6 types of compressed air filters

Compressed air contains various types of impurities, which may include oil, oil mist, sludge, water, water mist, particulate dust, micro-organisms, odours and other impurities in different forms, including granular, vapour and solid forms. In theory, one filter can achieve "full coverage" of filtration, but from the point of view of design, application and economy, it is not possible to achieve "full coverage" of filtration with one filter, for this reason, we need to classify different forms and types of impurities according to For this reason, we need to classify the different forms and types of impurities and design different types of filters to meet the actual needs according to the different application scenarios (the level of compressed air quality required). Otherwise, not only will the filter fail to perform its function, but it will also cause greater losses.

The impurities contained in compressed air can be divided into four categories, i.e. solid dust, moisture, oil mist and odour, for which the filters can also be divided into four main types, each of which is divided into several levels (e.g. Q, P, S, H) according to different filtering accuracy, thus forming a complete spectrum of compressed air filter products.

I. General-purpose filter

General purpose filter is suitable for general dust removal in pneumatic pipeline, suitable for removing solid particles above 5μm, its structure is shown in Figure 1.

The filtering process is that the compressed air enters the filter and is guided by the curved guide plate to do spiral operation, the centrifugal force generated will contain large solid particles and liquid water droplets thrown outward, hit the inner wall of the water collection cup, and leap down under the action of gravity and fall into the water collection cup over the baffle plate, the baffle plate will separate the dirty liquid and prevent the gasping air from bringing the dirty liquid downstream twice, then the compressed air passes through the filter element from outside to inside, and the filter will remove the fine solid particles. Then the compressed air passes through the cartridge from outside to inside, filtering out the fine solid particles, and finally the clean air is discharged from the centre of the cartridge through the filter outlet.

General cylinders, pneumatic instruments, pneumatic sensors, pneumatic gauges and pneumatic bearings for general use are suitable for general purpose filters.

Second, ash and water filter

Ash and water filter, also known as "pre-filter", is a pre-stage purification equipment in the compressed air post-treatment system. Filtration precision in 10 ~ 3μm, filtration efficiency ≥ 99%. As a pre-filter, the incoming air is saturated and condensation will occur inside the filter when the ambient temperature drops slightly.

In order to adapt to the work characteristics of high load, most of these filters use two-stage filtration, that is, after the compressed air enters the filter, the first encounter can produce inertia separation device, in the mechanical force (centrifugal, collision, etc.) to make the air in the larger particle size of dust, water droplets and air separation, and then flow into the water-repellent treatment of fluffy fiber main filter layer, continue to remove the remaining smaller particle size of impurities in the air.

Third, the main line filter

The main line filter plays a major role in the purification of compressed air, according to the design of the structure used, filter materials and different mechanisms, filtering precision in ~ 1μm, filtering efficiency ≥ 99%.

1, it can effectively filter out solid micro-particles larger than 1μm and liquid water mist in the air, and has a certain ability to remove oil mist. The compressed air filtered by it can be used directly in the pneumatic circuit of cylinders, reversing valves, wind tools, measuring instruments and various machines.

2, due to the compressed air entering the main road filter contains a large number of particle size between 0.1μm ~ 0.5μm micro-particles, because their penetration rate is the highest (according to most current theories), in order to improve its filtering efficiency, the cartridge material mostly uses high-efficiency fiber filter paper type material, such as glass fiber filter paper.

3, its filtration mechanism is based on inertial impact and interception and retention, the diffusion mechanism of oil mist particles with particle size less than 0.1μm plays a major role, but the filtration speed cannot be too high.

Four, in addition to oil mist filter

Compressed air contained in the oil impurities, except for a few larger particle size tar can be in charge of the road filter, most of the particle size of 0.01μm ~ 1.0μm aerosol and ultra-fine suspended droplets, ordinary forms of filters can not remove this type of ultra-fine oil mist aerosol. For this reason, these oil mist filters are made into a "coalescing" structure, i.e. a "coalescing filter".

In a coalescing filter, the compressed air must flow from the inside of the cartridge to the outside, and the cartridge material is so tight that it is able to rely not only on the narrow pores to capture the tiny particles, but also to capture the liquid and suspended particles that it has access to, which are smaller than the actual size of the airflow channel of the cartridge. The main object of filtration is oil mist particles with a particle size of less than 0.1μm, the filtration mechanism is based on diffusion and sedimentation, and the design filtration speed cannot be too high.

The filtration process is that the compressed air enters the internal space of the coalescing filter, and due to the sudden expansion of the volume, the airflow slows down, forming a laminar flow from the inside to the outside flowing through the filter layer of the cylindrical cartridge. During the outflow through the filter layer, the airborne particles are captured by a variety of different mechanical effects and led by the airflow into the foam outer layer.

The oil mist particles collide and coalesce with each other when passing through the foam layer and grow into liquid oil droplets of larger particle size, which settle to the bottom of the filter along the outer foam layer under the action of gravity and are then discharged from the drain.

V. Adsorption filter

(also known as deodorization filter)

The structure of the adsorption filter cartridge is basically the same as that of the oil mist filter cartridge, but it is used with an activated carbon cartridge, which can effectively remove the odour (organic chemical particles with a particle size of 0.002 to 0.003μm), certain bacteria and extremely small oil mist particles from the compressed air, so that the compressed air is purified for use in medicine and food, beer brewing, cosmetics production and diving, breathing air, etc. scenarios. Its filter cartridge precision reaches 0.01 to 0.001μm, with a filtration efficiency of 99.9999%.

VI. Sterilisation filter

Compressed air is also widely used in the processes of pharmaceutical and food industries, such as fermentation. Because compressed air contains a variety of microorganisms (generally attached to the dust, mostly in 5 ~ 20μm), as harmful bacteria in favourable conditions, its reproductive capacity is amazing, if not effectively filtered, let it into such processes, will bring unforeseen harm. For this reason, sterilisation filters use regularly sterilised media to intercept the micro-organisms contained in the air in order to obtain sterile air. At the same time, their construction is different.

1, the sterilisation filter must reduce the penetration rate by increasing the thickness of the filter layer, but the resistance will rise and the pressure loss will increase, therefore, the filter speed must be very low, generally below 0.1m/s.

2, the sterilisation filter must be able to withstand repeated sterilisation of high temperature steam close to 200°C, then its filter material must have a high moisture strength and high temperature resistance.

3, because completely sterile, then the sterilization filter in addition to the filter material has requirements, other auxiliary materials such as sealant, binder, etc., also to meet the same high temperature resistance, high moisture strength, not easy to ageing and other requirements. At the same time, the housing etc. must be stainless steel.

Note that the sterilisation filter must be certified by the medical and health, food and drug authorities, and that regular disinfection is strictly enforced in the course of use.

Lingyu ~ 1.0 0.01 micron precision moisture removal compressed air filter