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• Adhesive filtering for removing dust and pollen etc . They are made of various fibres such as glass , cotton , synthetic material and aluminium . The fibres are coated with an adhesive liquid or oil . Air is forced to change direction and lose speed as it passes through the filter . This results in trapping particles such as dust and pollen , as they make contact with the adhesive surfaces of the filter .
• Electrostatic filtering for removing very small particles . This type of filtering electrically charges the particles to be removed and adheres them to a surface having an opposite charge . Electrostatic filters are normally used as secondary filters to screen or adhesive filtering to clean the air for computer rooms and the like .
• Ultraviolet lighting will kill most bacteria in a fraction of a second . The lamps are installed in the return air duct in such a way that the rays cover the full cross-section of the duct to make it effective .
• Filters made of activated carbon will remove solid particles as well as certain gases that cause bad smells . They will also remove a limited amount of bacteria .
• Paper filters are sometimes used to remove finer dust or pollen particles .
More than one type of filter is sometimes used on one airconditioning system . For example , the return air in a hospital may first pass through a primary screen type filter made of a fibrous material to remove large particles . The air then passes through a secondary paper or electrostatic filter to remove fine particles . Finally , at the end of the duct where the air enters the room , the air passes through ultraviolet lighting to kill bacteria .
Air filtration supplies the means to obtain the level of particulate cleanliness required by any definition of ‘ airconditioning ’. It extends from the simple task of preventing lint and other debris from blocking heating / cooling coils to remove particles as small as 0.1 micron which could cause a short circuit on a microchip .
In addition to the reasons given above , air filters are used for a wide variety of purposes , some of which include :
• Protecting the general well-being of the occupants of spaces or buildings .
• Protecting the decor of occupied spaces by removing the staining portion of airborne dust .
• Reducing maintenance of building interiors by reducing the frequency of cleaning such as Venetian blinds , fluorescent bulbs and furnishings and equipment .
• Protecting other contents of occupied spaces including paintings , tapestries and other items of historic value .
• Elimination of fire hazards by removing lint and other materials which might accumulate in ductwork .
• Extension of shelf life of perishable dairy products by removing airborne mold during processing operations .
• Removing airborne bacteria from operating rooms air to help prevent postoperative infection .
In the ideal world , outside ( fresh ) air is free of all dust and gases which could affect a space ' s occupants , decor or contents . In the real world , it is rarely deserving of the description ‘ fresh ’ and must be filtered to remove the contaminants it would bring inside .
Return air contains the particulate material , which was generated within the space plus that which was not removed from the supply air by the filters , which are part of the conditioning system .
Supply air , as delivered to the space , should be of the cleanliness necessary to achieve the objectives of the space . However , once this supply air reaches the diffuser , its utilisation depends on the air diffusion system . If this clean air is not distributed properly throughout the space , comfort may not be achieved .
David , I hope that this increases your understanding of air filtration .
Thanks to everybody for the overwhelming response . I receive an average of over 60 questions a month and cannot publish all of them . But keep them coming , as I may answer you directly . Looking forward to hearing from you . RACA
Grant Laidlaw
REFERENCES |
1 . |
SETA training |
2 . |
ASHRAE |
3 . |
ACRA |
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RACA Journal I January 2024 www . refrigerationandaircon . co . za