Fundamentals to frontiers
Expansion tanks
By WILLIAM J. COAD
The fundamental components of a
hydronic system are the heat
source, load, circulator, piping, and
expansion tank. Strangly enough,
the most complex device of the five
is probably the one that would ap-
pear to be the least complicated -
the expansion tank. Considering the
fundamentals, the expansion tank
serves a dual purpose. It allows for
the volumetric changes in the flu-
id, resulting from temperature
changes, to occur between planned
pressure limits; and it establishes
the point of constant or known pres-
sure in the system. In many cases.
the tank serves the additional pur-
pose of being an integral part of the
air control subsystem.
It appears that very little has
been published regarding the design
afid sizing of a closed expansion
tank since the article "Compression
Tank Selection for Hot Watet Heat-
ing Systems," by H. A. Lockhart
and 6. F. Carlson, appeared in the
ASHRAE Journal Section of
Hecrtir~glPi~~rngiAir
Co~lditioiling in
April 1953! Yet, since that time,
there have been immense strides in
the application of hydronic sys-
tems.
The correct sizing of expansion
tanks is becoming ever more critical
as larger volume systems are being
employed. Not only are large vol-
umes a result of large capacity sys-
tems, but they are also an integral
part of solar systems and other
power conserving systems that
utillze thermal storage through liq-
uid phase temperature changes. The
very important observation made in
the article cited above was that the
sizing of a tank relates not only to
the volume of water in the system,
the temperature limits, and the
pressure limits, but also to how the
trulk is dc~sigi7rd it~to
t l ~ e
systrm!
The derivation of the formulas for
sizing tanks is rather fundamental if
it can be assumed that the air cush-
ion behaves as a perfect gas. For
such equations, all the necessary
values that are not established as
design parameters are readily avail-
able from a