requirements, in many instances existing equipment
problem. Nonmetallic piping has been used with
and processes will require the use of steam. Some
only limited success. For these reasons condensate
common examples of where steam may be required
was seldom returned from older systems. How-
on DoD installations are laundry facilities, mess halls,
ever, at current energy costs it becomes imperative
motor pools, and hospitals. A careful study should
to do so. Condensate drainage systems (steam traps,
be made of the alternatives before selecting the dis-
condensate pumps, and receiver tanks) have also
tribution medium and temperature. A list of com-
proven to be problematic, corrosion again being
mon attributes with a brief discussion of the relative
one of the major sources of problems. Although
merits of each medium is given below.
much developmental effort has been expended on
steam traps, the best designs today still have a rela-
Heat capacity
tively short (less than five years) life expectancy and
Steam has a distinct advantage as it relies prima-
will leak several pounds per hour of steam in the
rily on the latent heat of water rather than the sen-
closed condition even when new. In summary, prob-
sible heat. The net heat effect for saturated steam at
lems in condensate collection and return are the
6.9 bars (100 psig) (170C or 338 F) condensed and
major disadvantages of steam systems.
cooled to 82C (180F) is approximately 2.42 mJ/kg
(1040 Btu/lbm). For hot water cooled from 175 to
Pressure requirements
120C (350 to 250F) the net heat effect is 0.24 mJ/kg
Flowing steam and hot water both incur pres-
(103 Btu/lbm), or only about 10% as much. Thus, a
sure losses. Hot water systems may use intermedi-
hot water system must circulate about 10 times more
ate booster pumps to increase the pressure at points
mass than a steam system of similar heat capacity.
between the plant and the consumer. Pressure varia-
tions due to elevation differences within hot water
Pipe sizes
systems are much greater than for steam systems
Although much less mass of steam is required
due to the higher density of water. This can ad-
for a given heat load, steam usually requires a larger
versely affect the economics of a hot water system
pipe size for the supply line due to its lower density.
by requiring the use of a higher pressure class of
This is compensated by a much smaller condensate
piping and/or booster pumps.
return pipe. Piping costs for steam and condensate,
as opposed to hot water supply and return, are
Low temperature
comparable. With regard to the temperature level in
water design issues
hot water systems, given an equal temperature dif-
Because low temperature hot water systems are
ference between supply and return, the tempera-
the least common system type in the United States,
ture level of the supply will have no effect on the
designers are often not as familiar with these sys-
mass flow rate. Thus, aside from any minor effects
tems. For this reason we will address a few issues
caused by viscosity and density differences, a low
with hopes of dispelling some common myths.
temperature water system with a temperature dif-
From a designer's perspective low temperature
ference of 55C (100F) will have the same size pipes
water systems are no more difficult, and in most
as a high temperature system with a temperature
cases simpler, than steam and high temperature
difference of 55C (100F). Because the heat can be
water systems. Distribution system design is straight-
used only down to some practical lower limit of
forward and is less complicated than steam or high
temperature, low temperature hot water systems
temperature hot water because of the lower tem-
may not have as high values of temperature dif-
peratures and pressures involved. Because of the
ference between supply and return. Thus, if temper-
high degree of standardization of these systems, a
ature difference were the only consideration, higher
number of specialized components are available that
temperature hot water systems would be favored.
simplify the design process.
However, when the increased heat losses, mainte-
Low temperature water is well suited to the grad-
nance and system complexity that result from higher
ual change-over of the distribution medium from
temperatures is considered, lower hot water temper-
steam or high temperature hot water. When major
atures become more favorable. With proper design
distribution replacement projects are under consid-
they will not require significantly larger piping.
eration, or a new area of a base or community is
being expanded into, low temperature water can be
Return system
used simply by putting a heat exchanger station
Condensate return systems have proven to be
between the existing network and the new low tem-
very problematic. Corrosion of the piping is a major
perature system. In this way gradual conversion
3
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