AMCA InMotion - September 2019 - 24

PHOTO 2. Air distribution differs between perimeter spaces (left) and interior spaces (right).
ImageFlow/Shutterstock

of a diffuser except sound. The ADPI parameter L is half

controls having a fi nite ability to resolve low flow, VAV

the separation distance.

boxes began being set with a 30-percent minimum flow,

Knowing the separation distance, one can use ADPI to
determine the lowest acceptable airflow rate. Note that for
some ceiling air outlets, turndown may not be possible.

which became another rule of thumb.
Of course, nothing stays the same forever. Lighting loads
now are less than 1 W per square foot (10.8 W per meter

One also can use manufacturer data to determine

squared [W/m2]), as modern computers and screens draw

throw for any outlet and look at jets directly. An alterna-

far less current than older office equipment did and office

tive to using ADPI is to ensure the 50-fpm throw does not

spaces are more spread out. What's more, VAV-box probes

cause collisions at the midpoint between diffusers. The

have shown linearity well below any controller's ability

midpoint should not exceed the separation distance plus

to resolve low flow. The result is that interior-zone loads

the distance from the ceiling to the top of the comfort

likely are far less than the 22 Btuh (6 W per square foot

zone, usually 3 ft (0.9 m).

[64.6 W/m2]) that results with 1 cfm per square foot at 55°F
(12.8°C). When VAV boxes are set at 30-percent minimum

Changing Loads

flow, there is a likelihood the minimum flow will exceed

Rules of thumb. During the mid-1980s, variable air

the space load and the space will become subcooled.

volume (VAV) was becoming more and more electronically

A study of comfort and energy at the Sunnyvale, Calif.,

controlled and lighting was becoming stabilized at levels

campus of Internet-access provider Yahoo!4 showed that,

far below those during the 1960s, when many designs

at 30 percent of 1 cfm per square foot, spaces were being

were established. As a rule of thumb, a flow-rate set point

subcooled and employees were quite dissatisfied with a 68°F

of 1 cfm per square foot (0.47 l per second [l/s]) of interior

(20°C) workplace every afternoon. Reducing the minimum

space was established, making VAV design much easier. A

setting resulted in a measured load equivalent to about

second rule of thumb concerned acoustics, as NC 35 became

0.22 cfm per square foot (0.011 l/s per m2), very close to

the seemingly universal maximum desired noise level in

ventilation minimums in California. This has tremendous

specifications for interior spaces. Lastly, with pneumatic

implications, as the interior load in Sunnyvale, Calif., is

24

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

w w w. a m c a .o r g


http://www.amca.org

AMCA InMotion - September 2019

Table of Contents for the Digital Edition of AMCA InMotion - September 2019

Contents
AMCA InMotion - September 2019 - BB1
AMCA InMotion - September 2019 - BB2
AMCA InMotion - September 2019 - Cover1
AMCA InMotion - September 2019 - Cover2
AMCA InMotion - September 2019 - Contents
AMCA InMotion - September 2019 - 2
AMCA InMotion - September 2019 - 3
AMCA InMotion - September 2019 - 4
AMCA InMotion - September 2019 - 5
AMCA InMotion - September 2019 - 6
AMCA InMotion - September 2019 - 7
AMCA InMotion - September 2019 - 8
AMCA InMotion - September 2019 - 9
AMCA InMotion - September 2019 - 10
AMCA InMotion - September 2019 - 11
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AMCA InMotion - September 2019 - 15
AMCA InMotion - September 2019 - 16
AMCA InMotion - September 2019 - 17
AMCA InMotion - September 2019 - 18
AMCA InMotion - September 2019 - 19
AMCA InMotion - September 2019 - 20
AMCA InMotion - September 2019 - 21
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AMCA InMotion - September 2019 - 24
AMCA InMotion - September 2019 - 25
AMCA InMotion - September 2019 - 26
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AMCA InMotion - September 2019 - Cover3
AMCA InMotion - September 2019 - Cover4
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