= 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.

-- Then, check the assumed air film tempera-

the example above if installed at Barrow, Alaska,

ture:

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