Types, Filtration Engineering Ltd, Air Filtration, Air Quality Control Systems, UK

If we restrict ourselves to discussing air filters suitable for office building HVAC systems, then we must examine filters which are efficient at removing the very small "staining effect" particulate from the air. These filters are invariably of the mechanical media collection type, using either boro-silicate or synthetic fibres to capture and retain the small particles. Pre-filters may be fitted to control the effect of large coarse dust and extend secondary filter life. However, today many building air filtration systems operate without pre-filters.

Supply air ducts will remain clean for many years when using secondary filters. However, return air ducts can present a problem with dust build up, where quantities of larger dusts and debris such as hair, fibres and paper dust generated in the building must be removed, either with primary filters or the secondary filters before entering the HVAC equipment with the outside air quantity.

The design of secondary air filters is always of the extended media type with up to twelve pockets per 600x600 filter unit and approximately 10m² of media area. Over the past decade the rigid media or "rigid bag" filter has gained in acceptance, due to their superior initial and average efficiency ratings, rigid "safe" design and low operating pressure drop. All this adds up to extended service life. The only drawback seen by operators is the 'up-front' high price of rigid cell filters versus the pocket filter alternative. As prices come down we will see a much wider acceptance of this type of filter, where media area can be as high as 24m² per filter cell, 300mm deep. Pre-filter selection of minimum G4 is an advantage with rigid cell filters.

Gas phase filtration, if deemed necessary, would be located between a good quality pre-filter and the (minimum) F7 grade secondary filter. To protect the expensive gas phase filtration it is advisable to install primary pocket type filters of F5 grade for long service life with low pressure drop. The effective removal of odorous contamination can only be achieved by using sufficient media of the correct activity held in a well designed and leak free filter bed. The "dwell time" of the gas within the filter bed is critical for filter performance.

A critical point to consider and often overlooked is the use of a good filter framework system. A leaking framework will allow dust to pass into the AHU and ductwork. A new filter to frame seal should be fitted at every filter change. Side access filter rails for secondary filters should be replaced in preference for a front access installation. However space may prove a restricting factor.

One type of filter not discussed so far is the electronic air cleaner. Few systems have been installed in building HVAC projects, and their main area of use today is as self contained units in office areas, especially where smoking is allowed. The design and application of electronic air cleaners is a subject for a further technical document. There are a number of operating concerns which require careful consideration when discussing the use of this type of air filter system.

What types of air filters are best suited for the removal of dusts and fumes?