’s crucial is that the material itself be uniform and free from shedding fibers, and care must be taken to prevent dust from contaminating the material during transportation, storage, processing, and installation. ⑶ The main filter on the painting line is located above the damping layer; it is this filter that determines the cleanliness level of the painting environment. This filter is of the bag-type type with efficiency ratings of F5 – F7, and the commonly used models have dimensions of 592mm X 592mm (24″ X 24″). ⑷ On the baking production line in the painting workshop, the air fed in must be preheated to around 200°C. Since the heating equipment may generate dust, an air filter needs to be placed at the end where the hot air is located; this requires the filter to be able to withstand high temperatures of 200–250°C for extended periods. In such cases, the filters used are typically high-temperature resistant filters with partitions, having an efficiency rating of F7–F8. ⑸ Filters used in painting workshops must especially avoid silicones, as even a trace of silicone on the metal surface can cause the paint layer to bubble; automobile factories explicitly prohibit the use of any silicones. ⑹ For a medium-sized painting workshop, the annual cost of filters ranges from 2 to 4 million yuan. 4.8 Filters for the nuclear industry ⑴ Filters used in the nuclear industry operate on principles and have structures that are not very different from those of filters used in other industries. ⑵ Filters used in the nuclear industry must undergo more testing and receive additional certifications, and these tests and certifications are often carried out by specialized agencies related to the nuclear industry. 4.9 Filters in vacuums ⑴ Old-fashioned vacuums, due to the lack of proper filters, often end up acting as \"dust generators\" – sucking in debris on one end and releasing fine dust on the other. ⑵ A good vacuum cleaner is equipped with an exhaust filter; the efficiency standard for such filters is generally F7 (with an average filtration efficiency of 80%–90% for particles of 0.4 μm in size), allowing it to block most inhalable particles. ⑶ There are also vacuums on the market that use high-efficiency filters as exhaust filters. 4.10 Cleaning and Disposability of Filters ⑴ Most filters used in air conditioning systems and cleanrooms are disposable; some filters cannot be cleaned, while others are not worth cleaning from an economic perspective. ⑵ Cleaning agents may undermine the filtering efficiency of the filter media.