the site conditions and project requirements are
ever, with energy costs still low, they offered a good
known. Some of the basic issues that impact distri-
compromise because large temperature differentials
bution system efficiency are addressed below. For a
could be achieved and thus mass flow rates and
more complete discussion of the issues and alterna-
pipe sizes remained lower than was possible with
tives Phetteplace and Meyer (1990) and ASHRAE
water at lower temperatures. With low temperature
(1992) should be consulted.
hot water it would have been more difficult to
achieve temperature differentials of the same mag-
nitude with the heat exchanger technology of the
Distributed media selection
District heating began in the United States in the
time. In Europe, however, energy costs were higher
late 19th century. These early systems used steam as
and lower temperatures were shown to offer lower
the heat-carrying medium. The steam-based sys-
life cycle costs, particularly since the lower tem-
tems thrived with the inexpensive energy prices of
peratures were much more favorable for cogenera-
the time and many systems were built, with a large
tion of heat and electricity. A number of other mate-
number of them still operational today. The very
rial developments, such as suitable nonmetallic jacket
high load density of the densely developed down-
materials and polyurethane foam insulation, appli-
town areas these steam systems serve allows them
cable only to lower temperature systems, further
to tolerate the large distribution losses and even
widened the advantage these systems held. These
today remain competitive with alternative means of
materials either were not able to withstand the higher
building heating.
temperatures or degraded very rapidly under such
After World War II many district heating sys-
conditions. Currently nearly all systems in Europe
tems were built in European cities. Both steam and
use low temperature hot water. Such systems have
hot water-based systems were built. As the technol-
been so successful in Europe that the market pen-
ogy for the hot water systems evolved, it became
etration is very high, for example, approximately
clear that they were a good alternative to the steam
50% in Denmark and Finland. The major advantage
systems. The hot water systems eliminated the prob-
that low temperature water holds with regard to
lems of condensate handling and were easier to
heat losses is best illustrated by actual field mea-
control than the steam systems. Thus, most systems
surements made on low temperature hot water and
built in Europe after the war were hot water based.
medium temperature hot water systems installed
This technology was brought to the United States in
on Army bases. A study that made such measure-
the early 1950s and a number of hot-water based
ments is described later in this report.
systems were built in the United States, particularly
Regardless of whether steam or hot water is cho-
on DoD facilities. These systems used pressurized
sen, the temperature and pressures used should be
water at temperatures above 175C (350F) in most
only as high as required to satisfy the requirements
cases. Hot water systems are normally broken into
of the consumers. This cannot be overemphasized.
three temperature classes. Systems with supply tem-
Higher temperatures result in higher heat losses, as
peratures over approximately 175C (350F) are usu-
will be shown from the data presented later. In
ally considered to be high temperature hot water
addition, higher temperatures and pressures result
(HTHW) systems. Medium temperature hot water
in numerous other problems such as accelerated
(MTHW) systems have supply temperatures in the
corrosion, higher rates of leakage, and lower safety
range of 120 to 175C (250 to 350F). Low tempera-
and comfort levels for operators and maintenance
ture hot water (LTHW) systems have supply tem-
personnel. Higher temperatures also may require
peratures of 120C (250F) or lower. In practice, the
higher pressure ratings for piping and fittings and,
return temperature from all these types of systems
in addition, may preclude the use of desirable mate-
varies widely, with the higher temperature systems
rials such as polyurethane foam insulation and non-
tending to have higher return temperatures. Achiev-
metallic conduits. For hot water systems it is im
ing a large temperature difference between supply
portant to design for a high temperature drop at the
and return is desirable because this reduces the
consumer. This reduces the flow rate through the
amount of water that needs to be circulated for a
system and thus the pumping power required. High-
given heat delivered.
er temperature drops at the consumer also result in
While the high temperature hot water systems
lower return temperatures, which in turn result in
proved to be much less problematic than steam
lower return line heat loss, as the data to be present-
systems, they still suffered from relatively high lev-
ed will show.
els of heat loss, as well as other problems associated
Although hot water is widely regarded as the
with elevated temperatures and pressures. How-
best alternative where either medium will meet the
2
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