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 Inside Insights
Tales of the Paranormal:
PHEs, variable counter current flows and the “X-Files”
Steve Tredinnick, P.E.
Vice President of Energy Services
Syska Hennessy Group
Fourth Quarter 2007, District
Energy Magazine
Editor’s Note: “Inside Insights” is a column
designed to address ongoing issues of interest to building
owners, managers and operating engineers who use district
energy services.
Some things just aren’t what
they seem at first glance. An example is the trip my two
daughters took this summer to visit friends who had relocated
to southern Oregon.
One of the many tourist traps
– er…I mean roadside attractions – they visited was called the
Oregon Vortex and House of Mystery. I had never heard of such
a place, but apparently it has a number of optical illusions
that occur due to the local paranormal properties of the area:
balls roll up hill, brooms stand on their own, people change
heights – you know, stuff so strange that you go out of your
way to cough up $9 to see it. (Evidently, there are many sites
around the world with the same phenomena, such as
Pennsylvania’s Laurel Caverns and the Santa Cruz, Calif.,
Mystery Spot, so there may be one near you too!)
The Oregon site was so weird
that it was mentioned in an “X-Files” episode. It had me so
intrigued that I wished I had played hooky from work and
joined them on their paranormal sojourn. It also reminded me
of a similar mystical discussion that occurred at the office
recently regarding plate heat exchangers (PHEs) and varying
flows. Believe it or not!
My colleague and I are
currently studying the idea of pooling the reserve capacity of
several building cooling plants and interconnecting them into
one common chilled-water loop. There are six buildings to
connect of which four are high-rise buildings (more than 14
floors) and two are only five stories. A PHE will be used on
the low-rise buildings to protect the existing chilled-water
system from the high static pressures imposed by the adjacent
buildings. The low-rise buildings currently have constant flow
primary pumping with constant speed booster pumps at each air
handler. There are about 15 air handlers in each building
whose cooling coils are controlled by three-way valves. Our
plans also included retrofitting each building to variable
flow on the distribution pump (secondary) side, thus
eliminating the three-way valves and replacing them with
two-way valves.
I stated that converting to
variable flow will not only (1) save pump energy in the
building by reducing the distribution water flow, but will
also (2) keep the delta T high through the PHE. The first
concept was understood, but the second was not. I attempted to
explain that variable flow on both sides of the PHE was
essential to get a more linear exchange of heat and
proportional flow of water (and hence delta T) on each side.
My colleague was not convinced by my explanation of this
occurrence and paranormal phenomena and was convinced I was
smoking something mind-altering.
Hell hath no fury like an
engineer dissed (or something like that)! Energized by his
challenge, I unearthed a paper from my dusty archives that was
presented by Susan Purdue (now Susan Hammock) in 1999 at the
IDEA Cooling Conference in Atlantic City, N.J. Susan addressed
this very issue in “Understanding Design Implications of
Constant vs. Variable Flow Pumping on Plate Heat Exchangers.”
(To read the complete paper, visit
www.districtenergy.org
and search the archives for Hammock; the paper is
complimentary for members, a small fee for nonmembers.)
Using Alfa Laval’s ALfaQalc
PHE selection program, I recreated the data presented in
Susan’s paper. I pointed out to my coworker that the paper’s
conclusion stated the transfer of heat through a heat
exchanger is not a linear relationship and variable flow on
both sides of the heat exchanger is critical in maintaining
system delta T’s. The graphs were irrefutable evidence to this
fact when looking at the constant flow scenario and how much
additional cold-side (district-side) water flow was required
for an equivalent load as to variable flow.
- The transfer of heat
through a heat exchanger is not a linear relationship and
variable flow on both sides of the heat exchanger is
critical in maintaining system delta T’s.
As can be seen from figure 1,
variable flows on both sides have a more linear function
compared to the constant flow values indicated in figure 2. As
an example, at 50 percent of the building load, 51.3 percent
of peak capacity flow is required for a variable system, and
79 percent of peak capacity flow is required for a constant
flow system. This is further reinforced by the 56 degree F
district return temperature for variable flow and 49.6 F for
constant flow at these conditions.
Figure 1. Plate Heat
Exchanger Performance With Variable Customer Flow.
Source: Steve Tredinnick, Syska Hennessy Group.
PHE
selection by Susan Hammock, Alfa Laval.
Figure 2. Plate Heat
Exchanger Performance With Constant Customer Flow 
Source: Steve Tredinnick, Syska Hennessy Group.
PHE
selection by Susan Hammock, Alfa Laval.
Once this information was
presented, I persuaded my coworker of the virtues of variable
flow on both sides of the PHE. By varying the flow on both
sides of the PHE, you can not only save pump energy but also
increase the system delta T. Furthermore, I hope that I have
instructed and convinced District Energy readers regarding
this occurrence and illustrated that this phenomenon is not
from “The Outer Limits,” just basic, sound engineering
practice. Of course, relating this success story to my kids
was futile, since I was reminded once again that I was a
‘weird’ phenomenon and a geek to boot. With teenagers,
sometimes I just can’t win.
Author’s Note: The
author credits Susan Hammock of Alfa Laval for her technical
assistance with this column.
Based in Madison, Wis.,
Steve Tredinnick, PE, is vice president of energy services
for Syska Hennessy Group, which has 16 locations across the
U.S. He has more than 24 years' experience related to building
heating, ventilation and air-conditioning systems. The past 13
years of his work have been focused on district energy
systems. Tredinnick is a graduate of Pennsylvania State
University with a degree in architectural engineering. He is a
member of IDEA and ASHRAE and is currently chair of ASHRAE TC
6.2 District Energy. Tredinnick currently serves on IDEA’s
board of directors. He may be reached at
stredinnick@syska.com.
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