A brief definition is:
– A method to remove airborne impurities from the air.
The two most common methods of air cleaning are particulate filtration, and electrostatic precipitation. Air Cleaning can be included as a component within a ventilation system, or can be a stand-alone item. The difference is that the latter has an integral fan, and own means of control, whereas if it where a component of a ventilation system the fan would provide the airflow across the air cleaner. The function of the air cleaner or filter is to reduce the concentration of solid particles and gases in the air-stream to a level that can be tolerated by the process, occupancy purpose. Solid particles in the air stream range in size from 0.01 micrometer to objects that can be caught by ordinary fly screens, such as lint, feathers, and insects. The particles generally include skin cells, hairs, dust, soil, lint, and smoke, but may include almost any other organic or inorganic material, even bacteria and mould spores.
This wide variety of airborne contaminants, added to the diversity of systems in which air cleaners are used, makes it impossible to have one type that is best for all applications. The level of air cleaning also varies depending on the application and various products can be more effective than others. The different grades of filter have been standardised, to make it easier to specify the level of filtration. Known as the EU Classification. The classification determines:-
- Air Volume.
- Initial Resistance.
- Final Resistance.
- Maximum Operation Temperature.
- Dust Holding Capacity.
There are also different types of filter:-
- Panel – Constructed from various filter media depending on application, most common type.
- Pleated Panel – increased surface area provides improved efficiency.
- Bag – Greater filtration efficiency and generally used with a disposable panel filter.
- Grease – Used in the catering industry in kitchen canopies.
- Carbon – Used in applications where odours/gases/smoke are present.
- HEPA – Absolute filtration, very high efficiency used with Panel and Bag Filters, usual found in Clean Environments.
- Electrostatic – Provide excellent filtration levels, can be used with pre-filters, or in series to increase the efficiency, good for high air volume applications, provide little airflow resistance in comparison to the HEPA type filters, this reduces the capacity of the fan required. The electrostatic cells can be easily removed, cleaned and recycled, whereas HEPA’s are disposable.
Some of these filters are used in combination to reduce the size of particulates prior to being introduced to the fine filter. The fine filters can be used in banks to increase the cleaning efficiency.