(Btu/hr-ft)
(W/m)
110
100
100
90
90
80
80
70
70
60
60
50
50
40
40
OND
J
FMAM
J
J
A
SON
FMAM
J
J
JFMA
M
J
1986
1987
1988
1989
Figure 10. Heat losses for the MTHW trench site.
conduit averaged 54.1C (129.4F), while the undis-
The reduction in heat losses during 1988 and 1989
over the previous years is attributable to the re-
turbed ground temperature at approximately the
duced return temperature during that time period.
same depth as the centerline of the conduit aver-
aged 17.4C (63.3F). This illustrates the rather dra-
The cause of this temperature reduction is not
known. This is a fairly significant reduction and
matic effect of the buried conduit on surrounding
provides a clear example of the benefits of keeping
soil temperatures.
the temperature differential between supply and
The heat losses for the common conduit system
return as large as possible, thus resulting in lower
were calculated by three of the methods described
return temperature. Not only will heat losses be
earlier. The method referred to as the "soil method"
reduced by lower return temperatures, but pump-
uses the single buried pipe equation and the con-
ing costs are also reduced since less mass will need
duit outer surface temperature to calculate the heat
to be circulated. Of course, this assumes that the
flow. The thermal conductivity of the soil is taken as
1.08 W/m-C (7.5 Btu-in./hr-ft2-F) in this and the
thermal load is constant and that some method of
reducing pumping power input, such as variable
CEGS-02695 method, which is believed to be a real-
speed drives or multiple pumps, is available.
istic average value based on the observed soil type
and moisture content (Phetteplace et al. 1991) and
published data (Kersten 1949). Because of the in-
MTHW common conduit site
Figure 11 shows the heat loss from the MTHW
homogeneous nature of soil and the difficulty in
common conduit site over the study period. The
making thermal measurements on soils, this value
temperature of the supply at this site during the
is considered to be accurate only to within 25%.
study period averaged 164.8C (328.7F) and the
The average of the values computed by the three
return averaged 120.8C (249.4F) for the same pe-
methods is 108.8 W/m (113.2 Btu/hr-ft). The high-
riod of time. The supply temperature is somewhat
est of the methods (CEGS-02695) was approximately
higher (1.3C, or 2.3F) than the supply temperature
7.6% greater than the average value and the lowest
observed at the trench site. The return temperature
(insulation method) was 4.3% below the average.
averaged about 130C (23.4F) higher at this site as
Considering the difficulty involved in making ther-
well. This would tend to make heat losses higher at
mal measurements of this nature, we believe this
this site if all else were equal, as of course is not the
agreement is very good. This is particularly true
case. The temperature of the outer surface of the
when one considers that the CEGS-02695 method
18
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