Duct is a 8" x 8" steel square tube with 0.3125" wall thickness, 200" long.
Duct weight 742 lbs.
One end is closed off and the other end is closed with a 4" OD inlet opening.
An external heat source will be used to pump heated air through opening in the tube.
The duct is place in -20 degrees F outside still air.
Solve for:
Determine overall BTu's per hour to heat the steel from inside to out to bring the outer surface from the -20 to 32F and determine the required forced air temperature of heat source to achieve this.