ASHRAE Journal - June 2021 - 22
IEQ APPLICATIONS
Column
Wade H. Conlan
Strategies from ASHRAE's Epidemic Task Force
Managing Aerosols
Using Space Flushing
BY WADE H. CONLAN, P.E., BCXP, MEMBER ASHRAE
The concept of flushing a space to remove contaminants between uses is not
new, but applying that strategy to remove a specific aerosol, such as SARS-CoV-2,
requires a process to target a specific particle size and range. Since the beginning
of the COVID-19 pandemic, ASHRAE's Epidemic Task Force (ETF) has
suggested flushing spaces between occupancy, and flushing is currently a Core
Recommendation.1 The ETF's Building Readiness guide2 recommends that the
pre- or post-occupancy flush should be no less than two hours if space-specific
flushing calculations are not performed. Following the process outlined in that
guidance, as described in this column, will typically result in a flushing time that
is a fraction of that two-hour minimum and, therefore, that requires much less
additional energy use.
The prime question with fl ushing is, " What airfl ow
should be used in that calculation? " This leads to potential
answers, phrased as questions, that include " Is it
only outdoor airfl ow rate?, " " Is it the total supply airfl ow
rate?, " and " Should it be the 'uncontaminated airfl ow'
calculated using the Wells-Riley equation? " The answer,
for removing aerosols, is the equivalent outdoor airfl ow.
Equivalent outdoor airfl ow is known as the noninfectious
air delivery rate to the space or equivalent clean air
delivery rate. This equivalent outdoor air is the combination
of the outdoor air plus the percentage of fi ltered
or inactivated recirculated air (i.e., air that contains
broken RNA of the virus).
As an example, consider a variable air volume
22
ASHRAE JOURNAL ashrae.o rg J U N E 2 0 2 1
air-handling unit with mixing box fi lters, cooling coil,
potential airstream ultraviolet germicidal irradiation
(UVGI) and a supply fan delivering the air to the spaces
as shown in Figure 1. We assume that no people, and
therefore no aerosol generation, are in the space during
the fl ushing process.
Because they are in such widespread use, the system
type shown in Figure 1 is used as an example in the
Building Readiness guidance, including in the Google
sheet3 linked in that document to help automate these
calculations. But as my fi rst HVAC professor, Gren Yuill,
Ph.D., P.E., Fellow/Life Member ASHRAE, said to our
Wade H. Conlan, P.E., is the commissioning and energy discipline manager for Hanson
Professional Services, Inc., in their Orlando, Fla., offi ce.
https://www.ashrae.org/
ASHRAE Journal - June 2021
Table of Contents for the Digital Edition of ASHRAE Journal - June 2021
Contents
ASHRAE Journal - June 2021 - Intro
ASHRAE Journal - June 2021 - Cover1
ASHRAE Journal - June 2021 - Cover2
ASHRAE Journal - June 2021 - 1
ASHRAE Journal - June 2021 - Contents
ASHRAE Journal - June 2021 - 3
ASHRAE Journal - June 2021 - 4
ASHRAE Journal - June 2021 - 5
ASHRAE Journal - June 2021 - 6
ASHRAE Journal - June 2021 - 7
ASHRAE Journal - June 2021 - 8
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ASHRAE Journal - June 2021 - Cover3
ASHRAE Journal - June 2021 - Cover4
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