Pipe Temp - Wind Outside & Hot Gas Inside 1

[meca]

I have a pipe that has a hot gas (air at low velocity) on the inside (800 Deg. F). On the outside of the pipe I have a 90 mph wind blowing across the pipe (70 Deg. F). I’m trying to determine the average steel temperature on the pipe itself. The pipe itself is Carbon Steel 70" OD and 0.3125" wall, with no insulation or refractory.

Can someone point me in the right direction for solving this type of problem? I'm a Mechanical Engineer, but my work is more in the stress and strain arena.

 

[jte]

meca-

First assume that the temperature gradient within the steel is neglegable. Chances are that it will be on the order of 2 to 20 degrees. You need to get some values for the convection coefficient h, I'd expect to see somewhere on the order of h=2 to 10 BTU/hr*sqft*°F for the slow air on the inside. The 90 mph wind will be more of a challenge to find data for. Figure on the order of 100 for starters.

Now you have to set the heat transfer from the fluid inside the pipe to the wall of the pipe equal to the heat transfer from the wall of the pipe to the ambient air. For a one hour period over one square foot of pipe, q=h*deltaT.

So h(inside) times (800-Tpipewall) is equal to h(outside) times (pipewall-70).

Given the arguments you can make for the huge variation in convection coefficient, and the reasonableness of neglecting radiation or bumping up the h to account for radiation, you can understand why assuming that the pipe wall has a uniform temperature from inside to outside. Especially for such a thin wall and large diameter.

But who does heat loss calc's for a sustained 90 mph wind? Typically you'd want to maximize your design temp to be conservative - assume a 100°F day with no wind so the pipe is at something like 795°F. Is this ducting off a furnace or something similar?

jt

 

[stanier]

Download a copy of AFT Arrow Demo program. This allows you to model 5 pipes. You get a full help file. This will help you understand the problem.

Then invest in the program for your future work.

http://www.aft.com

Geoffrey D Stone FIMechE C.Eng;FIEAust CP Eng

http://www.waterhammer.bigblog.com.au