It`s got me under pressure

Expansion vs.
It’s got me under pressure
Steve Tredinnick, PE, Vice President of Energy Services, Syska Hennessy Group
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.
oday it seems like everyone is overextended and constantly under pressure. There is pressure from our jobs,
our bosses, our spouses, our families and so
on. Pressure here, pressure there and pressure everywhere. Pressure! (I think I just
heard a Billy Joel riff.)
Most people have some sort of outlet
for relieving stress-induced pressure. They
‘decompress’ by having a vacation, exercising,
listening to music, kicking the dog (not
recommended or endorsed), being a couch
potato and even enjoying an adult beverage or two (or four if they live in Wisconsin
like me. No pressure here!).
Pressure relief is a good thing. Without
it, things will literally blow. Too much stress
and your blood pressure gets too high and
you will bust an artery. The same goes for
your heating or cooling water systems. Is
your water system under pressure and close
to the breaking point? Is the safety valve
ready to blow? Luckily, engineers long ago
prescribed the perfect pill for this: the expansion tank/compression tank or expansion
chamber, one of the really simple set-it-andforget-it devices.
Technically speaking, I personally think
District Energy
we find more actual compression tanks
than expansion tanks used nowadays.
Since this may be a lexicon question, we
should set the record straight as to what
the differences are, per Roy Algren of ITT
Fluid Handling and ASHRAE’s viewpoint in
“Tanks,” January/February 2003 Plumbing
Systems & Design magazine.
● Expansion Tank – “a partially filled tank,
operating at atmospheric pressure, at the
top of a water system for the accommodation of volume expansion.” Expansion
tanks are mostly found in older hydronic
systems and rarely seen in modern designs.
● Compression Tank – “a pneumatic
cushioning device, operating at system
pressure, which absorbs fluid expansion
as a result of temperature change.” Fundamentally, a compression tank is a partially
filled tank under pressure, usually having a
diaphragm or bladder separating the system
water from a gas cushion (air or nitrogen).
As you can see, what we normally
construe as expansion tanks are really
compression tanks. The term ‘expansion
tank’ has been used for so long it has
become the universal brand name for this
device, similar to how the Kleenex® brand
name is used universally. I dropped the
term ‘compression tank’ at a meeting a
few weeks ago and received a baffled
expression from a contractor. After I
explained the device and its function, he
said, “Oh, you mean an expansion tank!”
Sometimes I don’t have the heart to lecture
and have to pick my battles. Comprende?
Compression tanks have a few advantages over expansion tanks. For one, they
are about 80 percent smaller than conventional expansion tanks due to the fact that
they are under pressure and not atmospheric. Also, they are pretty flexible as far
as where they can be located. Although
the top of a building may provide the lowest pressure point for locating compression
tanks, they may actually be connected anywhere in the system. Usually, however, they
are placed close to the distribution pump
suction. Close to the pump is ideal, since
that is most likely in a mechanical room and
therefore more serviceable than a tank that
is tucked away in a ceiling space or closet
on the highest floor of the building.
What we normally construe
as expansion tanks are really
compression tanks.
Although compression tanks require
little maintenance, they should still be
accessible just in case they need recharging – because if your memory is like mine
and you set it and forget it, you may forget
where you set it in the first place.
So what do these tanks actually do?
● They provide a cushion in the system to
absorb the increased water volume as
system temperature rises. If not for the
expansion device, the increasing volume
could damage many piping system components since water is not compressible.
● They provide a point of ‘no pressure
change’ or act as a reference point of
pressure for the system, analogous to a
ground in electrical systems. This is important, especially if you want to locate the
most appropriate place to fill – or makeup water to – the system.
● They positively pressurize the water system
above atmospheric pressure to prevent
the intrusion of air and permit air-venting
devices to work properly. This typically
requires a minimum of 4-5 psig at the
top of the hydronic system.
● They maintain enough net positive suction head required to satisfy pumps and
prevent cavitation in control valves – mostly
an issue for heating systems and not for
cooling systems.
Reprinted from Second Quarter 2007 District Energy magazine with permission of IDEA.
If having one compression tank is so
good, are multiple tanks even better? Not
really. Multiple tanks can be accomplished
using adequate controls and pressure
monitoring, but usually it is not a contest
where having the most toys wins. Having
multiple tanks can create unexpected pres-
Multiple compression tanks can
create unexpected pressure
changes in your hydronic system.
sure changes in your hydronic system,
especially at the pump suction. This is particularly important if your existing facility
just connected to a district energy hydronic
system with a direct connection – or without a heat exchanger acting as a pressure
interceptor. The building’s existing compression tank should be valved off or preferably removed, unless there are extenuating
circumstances such as valving off your
building and using existing boilers and
chillers to self-generate in an emergency or
planned dispatchable condition. In this case,
the compression tank may be re-energized
to keep the system operating correctly.
So now that I have ‘expanded’ your
knowledge (pun intended) and unconfused
you about compression tank use, I can get
back to decompressing with my favorite
adult beverages (I am in Wisconsin, ya
know!) and listen to an appropriate ZZ Top
song with another great guitar riff: “It’s
got me under pressure.”
Steve Tredinnick, PE, is vice
president of energy services
for Syska Hennessy Group in
Madison, Wis. 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 may
be reached at
Column and previous columns available at
Reprinted from Second Quarter 2007 District Energy magazine with permission of IDEA.
Second Quarter 2007