AMCA InMotion - September 2019 - 23

PHOTOS 1A AND 1B. Based on the ratio of isothermal throw at 50 fpm (0.25 m/s) to the midpoint between
diffusers, ADPI aids design for comfort. An ADPI of 80 percent or greater indicates a space is well-mixed and that
drafts and stratiﬁcation are unlikely. If ADPI is less than 80 percent, occupant discomfort and complaints can
be expected. Above, on the left, is an illustration of optimal ADPI; on the right is an illustration of poor ADPI.
WR36/Shutterstock

ducted HVAC systems. Supply of system air includes both

Handbook-Fundamentals,3 usually using manufacturer

applied (chilled water) and unitary (compressor-equipped

software. An ADPI of 80 percent or greater is recommended

units) systems. The delivery of hot and cold air to building

for most office-type spaces and can be used to prove

occupants is the focus of the article.

compliance with ANSI/ASHRAE 55, Thermal Environmental
Conditions for Human Occupancy's vertical-temperature-

ADPI and Jet Mapping

stratiﬁcation limit of 5.4°F (3°C).

ADPI is a single-number rating of temperature and air

Once a diffuser's performance at a design load and

speed in the occupied zone
of a space. It can be measured
using ANSI/ASHRAE Standard 113, Method of Testing
for Room Air Diffusion. In
a sense, ADPI is a measure
of how well-mixed a zone
is (photos 1A and 1B). It is
limited to open spaces with
multiple diffusers and an
8-to-10-ft (2.4 to 3.0 m) ﬂoorto-ceiling height, such as a
typical open ofﬁce. It is not
applicable to perimeter or
closed (walls extending to the
ceiling) ofﬁces or to heating

spacing is determined, the

What Is a HPAS?
Usually consisting of a chiller, an air handler, and terminal
units or a chiller and fan coils, a modern high-performance air system (HPAS) provides advantages such
as individual temperature zoning and high-efficiency
filtration of 100 percent of both outside air and recirculated air. It radically improves energy efficiency with
features including aggressive supply-air-temperature
and chilled-water-temperature reset, demand-control
ventilation, integrated economizing with heat recovery,
separation of return-air and outside-air treatment
in humid climates, exhaust energy recovery where
appropriate, and heat-pump-chiller buildingwide heat
recovery and optional 100-percent electric operation.

evaluations.

lowest possible airﬂow rate
that will result in a calculated
ADPI greater than 80 percent
can be determined. From
that, the optimum separation distance and minimum
recommended ﬂow rate easily
can be determined.
It is important to note that
most acoustical speciﬁcations
require a noise-criteria (NC)
level of 35. ASHRAE recommends increasing published
NC levels by 5 to account for
real-world room acoustics and
less-than-ideal inlet condi-

An ADPI prediction considers air-outlet type, diffuser

tions. For a given airﬂow through a four-way outlet, the

spacing, throw of 50 fpm (0.25 m/s) at the desired airﬂow

separation distance simply is the square root of the sum of

(usually based on sound generation), and room load.

the airﬂow divided by the airﬂow/unit area. Note that this

One then enters data from the ADPI table in ASHRAE

calculation is independent of any performance parameter

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

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

http://www.amca.org

AMCA InMotion - September 2019

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