A 1 ½ inch pipe, carrying 100 psia saturated steam is
exposed to the surrounding air at 50 degrees Fahrenheit. If emissivity factor is 0.95 and
free-convection coefficient is 2.0 BTU per hour per square foot per degree
Fahrenheit, how much steam is condensing in the pipe. Use inside diameter of the pipe as 0.158 foot.
Solution:
Heat loss due to radiation is given by Fe time
sigma time A times (T14 minus T24). Please note that sigma is Stefan-Boltzmann
constant, and is equal to 0.173 times 10-8 BTU per hour per square
foot per degree Rankine4.
Assuming length of the pipe, L, is 1 foot, surface area, A, is
calculated as pi d L or 0.497 square foot.
Radiation losses are found to be 256.9 BTU per hour. Make sure that you are using absolute values
of temperature. In this case, T1
is 788 and T2 is 510 degrees Rankin.
Heat loss due to free-convection, is h A (T1
minus T2) or 276.6 BTUs per hour.
Combined heat loss is 533.5 BTUs per hour. Latent heat of vaporization of steam is
888.8 BTU per pound. It implies that
533.5 over 888.8 or 0.6 pounds per hour of steam are being condensed.