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Heat Transfer Calculations

by PWoolfall
Tags: calculations, heat, transfer
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Jan25-11, 01:33 PM
P: 1
I would be grateful for your help with a small project I am involved in at work. I would like to set up an excel spread sheet to calculate either the thickness of insulation required to reduce the temperature of say a furnace wall from the inside to the outside, or calculate the resultant outside wall temperature, knowing the insulation details and inside temperature. I have come across several different formulas on the net, which unfortunately have left me a little confussed.
An application I am currently looking at involves adding a piece of equipment to a furnace roof. This equipment consists of an outer stainless steel cylinder having an outside diameter of 750 mm, a wall thickness of 3 mm, and externally subjected to a temperature of 650 deg C. Inside the cylinder is another cylinder again constructed with stainless steel with a wall thickness of 5 mm. I need to sandwich a sufficient thickness of insulation between the cylinders, to achieve a temperature within the internal cylinder of +/- 60 Deg C.

Any help would be much appreciated.
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Jan26-11, 06:29 AM
P: 1,235
Hello PWoolfall,

If I understand correctly:
  • you want to build a system made of two coaxial cylinders
  • the gap in between the cylinders would be filled by some insulation
  • the external cylinder would be in contact with hot air at 650C
  • you want to keep the internal cylinder below 60C
Obviously, if there is no cold air flowing inside the inner cylinder, then you will never be able to keep it below 60C. Regardless of the amount and quality of the insulation, after some time, the inner cylinder will reach 650C.

If there is cool air flowing in the inner cylinder, then the design is possible but needs more information:
  • composition of the fluid inside the inner pipe
  • temperature of the fluid flowing in the inner pipe
  • flowrate inside the inner pipe
  • similar information regarding the outer pipe:
    is it in contact with some hot fluid?
    what is the composition of this fluid?
    is this fluid moving?
    where does the heat come from?
    some combustion?
  • which kind of insulation material are you considering?
  • what are you geometrical constraints?

If you chose a insulation material with heat conductivity of 0.04 W/K.m,
if you use an insulation thickness of 0.1 m,
then the cold air inside the inner tube should take out an amount of heat of at least q=236 W/m:

since q = 0.04/0.1 * (650-60) W/m

if you pipe is 1 meter long, its surface would be 4.7 m, and this would imply a cooling power of

Q = q*4.7 = 1100 W = 1.1 kW

This would only be possible if the cooling fluid is cold enough and has a sufficient velocity.

Before further calculations, please check these comments.

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