AMCA InMotion - September 2019 - 26

from a short drop. Most round diffusers have a slightly
upward discharge pattern with an open ceiling, usually
compensating for the lack of Coanda effect to avoid an
excessive amount of cold air falling into the occupied zone.

Linear diffusers. In deﬁning the area to be served
by linear diffusers, end-to-end spacing needs to be considered. At lower design airﬂows (less than 1 cfm per square
foot), a spacing of 8 ft (2.4 m) is typical with 4-ft (1.2 m)
linear diffusers, resulting in a 12-ft- (3.7 m) wide zone.
Dividing this into (q/r) will yield the length of the space to
be conditioned. For one-way discharge, this is the distance
to the next outlet. For two-way discharge, it is halfway to
PHOTO 5. Single-duct terminal unit.

the next diffuser (but the same total distance apart). ADPI

Photo courtesy of Krueger

for linear slot diffusers uses 100-fpm (0.51 m/s) terminal
velocity (instead of the 50 fpm used for everything else),

9 ft (2.7 m) from the ﬂoor, ADPI will be a good tool.

ADPI graph. Using device throw performance and
Handbook-Fundamentals3

but the analysis is similar.

Grilles. Grilles seldom are used in open-plan ofﬁces.

correlating

When they are used, ADPI typically is not a valid calcula-

throw to ADPI, one can determine the effective range (ADPI

tion, as the ceiling usually is much higher than 9 ft and

greater than 80 percent) of an air outlet as a function of

often there is no suspended ceiling. As with linear slot

ﬂow rate, ﬂow rate per unit area, and location in an array.

diffusers, the spacing between outlets is used to determine

For an open-plan ofﬁce with four-way or circular-pattern

the area served at a given design ﬂow rate. Grille throws

outlets, separation distance can be determined using a

can be shortened by increasing the spread, while exces-

simple equation:

sive drop can be controlled by adjusting horizontal blades.

data in ASHRAE

To avoid drafts, throws to 100 fpm from opposing grilles
Separation distance =

(q/r)0.5

never should be allowed to meet.

Perimeter zones. Perimeter loads are far less today
where:

than they were when overhead heating was introduced in

q = ﬂow at desired NC, cubic feet per minute

the 1970s. Nevertheless, data indicate effective overhead

r = ﬂow rate per square foot

heating requires that room-to-discharge delta-T never exceed
15°F (9.4°C). What's more, the downward convective airﬂow

Minimum ﬂow rate. Next, using the ﬂow and half

generated by the cold surface of a window is best treated

of the separation distance, we can consult the ADPI table

by being mixed with warm (not hot) air directed down

in ASHRAE

Handbook-Fundamentals3

to determine the

minimum rate of ﬂow.

the window. ANSI/ASHRAE Standard 62.1, Ventilation for
Acceptable Indoor Air Quality, says that, if discharge delta-T

Open ceilings. Most manufacturers provide data for

exceeds 15°F or a 150-fpm (0.76 m/s) throw does not come

throw measured along a surface. With an open ceiling,

to within 4.5 ft (1.4 m) of the ﬂoor, ventilation air will

however, throw is shortened by about 30 percent.

short-circuit into ceiling returns and additional ventilation

An open-plan space with open ceiling is best served with

air will be required. ASHRAE Handbook-Fundamentals3

round diffusers on spiral-duct drops. The larger the diam-

indicates excessive vertical stratiﬁcation also will result,

eter of the supply duct, the more uniform the discharge

preventing compliance with ANSI/ASHRAE Standard 62.1.

20 19 A M CA i n m o t i o n

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AMCA InMotion - September 2019

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