ASHRAE Journal - December 2020 - 23

TECHNICAL FEATURE

FIGURE 1 Economizer diagram.1

Mixed Air
(MA)

FIGURE 2 Test setup from ASHRAE Research Report RP-1045.5

Atmospheric
Pressure

Supply Air Cooling
Coil
Fan
(CC)
(SAF)
Supply Air
(SA)

Outdoor Air
(OA)
Filter
(F)

OA Damper
1
18 16 in.

3
54 16 in.

12 in.

Heating Coil
(HC)

Airﬂ
Recirculated Air (CA)

5
22 8 in.

ow

RA Damper

Dampers (D)
Exhaust Air
(EA)

7
67 8 in.

38 in.

Return Air
(RA)

144 in.

117 in.

Return Air Fan (RAF)

lead to coil freezing.2 The other effect of poor mixing is
causing the economizer controls to operate to the detriment of system efﬁciency since the sensors are sensing nonrepresentative conditions.3 ASHRAE Standard
90.1-20194 makes recommendations for the high limit
settings in various climate regions, but this assumes a
representative mixed air temperature is sensed.
Several CFD simulations were run to calibrate the
CFD model to test data published in ASHRAE Research
Report RP-1045, " Verifying Mixed Air Damper
Temperature Control and Air Mixing Characteristics. "
by Finaish, et al.5 The report tested a wide range of mixing box conﬁgurations and damper sizes, so a single
test conﬁguration was chosen that was close in size and
conﬁguration to a commercially available mixing box
for use with a 20 ton (70 kW) to 25 ton (87.5 kW) air handler. The test setup is shown in Figure 2. Figure 3 shows the
commercial mixing box product. The test data available
is for a ﬂow rate of 16,000 cfm (7551 L/s) at 15% outdoor
air (OA) and 30% OA. This paper focuses on the 30% OA
case.
The steps used to proceed from the validated CFD
simulation against test data of the setup in Figure 2 to the
commercial product in Figure 3 were as follows:
1. Adjust the CFD model of dimensions in Figure 2 to
match test data within 10% including a mesh sensitivity
study.
2. Make the return air (RA) damper the same size
as the outdoor air (OA) damper, changing it from
54.2 in. × 18.2 in. (1377 mm × 462 mm) to 67.9 in. ×
22.6 in. (1725 mm × 574 mm).
3. Change the box size to that shown in Figure 3
(90.5 in. × 28.375 in. [2299 mm × 721 mm]).

FIGURE 3 Commercially available 20 ton to 25 ton mixing box.

6 38 in.

88 in.

33 3 in.
4

24 in.
8 in.
90 12 in.

28 38 in.

4. Change the RA and the OA damper size to that
shown in Figure 3 (24 in. × 88 in. [610 mm × 2235 mm]).
5. Drop the total ﬂow to 10,000 cfm (4719 L/s) to match
the 25 ton (87.5 kW) rating.
Mixing effectiveness was described in the ASHRAE
Research Report5 as both range effectiveness and thermal statistical mixing effectiveness. Robinson6 has
deﬁned range effectiveness as:
 RangeDS 
ERange = 1 −
× 100%
 RangeUS 

(1)

where
RangeDS = (Tmax - Tmin )Downstream
RangeUS = (Tmax - Tmin )Upstream
And statistical effectiveness as:
 SD 
EStatistical = 1 − DS  × 100%
 SDUS 

(2)

where
SDDS = Temperature standard deviation downstream
of the mixing device
SDUS = Temperature standard deviation upstream of
the mixing device
D ECEM BER 2020

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ASHRAE Journal - December 2020

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