ASHRAE Journal - October 2021 - 58

COLUMN ENGINEER'S NOTEBOOK
the system was handling return air mixed with the MOA.
OAT Above Economizer High Limit Setpoint
When the outdoor air temperature reaches the high
limit setpoint, the economizer-equipped system should
revert to MOA. From this point forward, the coil load is
tied to the space load because the space heat and humidity
will be recirculated and mixed with the MOA.
In contrast, the load on the 100% OA system will always
be divorced from the load in the space (although it will
likely be numerically equal at some point). But the coil
will never see the heat and humidity generated in the
space, just the outdoor air heat and humidity.
Economizer High Limit
A crucial parameter associated with a working economizer
is the high limit setting used to terminate the
economizer process and revert to MOA. The exact setting
will vary with climate and the nature of the load.
Ideally, it should be the outdoor air condition where
the amount of energy required to cool 100% OA is exactly
equal to the amount of energy required to cool the
mixed airstream operating on MOA. Achieving that is
not as simple as it sounds.
Energy codes will provide a recommended setting for a
given climate zone. While there are quite a few strategies
that can be used, it has been demonstrated that the OA
dry-bulb strategy tends to be the best option.#
That is the approach I typically use, but I like to finetune
it, as illustrated via the link included with Figure 2.II
Concept Application
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Applying this concept requires OAT and cooling load
data, which can be obtained via trending and functional
testing. The data is then superimposed on the perfect
system lines as a scatter plot. If everything is perfect:
* The data cloud will follow the perfect lines.
* There will be scatter around the perfect lines due
to sensor accuracy and factors other than OAT that drive
the load.
* Data points that stray into the areas highlighted in
green or yellow in Figure 2 may represent opportunities
to improve performance and/or save
resources.
To demonstrate this, I will use a
spreadsheet that takes the model of
perfection to the next level using:
* Hourly weather data;
* Cooling coil performance
metrics;
* Design conditions;
* Psychrometrics;
* Fundamental HVAC equations;
and
* Logic.
I will use it to project a coil load
for each hour in the weather data
file. The output is superimposed
on the lines of perfection, creating
a " cloud " providing additional
insight.
#See " Economizer High Limit Controls and
Why Enthalpy Economizers Don't Work, "
https://www.cmfnh.com/documents/
ASHRAE_Journal_QA.pdf
IISee Chapter 3 of the " Functional Testing
Guide-Air Handling System Reference
Guide. "
https://tinyurl.com/FunctionalTestingGuide
58
ASHRAE JOURNAL ashrae.o rg O CTO B E R 2 0 2 1
http://www.VERSABLOCK.COM https://www.cmfnh.com/documents/ASHRAE_Journal_QA.pdf https://www.cmfnh.com/documents/ASHRAE_Journal_QA.pdf https://www.tinyurl.com/FunctionalTestingGuide http://ashrae.org
ASHRAE Journal - October 2021

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