(Btu / hr - ft) (W / m)
50
50
45
45
40
40
35
35
30
30
25
I
Soil Method
25
nsulation Method
20
06 - 01 - 89
12 - 01 - 89
06 - 01 - 90
12 - 01 - 90
06 - 01 - 91
12 - 01 - 91
Date
Figure 13. Heat losses from the LTHW site 1 system.
the FRP jacket averaged 37.6C (99.7F) and 37.1C
difference compared to the common conduit site is
(98.7F) for the supply and return, respectively. The
the increased insulation resistance at this site. Be-
undisturbed ground temperature at approximately
cause the pipes at this site are buried very deep (3.0
the same depth as the centerline of the pipe aver-
m or 9.7 ft) this site has a relatively significant ther-
aged 25.5C (78.0F). Here the average temperature
mal resistance in the soil which results in significant
difference between the outside of the jacket and the
temperature drops in the soil. Thus, the tempera-
undisturbed soil temperature at the burial depth is
ture drop between the jacket and the undisturbed
11.7C (21.2F), the lowest of any of the direct bur-
soil at this site is slightly more than at the individual
ied systems in this study. Despite the fact that this
conduit site even though the heat losses are much
system is buried deep (2.5 m or 8.2 ft.), the relatively
lower.
high level of insulation results in less significant
The heat losses for the LTHW site 1 system were
temperature drops in the soil compared to the LTHW
calculated by the insulation and soil methods de-
system site 1, where the same thickness of insula-
scribed earlier. The thermal conductivity of the soil
was taken as 1.46 W/m-C (10 Btu-in./hr-ft2-F) for
tion is used on a pipe nearly twice as large in outer
diameter. Thus, the temperature drop between the
the soil method. This value is based on published
jacket and the undisturbed soil at this site is con-
data (Kersten 1949) and measured soil moisture and
siderably less than for site 1 on the LTHW system.
classification. The average of the heat loss values
As was done for site 1 on the LTHW system, heat
computed by the two methods is 37.9 W/m (39.4
losses from site 2 were calculated by the insulation
Btu/hr-ft). The highest of the methods (soil method)
and soil methods described earlier. Again, the ther-
was approximately 10.1% greater than the lowest
mal conductivity of the soil was taken as 1.46 W/m-
(insulation method). The heat losses for the LTHW
C (10 Btu-in./hr-ft2-F) for the soil method. The
system site 1 for the study period are shown in
Figure 13.
average of the heat loss values computed by the two
methods is 24.7 W/m (25.7 Btu/hr-ft). The highest
of the methods (soil method) was approximately
LTHW preinsulated system, site 2
For the LTHW system at site 2, the temperature
6.4% greater than the lowest (insulation method).
of the supply during the study period averaged
The heat losses from site 2 of the LTHW system
83.5C (182.3F) and the return averaged 80.1C
over the course of the study period are shown in
(176.2F). The temperature of the outer surface of
Figure 14.
21
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