| In 2002, the European Committee for
Standardization, Technical Committee 195, Work Group 1 (CEN/TC195-WG1)
established a new standard for general ventilation filters. The
introduction of this standard under document name EN 779:2002 will
supersede the existing EN779:1993. Member countries of CEN* are obliged
to issue their own national version of this standard within the existing
framework of their own national standards organizations. The procedures
described in this standard have been developed from those given in
EN779:1993 and Eurovent 4/9:1997. The basic design of test rig given in
EN779:1993 is retained with the exception of the "dust-spot"
atmospheric aerosol opacity test equipment. Instead, a challenge aerosol
of DEHS (or equivalent) is dispersed evenly across the duct upstream of
the filter being tested. Representative upstream and downstream samples
are analysed by an optical particle counter (OPC) to provide filter
particle size efficiency data. Classification
The EN779:1993 classification system (comprising groups F and G filters)
has been retained; classification is now determined from the average
filtration efficiency with respect to liquid DEHS particles of 0,4 µm
diameter. Classification of F filters is based on performance with
respect to 0,4 µm particles because of practical evidence that the EN779:1993
classification based on the "dust-spot" opacity test is very
closely matched. Filters found to have an average efficiency value of
less then 40% will be allocated to group G and the efficiency reported
as "<40%".
The classification on G filters is based on their average arrestance with the loading dust.
Initiatives to address the potential problems of particle re-entrainment,
shedding and the in-service charge neutralisation characteristics of
certain types of media have been included in annexes A and B.
|
Certain types of filter media rely on electrostatic effects to achieve
high efficiencies at low resistance to air flow. Exposure to some types
of challenge, such as combustion particles in normal atmospheric air or
oil mist, may neutralise such charges with the result that filter
performance suffers. It is important that the users are aware of the
potential for performance degradation when loss of charge occurs. It is
also important that means be available for identifying cases where the
potential exists. The normative test procedure, described in annex A,
provides techniques for identifying this type of behaviour. This
procedure is used to determine whether the filter efficiency is
dependent on the electrostatic removal mechanism and to provide
quantitative information about the importance of the electrostatic
removal.
Filters are classified according to their efficiency (arrestance) under
the following test conditions:
- the air flow shall be 0,944 m3/s (3400m3/h) if the manufacturer does
not specify any rated air flow rate;
- 250 Pa maximum final pressure drop for Coarse (G) filters;
- 450 Pa maximum final pressure drop for Fine (F) filters.
If the filters are tested at 0,944 m3/s and at maximum final pressure
drops, they are classified according to the table below. For instance
G3, F7.
Filters tested at airflows and final pressure drops different from those
above shall be classified according to the table. However the
classification shall be qualified by test conditions in parentheses,
e.g. G4 (0,7m3/s, 200 Pa), F7 (1,25 m3/s). |
