Page 393 - The Engineering Guide to LEED-New Construction Sustainable Construction for Engineers

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the airstream that are circulated within the facility and preventing buildup of
contaminants within the HVAC system which can reduce equipment efficiency, lead to
reentrainment of contaminants in the airstream, and serve as a breeding ground for
other microbes. Some technologies that remove contaminants from air are particulate
filtration, electrostatic precipitation, negative ion generation, and gas sorption. The first
three technologies are designed to remove particulates, and the fourth is designed to
remove gaseous contaminants.
Particulate filtration involves the use of mechanical devices placed in the airstream
often using tightly woven fibers to trap the particulates as air flows through the filter
medium. Filters are classified by their minimum efficiency reporting value (MERV)
which relates to the efficiency the filter has for removing particles from 0.3 to 10 μm.
High-efficiency particulate air (HEPA) filters and ultra-low penetration air (ULPA) filters
are also available for the removal of even smaller particulates. The disadvantage of
particulate filtration systems is that the higher the efficiency of the filter, the more it will
increase the pressure drop within the air distribution system and reduce total airflow.
This in turn can overwork fan motors causing increase in energy usage and can reduce
the air exchange rate critical to dilution of indoor air pollutants. Also, particulate filtration
systems may require frequent maintenance for replacement or cleaning of filters.
Electrostatic precipitation and negative ion generation are similar in principle
because they apply a charge to particles to enable removal from the airstream. With
electrostatic precipitators (ESPs), the charged particles become attracted to an oppositely
charged surface on which they are collected. These systems provide relatively high-
efficiency filtration of small respirable particles at low pressure losses, although ESPs
must be serviced regularly and produce ozone. Negative ion generators (ionizers) apply
a static charge to remove particles from the indoor air. The charged particles then
become attracted to surfaces such as walls, floors, draperies, or even occupants. This is
not always an ideal method since it requires deliberate cleaning of the space to actually
remove the particles, although more effective designs include collectors to attract
particles back into the unit. Like ESPs, ionizers require frequent maintenance and may
produce ozone.
Gas sorption is a process for the control of gaseous contaminants, such as
formaldehyde, sulfur dioxide, ozone, or VOCs. It involves the use of a sorption material
(e.g., activated carbon, chemically treated clays) and processing of the gaseous
contaminant, such as chemical reaction with the sorbent, binding of the contaminant
with the sorbent, or diffusion of the contaminant from areas of higher concentration to
areas of lower concentration. There are currently few standards for rating the performance
of these systems, which makes design and evaluation problematic. Also, operating
expenses of gas sorption systems can be quite high, and sorbent filters can reach
breakthrough conditions at higher temperatures and relative humidity which causes
them to desorb VOCs back into the airstream. Recent system development and testing
for gaseous contaminant removal have been conducted with photocatalytic oxidation
(PCO). PCO is a process involving a photocatalytic filter and UV light for the elimination
of VOCs through an oxidation reaction producing carbon dioxide and water.
Air cleaning through filtration systems can be a critical link to long-term maintenance
of good IAQ. It is not likely that sources of contaminants can be completely avoided
through product selection, and dilution through increased ventilation does not fully
eliminate contaminants; therefore, filtration can drastically improve the effectiveness
of removing indoor pollutants as the third leg of the IAQ strategic triad. Currently,
LEED-NC 2009 provides no specific credits for application of air cleaning or filtration