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Fig. 5.26: Filtrations mechanism of deep-bed filters | Microfilters are used when high quality compressed air is required. They deliver technically oil-free compressed air. Microfilters reduce the residual oil content of compressed air to 0,01 mg/m³. They filter out dirt particles with a separation rate of 99,9999 % relative to 0,01 µm.
Operating principle
Microfilters, also called high-performance filters, are deep-bed filters. The filter the water and oil condensate phase from the compressed air in the form of fine and ultra-fine droplets.
The deep-bed filter is a fibrous web consisting of a tangle of very fine individual fibres. The fibres are randomly intertwined and thus form a porous structure. Between the fibres there is a labyrinth-like system of passages and openings. This system has flow channels that are sometimes much larger than the particles than the particles to be filtered. Filtration occurs along the entire path travelled by the compressed air on its way through the filter element.
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Fig. 5.27: Pleated and wound filter material | Microfilters work with pleated filter material. This enlarges the effective filter surface by approx. 1/3 in comparison to wound filters. The pressure drop Dp is also considerably reduced. There are several advantages in this:
- Increased through-flow rate.
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Fig. 5.28: BOGE-Microfilter, Series F | Air passes through deep-bed filters from the inside towards the outside. The liquid phase from oil and water deposits on the fibrous web when passing through the filter. The flow of air then drives the condensate and growing droplets further on through the filter towards the outside. A part of the condensate leaves the filter element again as a result of this effect. Following the laws of gravity, the condensate collects in the collection chamber of the filter.
The working lives of the filters are longer because the condensate filtered out is no longer a burden to the element with this direction of flow.
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Fig. 5.29: Mechanisms of deep-bed filtration | Filter mechanisms
Three different mechanisms operate together to separate fine particles from the air.
- Direct contact.
Larger particles and droplets hit the fibres of the filter materials directly and are bound.
- Impact.
Particles and droplets hit the randomly arranged fibres of the filter material. There they bounce off, are directed out of the path of flow and are absorbed by the next fibre.
- Diffusion.
Small and ultra-fine particles coalesce in the field of flow
and following Brown’s law of molecular movement come
together to form ever-growing particles. These particles are
then filtered out.
Borosilicate fibre in the form of fibreglass layers is the most widespread material in high performance filters. It is used as a material for deep-bed filters. The following are also used:
Features
- Separation of oil in the liquid phase. Hydrocarbons are found in two aggregate conditions in compressed air:
- in gaseous form as oil gas.
- liquid in the form of droplets.
A high-performance filter removes almost 100% of the oil droplets. The oil gas can not be filtered out.
- Low operating temperatures. The efficiency of the filter drops when the operating temperature rises. Some of the oil droplets vaporise and go through the filter. With a rise in temperature from + 20° to
+ 30° C, 5 times as much oil passes through the filter.
- Recyclable. The materials used are chosen with ecological aspects in mind.
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