The function of compressed air filters and the selection of filter media

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1 The function of the compressor air filter: In a compressed air system, contamination can cause serious problems. Because air can be compressed, pollutant particles are also drawn into the compressor during the compression process. In a system at 0.8 MPa, the amount of pollutants is amplified by 8 times within the compression system. At this point, these pollutant particles can cause severe damage to products manufactured using compressed air. These pollutants are pipe dust, worn particles, soot from the combustion process, and microorganisms. They can be roughly divided into three categories: large dust, with particle sizes of 10μm or more. Fine dust, with particle sizes ranging from 10 to 1 μm. The smallest dust particles are those smaller than 1μm. For larger dust particles and impurities, it is relatively easy to remove them given the current level of technology. However, it is more difficult to eliminate those dust particles that are invisible to the human eye, with a minimum size threshold of 0.3~5μm. This requires the use of filtering and removal methods within the system to eliminate this dust. The purpose of using filters is to reduce pollution, but filters themselves can also be a source of pollution for the system. With the advancement of clean technology in recent years, a large number of high-quality filters, filter media, and production systems from abroad have been introduced. Although some existing filters do not cause contamination on their own, it is only by understanding and being familiar with the distribution of particles as well as particle filtration techniques, and by knowing and mastering the properties of various filter media, that it is possible to improve the cleanliness of the system and produce filters that do not generate contamination. This understanding is important for both users and manufacturers. What kind of filtration device and what type of filtering material should be used to effectively remove these dust particles is also an issue that concerns us all. 2 Selection of compressed air filters. There are mainly two types of contaminants in compressed air: one is the atmospheric pollutants drawn into the compressor, 80% of which have a diameter of less than 2 μm; the other is the emissions from the compressor, which appear as smoke and, upon dispersing, turn into aerosols with diameters ranging from 0.01 to 0.8 μm. General-purpose filtration can remove most liquid and solid particles, which generally have a particle size of more than 1 μm. To eliminate extremely small solid particles as well as oil and water aerosols, high-efficiency filters are required, which is technically quite complex. In terms of its methods and mechanisms, it is impossible to make any rigorous physical classification. For example: gravity separation, centrifugal separation, inertial impact, direct interception, Brownian diffusion, vortex diffusion, thermal aggregation, electrostatic sedimentation, magnetic precipitation, Brownian aggregation, acoustic aggregation, turbulent sedimentation. In certain situations, for specific types of particles, only one mechanism plays a dominant role; however, in most cases it is the combined effect of multiple mechanisms. Generally speaking, it relies mainly on direct interception, inertial impact, diffusion or Brownian motion, as well as thermal aggregation. Direct interception is primarily aimed at larger particles in the airflow (typically with a particle size of 1 μm or more). During this process, inertial impacts also occur; as the airflow carrying solid and liquid particles passes through the filter media, the particles, due to their weight and velocity, cannot cause the airflow to change direction rapidly in sync. As a result, they are unable to pass through the winding paths within the filter media and instead collide with it, allowing the filter media to capture the particles. Thus, the journey of the solid particles in the airflow comes to an end at this point. Liquid particles, on the other hand, behave differently: as they pass through the filter material, the tiny particles tend to gather together, eventually converging at one point to form larger droplets. These droplets are then pushed outward from the filter material, creating a saturated liquid zone that is subsequently discharged outside. This is what is known as coalescing filtration. This type of filtration can remove solid particles larger than 0.01 μm; if contaminants account for less than 0.01/106 of the total weight, then all solid and liquid contaminants in the airflow can be eliminated. A good filtering device should meet the following requirements: it must have a high filtration efficiency, generally exceeding 99.99%; at the same time, it should present low resistance to ensure that there are minimal changes in the pressure and flow rate of the gas being filtered. The structure of the filtration device itself should not only be able to withstand the corresponding pressure but also have good airtightness. To ensure filtration efficiency, the filter material itself must possess a certain degree of mechanical strength to withstand pressure, as well as resistance to air flow impacts; it should not crack, become fuzzy or shed particles during use, nor should it experience oxidation or surface peeling. Additionally, the filter material should have a long lifespan, and it should be easy to replace and clean. Compressed air filters can be classified according to the structure of their filter media into \"deep\" filters, \"mesh\" filters, or