Function of compressed air filter and selection of filter material
1.Function of compressor air filter
In the compressed air system, pollution can cause serious problems. Because air can be compressed, pollutant particles will also be sucked into the compressor during the compression process. In the 0.8MPa system, the amount of pollutants in the compression system increased by 8 times. At this time, these pollutant particles will cause serious damage to the products produced using compressed air. These pollutants include pipe dust, wear particles, smoke and microorganism during combustion. It can be roughly divided into three categories:
Large dust with particle size of 10 μm Above.
Fine dust with particle size of 10-1 μ M.
The smallest dust with particles less than 1μm
For large dust and impurities, it is easy to remove from the current technical level, while the removal of invisible dust and impurities is usually 0.3~5 μ m. It is difficult to measure dust particles with m as the lower limit. This requires filtering and cleaning measures to remove dust from the system
2.Selection of compressed air filter
There are two main pollutants in compressed air. One is the air pollutant inhaled into the compressor, 80% of which has a diameter of 2 μ Below m; The other is the discharge from the compressor, which is in the state of smoke, when it dissipates μ M aerosol, the emission will be 0.01~0.8.
General filtration can remove most liquid and solid particles, and the particle size is generally greater than 1 μ m。 To remove very small solid particles and oil-water aerosols, it is necessary to use high-efficiency filters, which is technically very complex. In terms of its methods and mechanisms, it is impossible to carry out any strict physical classification. For example, gravity separation, centrifugal separation, inertial impact directly intercept Brownian diffusion, eddy current diffusion, thermal condensation, electrostatic precipitation, magnetic precipitation, Brownian condensation, acoustic condensation, turbulent precipitation. In some cases, for special types of particles, only one mechanism plays a leading role, but in most cases, it is a combination of multiple mechanisms. Generally speaking, it mainly depends on direct interception, inertial collision, diffusion or Brownian motion and thermal condensation. Direct interception is mainly aimed at larger particles in the air stream (generally the particle size is 1 μ M) Inertial impact will also occur during the interception process, because when the air flow containing solid and liquid particles flows through the filter material, the particles cannot change the direction of the air flow synchronously and rapidly due to the influence of weight and speed, so it is also impossible to impact the filter material through the bending path of the filter material, so that the filter material catches particles, so the process of solid particles in the air flow is over. However, the liquid particles are not exactly the same. When flowing through the filter material, tiny particles will gather and finally gather in one place to form larger droplets, which are pushed to the outside of the filter material to form a saturated liquid zone and then discharged from the filter, which is called condensation filtration. This filtering method can remove 0.01 μ If the solid particles above M are less than 0.01/106 of the total weight, the solid and liquid pollutants in the air stream can be completely removed.
A good filtration device should meet the following requirements: high filtration efficiency, generally more than 99.99%; At the same time, the resistance is small to ensure that the pressure and flow of the filtered gas will not change too much.
The structure of the filter device shall not only bear the corresponding pressure, but also have good air tightness. In order to ensure the filtration efficiency, the filter material itself should have a certain mechanical strength, bear a certain pressure, and be able to resist the impact of air flow. During use, there shall be no cracking, bulkiness, scum, oxidation or surface peeling. In addition, the service life of the filter material should be longer, and the replacement and cleaning should be convenient.
According to the structure of filter material, compressed air filter can be divided into "deep" filter, "mesh" filter or combination of deep and mesh filter.
The filter materials of "deep" filter are mostly disorderly filled fibers, sintered powder metallurgy and porous ceramics. Its function is to create a curved flow path to catch dust particles when the air flow passes.
The "mesh" filter is composed of porous metal, plastic, various fiber fabrics and microporous membrane as the main filter elements. The pore size of this filter material is relatively uniform and the porosity is relatively high. All dust particles larger than pores can be filtered out. For example, the filter element made of superfine glass fiber fabric and polyester fiber woven fabric is greater than 0.9 μ M dust particles can be basically removed.
2.1 Wound filter element
At present, this filter element is widely used, and reverse purging and reuse are usually not considered. It is basically a disposable filter element. The main materials are rayon, nylon, polypropylene fiber, glass fiber, etc. Due to the use of mechanical winding and no adhesive, it has strong adaptability to media and is cheap. Because this form belongs to deep filtration, the dust content is large, but the filtration accuracy is not very high. Foreign products can remove 0.8% μ For particles above m, China can remove 1 μ M Due to the winding quality problem, the optical fiber may be taken out under the influence of air flow when it is initially put into operation.
