ASHRAE Journal - October 2021 - 55

COLUMN ENGINEER'S NOTEBOOK
and their real-time interactions with the occupants and
climate.
It's All About the Load Profi le
The models I cut my teeth on ran on bin weather data
algorithms due to the available computer technology,
which involved a machine as big as a house at a remote
location-a crude approach by today's standards. But
crude as they were, they provided insight into the load
profi le, and that information is golden.
It's one thing to select/confi gure equipment to handle
the design condition in an effi cient, repeatable, reliable
and robust manner. It's an entirely different-and far
more critical-thing to select/confi gure the equipment to
deal with the nuances of the load profi le.
Therein lies the challenge (and I would suggest, the
joy) of our occupation.
Cooling Load, Cooling Coil Load,
And Outdoor Air Temperature
If you asked the " average person " if cooling load was
related to OAT, they'd probably say " absolutely. " But if
you work with complex buildings, you know there is a lot
more to it.
An eye opener for me early in my career came on a
sunny but wintery (5°F [-15°C]) St. Louis day when I
realized that without cooling, the core of the high-rise I
was troubleshooting would overheat.* Clearly, the core
load was related to more than outdoor air temperature.
I subsequently realized that:
* The coil load (vs. space load) is very much tied to the
OAT;
* For a 100% outdoor air (OA) system, the coil load is
an OAT load; and
* Integrated economizer-equipped systems are 100%
OA loads much of the time.
Granted, space and coil loads are related:
* The space sensible heat ratio (SHR) likely set the coil
leaving conditions.
* For variable fl ow systems, the sensible gains in the
space will drive the fl ow profi le.
But, if you gain a sense of the fl ow profi le for a system
(using trending or engineering judgement) and
determine the coil's design performance metrics, you
can understand a lot about the design intent, space
loads, ventilation loads and how the system would use
resources if it were functioning as intended. Comparing
that to reality allows you to identify issues (aka
opportunities).
Cooling Coil Insights
You can deduce a lot from cooling coil performance
specifi cations. Ideally, you will fi nd them on a drawing
or equipment submittal. Failing that, other options
include using a coil program to build a geometrically
similar model of your coil, and/or functional testing.
Minimum Outdoor Air Percentage
For an economizer-equipped system, the cooling coil is
typically selected for the design fl ow rate on the design
day with the system using minimum outdoor air (MOA)
percentage. The entering air condition represents the
result of mixing return with outdoor air and will be proportional
to the MOA fl ow and the properties of the two
airstreams.
In EBCx, uncertainty can exist about what the original
design conditions were. But if you plot the return condition
and coil entering condition on a psych chart and
project a line through those points, the design condition
should lie on that line.
You can contrast known ASHRAE conditions with your
projection. In a perfect world, the line will intersect one
of them. In an imperfect world it won't, but it will come
close enough to one to allow you to make an informed
decision about the designer's thinking.
From there, you can project the MOA percentage
(Figure 1) or deduce it mathematically (Equation 1.†)
%Outdoor Air
()−
−
t
t
%Outdoor Air =
=()
Mixed Air Return Air
Outdoor Air R
t
t
()−
()−
78 2 73 0
938 730
..
..
eturne Air
= 0.252 25 %()
Having established the design OA and MOA conditions,
you can use similar techniques to " mine " the coil performance
for additional information. One thing will lead
to the other, starting with the ventilation load, the total
*The operating team had disabled economizers to keep coils from freezing due to poor mixing design.
†Derived at " Economizers-The Physics of a Mixed Air Plenum. " https://tinyurl.com/OAPctDerived
O CTO B E R 2 0 2 1 ashrae.o rg ASHRAE JOURNAL
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ASHRAE Journal - October 2021

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