ASHRAE Journal - January 2021 - 62

COLUMN IEQ APPLICATIONS

Ventilation System. The model implements a simple
air-handling system (AHS) that provides supply air to
and removes return air from the zone. The supply and
return airflow rates, Q s and Q r, respectively, the outdoor
air fraction of the AHS and the local exhaust Q lx are user
inputs. Q exh in Figure 1 is the air leaving the system, which
is calculated by FaTiIMA from the other systems airflows.
The AHS model calculates the zone air balance, including the system outdoor air intake Q oa and the infiltration
rate Q inf. It also calculates Q rc, which is the return air from
the space that is recirculated and mixed with the outdoor
air. These three flows, Q oa, Q inf and Q rc, are displayed as
Calculated Airflows.
System Filters and Portable Air Cleaners. FaTIMA
accounts for filtration of particles both within the ventilation system and by a room air cleaner. Ventilation
system filters (outdoor air and recirculation) can be
selected from a predefined set of filters specified according to the minimum efficiency reporting value (MERV)
(per ANSI/ASHRAE Standard 52.2-2017). These filters
are based on filter models correlated with specific filters
and may not be representative of different filters of a
given rating or account for variations in installation that
affect actual efficiency.7 Details of the MERV filter models are provided in the FaTIMA documentation.1
Other factors, such as airflow that bypasses the filter,
can reduce the effective efficiency of a filtration system.
FaTIMA can also account for the impact of a portable
room air cleaner, requiring the user to define room air
filter efficiencies and the airflow rate through the air
cleaner, Q ac, as shown in Figure 1.
Particle Properties. The fate of a microbiological aerosol within the built environment depends on the aerosol
properties, which must be input by the user, including
diameter, density, generation rate, deposition rate and
deactivation rate. Aerosols generated by building occupants and other sources consist of a range or distribution
of particles of varied sizes, shapes and constituent materials. However, the current version of FaTIMA only considers a single particle size in each simulation. Therefore, the
simulation results will be specific to that size based on the
input assumptions and the modeling strategies used.
Sources. FaTIMA accounts for two types of sources to
generate particles within the zone: continuous and burst.
Tidal breathing is an example of a continuous source of
aerosols from a human occupant. Burst sources such as
coughs or sneezes can be modeled either as a single event
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JAN UARY 2021

or as intermittent events. FaTIMA assumes all particles
generated are instantly distributed throughout the entire
zone volume. For FaTIMA to model particles emitted, the
user must input the rate of particle generation (depicted
in Figure 1 as G) of the specified aerosol size. For assistance
in estimating this input, users are referred to studies that
have quantified the generation of pathogen-containing
aerosols emitted by people infected with various illnesses
when breathing, coughing and sneezing.8-11
If the modeling effort involves simulating viable virusladen aerosols, then the generation rate of the aerosol containing viable viruses must be input. In addition, one may
also choose to account for deactivation of viable viruses.
FaTIMA allows input of a half-life (deactivation rate) to
enable tracking of particles containing viable viruses that
have been deactivated during the simulation period. In
FaTIMA, deactivated particles are modeled as if they are
removed from the air; therefore, they do not accumulate
on surfaces, get removed by filters or count toward occupant exposure.
Particle Deposition Velocities. FaTIMA requires the
input of deposition velocities (shown as vd in Figure 1) for
the particle size to be modeled. Particle deposition velocities have been measured within various environments,
including occupied spaces and test chambers, and models of deposition velocity have also been developed. One
such model12 is presented in the user guide, along with
some empirically estimated deposition rates for various
particle sizes and types in residential buildings. FaTIMA
calculates an " effective deposition rate " based on the
combination of all the deposition velocities and surface
areas entered for a given simulation.
Occupant Exposure. The user inputs the time during
which the exposed occupant occupies the space, which
can be continuous or intermittent at regular intervals.
FaTIMA bases the integrated occupant exposure based
on this information.

Outputs
Figure 3 shows FaTIMA's results web interface, which
displays average and maximum airborne concentration for the exposure period and for the full 24-hour
simulation, as well as the integrated exposure during
occupancy. Time histories of the zone airborne concentration, exposure concentration and surface loading
are also provided. These concentration plots also show
the average concentrations associated with both the

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ASHRAE Journal - January 2021

Table of Contents for the Digital Edition of ASHRAE Journal - January 2021