Dust has been described as matter in the wrong place, and the object of air filters is to prevent as much dust as possible from entering spaces where it is not wanted by separating it from the air that is carrying it.

These dusts have to be separated from the air because of their effect on the health of persons occupying a room to which air is being supplied. Their effect on the operating efficiency of machines or appliances (e.g. computers), and on the appearance of valuable stock (jewellery); their nuisance potentialities (discharged dust spreading over adjoining goods or property); their economic value (collection for sale or re-use).

Types of Air Cleaners

The types of air cleaning equipment is common use are settling chambers, cyclones, filter panels of various types, electrostatic precipitators, carbon filters and air sterilisers for (for bacteria). The selection of equipment suitable for a particular job depends on the degree of fineness of the dust (particle size), the required efficiency of collection (the percentage of the finer particles to be removed), and the amount of dust to be collected.

Dust Sizes

The size of dust particles is measured in microns (1 micron = 1/1000 of millimetre, approx. 1.25000 of an inch). It is obvious therefore, that the smaller the particle to be collected, the finer must be the mesh of the filter, and therefore, for a given size of filter panel, the greater will be the resistance to airflow, and consequently, also the power required to push air through it.

With the comparatively low pressures and motor power of small axial fans (quiet and economical running), we must use fairly large filtering areas for the size of fan, and must limit the size of particle collected and the efficiency of collection to about 99% of dust down to 5 microns, which is ideal for normal ventilation work. Another advantage using larger filter panels is that they will give useful and effective service for a longer period before becoming loaded due to the larger area through which the dust-laden air can pass.

A rough idea of particle sizes are as follows:

Smoke	up to 1 micron
Bacteria	1 to 10 microns
Dust	1 to 1000 microns and over
Spores, moulds	3 to 50 microns
Pollens	9 to 80 microns
Fog	1 to 60 microns

Specialist ethnic cooking (Chinese Woks, etc.,) will require special treatment. The temperatures are very high and the volatile oils used will need special grease eliminators. Fans should be rated for a minimum 100°C ambient air temperature.

Fish and chip cabinet fryers usually have ducted extract provided by the cabinet manufacturers. The air/oil mixture can be an explosive mixture and require appropriately rated fan motors.