Performance and costs of particle air filtration in HVAC supply airstreams Page: 2 of 8
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well-mixed indoor space and integrates over particle size, accounting for the size distributions of source-
specific particles and the size-dependent variations in both filter performance and particle deposition
losses to surfaces. The modeling also accounts for particle entry via infiltrating outdoor air that does not
pass through the filters in the HVAC system. The percentage reduction in indoor particle mass
concentration was used as the performance metric, with the reference case typically being no filter. A
detailed description of the model and the model input parameters is provided in Fisk et al. (2002).
Calculations were performed for non-reacting outdoor fine-mode particles (smaller than 3.1 m) and for
three types of indoor-generated particles - those with dust mite allergen, with cat allergen, and from
environmental tobacco smoke (ETS). Our calculations do not apply for the many gaseous compounds
within ETS, which will not be removed by particle filters. The detailed particle size distribution data used
in the calculations are provided in Fisk et al (2002). For cat allergen, 60% to 70% of the mass is within
particles larger than 5 m in aerodynamic diameter. For dust mite allergen, 80% to 90% of the allergen
mass is in particles larger than 4 m. ETS and outdoor fine mode particles are much smaller.
Approximately 97% of the mass of ETS particles is within particles smaller than 1 m and approximately
93% of the mass of outdoor fine mode particles is in particles smaller than 1 m in aerodynamic diameter.
As an overall (i.e., single-number) efficiency rating, we have characterized filters according to the
ASHRAE Dust Spot Efficiency Rating (ASHRAE 1992), the standard rating used until recently by U.S.
industry. To denote the ASHRAE Dust Spot Efficiency, we will use the notation "ASHRAE nn%" where
"nn" refers to the Dust Spot Efficiency rating. In parenthesis, we provide the corresponding MERV filter
efficiency rating, based on ASHRAE's new filter testing standard. There is no unique filter efficiency
curve, i.e., curve of particle removal efficiency versus particle size, for each Dust Spot Efficiency Rating;
hence, we have performed calculations with example efficiency curves based on data provided by Hanley
et al. (1994) and provided by manufacturers. For some filters, we extrapolated from the reported data to
estimate filter efficiency for the largest particles. These example efficiency curves are provided in Figure
1. To maintain readability, we have not shown the curve for high efficiency particulate air (HEPA) filters
which have a rated minimum efficiency of 99.997%.
We neglected air bypass, which is the leakage of air between adjacent filters and between filters and the
framework holding the filter. Insufficient information is available on typical rates of bypass, although the
limited information available indicates that bypass rates will often exceed 10%. By neglecting bypass, we
overestimate the particle removal efficiencies of the filtration systems. However, we have also used the
particle removal efficiencies of new filters, and efficiency normally increases as filters load. This
simplification causes an underestimation of time average particle removal efficiencies.
We assumed that the rates of outside airflow and recirculation airflow were one and four indoor volumes
per hour respectively (denoted by 1 h-' and 4 h-1). Most calculations assumed an air infiltration rate of
0.25 h-1. For some calculations, we assumed no infiltration or no recirculation.
Costs of Air Filtration
The detailed method used to calculate costs is provided in Fisk et al. (2002). As background for the
filtration cost calculation, we note that filters vary a great deal in the degree of pleating (i.e., folding of
the filtration media) and in the depth in direction of airflow. Depth may vary from approximately 2 cm to
30 cm (1 to 12 inch). With an increase in depth and pleating, the area of filtration media increases, price
usually increases, and the ratio of pressure drop to efficiency usually decreases. To limit pressure drops,
more efficient filters tend to have increased pleating and depth. Because of their increased surface area,
these larger filters will often have an increased lifetime before excessive pressure drops make it necessary
to replace the filter.
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Fisk, William J.; Faulkner, David; Palonen, Jari & Seppanen, Olli. Performance and costs of particle air filtration in HVAC supply airstreams, article, June 1, 2003; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc738204/m1/2/: accessed October 22, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.