ASHRAE Journal - July 2021 - 22

IEQ APPLICATIONS
Column
Brendon J. Burley
Designing Beyond
The Well-Mixed Space
BY BRENDON J. BURLEY, PH.D., P.E., MEMBER ASHRAE
The well-mixed space is one of HVAC design's core assumptions. It is the basis of the
equation used for calculating required airfl ows for thermal loads. And it provides the
rational underpinnings for dilution-based ventilation. But sometimes well-mixed
spaces do not behave as ideally mixed spaces. So, designers should use ASHRAE
standards and other guidance, which address the nonuniform nature of the built
environment.
The idealization of a well-mixed space is convenient
because it posits that all physical properties of the space
are equivalent. The implications are that the location
of inlets and outlets, the velocity and direction of airfl
ows, and the location and intensity of heat sources
and of contaminant sources are immaterial because the
space's environmental conditions are all uniform. Any
experienced designer or occupant knows all these factors
have an infl uence on the overall quality of the built
environment.
Chapter 58 of the 2019 ASHRAE Handbook-HVAC
Applications has an extensive discussion on room air distribution
and addresses the different characteristics and
design considerations for spaces that are fully mixed to
spaces that are fully stratifi ed. The discussion of fullymixed
spaces includes a description of the air distribution
performance index (ADPI), which is the percentage
of points within the space that are within -3°F and +2°F
(-1.6°C and +1.1°C) of the draft adjusted cooling setpoint
and -4°F and +3.6°F (-2°C and +2°C) of the draft
adjusted heating setpoint, with a rating of 80 or higher
indicating a well-mixed space.1 By this defi nition, a
well-mixed space is clearly not uniform.
Studies have shown how the position of air devices,
discharge patterns and air speeds and air temperature
can infl uence the temperatures and air velocity
22
ASHRAE JOURNAL ashrae.org J U LY 2 0 2 1
distribution within a space.2,3 Burkett recently discussed
the relationship between airborne transmission
of viruses and air distribution, noting local effects
such as relative position and motions of occupants and
heat sources that are more signifi cant than the overall
airfl ow pattern.4 Well-mixed environments have variations
in physical properties, which means that real environments,
even well-mixed ones, are not ideally mixed.
Examples of How ASHRAE Standards and Other Guidance
Address Nonuniform Characteristics of Spaces
ASHRAE Standard 55-2020 includes temperature
asymmetry and air velocity as factors that impact
thermal comfort.5 ASHRAE/ASHE Standard 170-2021
requires certain types of diffusers in different patient
care areas and even prescribes the location and velocities
of supply diffusers and return diffusers in operating
rooms.6 Another example of this level of specifi city is
that exhaust outlets in airborne infection isolation (AII)
rooms must be located at the patient headwall or above
the patient bed.7 The basis for this recommendation is
in a study that used computational fl uid dynamics to
show the position of air inlets and outlets had greater
Brendon J. Burley, Ph.D., P.E., is the healthcare practice leader for Burdette, Koehler,
Murphy & Associates, Inc., in Baltimore. He is the chair of the ventilation subcommittee
of ASHRAE SSPC 62.1 and a member of ASHRAE's Environmental Health Committee.
This peer-reviewed column does not represent offi cial ASHRAE guidance. For more information on ASHRAE resources on COVID-19, visit ashrae.org/COVID19.

http://www.ashrae.org/COVID19 http://www.ashrae.org

ASHRAE Journal - July 2021

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