filters that combine both deep and mesh types. “The filter media of “deep” filters are typically randomly arranged fibers, sintered powder metallurgy materials, and porous ceramics; their function is to create a tortuous flow path that allows dust particles to be captured as the air flows through. “The “mesh” filter uses porous metals, plastics, various fiber fabrics, and microporous filter membranes as its main filtering elements. The pores of this filter material are of relatively uniform size, and its porosity is also high; any dust particles larger than these pores can be filtered out. Filter elements made from ultra-fine glass fiber fabrics or polyester fiber woven materials, which are commonly used, can basically remove dust particles larger than 0.9 μm. 2.1 Wound-type filters: These types of filters are currently widely used; they are not designed for reverse flushing or reuse, and are essentially disposable filters. The main materials include rayon, nylon fibers, polypropylene fibers, glass fibers, etc. Due to mechanical winding without the use of adhesives, it has strong adaptability to different media and is also inexpensive. Since this type belongs to deep filtration, it has a high dust content, but its filtering precision is not very high; foreign products can remove particles larger than 0.8 μm, while domestic ones can only remove particles of 1 μm size. Due to winding quality issues, at the time of initial operation, there is a possibility that fibers may be carried away under the impact of air flow. 2.2 Non-woven filter elements: These filter elements are made by using porous films of vinyl cellulose, polypropylene, and polytetrafluoroethylene, along with reinforcing materials, to create cylindrical filter elements. Most filters available on the market today are combinations of such filter elements; for example, microporous filters made from cellulose acetate, cellulose nitrate, or a mixture of these two materials. They are resistant to corrosion by dilute acids, dilute bases, and non-polar liquids, but their operating temperature should not exceed 75°C. Polytetrafluoroethylene filters are made from pure Teflon; they are suitable for use with organic solvents, strong acids, and strong bases. They are also chemically inert, and remain stable at temperatures ranging from -100 to 260°C. Polyvinyl chloride filters have high strength and toughness, as well as resistance to acids and bases of moderate strength. Their operating temperature should not exceed 65°C; otherwise, they will soften and become toxic. Therefore, caution should be exercised when using them in the food and pharmaceutical industries. The filter elements of this type of filter can remove dust particles larger than 0.5μm, whether it is a liquid or a gas; the operating pressure is generally between 0.8 and 1.0 MPa. A major advantage of this type of filter element is that, as gas passes through it, the filtering element acquires an electrostatic charge. This static charge prevents suspended particles from penetrating deep into the pores of the filter element, thus allowing dust to settle on the surface of the element in a loose state, making it easier to remove and clean. 2.3 Hollow fiber composite filters: Due to the much denser distribution of micropores compared to ordinary filter membranes, the number of micropores per square centimeter of filtering area is quite high, which means that the maximum number of micropores is reduced. Conventional membrane filters have a maximum pore size of 0.3 μm, whereas those made of hollow fibers have a maximum pore size of 0.1 μm; this implies a longer service life for hollow fiber filters as well as greater safety in gas filtration. Although hollow fiber membranes are made from 100% polypropylene, their good water permeability allows them to be used occasionally as components in dryers for gas drying, namely to remove moisture from gases. However, this type of filter element has defects: first, its manufacturing is complex; second, it has to withstand continuous flow while in a hollow state, and sudden changes in operating conditions often cause the hollow fiber membranes to rupture, resulting in the failure of the filter. Furthermore, ensuring the tightness at the junctions between multiple bundles of hollow fibers and the fixed ceiling, as well as maintaining their strength, is also a challenge; of course, these issues are constantly being improved upon and overcome. As such, it remains an excellent filtering device. 2.4 Powder metallurgy porous filter media: These filter media are porous metals or alloys produced using powder metallurgy techniques. They possess the filtering properties inherent to ordinary porous materials due to their porous structure, while also having all the properties of metals; they represent an important category of filter materials today. 