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Filtration Table of Contents
HEPA Air Filter Supplier
By design, HEPA air filters will create resistance
to flow, and a high differential pressure drop will exist
across the filter. Most HEPA air filter manufacturers’
literature suggests replacement of HEPA air filters when the
resistance due to dust loading has reached 2 in.wg.
The advantages of operating to high pressure
drop must be weighed against initial costs
(higher-static-pressure fans, larger motors, heavier
ductwork), higher power costs, and less efficient fan
operation. The installed fan and motor must have sufficient
capacity to deliver the design airflow at the maximum
differential pressure under which the system will operate,
with the filters at maximum dirty-filter pressure drop prior
to change. Therefore, consideration must not only be given to
the increased installed capacity required to operate to the
higher pressure drop, but also to the fact that the fan
operates at a penalty much of the time to provide the
required airflow over the wide span of pressure drop between
installation and replacement of filters.
The cost of ductwork, on the other hand, may
not be significantly affected by operation to a high pressure
drop because there is a minimum sheet-metal thickness for
effective welding, regardless of pressure. The cost of fans
and motors is a function of the maximum total pressure that
must be developed. Fan horsepower can be estimated from the
following equations.
Motor horsepower can be estimated from the
equation:
Annual power costs can be estimated from the
following equation:
Although investment and power costs will be
lower for systems operated to 2-in.wg pressure drop, the
total annual cost of owning a system, including materials and
labor costs for filter replacement, may be less for a system
in which HEPA filters are replaced at pressure drops on the
order of 4 in.wg. Total savings for the facility as a whole
may be even greater when the reduced interruption of building
or facility operations, such as clean rooms, due to the
reduced frequency of filter change is taken into
consideration.
Some pre-filters can be operated to higher
pressure drops than recommended by their manufacturers (but
such overuse must be supported by operating experience). This
results in less frequent pre-filter changes than when
pre-filters are changed at a pressure drop of only two or
three times the clean-filter pressure drop, as recommended by
most manufacturers. Care must be taken in selecting
pre-filters. Because of the many types, efficiencies,
configurations, and constructions available, the designer
must specifically investigate the safe overpressure allowance
for the particular model under consideration. Some benefit
could also have been obtained by installing a screen or
expanded metal grille on the downstream face of the
pre-filters against which the filter cores could bear; in any
event, screens or grilles would have prevented damage to the
HEPA filters when pieces of pre-filter struck them.
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