Biomedical Engineering Reference
In-Depth Information
not effective for gases and vapors typical of
chemical attacks. These types of compounds
require adsorbent filters (i.e., activated carbon
or other sorbent-type media) and result
the pressure loss of the filters. While higher
efficiency filters tend to have a higher life
cycle cost than lower efficiency filters, this
is not always the case. With some higher
efficiency filter systems, higher acquisition
and energy costs can be offset by longer
filter life and a reduced labor cost for filter
replacements. Also, improved filtration gener-
ally keeps heating and cooling coils cleaner
and, thus, may reduce energy costs through
improvements in heat transfer efficiency.
However, when high-efficiency particulate air
(HEPA) filters and/or activated carbon adsor-
bers are used, the overall costs will generally
increase substantially.
3.
Ducted and non-ducted return air systems
.
Ducted returns offer limited access points to
introduce a CBR agent. The return vents can
be placed in conspicuous locations, reducing
the risk of an agent being secretly intro-
duced into the return system. Non-ducted
return air systems commonly use hallways or
spaces above dropped ceilings as a return-air
path or plenum. CBR agents introduced at
any location above the dropped ceiling in a
ceiling plenum return system will most likely
migrate back to the HVAC unit and, without
highly efficient filtration for the particular
agent, redistribute to occupied areas. Build-
ings should be designed to minimize mixing
between air-handling zones, which can be
partially accomplished by limiting shared
returns. Where ducted returns are not feasible
or warranted, hold-down clips may be used
for accessible areas of dropped ceilings that
serve as the return plenum. This issue is
closely related to the isolation of lobbies and
mailrooms, as shared returns are a common
way for contaminants from these areas to
disperse into the rest of the building. These
modifications may be more feasible for new
building construction or
in
substantial initial and recurring costs.
Upgrading filtration is not as simple as
merely replacing a low-efficiency filter with a
higher efficiency one. Typically, higher effi-
ciency filters have a higher pressure loss,
which will result in some airflow reduction
through the system. The magnitude of the
reduction is dependent on the design and
capacity of the HVAC system. If the airflow
reduction is substantial, it may result in inad-
equate ventilation, reductions in heating and
cooling capacity, or potentially frozen coils.
To minimize pressure loss, deep pleated filters
or filter banks having a larger nominal inlet
area might be feasible alternatives, if space
allows. Also, high-pressure losses can some-
times be avoided by using prefilters or more
frequent filter change-outs. Pressure loss asso-
ciated with adsorbent filters can be even
greater.
The integrity of the HVAC system's filter
rack or frame system has a major impact upon
the installed filtration efficiency. Reducing
the leakage of unfiltered air around filters,
caused by a poor seal between the filter and
the frame, may be as important as increasing
filter efficiency. If filter bypass proves to
be significant, corrective actions will be
needed. Some high-efficiency filter systems
have better seals and frames constructed to
reduce bypass. During an upgrade to higher
efficiency filters, the HVAC and filtration
systems should be evaluated by a quali-
fied HVAC professional
to verify proper
performance.
While higher filtration efficiency is encour-
aged and should provide indoor air quality
benefits beyond an increased protection from
CBR terrorist events, the overall cost of
filtration should be evaluated. Filtration costs
include the periodic cost of the filter media,
the labor cost to remove and replace filters,
and the fan energy cost required to overcome
those undergoing
major renovation.
4.
Low-leakage, fast-acting dampers
. Rapid
response, such as shutting down an HVAC
system, may also involve closing various
