ASHRAE Journal - February 2020 - 59

COLUMN ENGINEER'S NOTEBOOK

FIGURE 5 IR image of preheat coil.

FIGURE 6 OA and RA combined prior to air handler.

AHU-3 Preheat Coil (Upper Portion)
OA

Control Damper (TYP)

Intake
Plenum

IR000230.IS2
Average Temperature 56.1°F
Image Range
48.1°F to 64.3°F
Image Time
March 14, 2011 2:20:49 p.m.

to perform adequately.
* Combine airstreams in the ductwork upstream of
the air handler if possible. Install control dampers in the
ductwork.
* When using a mixing box, use a top/back connection conﬁguration.
* If possible, do not use a side/back, side/side or side/
top connection conﬁguration. If you use this conﬁguration, study the layout, dimensions and relative location
of the connections carefully. Consider adding a stationary mixing device.
* When using a mixing box, design return air and outside air openings to be the full width of the mixing box.
* If the unit is variable volume and has an economizer, study the range of relative expected RA/OA volumes
and total airﬂow volumes.
A design to adequately mix OA and RA airstreams
requires careful thought and attention. To help improve
the performance of mixing boxes and stationary mixers,
I think the industry needs a standard method for testing
and rating the performance of these devices.

References
1. Delaney, T.A., T.M. Maiocco, A.G. Voge. 1984. "Avoiding coil
freezeup." Heating, Piping, & Air-Conditioning (12).
2. Environmental Protection Agency. 1991. "Building air quality:
a guide for building owners and facility managers."
3. Kao, J.Y. 1985. "Sensor errors in VAV systems." ASHRAE Journal.
4. 2016 ASHRAE Handbook-HVAC Systems and Equipment, Chapter 4.
5. Robinson, K. 2000. "Rating air-mixing equipment." ASHRAE
Journal (2).

Air Handler

6. Robinson, K. 1998, "Damper control characteristics and
mixing effectiveness of an air-handling unit combination mixing/
ﬁlter box." ASHRAE Transactions 104(Pt. 1).
7. S.C. Lee, H.J. Sauer, F. Finaish. 1996. "Computer Simulation
of Turbulent Mixing Between Outdoor Air and Return Air in HVAC
System." ASME, FED-Vol. 239, pp. 497-502.
8. F. Finaish, H. Sauer, B. Van Becelaere. 2002. "ASHRAE 1045TRP: Verifying Mixed air Damper and Air Mixing Characteristics
Final Report."

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FEBRUARY 2020

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ASHRAE Journal - February 2020

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