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The types of filters that are applicable to the HVAC sector generally are :
• Primary filters . These are your first line of defence and is typically a consumable that is thrown away after being cleaned or washed a couple of times . Primary filters only stop coarse or bigger particles such as leaves or feathers .
• Secondary filters . These come in either a pocket or a cassette design . º Pocket filters are normally used in shopping malls , office buildings and data centre applications . They reach a point of particle saturation , and generally cannot be cleaned as the material deterorates . º Cassette filters have a much better dust holding capacity than the pocket filter . Cassette filters can ’ t be cleaned and must be thrown away when expired .
• HEPA filters . These filters are typically used in most medical applications , clean rooms and production facilities where pure air is required such as food processing and packaging .
• ULPA filters . These filters come from the same line of materials to make HEPA filters and as the manufacturer tests the material the sections that are particularly good are allocated to the ULPA range . These are very expensive products .
• Carbon filters . Carbon filters are capable of handling fumes , smells and gases , and come in two different types : a cassette-type filter whereby the media has been impregnated with carbon allowing this type to be used in a variety of applications . The second type , and for more specialised applications , carbon filters also come in modules that are essentially filled with carbon pellets that absorb all of the odours , gases or chemical to a point of saturation and are then disposed of through incineration . º Carbon can be blended to suit particular applications in removing gases , odours or acids , which would be different to normal air ‘ smells ’. This blend can then be impregnated in the filter media .
All filters come in various configurations , sizes , airflow ranges , pressure capacities , particle size capabilities and efficiency ratings . They are also produced in a variety of materials such as galvanised sheet metal , plastic , aluminium and wood ( MDF ). Custom sized products are also available to be manufactured for special applications .
“ Sizes of filters in a design are determined by the amount of air pushed through the system . One filter can only handle so much air at a time before it bursts . Generally one full-sized filter ( 600 x 600 ) can handle 1 cubic meter of air per second . Obviously , a 1m diameter fan with a small fiter cannot work and will pull the filter apart . Incorrect sizing can result in major
Doublewin Co Ltd Mann and Hummel
A HEPA pleating machine used in filter manufacturing .
An image showing the pleating process of the glass fibre material .
damage to the filters , equipment and AHUs , especially when you are working powerful fans . This element is particularly important because if you spec the wrong product for the system , you pay the price with a destroyed system ,” says Rean van Wyk , managing director at Airbox .
What is known to the South African market are filters that are classed according to EN779 standards . EN779 is the ASHRAE standard establish whereby dust was taken out of Las Vegas , in America and analysed . This analysis established how much of each type of particulate was in the dust , but in other countries the dust composition is vastly different .
“ A new ISO standard on filter selection was established – ISO1689 . The major difference is that we are now no longer looking at a particular dust composition , but rather working on the principle that a particular filter must stop a particular size of particle , looking at the fractional efficiency of the filter . This is broken down into four categories – Course , 10 , 2.5 , and 1 . As an example an ISO-1 90 % rated product needs to filter 90 % of all possible air compositions with particles of 1micron or less . The new standard was implemented as of 2018 but has not been well used and quoted to-date ,” says Verster .
ALL ABOUT HEPA FILTERS The idea of any application using HEPA filters is to protect this filter type because they are an expensive product . HEPA filters have typically only been broadly used in the medical , chemical
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www . hvacronline . co . za RACA Journal I November 2020 15