ASHRAE Journal - February 2020 - 57

COLUMN ENGINEER'S NOTEBOOK

FIGURE 1 Mixing box configurations.

RA

FIGURE 2 Air handler elevation.

Mixed
Air

RA

Mixed
Air

SA

OA

OA

OA

RA
RA

OA

Mixed
Air

RA

Mixed
Air

OA

in the mixing box are dynamic due to moving dampers and supply fan speed changes in variable air volume (VAV) units.
One rule that I was taught when I ﬁrst started in the
industry is to always locate the outside air opening
on the top and the return air opening on the back of
the mixing box. The theory is that the warmer return
air will tend to rise, and the cold air will drop due to
differences in temperature/density. This will result
in more thorough mixing. Breaking this rule was forbidden. What I have since come to learn is that this
scenario rarely occurs. The momentum of the return
air stream is usually greater than the force due to the
density difference. The result is that the airstreams do
not mix but are simply carried downstream in a stratiﬁed condition.6
Another common misconception that I was taught as a
young engineer is that ﬁlters installed in a combination
ﬁlter/mixing box will enhance airstream mixing. This
was dispelled by experimental work done in the late
1990s by Keith Robinson.6
There is plenty of technical literature, especially in
manufacturer's application data, that suggests pointing
parallel blade outside air and return air dampers toward
each other in the mixing box will help facilitate mixing. No deﬁnitive data exists to show that this strategy
is effective, but many engineers still insist on using it.7
Many other variables contribute: opening size, opening conﬁguration, relative temperature and volume of
the two airstreams, and distance from openings to the
downstream water coil being just a few.

The ASHRAE Handbook-HVAC Systems and Equipment,
Chapter 4 lists rules of thumb to improve mixing but
acknowledges that there is little supporting data and
states that the rules of thumb are based largely on anecdotal evidence.4 ASHRAE research has shown that a top/
back opening conﬁguration produces better mixing
than any conﬁguration using side openings. Relative
ﬂow momentum between the two airstreams also has a
signiﬁcant effect.7,8 This information does not appear to
be reﬂected in manufacturers' mixing box literature.
Stationary air mixing devices, also called static mixers,
are devices designed to improve mixing of airstreams.
However, there are no performance standards for these
devices.4,5 Furthermore, static mixers create turbulence
to improve mixing and thus increase pressure drop and
energy consumption. They also require additional air
handler length.
Mechanical rooms are generally tight on space. Air
handlers must be squeezed into a space that may not
be adequate. The engineer may be tempted to shorten
the mixing box or eliminate the static mixer to make
the unit ﬁt and this contributes to the stratiﬁcation
problem.

Case Studies
Case Study 1: University Academic Building

This stratiﬁcation problem occurred in an air handler that has a mixing box with a back outside air
inlet located near the top of the box, and a side return
inlet. The ﬁlters are in the mixing box. The unit had a
low design outside air percentage. Mixed air calculations showed that the average mixed air temperature
would never approach freezing, so the unit did not
include a preheat coil. The chilled water cooling coil
is approximately 6 ft (1.8 m) downstream of the outside air intake. Refer to Figure 2 for unit conﬁguration.
The unit had been shutting down on low temperature
safety trips. Over winter break, there was a cold spell
FEBRUARY 2020

ashrae.org

ASHRAE JOURNAL

57
https://www.ashrae.org/
ASHRAE Journal - February 2020

Table of Contents for the Digital Edition of ASHRAE Journal - February 2020
