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literature in equipment specifications sections, above in this column, & on
Indoor Air Cleaners
installation in an HVAC system.
Advantages of a central air cleaner system
over individual stand alone units include:
- better clean air distribution,
- less noise,
- keep central heating/cooling equipment clean and efficient
- ability to filter outside air before it enters
the occupied area,
- ability to place occupied space under positive pressure
to reduce infiltration of contaminants
- less maintenance than with
many portables around the facility,
- more cost effective than portables for larger areas
- Inline, In-duct.
The most direct way to install air filtration equipment in
a central HVAC system is to place the equipment directly in the return duct at
a common point where all the air is flowing back to the heating and cooling
When the air filtration equipment is directly in the duct at a common point
where all the air is flowing, the air must pass through the air filtration
equipment in order to return to the heating and cooling equipment and
therefore, all the circulated air is filtered.
High efficiency air filters have very high air resistance. As a
result, placing high efficiency filters, such as HEPA filters, directly in
the duct, imposes loading on the existing HVAC air handler which the air
handler may not be able to adequately overcome. For this reason, lower
efficiency, lower loading, ASHRAE type efficiency filters are generally used
in line, in duct, as they impose less loading on the existing air handler.
The use of high efficiency filters could result in the need for an air
handler upgrade and additional cost. An alternative to ASHRAE filters
is the use of electronic air cleaners. Electronic air cleaners
generally impose relatively low loads on the HVAC air handler making them
popular alternatives to high efficiency filters. Additionally, electronic
air cleaners require routine cleaning rather than filter replacement, which
is seen by some as an additional advantage. However, electronic
air cleaners will not provide equivalent efficiency as high efficiency
filters such as HEPA filters, as electronic air cleaners usually are rated
on a ASHRAE efficiency scale. NOTE: More information about
efficiency standards is available.
The By-Pass installation of air filtration equipment into
an HVAC system circumvents the HVAC air handler loading issue noted in the
inline-induct disadvantages above. In the By-Pass installation, the air
filtration equipment intake is connected to a port on the return duct at a
common point where all the air is flowing back to the heating and cooling
equipment. After being diverted into the air cleaning equipment and
filtered, the clean filtered air is then exhausted back into the HVAC system
return duct downstream (closer to the heating and cooling equipment). If
the air cleaning equipment is properly matched to the air flow in the HVAC
return duct, a large portion of the return air will pass through the air
cleaner and be filtered.
The By-Pass installation avoids the loading effect of air cleaning equipment
on the HVAC air handler and also avoids the expense of an HVAC air handler
Since air is being diverted from the return duct to the air cleaning
equipment, there remains a path for the air to return to the heating and
cooling equipment without being filtered. This is generally not an
issue unless the application is critical in nature where the air
contaminants must be removed on a single pass through the system.
Although only a portion of the return air is being filtered on each pass
around the facility, the air is becoming cleaner on each pass and eventually
will reach a steady state value of cleanliness after several passes.
Waiting several passes for the air to reach the desired level of cleanliness
is generally not an issue in most applications.
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