2.2 Non-woven filter element
This filter element is mainly processed into a porous membrane cylindrical filter element, and the reinforcing materials are vinyl cellulose, polypropylene and polytetrafluoroethylene. At present, most of the filters sold on the market are combinations of such filter elements, such as cellulose acetate and cellulose nitrate, and microporous filters composed of two mixed materials. It shall not be corroded by dilute acid, alkali and non-polar liquid, but the service temperature shall not be higher than 75 ℃. Teflon filter element is made of pure Teflon and is suitable for organic solvents, strong acids and strong alkalis. When used at - 100~260 ℃, it also has chemical inertness and stability. The polyvinyl chloride filter element has high strength, strong toughness, resistance to medium strength acid and alkali, and the use temperature should not be higher than 65 ℃, otherwise it will soften and toxic, so it should be used carefully in the food and pharmaceutical industry.
The filter element of this filter material, whether liquid or gas, can remove 0.5 μ For fine dust particles above M, the working pressure is generally 0.8~1.0MPa.
One of the advantages of this filter element is that when the gas passes through, the filter element often carries static charge, which can prevent the filter element from being blocked due to the small suspended particles invading the hole depth, and make the dust deposit on the surface of the filter element in a loose state, which is easy to discharge and clean.
2.3 Hollow fiber combined filter element
Because the distribution of micropores is much larger than the density of ordinary filter membranes, the number of micropores per square centimeter of filter area is considerable, which means that the maximum number of micropores is reduced. The maximum pore diameter of ordinary membrane filter is 0.3 μ m. 0.1 for hollow fiber μ m。 This also means long service life of hollow fiber and safe gas filtration. Although the hollow fiber membrane is made of 100% polypropylene, it has good water permeability, so it is sometimes used as a part of the dryer for gas drying and removal of moisture in the gas. However, this filter element has defects. First of all, it is complex to process. Second, it can withstand continuous flow in a hollow state. However, due to the sudden change of working conditions, the hollow fiber membrane often breaks, resulting in filter failure. In addition, it is also difficult to ensure the tightness and strength of the connection between the multi-bundle hollow fiber and the fixed pattern plate. Of course, these are also being improved and overcome, so it is an excellent filtering device
2.4 Powder metallurgy porous filter material
The filter material is porous metal or porous alloy made by powder metallurgy process. Because of its porosity, it has not only the filtration characteristics of ordinary porous materials, but also all the properties of metals. It is the main category of current filter materials.
2.4.1 Characteristics of powder metallurgy porous materials
Powder metallurgy porous materials have excellent permeability and are suitable for filtration, fluid uniform distribution and permeation devices. When used as a filter, the filtration rate is high. For example, when the filter made of sponge titanium powder sintered material is filtered with zinc sulfate solution for electrolysis, the filtration rate is 6 times that of ceramic filter; The flow rate of the filter made of sintered raw steel powder can be 4 times that of the paper pleated filter and 6 times that of the cotton yarn filter. However, the filtering area of wrinkles and cotton yarn is much larger than that of powder metallurgy materials.
As a filtering material, it can control the pore size and porosity, and has high filtering accuracy. When used for gas separation, the separation effect is good. When used for instruments and meters, the fluid flow can be accurately controlled. For example, the familiar oil circuit system of centrifugal compressor uses 40 μ M stainless steel powder metallurgy filter material replaces the previous metal mesh filter material. Because it effectively prevents the invasion of solid particles, the service life of the centrifuge bearing can be increased to 10-20 years, and the filtration efficiency is more than 3 times that of the screen filter.
Large specific surface area can improve the heat exchange effect when used as heat exchange material.
It can absorb energy, so it can be used as muffler, shockproof material and buffer material for dryer exhaust.
Maintain certain metal and alloy properties, such as heat conduction, electric conduction, welding processing, etc. Because of its strength and toughness, it can work under high pressure.
2.4.2 Specific application
The application fields of powder metallurgy filter materials are extremely wide and are expanding:
Filter solid particles in the liquid and separate various media. In atomic energy technology, filter aircraft, tanks, ships, etc（ μ m) Liquid fuel, oil and lubricating materials, filtered liquid metal sodium and lithium need fine purification (5~10). Neutron emission devices, as well as medicine and hygiene, are used to filter viruses and bacteria. During the preparation of penicillin and streptomycin, it is used to filter and produce sterile air, and separate some antibacterial crystals, such as penicillin and streptomycin, from the mother liquor. Separate the liquid in the gas, such as water and oil mixed in compressed air.
Filtered gas is used to collect dust, drain water and control gas pressure. For example, it is used for gas purification of precision instruments and meters. Recovery of radioactive dust; The blast furnace exhaust dust collector can also be used as a buffer device for high-pressure oxygen, nitrogen, hydrogen, air and other gases.
The gas and liquid passing through the material become uniform, and the liquid boiling phenomenon after passing through the material and the liquid backflow phenomenon when stopping the gas can be prevented.
Some factories in our department use this material to purify compressed air, and some antibiotic pharmaceutical factories use it to clean 0.5~1% of the air μ M dust and bacteria make it a dust-free and sterile compressed air, achieving ideal results.
