Analysis of compressed air filter design points and methods

In the compressed air system, although the economic cost of the compressed air filter is not high, the cost of its proportion of the total cost of the system is very small, but its role is not to be ignored. It is equivalent to the human body's "liver", although not as familiar as the brain, the five senses, the heart, but it is the largest detoxification organ in the human body, the body produced by the toxins, waste, eaten poison, health-damaging drugs and so on must rely on the liver detoxification. The role of the filter is to detoxify the "toxins" contained in the compressed air - water mist, oil mist, microscopic dust particles and micro-organisms, etc., so that clean compressed air can be used in pneumatic equipment, instruments, production processes, and even as a human breathing air source (in the medical industry).

　　Different industries and applications have different requirements for the quality of compressed air, so how do you design a compressed air filter from the point of view of performance, price and technical rationality? We briefly analyse the choice of filter material, the calculation of the filter area, the determination of the filter speed and the choice of filter housing.

　　First, the choice of filter material

　　Compressed air contains a variety of impurities, different gas scenes on the quality of the compressed air used requirements are also different, and with the development of science and technology filter material is also diverse, each has its own characteristics and features. It is impossible to have a filter material that can meet the effective filtration of all impurities, so when choosing a filter material, we should try to consider the following two basic requirements:

　　1. The filtration precision and filtration efficiency of the actual use occasion

　　The filtration precision and mechanical properties of different filter materials are different, and the filtration efficiency of impurities for different filtration objects is very different. For example: there are suitable for filtering impurities of size ≧ 5μm powder metallurgy filter material and ceramic filter material; there are also suitable for filtering impurities of size <5μm, filtering precision ≧ 99% of the fiber filter material. But when the object of filtration for oil mist, the above two filter materials are not suitable for selection, because the size of the oil mist is usually <0.1μm, then you can not use the fluffy fiber filter material to do cartridge materials, and should be used to do the high nature of the glass fiber filter paper cartridge. And to further improve the comprehensive performance, the cartridge is generally made into a composite structure, that is, a variety of filter materials are used together with the structure according to the scientific design.

2、Economy

　　For occasions where the quality of compressed air is not so demanding, it is not necessary to choose high-performance filter media. If you choose, it will not only cause economic waste, but also the filtration effect may not be good. For example: high precision filters in dusty occasions, it is easy to clog, and in technical terms should not use such filter material.

　　Second, the calculation of the filter area

　　The filter area is the effective area of the filter material perpendicular to the direction of airflow. The filter speed is an important parameter when designing the filter, and it is the main basis for deciding the filter area of the cartridge.

Three, the principle of determining the filter speed

When determining the filtration speed of fiber-like filter materials (including filter paper), the velocity characteristics of the filter material should be considered, i.e. the relationship between the filtration efficiency and the filtration speed. Diffusion effect decreases with the increase in filtration speed, inertia impact effect increases with the increase in filtration speed, while other effects basically have little to do with the filtration speed. When the object of filtration dp ≥ 0.3μm, inertial impact plays a major role, the design of the filter speed C should be greater than the calculated C0 as far as possible. df can be seen in the formula and C0 proportional relationship, then the fiber diameter should not be too large; when the object of filtration dp for 0.01μm ~ 0.3μm fine particles (such as oil mist particles), diffusion deposition mechanism plays a major role, the design of the filter speed C should be as far as possible, less than the calculated C0. can be seen in the formula, df and C0 mentioned in a proportional relationship, then the fiber diameter should not be too small.

　　Fourth, the filter housing

　　Compressed air filter needs to withstand the pressure of compressed air, generally made of tubular structure, the main body of the filter - the filter element is sealed in the shell. As the pressure drop on the filter element is negligible compared to the pressure of the air being treated, the filter housing is subjected to the full pressure of compressed air. The shell thus belongs to the pressure vessel and must have the pressure vessel production qualification to produce.

　　1、Shell material. General metal materials, small filter shells are commonly used in high-strength zinc-aluminum alloy die-casting; high flow filter shells are mostly made of boiler steel plate welded with good pressure-bearing properties. With the development of technology and changes in demand, now also useful stainless steel materials manufactured from. Regardless of what materials are used, the finished shell must be tested for sealing and hydrostatic test.

2、Shell structure. As in Figure 2, generally composed of two parts, the upper part is the "filter cover", the lower part is the "filter cartridge". There is a compressed air inlet and outlet channel on the filter cover (inlet and outlet, the same calibre as the external pipe diameter), and the filter element (as shown in Figure 7 on the right) is also suspended and fixed on the filter cover. After the compressed air enters the filter, it can only be discharged from the exhaust port through the cartridge. For this reason, there can be no leakage points between the inlet and outlet, otherwise the compressed air will be short-circuited, thus not playing a filtering role.

　　The role of the filter cartridge is, firstly, to seal and restrict the compressed air to flow within the filter housing and in the prescribed direction; secondly, to accept the filtered water, oil and other filtrate (in order to exclude the filtrate dirt in time, there is a manual or automatic drainage device at the bottom of the filter cartridge). Generally, the diameter of the filter cartridge is twice as large as the diameter of the filter element, and the length of the filter cartridge should be about 1.25 to 1.5 times longer than that of the filter element to ensure that the filter element is not submerged by the filtrate.

　　The connection between the filter and the external pipeline is generally threaded for small flows and flanged for large flows.

　　3. The structure of filters with large flow rates is somewhat special, and sometimes several to dozens of filter elements are installed in the filter cartridge, at which point the arrangement of the filter elements becomes very important. It is usually required that any three adjacent cartridges should be arranged in an equilateral triangle, while the centre distance between two adjacent cartridges should be not less than 1.25 times the diameter of the cartridge, in order to maintain sufficient circulation space between the cartridges.

Lingyu Machinery - coalescing compressed air filter