ASHRAE Journal - December 2023 - 39

TECHNICAL FEATURE
such as those serviced by mechanical ventilation.
The importance of maintaining healthy indoor air
quality (IAQ) within buildings is well established, with
building rating schemes such as the U.S.'s LEED, United
Kingdom's BREEAM and Australia's Green Star rating
systems acknowledging the importance of indoor
environmental quality. One method to improve IAQ is
through supplying a conditioned space with outdoor
air in excess of the minimum requirements (i.e., overventilation).
However, this may have a signifi cant
penalty on energy consumption, as the outdoor air is
typically conditioned when brought into the building
where the outdoor air is intemperate. Furthermore, this
may require larger fans and ductwork, which in turn
may signifi cantly increase capital cost, particularly for
larger buildings where long duct runs may be required.
Another method to improve IAQ is to use high effi ciency
fi ltration systems. A commonly used classifi cation for
high effi ciency fi lters is HEPA, which is defi ned slightly
differently for different standards (Online Table 1, http://
tinyurl.com/JournalExtras).4,5 In most cases, true HEPArated
fi lters should have a minimum particle collection
effi ciency of 99.95% (or 99.97% according to some
standards, e.g., U. S. Department of Energy6) for the most
penetrating particle size (see Background section).
HEPA fi lters are typically made of tightly woven,
random arrangement of fi bers or porous materials,
which allows them to capture small particles much
more effi ciently than conventional fi lters. However, the
disadvantage of using HEPA fi lters is that they introduce
a signifi cant pressure drop within the mechanical
ventilation system, which signifi cantly increases fan
energy consumption and potentially fan size and cost.
For this reason, HEPA fi lters are typically only used
for specifi c applications where the removal of PM is
paramount (e.g., intensive care units).
The new generation non-thermal plasma air
purifi cation devices typically rely on passing the
particle-laden airstream through a high intensity
electric fi eld generated by applying a high voltage
between discharge and ground electrodes, separated
by a small gap of dielectric material.7 The electric fi eld
can generate a large volume of high-speed, low-energy
electrons, which are then introduced into the airfl ow.
Recently this technique has been used to remove PM,
as the free electrons can increase the propensity of the
particles to agglomerate or to stick to other surfaces.8
This technology is similar to electrostatic precipitators
(ESPs), with the major difference being that in ESPs
particles are collected on collection plates within the
device, while plasma purifi cation systems typically use
porous media fi lters in conjunction with the plasma
fi eld to collect the particles.
In the latter, the plasma fi eld can potentially increase
overall particle collection effi ciencies by causing the
particles to stick to the post-fi lter or to increase the
effective particle size so they are more easily captured
by the post-fi lter. The advantage of this method relative
to HEPA fi lters is that it potentially reduces harmful
particles from the airstream with signifi cantly less
pressure drop and with only minimal electricity input.
However, while this technology offers signifi cant
advantages as a potential replacement for porous
media fi lters in ducted HVAC systems, it has not been
widely used because there is a lack of data clearly
demonstrating its effi cacy under practical settings.
Background
The effi ciency and effectiveness of fi ltration systems
can be classifi ed and/or rated in different ways,
depending on the application. Arguably the most
straightforward method to rate a fi lter is through its
single-pass particle collection effi ciency, defi ned as
=
CC−
C
oi
i
where Ci and Co are the particle concentration (count
per unit volume) upstream and downstream of the
fi lter, respectively. The collection effi ciency not only
depends on fi lter characteristics, but may also be a
function of particle diameter and fl ow conditions, such
as face velocity. Therefore, collection effi ciency is usually
specifi ed either at a specifi c set of conditions or as an
average value.
Filter effi ciencies can also be measured by introducing
instantaneous particulate matter load in the room and
measuring the time it takes to return the PM levels
to a specifi ed level based on the preload conditions.
Using this method, we can defi ne the particle removal
effi ciency, ρ, defi ned as
ρ(t) = C(t)/Cinit
where C(t) is the PM concentration after a certain time, t,
and Cinit is the initial PM concentration.
As previously noted, all these measures of effi ciencies
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ASHRAE Journal - December 2023

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