ASHRAE Journal - December 2022 - 50
TECHNICAL FEATURE
Practical Diffuser
Selection and Layout
Procedure
BY KEVIN LAWSON, P.E., MEMBER ASHRAE
The selection and layout of diffusers to maximize occupant thermal comfort can be
complicated and time consuming. Given the sheer quantity of diffusers that must
be selected and laid out on every project, developing a streamlined and consistent
process can help firms remain profitable in a competitive environment. This article
describes a practical selection and layout procedure that provides occupant comfort,
minimizes construction cost and can be applied to most spaces quickly and effectively.
Constraints and Assumptions
No single procedure could encapsulate the engineering
design required to adequately distribute air in any
space that an architect can envision. Edge cases and
limitations will always exist. As such, we will work with
the following constraints and assumptions:
1. The investigation will be limited to spaces where
overhead air supply is possible. We won't include, for
example, gymnasiums, which typically require a large
space between the court or field and the nearest overhead
obstruction.
2. The focus will be on spaces designed primarily for
human comfort. We won't include, for example, surgery
suites, where the primary intent of the air distribution
system is to create an envelope of cold clean air around
the staff and patient.
3. A consistent diffuser look will be maintained by
Kevin Lawson, P.E., is the founder of Ripple Engineering Software.
50
ASHRAE JOURNAL ashrae.o rg D ECEM BER 2 0 2 2
assuming all diffusers in a space will be the same model.
This will limit our coverage options, but consistency is
key when designing the aesthetics of a space.
4. The assumption will be made that windows don't
need " washing. " Historically, linear slot diffusers have
been used to blow warm air onto windows in the winter-when
the delta T across the window is highest-in
an effort to keep the interior window surface temperature
to within 18°F (10°C) of the space temperature to
satisfy the allowable radiant temperature asymmetry in
Table 5.3.4.2 of ASHRAE Standard 55-2017.1
With the proliferation of the International Energy
Code, washing is not as important because code minimum
windows have high enough R-values that the
internal surface temperature stays above the allowable
temperature in the coldest regions of each climate
zone. Table 1 shows the calculated window inside
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ASHRAE Journal - December 2022
Table of Contents for the Digital Edition of ASHRAE Journal - December 2022
Contents
ASHRAE Journal - December 2022 - BB1
ASHRAE Journal - December 2022 - BB2
ASHRAE Journal - December 2022 - Cover1
ASHRAE Journal - December 2022 - Cover2
ASHRAE Journal - December 2022 - 1
ASHRAE Journal - December 2022 - Contents
ASHRAE Journal - December 2022 - 3
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ASHRAE Journal - December 2022 - Cover3
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