Page 3 - Vira Filter Digital Catalouge

INTRODUCTION TO
AIR FILTRATION

The term ‘air filtration’ refers to all applications in which contaminants
are removed from an air flow. Having a filtering system, and
therefore obtaining cleaner air, is useful for more than just
comfort in homes, offices and hotels; there are other reasons
for filtering the air.
Technology increasingly requires rooms or work areas free of
dust, smoke and odours, making it necessary to filter air in
numerous industrial activities.
Pure air is essential in operating the theatres, pharmaceutical,
laboratories, cleanrooms for electronics, data processing centers,
museums, libraries, food industries and public facilities.
PRINCIPAL FEATURES
Before selecting a filter, it is important to analyse the following points:
efficiency, dust accumulation capaticy, pressure drop, test methods
Efficiency: This is the most important factor when selecting an air
filter: measurement of the quantity of contaminant that the filter
manages to remove from the air flow. It is expressed as a percentage
and determined using various testing methods, described below.
Preessure Drop: This is the resistance the filter offers to the air flow,
measured in water column millimetres or pascals (Pa). The value
varies from filter to, depending on the efficiency.
Dust Accumulation Capacity: This characteristic indicates how
much dust a filter can collect in the course of its lifetime, before it
must replaced. It is another important factor in evaluating a filter.

TEST METHODS
The following test methods may be used to determine the efficiency
of various stages in filtering:
Gravimetric Method (sec.AFI, ASHRE 52/76):
A know quantity of synthetic dust is injected into the air flow passing
through the filter to be tested. Downstream of the filter being tested
is an absolute filter. The increase in the absolute filter’s weight
indicates the quantity of dust that has passed through the test filter,
which may be subtracted to calculate the quantity of dust stopped.
D.O.P (DESH, DOS) Optic Method: A number of aerosols contaning
particles of uniform diameter measuring 0.30 microns are used as
an air flow. The difference in the concentrations of these aerosols
upstream and downstream of the filter to be tested, meansured by
a photometer, determines efficiency.
Colorimetric, Atmospheric Method (sec. AFI-DUST SPOT,
ASHRE 52/76): A colorimeter is used to analyse the colouring of
two filter paper probes placed in a flow of atmospheric air, one
downstream of it. Efficieny is calculated on the basis of the ratio
between the volumes of air required to obtain the same colour in
the two probes.
Sodium Flame (Na CI) Optic Method: Dehydration of a 2% sodium
chloride solution in water provides the test aerosol.Un like in the
D.O.P. method, the size of the particles is not uniform, but varies
from 0.1 to 1. 7 microns. Efficiency is determined by using a
photometer to measure the difference in the intensity of the colour
of the hydrogen flame in contact with the aerosol upstream and
downstream of the test filter.
CURRENT CLASSIFICATIONS
The test methods listed above and applicable international standards
have been adopted by the European organisations EUROVENT,
which have established the following:
Classifications for Standardisation:
EN 779-2012 classifies filters into two groups on the basis of
efficiency: Grade G (efficiency<20% ) Grade M & F (efficiency from
40% to 98 %).

Better Air Every Where