TM 5-852-5/AFR 88-19, Volume 5
= 0.139 hr ft F/BTU
-- Thermal resistance of insulation
RI = 1n(rout/rin)2BkI:
Inside radius, rin = 3 inches
Outside radius, rout = 5 inches
-- If the values did not check it would be
necessary to repeat the calculation with another
= 6.115 hr ft EF/BTU.
assumed Ts until a reasonable check is attained. The
combined thermal resistance (Rc) is
-- To determine the thermal resistance of the air
film (RA) it is necessary to estimate the surface
Rc = RA + RI + Rp
= 0.0504 + 6.116 + 0.1296
= 6.306 hrftEF/BTU.
-- From figure 12-4a, N = 0.23 and w =
(12.5V+1) where V + windspeed, mph:
--The rate of heat loss (Q) is
W = ((12.5)(15)=1)
= 13.73
-- In this case Rp is to the outer surface of the
insulation = 5/12 = 0.417 ft. For the first iteration
one must assume a surface temperature (Ts). This
will be close to air temperature. Assuming Ts =
-39EF, then:
--Figure 12-8 can be used to obtain an estimate
of heat loss if it is assumed that the thermal
resistance of the air film and of the pipe material are
negligible:
= 7.573 BTU/hr EFft
-- Then, to calculate thermal resistance of air
film:
K1 = 0.0133 BTU/fEFhr.
--Then, enter figure 12-8 with these values and
R =6.4.
=12.5 BTU/hr linear foot (LF) of pipe.
e. Compare the heat losses for the water pipe in
-- Then, check the assumed air film tempera-
the example above if installed at Barrow, Alaska,
ture:
12-18
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