ASHRAE Journal - February 2014 - 49

COLUMN HVAC APPLICATIONS
Dan Int-Hout

Conditioning Challenges:
Lobbies and Atriums
BY DAN INT-HOUT, FELLOW ASHRAE

Conditioning spaces with large vertical walls is a challenge for design engineers,
especially when it has to be coordinated with an architect's aesthetic concept. As
is often the case in lobbies or atriums, these areas are composed of glass and can
be anywhere from 12 to 18 ft (3.6 m to 5.4 m) high. While people do not typically
occupy the space next to the window for long lengths of time, it is still important
that proper air distribution and comfort conditions be maintained.
ASHRAE's thermal comfort standard (Standard
55-2013) states "the occupied zone is to be between the
floor and 6 ft (1.8 m) above the floor and more than 3.3
ft (1.0 m) from external walls/windows or fixed heating,
ventilating, or air-conditioning equipment and 1 ft (0.3
m) from internal walls."
While this implies that the space close to the window
is outside the "comfort zone," it does not mean that it
may not affect or impact the airflow patterns within
the occupied area, nor does it mean that occupants
will not pass through or spend shorter durations in the
space.
To adequately offset such heating and cooling
demands in these applications, one must understand
that hot air rises and cold air falls. Surprisingly, this
basic notion is largely overlooked by many design
engineers. It is not only what allows one to control air
delivery, but also take advantage of it. Listed here are a
couple of "rules of thumb" that can be used to assist in
the understanding of air distribution dynamics and may
help save energy in the process.
1. A jet of air delivered by a ceiling diffuser is typically
affected by buoyancy and can be described through this
simple equation: The throw to 75 fpm (0.37 m/s) is increased or decreased by 1% divided by degree DT. (DT beT
ing the difference between the thermostat setting, which
is the average of the occupied zone temperature and the
jet discharge temperature, in °F).

2. The direction of the effect is determined by buoyancy, so cold air that is directed down will travel farther,
but hot air will not travel as far. On the other hand, hot air
directed along a horizontal surface will travel farther, and
a cold jet will travel less distance. A horizontal jet with no
adjacent surface will rise or fall at the isothermal 75 fpm
(0.37 m/s) throw distance by about 1% divided by °F.
Pretty simple, huh? These rules of thumb work surprisingly well in practice, but there are a couple of other
details that need to be considered.
1. The throw to 150 fpm (0.76 m/s) is not measurably
affected with less than 25°F (14°C) DT.
2. A return slot located at the top of the window will
remove the heat that rises from the window and will not
have to be included in the room load that the HVAC system must handle. (Of course, the warmer plenum air will
likely be returned to the A/C equipment.)
To deliver air to the space without creating any
unwanted drafts, it is important to understand the vertical air patterns from overhead. In most manufacturers'
catalogs, a downward jet will have published isothermal
throw values at 50 fpm and 100 fpm (0.25 m/s and 0.51
m/s) terminal velocities. The 75 fpm (0.38 m/s) terminal
velocity will be in between these values.
If a 20°F (11°C) DT cooling is assumed, this value will
be increased by 20% because cold air wants to fall. If
Dan Int-Hout is a chief engineer at Krueger in Richardson, Texas. He is a member of
SSPC 55, SPC 129 and consultant to SSPC 62.1.
FEBRUARY 2014

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