small portion of the delivered energy (25%), so this
temperature hot water is a feasible, and in most
additional cost is insignificant when compared to
cases, desirable heat distribution medium for DoD
the greatly reduced heat losses.
facilities. When new systems or major replacement
Low temperature systems require larger heat exchang-
projects are planned, serious consideration should
ers. This is true; however, it does not become a
be given to this technology.
major disadvantage for several reasons. Because of
To illustrate the efficiency of several types of
the lower temperatures, it is often possible to elimi-
heat distribution we will present results from sev-
nate the need for heat exchangers for building heat-
eral field studies conducted by CRREL over the last
ing altogether. Since higher temperatures must ulti-
few years. First we will discuss an efficiency analy-
mately be reduced within the building systems un-
sis conducted on a steam system at Hawthorne AAP.
der most circumstances, lower supply temperature
is seldom a problem in space heating applications.
If heat exchangers are needed or desired for isola-
EFFICIENCY ANALYSIS OF A STEAM
tion purposes, larger buildings can use plate-and-
HEAT DISTRIBUTION SYSTEM
frame heat exchangers which, because of their large
surface area to volume ratios, are economical for
The following efficiency analysis was conducted
LTHW applications. For domestic hot water pro-
on data obtained from Hawthorne Army Ammuni-
duction, careful design of the heat exchange system
tion Plant (Hawthorne AAP). This facility is located
can yield a very close approach temperature be-
in Mineral County, Nevada, about 220 km (135 mi)
tween the LTHW supply and the delivered domes-
southeast of Reno. Hawthorne AAP is located in a
tic hot water, again eliminating any problems with
high desert region 1276 m (4,186 ft) above sea level.
inadequate temperature at the end user. In smaller
The 99% dry bulb heating design temperature is
13.9C (7F) and the 97.5% dry bulb heating design
buildings, "brazed" heat exchangers, which are very
temperature is 11.7C (11F) (U.S. Army 1978). This
similar to plate and frame units except they cannot
climate accumulates 3060C-days (18.3C base)
be disassembled, are becoming very popular in Eu-
(5508F-days, 65F base) during an average heating
rope because of their low cost and extreme com-
pactness. For example, a stainless steel brazed heat
season (U.S. Army 1978).
exchanger including preformed insulation, for a res-
The compound of Hawthorne AAP covers about
5.87 108 m2 (145,000 acres) and has 2,873 build-
idential size application is about the size of a lunch
box, and would cost about 0.
ings. The land and buildings can be divided into
The large surface area to volume ratios which
two categories: industrial and ordnance. This study
can be achieved in plate type heat exchangers make
addresses only the buildings in the industrial area
of Hawthorne AAP. These buildings consist of of-
their performance impressive. Consider, for exam-
fices, shops, housing units, and recreational and
ple, a typical design for a brazed plate unit that
other facilities. Most of these buildings are heated
might be used on a small space heating or domestic
by steam from a central distribution system. The
water heating load of about 20 kW (70,000 Btu/hr).
With a primary water supply temperature of 120C
steam is generated in a central plant using fuel-oil-
(250F) and design secondary side temperatures of
fired boilers. The conversion of this central heat
90C (195F) supply and 70C (160F) return, the
distribution system from steam to low temperature
approach temperature would be less than 0.5C (1F).
hot water was the subject of a detailed study
(Phetteplace 1991a).
This means that the primary return water would be
at about 70.5C (161F), within 0.5C (1F) of the
theoretical minimum of the entering secondary wa-
Heat generation and distribution
ter temperature. The flow rates through the heat
at Hawthorne AAP
The existing steam distribution system at Haw-
exchanger would be about 6 liters/min (1.6 gpm)
thorne AAP consists of a steam supply line and a
on the primary side and 14.8 liters/min (3.9 gpm)
condensate return line. The distance along the steam
on the secondary side. Pressures losses would be
line from the central plant to the most distant con-
about 34.5 kPa (0.5 psi) on the primary side and 145
sumer is less than 1 mile. The steam pressure is
kPa (2.1 psi) on the secondary side. The heat ex-
normally around 6.9 bars gage (100 psig) at the
changer used for this sample design would be about
entrance to the distribution system. This pressure is
53.3 cm (21 in.) long, 11.4 cm (4.5 in.) wide, and 4.1
reduced in the buildings to a nominal 1 bar (15
cm (1.6 in.) thick with 14 plates and an empty weight
psig). In the housing area, a central pressure reduc-
of less than 5.4 kg (12 lb).
ing station reduces the pressure to approximately
The above discussion clearly illustrates that low
5
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