ASHRAE Journal - December 2022 - 53

TECHNICAL FEATURE
ATW = ATW + 1
ATL = ATW × [RS]
ATR = ATL / ATW
Until: desired diffuser count, ATW × ATL is reached
where SR is the space aspect ratio, calculated by dividing
the space length, SL, by the space width, SW.
In this example it is assumed that SL is the larger
dimension. ATR is the diffuser aspect ratio, calculated by
dividing the diffuser count in the larger dimension (ATL,
by ATW), which is the diffuser count along the shorter
dimension. To ensure the diffuser grid follows the space
shape as closely as possible, initially and every time we
add a diffuser in the shorter dimension, we multiply
the grid count in the shorter dimension (ATW) by the
rounded space aspect ratio, [SR] .
When the diffuser aspect ratio diverges too signifi cantly
from the space aspect ratio, as indicated by the absolute
value of the difference of the two aspect ratios being
greater than 0.5 (|ATR - SR| > 0.5), then a diffuser in the
shorter dimension must be added and the grid reset to
match the space aspect ratio as closely as possible.
This algorithm allows the user to add diffusers in the
smallest increment possible while maintaining the
tightest characteristic length. In the case of our 20 ft ×
40 ft (6 m × 12 m) space, the diffuser counts would progress
from 2 to 3 to 4 to 4 to 6 to 8 as shown in Figure 7.
Length to 50 fpm Terminal Velocity, T50
As previously mentioned, T50 is the diffuser throw
length to a terminal velocity of 50 fpm (0.25 m/s). These
values can be determined for a given diffuser size and
throw pattern at a given cfm by reviewing the catalog
data. An example for a louvered face diffuser is given in
Figure 8.3
Regardless of what the ideal T50/L is, each diffuser is
going to ideally " cover " some area for a given cfm based
on the T50 throw of the diffuser. If the ideal T50/L is 1,
then the area covered will be (T50 × 2/1).2 If the desired
T50/L is 2, then the area covered is (T50 × 2 / 2).2 For an
example from Figure 8, an 8 in. (203 mm) four-way louvered
face diffuser with 314 cfm (148 L/s) of supply air
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D E C E M B E R 2 0 2 2 ashrae.o rg ASHRAE JOURNAL
53
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ASHRAE Journal - December 2022

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