ASHRAE Journal - August 2024 - 72

REFRIGERATION APPLICATIONS
COLUMN
Andy Pearson
What's in a Nomograph?
BY ANDY PEARSON, PH.D., C.ENG.,FELLOW ASHRAE
Shakespeare wrote " What's in a name? That which we call a rose, by any other name
would smell as sweet. " When it comes to selection software, apps and computer-aided
design tools, however, there are some that are good, some that are tolerable and some
that stink.
Before there were smartphones we had laptops, and
before laptops there were personal computers. Even
further back, when detailed calculations were required
so that competing factors in a design would be
balanced, we had scary looking graphs that presented a
visual path through complex calculations. These might
be showing a pipe sizing calculation given volume
flow rate, density, friction factor and
pressure drop, or a fan calculation to
deliver power requirements and air
pressure developed.
It was quite common to have two or
three graphs arranged on a page so
that a line could be drawn vertically
on one graph to a point of intersection
on a diagonal on the second graph
and then horizontally from there to a
second diagonal on the third graph.
Great ingenuity was required to create
the appropriate layout, and when it was
done well, it created a thing of rare beauty. When the
system offered the calculation in both I-P and SI units, it
got even more convoluted, with axes on all four sides of
each graph, and sometimes colors for additional layers
of complexity. This was problematic in the days before
easy color photocopying because a good nomograph
would be passed from trainer to apprentice, from
master to novice, from generation to generation until
the copies became almost unreadable.
I reminisced about this with a friend at the ASHRAE
conference in Chicago last January, and we realized
that everyone had their favorite nomograph and would
go to great lengths to defend it. My personal gem was
for calculating freezing time for different sizes and
shapes of foods under different types of wrapping in
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ASHRAE JOURNAL ashrae.org A U G UST 2024
The world's daftest app?
different styles of plant and at different temperatures.
This remarkable Rosetta Stone of freezing served me
well until we had to design a plate freezer plant with
carbon dioxide as the refrigerant. I learned then that
the knowledge of the ancients in the nomograph used
some substantial assumptions in the foundations,
particularly relating to the drop in saturated
temperature caused by the hoses that
connect the plates of the freezer to
the suction header. The unusual (and
beneficial) pressure-temperature
characteristic of carbon dioxide
produced a freezing performance that
was literally " off the chart. "
The advantage of a nomograph
over an app is the visual impression
created by the drawing exercise shows
whether the answer lies in a sensible
place or not. These days we are used
to " computer says no, " and if we don't
crash the server or get a terse error message, then it is
natural to assume that all's well with the world. This is
not always the case.
As another example of the visual feedback that is
lacking in many modern selection programs, I used
to have on my smartphone a beautiful example of the
crossover between these two worlds. It was an app that
simulated the function of a traditional slide rule-even
to the lovely wood grain of the frame and the ivory of
the slider. I held in my hand a computer with more
calculating power than the Apollo 11 moon landings,
and I used it to do logarithmic approximations on a
simulation of 400-year-old technology. Crazy, but fun.
Andy Pearson, Ph.D., C.Eng., is group managing director at Star Refrigeration in
Glasgow, UK.
WWW.ISTOCKPHOTO.COM/PORTFOLIO/BALLUN
http://www.ashrae.org
ASHRAE Journal - August 2024

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