ASHRAE Journal - February 2020 - 58

COLUMN ENGINEER'S NOTEBOOK

FIGURE 3 Air handler plan view-stratification in mixing box.

FIGURE 4 Air handler plan view-sheet metal baffle in mixing box.

RA

RA

Maintain 57 in. Clear
From Edge of Baffle to
Side of Mixing Box

Cooling

Warm
Zone
Mixed
Zone

Install Floor to Ceiling
Baffle in Mixing Box

OA

Cold
Zone

the openings. A hot water preheat coil and chilled water
cooling coil are located downstream. At some point
when the unit went into economizer mode, it began
with sub-zero temperatures, and the chilled water coil shutting down due to low temperature detector (freezefroze and broke, flooding the floor. The low limit temstat) trips. Upon further investigation, we determined
perature detector was supposed to close the outside
that the outside air was dropping into the 10 in. (254
air damper, open the return damper, and keep the
mm) of dead space on both sides at the top of the mixing
fan running if it sensed a temperature of 42°F (5.5°C).
box, and the stratified airstream was being sent forward
However, it was not programmed correctly. The chilled (Figure 5). The fuchsia color indicates the coldest area.
water pumps were supposed to start, and the cooling
The manufacturer recommended adding sheet metal
coil valve was supposed to fully open, but this also did
panels on both sides inside the mixing box to eliminate
not happen.
these dead spots. This effectively
PHOTO
2
University
student
union
mixing
box.
When the coil was repaired and
made the interior width of the mixing
the unit put back in operation, the
box the same as the OA and RA openbalancer took temperature readings
ings. This eliminated the problem.
in the mixing box, and the results
Case Study 3: County Services Building
showed significant stratification.
OA
This air handler does not include a
The airstream on the side of the unit
mixing box. It has an intake section
opposite the return damper was the
with a single duct connection on top.
same temperature as the outside air
The outside air and return air duct(Figure 3)!
work are combined in the mechanical
The manufacturer recommended a
room above the air handler. OA and
field baffle be installed in the mixing
RA dampers are located in the ductbox to force the return air to mix with
RA
work upstream of the unit (Figure 6).
the outside air. After some field experiThis approach provided thorough
mentation with varying baffle arrangements, a permanent sheet metal baffle was installed, which mixing and no stratification occurred.
solved the stratification problem (Figure 4).

Conclusions and Recommendations
Case Study 2: University Student Union

This air handler (Photo 2) includes a mixing box with
an outside air opening on top, a return opening in the
back and a set of filters. The return air and outside air
openings are both 117 in. (2972 mm) wide. The overall
dimension of the mixing box is 137 in. (3480 mm). This
left 10 in.(254 mm) of "dead air space" on either side of

58

ASHRAE JOURNAL

ashrae.org

FEBRUARY 2020

There are many reasons why mixing boxes may not
provide adequate OA and RA mixing. Stratified airstreams in an air handler can have disastrous consequences, including broken coils and flooded buildings.
My suggestions for minimizing stratification in air handlers are as follows:
* Don't rely blindly on the manufacturer's mixing box
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ASHRAE Journal - February 2020

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