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Filtration is a solution to dealing
with external pollutants. It is almost universally used in conjunction
with mechanical ventilation systems associated with fully air-conditioned
buildings. In the European Union, filtration performance is governed by a
Standard ‘EU’ rating which categorises filtration performance by means
of the efficiency with which it can trap particles of varying size. The
classification system is presented in the Figure.
Typically, an EU3 filter would be used for pre-filtering,
coupled to an EU6 or EU7 main filter. This gives approximately 97%
efficiency down to 2.5 mm and
between 44% (EU6) and 55% (EU7) at 0.1mm.
Subject to good design and building airtightness, this filtration approach
is therefore potentially effective at reducing the higher end of
respirable particle concentration. To reduce fine particle concentration
(e.g. below 2.5 mm) by a greater
amount, however, high efficiency (HEPA) filters in the EU10-14 range must
be considered.
Gaseous pollutants must be controlled using gas
adsorption filters. In comparison to conventional filtration, gaseous and
HEPA filtration is expensive.
All filter assemblies suffer from pressure drop
(from a few 10's of Pascals up to, possibly, several hundred Pascals).
Generally, the more demanding the filtration requirement, the greater will
be the energy load needed to drive the filtration process.
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Typical filtration performance, European 'EU'
specification
Filter Specification: United States
In the United States, filtration is
covered by ASHRAE Standard 52.2-1999. This classifies performance by
particle removal efficiency using a standard Minimum Efficiency Reporting
Value (MERV). There are a total of 16 performance ranges covering
efficiency in three particle size ranges (i.e. range 1: 0.3-1.0mm,
range 2: 1.0 - 3.0mm and range 3:
3.0 - 10.0mm). A MERV value of 1
covers the lowest performance filters with an efficiency of < 20% for
range 3 particles. A MERV value of 10, equates to a filter with a 50 - 65%
efficiency for rang 2 (>85% for range 3). A MERV value of 16 equates to
a filter with > 95% performance in all three ranges.
Despite the benefit of filters they
should not be seen as an excuse to accept poor outdoor air quality.
Filtration solutions are expensive and cannot be applied to the many
buildings that are naturally ventilated (since there is insufficient
driving pressure through natural air inlets) or excessively leaky (since
filtration systems will be bypassed by air infiltration).
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