ASHRAE Journal Supplement - AMCA InMotion - September 2019 - 25

PHOTO 3. Round diffusers installed in an open
ceiling. tavizta/Shutterstock

PHOTO 4. Square diffuser installed in a closed
ceiling. imnoom/Shutterstock

very similar to the one in Saskatoon, Saskatchewan.

temperature and humidity, except for the conditioning

What to do. Some engineers are lowering design

of ventilation air.

interior airﬂow to more realistic levels, while others are

Additional recirculated air typically is provided to take

reducing it to code-minimum values. Although this is

care of sensible loads, including lighting loads. When

helping to reduce incidents of subcooling, issues with air-

VAV ﬁrst was employed in the early 1970s, lighting loads

outlet performance and objectionable drafts remain. We,

were quite high-as high as 6 W per square foot. With

therefore, need to look at air outlets.

increasing energy costs and the addition of computers,
necessary lighting levels have dropped.

Diffusers

Ceiling air-outlet performance. Open-plan ofﬁces

Selection and location. A common question concerning

have been conditioned from the ceiling, with ceiling returns,

open-plan ofﬁces the author has encountered over the last

for many years. Ceiling air outlets, properly referred to as

40-plus years is, "Where do I place my air outlets?" The

air diffusers (photos 3 and 4), are designed to distribute

answer, of course, is, "It depends"-on cost, performance,

cool air along a ceiling surface, mixing room air by means

and aesthetics.

of induction (a jet of air has lower static pressure than the

Often, there is a trade-off between cost and perfor-

slower air around it) and preventing cold air from falling into

mance, as, typically, the lower the (ﬁrst) cost, the poorer

the occupied zone through reliance on the Coanda effect.

the performance (noise, pattern control, durability, etc.).

Performance is characterized by throw distance, generated

An understanding of the actual performance character-

noise, and pressure required over a range of airﬂow. ADPI

istics of air outlets can lead to better selection and value.

can be calculated using the throw, diffuser-spacing, and

Interior vs. perimeter. Air distribution differs

load data and tables in ASHRAE Handbook-Fundamentals.3

between interior ofﬁces and perimeter ofﬁces (Photo 2). With

Open vs. closed ofﬁces. In closed ofﬁces, air typi-

interior ofﬁces, only interior loads need to be considered.

cally is distributed with a single centrally located diffuser.

When at least one of an ofﬁce's walls is an outside wall,

ADPI generally is not useful at design loads in single-outlet

both interior and perimeter loads need to be considered,

closed ofﬁces, as the throw will hit the walls. If such a

which affects both diffuser selection and diffuser location.

space has a window, however, ADPI needs to be consid-

Interior zones. Spaces more than 15 ft (4.6 m) from

ered. When designing air distribution for an open-plan

a perimeter surface usually are considered "interior." For

ofﬁce, jet collisions at high ﬂow and excessive drop at low

the most part, interior spaces are independent of outside

ﬂow have to be considered. If the suspended ceiling is near

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

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

25
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ASHRAE Journal Supplement - AMCA InMotion - September 2019

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