Filtration
Air carries with it
a large quantity of suspended particles including dust, fibers, bacteria, mould
and fungal spores, viruses and smoke. It also carries gaseous pollutants such
as benzene, NO2, SO2 and other odours emitted from vehicle exhausts. This is
especially so where buildings are sited in urban areas where there is typically
twenty times as much dust per cubic meter of air than is found in rural areas.
The particles in the airstream vary in size from the visible such as hair and ash
to the microscopic such as bacteria and viruses (see IP1: Air Filter Characteristics). It is essential that
air passing through an air handling unit is filtered to remove these
impurities. Inadequate removal of these particles leads to problems of poor air
quality and ill health. Dust particles accumulate on heating/cooling coils
reducing their effectiveness. Ductwork surfaces become coated in dust providing
a breeding ground for bacteria. Finally, the presence of dust in the airstream
leaving an air outlet causes unsightly dirty streaks on adjacent surfaces. In 1996
health and safety regulations concerning ventilation system maintenance and
cleaning came in to force requiring that ductwork be cleaned on a regular basis.
There are two
methods by which air can be filtered these are mechanical filtration and
electrostatic filtration. Gases and vapours are removed from the airstream air using
the process of adsorption.
Mechanical Filters
Mechanical
filtration involves passing the airstream through a porous material known as
the filter media. The materials are usually fabrics, glass fiber, non-woven synthetic
materials or paper. Each is held across the airstream by a supportive framework.
The capture process involves three mechanisms; Direct interception, inertial
impaction and diffusion. These involve either directly stopping large particle
as a result of a single collision or by gradually slowing down smaller
particles by multiple collisions with successive fibers. Eventually the small
particle loses energy and comes to rest. A fourth capture mechanism for some filters
is to give the fibers an electrical charge during manufacture. This will attract
dust out of the airstream but the effectiveness will reduce with time as the
charge is lost.
Since filters are
required to be changed regularly it makes sense to construct them in a robust
and effective way but cheap enough to be disposed of at the end of their life.
It also makes sense to utilize the grades of filters in such a way as to extend
the life of the most expensive filters for as long as possible. The effectiveness
of lower grade (G2-G4) (see IP1: Air Filter Characteristics) pad and panel filters can be enhanced by
using a filter media of graduated density. This means that the back of the filter
will have smaller pores than the front, and therefore will be able to capture a
broader range of particulate sizes.
Pad Filters are used as the first bank of filters to
prevent large particles from entering the system. A single flat sheet of
material, they are held in a card or re-usable steel or lightweight aluminum
frame. They protect the higher grade, and hence more expensive filters, next in
line. They are normally referred to as Primary Grade Filters ranging from G2 to
G4 in classification.
Panel Filters. As shown in figure, the filter media is folded into pleats.
This extends the media surface area when compared to the
flat pad of material in a pad filter. The filter media is
backed by an open strengthening grid and is then
sandwiched into a card, wire-mesh or plastic frame. This
panel can be easily slid into position in a metal holding
frame in the air handling unit (AHU). They are
particularly suited to AHU's which have insufficient
depth to accommodate a bag filter (see later). When
the filter requires changing it is
removed, disposed of
according to regulations, and a replacement inserted
in the same manner. Some filters of this type are
available as primary filters (usually G4) but more commonly
are used for grades F5-F8 and as a secondary filter.
Anti-Microbial Filters. This is a panel filter surface sprayed or impregnated with biocides. The biocides used fall into one of two categories: inhibitor or eradicator. Inhibitors simply prevent the microorganism from reproducing an eradicator kills it completely. The range of micro-organisms against which the biocide will be effective includes various types of bacteria, algae and yeasts. Anti-microbial filters are most commonly used in hygiene sensitive areas such as hospitals or food processing outlets. They are however being increasingly used in office environments to improve general indoor air quality and health. All grades of filter are available from G2 to F9.
Bag Filters have a filter medium which is formed into a bag and held in
place by a metal or plastic frame (as shown in figure).
The seams are well sealed and it is mounted so that the
open end of the bag faces the oncoming airflow. Providing
that the bag has sufficient depth this form greatly extends
the surface area over which filtration can take place and
as a result bag filters have a long life, a high
dust carrying capacity and offer a low resistance to
airflow. To minimize the risk of sagging or collapse, some bag
filters are manufactured with spacers to help the
individual pockets remain open even at reduced
airflows. Bag filters are usually available
in grades from G4 to F9.
High
Efficiency Particulate Air (HEPA) Filters are panel filters with extremely fine filter media with collection
efficiencies ranging from 99.95 to 99.999%. To extend the life of a HEPA filter
it must be used in conjunction with at least one pre-filter. HEPA filters may
be included in the main area of the AHU or only at the air inlet grilles serving
those rooms which require the cleanest air. HEPA filters are used where a very clean
environment is required such as in microelectronics and pharmaceutical
manufacturing and storage areas for documents and artifacts. An even higher
grade of filter, the Ultra-Low Penetrating Air (ULPA) filter is used in environments
where ultra clean air is required such as the nuclear or space industries.