2.4.1 Characteristics of powder metallurgy porous materials Powder metallurgy porous materials possess excellent permeability, making them suitable for use in filtration, as well as in devices for the uniform distribution and penetration of fluids. When used as filters, they exhibit a high filtration rate. For example, filters made from sintered titanium sponge powder can achieve a filtration rate 6 times that of ceramic filters when used to filter zinc sulfate solutions in electrolysis processes. Filters made from sintered green steel powder can have a flow rate 4 times that of folded paper filters and 6 times that of cotton yarn filters; yet the filtration areas of these folded paper and cotton yarn filters are much larger than those of powder metallurgy materials. The pore size and porosity can be controlled, resulting in high filtration precision when used as a filtering material. When used for gas separation, it achieves good separation results; when applied in instruments and meters, it allows for relatively precise control of fluid flow. For example, in the oil circuit systems of centrifugal compressors, 40μm stainless steel powder metallurgy filter media are used in place of the traditional metal mesh filters. By effectively preventing the intrusion of solid particles, the lifespan of the compressor bearings can be extended to 10–20 years, and its filtering efficiency is more than three times that of screen filters. It has a large specific surface area, which allows it to improve heat exchange efficiency when used as a heat exchange material. It can absorb energy, and thus can be used as a sound-damping, shock-absorbing, and cushioning material for dryer exhaust gases. It retains certain properties of metals and alloys, such as heat and electricity conductivity as well as weldability. Thanks to its strength and toughness, it can operate under high pressure conditions. 2.4.2 Specific Applications The fields of application for powder metallurgy filter media are extremely wide and continue to expand: filtering solid particles from liquids, separating various media. Such as the filtration of liquid fuels, oils, and lubricants that require high purity (5–10 μm) for use in aircraft, tanks, ships, etc., as well as the filtration of liquid metals like sodium and lithium in nuclear energy technology. Neutron emitters, as well as those used in medicine and healthcare to filter viruses and bacteria. It is used for filtration in the production of penicillin and streptomycin, to generate sterile air, and to separate antibacterial crystals such as penicillin and streptomycin from the mother liquor. Separate liquids from gases, such as water and oil mixed in compressed air. Filtering gases is used for dust collection, water drainage, and controlling gas pressure. Such as the purification of gases for precision instruments and meters. Recovers radioactive dust; collects dust from blast furnace exhaust gases, and can also serve as a buffer for gases such as high-pressure oxygen, nitrogen, hydrogen, and air. The gas and liquid passing through this material become uniform, and it also prevents boiling of the liquid after passing through the material as well as backflow of the liquid when flow is stopped. Some of my factories use this material to purify compressed air, and some antibiotic manufacturing plants employ it to remove dust and bacteria with sizes of 0.5–1 μm from the air, resulting in dust-free and sterile compressed air – achieving ideal outcomes in both cases. The powder metallurgy porous materials currently in use are mainly made of bronze, low-carbon steel, stainless steel, titanium, nickel, and their alloys. Whether in flake or tubular form, its pressure resistance strength ranges from 0.98 to 8.34 MPa. 2.4.3 Installation of powder metallurgy filter media: Since these filter media possess all the properties of metals, when larger sizes, greater areas, or more complex shapes are required, they can be achieved by combining smaller filter media together. Assembly methods can include crimping, pressing, bolt fastening, riveting, welding, threaded connection, and adhesive bonding. 2.5 Porous ceramics and other filtering materials: Porous ceramics are a type of filtering material that appeared abroad quite early on; they also have a high pressure resistance. In recent years, their use has been on the rise. The performance of these materials varies depending on the operating conditions, and the shortest service life for domestic products is 6 to 8 months. Therefore, to ensure reliability, a two-stage design is often used, and it is more commonly applied in high-pressure scenarios. Parker filters from the United States feature a housing made of 316L stainless steel, with an inner wall roughness of only 0.8μm. It can withstand a pressure of up to 3000 lbf/in2 (1 lbf/in2 = 6.8948×10-3 MPa), and it belongs to the ultra-clean series designed for end-use applications. Domestic products have not yet reached the level of high precision and ultra-purity; moreover, after being used for a period of time, the outer surface of the filter element often becomes rough, may even develop debris, show honeycomb-like spots, and become brittle. Filter materials such as porous ceramics and glass are not widely used due to their brittleness, relatively poor thermal shock resistance, and the inability to be welded. To date, no domestic ceramic filter products in the ultra-clean series have been found. Other filter media such as various woven metal meshes and etched meshes, although strong and having good permeability, are prone to mesh deformation at high temperatures, which affects the filtering accuracy. Moreover, their manufacturing processes are complex and they are expensive, so they can only be used in a few specialized applications. Various filter materials are used in different departments and scenarios. As for which filter material is the most suitable, it must be determined through technical and economic comparisons taking into account factors such as gas pressure, flow rate, temperature, the required level of gas purity, and operating conditions; there is no one-size-fits-all answer. With the introduction of foreign equipment and technologies, and through their digestion and adaptation into something that belongs to us, I believe that it won’t be long before well-known domestic brands emerge, providing convenient and superior conditions for clean air compression technology.