Efficiency of Steam and
Hot Water Heat Distribution Systems
GARY PHETTEPLACE
sions. If we assume an optimistic value for average
INTRODUCTION
heat loss from current systems of 48 W/m (50 Btu/
Most major Department of Defense (DoD) facili-
hr-ft) (for older systems and those with large pipes,
ties are heated using central heat distribution sys-
greater than 4 in. or 100 mm, a value of several times
tems. The heat from the central heating plants usu-
this is likely) and a cost of .48/GJ (/million
ally is distributed to the buildings as high-tempera-
Btu's) for heat energy, we find that heat losses cost
ture hot water or steam through buried piping sys-
the Army around million per year. Before giv-
tems. DoD has approximately 10,000 km (6000 mi)
ing details on several of the heat distribution effi-
of heat distribution piping systems in service (Segan
ciency studies that CRREL has done, we will outline
and Chen 1984). Many of these systems are old and
some of the basic design considerations for these
in need of major repairs or replacement. To replace
systems.
these systems currently costs about 00/lineal m
(0/lineal ft). Thus, the DoD is facing monumen-
tal costs for replacement. In addition, the technol-
DESIGN CONSIDERATIONS FOR
ogy currently being used by DoD is problematic.
HEAT DISTRIBUTION SYSTEMS
Many systems that have been recently replaced have
failed prematurely. A previous study done by the
The design of heat distribution systems is much
Corps of Engineers (Segan and Chen 1984) identi-
more complicated than the design of other piped
fied many problems caused by improper design,
utilities such as potable water and sewage systems.
installation, and maintenance. Most of these prob-
Because of the need to conserve thermal energy as
lems led to premature failure of the systems. For the
well as the hot water or steam itself, the systems
Army's 1992 fiscal year the annual maintenance
become much more complicated and costly, from
costs were over million (U.S. Army 1992). This
both the installed cost and operations and main-
does not include any of the larger replacement
tenance standpoints. The major challenge in the de-
projects. In addition, as will be shown later, the cost
sign of heat distribution systems is keeping the ther-
of heat loss is much greater.
mal insulation dry so that it remains effective. This
In an effort to reduce the installation and operat-
becomes a formidable task because the system is
ing cost of these systems, the Army has an active
usually buried in the ground, which is frequently
research and development program in heat distri-
saturated with water, and it is carrying water inside
bution technology. The objective of DoD heat distri-
the pipe. Fortunately, the heated pipes tend to drive
bution research is to identify improvements in meth-
moisture out of the insulation. Unfortunately, the
ods and systems that will prove to be less costly and
elevated temperatures accelerate corrosion as well,
problematic. This report gives sample results from
which tends to increase the frequency of leaks. Leaks
several of the Army's research projects.
are very costly in heat distribution systems because
One of the major components of the Army's heat
not only is the treated water itself costly, but the
distribution research is the accurate assessment of
thermal energy contained in the water is also lost
heat losses. As well as being an essential part of the
when a leak exists.
design of these systems, determination of heat losses
Thus, the design of heat distribution systems
is a primary factor in making repair/replace deci-
requires careful examination of the alternatives once
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