ASHRAE Journal - April 2022 - 46

ENGINEER'S NOTEBOOK
COLUMN
David Sellers
The Perfect Economizer
BY DAVID SELLERS, P.E., MEMBER ASHRAE
My previous column* in the October 2021 issue of ASHRAE Journal explored a technique
that compares a chilled water plant's performance to perfection (Figure 1). Data from
a near-perfect plant will create a cloud around the solid lines if plotted against them.
A hazy blue cloud nowhere near the lines, like the areas outlined in red, yellow and
green, indicates a potential problem. A common reason for the unnecessary chilled
water use in the areas outlined in red and yellow is dysfunction in the preheat and
economizer processes. Thus, the team I was working with at the facility decided to use
a similar idea that I call " The Perfect Economizer Concept " to assess their air-handling
units (AHUs).† That analysis technique is the focus of this column.
Nothing New
The " Perfect Economizer " concept is similar to the
" Perfect Load " concept from my last column in that you
create a chart that illustrates perfection and then plot
real data against it to see how closely reality matches it.
The idea is not particularly new. For example, the
(free) Universal Translator application§ includes a module
that uses this approach.
This column will illustrate how the concept works, and
how it can be used to perform diagnostics founded on
field data. If I am successful, you should be able to take
what I write and build a spreadsheet in Excel or similar
application that will perform the analysis.# When I
first started using this approach, we did the math with
* " Modeling Perfection " ASHRAE Journal, October 2021.
† As a clarifying point, I am going to focus on a perfect airside
economizer. But the fundamental principles can be extrapolated to
waterside economizers.
§ https://tinyurl.com/UTranslate
#For those who would like a starting point, you can download the
spreadsheet behind the images in the article at https://tinyurl.com/
PerfectEconomizer
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paper, pencil and calculator using a handful of manually
measured data points. The evolution to computers
has opened the door to much more powerful data visualization
capabilities.
The analysis bottom line is based on how well the system
in question mixes outdoor air (OA) and return air
(RA) relative to a theoretical requirement for mixed air
(MA), i.e., " perfection. " It is important to recognize that
while mixing is a key goal for an airside economizer,
there are other important, related processes, including
minimum outdoor air (MOA) regulation, integration
with mechanical cooling, preheat, humidification and
dehumidification, and building pressure control.
The technique will flag issues with MOA percentage
and preheat and cooling integration but does
not directly address building pressure control.
Dehumidification and humidification integration are
somewhat addressed by the various settings used to
define the inflection points in the lines of perfection.
David Sellers, P.E., is senior engineer at Facility Dynamics Engineering's office in
Portland, Ore.

https://www.tinyurl.com/UTranslate https://www.tinyurl.com/perfecteconomizer https://www.tinyurl.com/perfecteconomizer http://ashrae.org

ASHRAE Journal - April 2022

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