A porous material through which a fluid stream is passed for removal of solid particles is called a filter.

Types of air filters:

(a) Cell types:

Filtering medium is thrown away when dirty and a clean medium is in­stalled. e. g. spun glass mats contained in card board frames.

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(b) Cell type cleanable type:

Medium is cleaned cell by cell and reused. Metal mesh included in a metal frame which washed when dirty and recharged with a sticky surface by dipping in suitable oil.

(c) Continuous cleaning type:

A series of over lapping perforated plates moving as a belt over upper & lower drums. The lower drum is mounted in a tank of oil. Each plate is returned with a snap into the oil bath where accumulated dust is washed away.

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(d) Electrostatic filter:

Dust particles are charged electrically and accu­mulated on a plate of opposite electric charge.

Electrostatic cleaners might seem to require more energy than the others, this may not be the case, and even passive dry filters use energy because they in­crease air friction in the duct, which in turn means that the fans must use addi­tional power to move the air.

Many applications use two layers of throw away cells in series to allow air flow.

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(e) Oil & water filters:

Maintenance:

(a) Some economy may be affected by replacement of the cells on the air leaving side with new cells and reuse of the cells from the leaving side on the entering side.

(b) Installations using cleanable oiled cells –

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Duty cells should be thoroughly washed with a strong solution of washing soda, allowed to dry, dripped in an oil bath and drained for several hours before reuse.

(c) The continuous cleaning filter requires routine removal of sludge from the oil basin, oiling operating motors annually and greasing of the chain drive.

The filter must be put into the operation at least 12 hours before the fan is started so that the filter gird will have to complete one circuit through the oil bath. The filter should not be turned off overnight.

(d) Electrostatic filters:

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Cleaning is usually done with a water hose.

(e) Oil & water filters:

(i) Multi plate type is equipped with a lever for rotating alternate plates to clear the strainer.

Filters should be checked weekly.

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The operating lever turned several revolutions to dislodge the collected dirt from the surface of the plates.

(ii) Felt cartridge type