Thermal conductivity of metals

[whereami]

Hello,

Assume that we have a pipe flowing with air of around 80 degrees F surrounded by air of approximately 140 degrees F. The pipe can be made out of three materials: Aluminum, Mild-Steel, and Stainless Steel.

What material would it be best to use for the pipe in order for the air flowing through the pipe not to get heated as much (provided that wall thickness is the same)? In case I didn't make myself clear, http://www.ivdstudios.com/Misc/Turbo_pipe_question.png of what I am talking about.

For reference, thermal conductivity of aluminum is around 230 degrees. Thermal conductivity of mild-steel is around 50 degrees, and the thermal conductivity of stainless steel is around 15 degrees. I have no clue on how to interpret those numbers. :uhh: My confusion rises because, even though I think mild-steel won't heat up as fast, it will heat up more than aluminum over time overall and will thus pass more heat.

Thank you in advance for your input. :)

P.S. To those wondering, this question is in regards to the intercooler piping on a turbo'd car. I know it won't make a whole lot of difference, but this debate has been going on for way too long within the turbo community.

 

[vanesch]

You want to minimize the thermal flux from the outside to the inside, and therefor you have to take the material with the highest thermal resistivity (the lowest thermal conductance). The thermal capacity won't matter, because this is a problem in steady state, so once everything is at its stationary temperature, it won't change anymore.
(the thermal capacitance might play a role for finite-duration tests, in which thermal steady state is not yet reached of course).

I don't understand the units of the thermal conductivity. Normally, it is specified in Watts / degree meter

meaning, if you have a difference in temperature of one degree over one meter, then there are so many watts of thermal power that flow per square meter of material where these conditions hold.

 

[whereami]

You want to minimize the thermal flux from the outside to the inside, and therefor you have to take the material with the highest thermal resistivity (the lowest thermal conductance). The thermal capacity won't matter, because this is a problem in steady state, so once everything is at its stationary temperature, it won't change anymore.
(the thermal capacitance might play a role for finite-duration tests, in which thermal steady state is not yet reached of course).

I don't understand the units of the thermal conductivity. Normally, it is specified in Watts / degree meter

meaning, if you have a difference in temperature of one degree over one meter, then there are so many watts of thermal power that flow per square meter of material where these conditions hold.

You are absolutely right. The numbers are correct, but I've messed up on the units of measurement themselves.

Thank you very much for your answer. It was exactly what I was looking for.

EDIT: What if I would http://www.teegardenmotorsports.com/thermo.htm the stainless steel pipe. Would the pipe eventually reach the underhood temperatures (by convection) or would the heat wrap prevent it from reaching such a high temperature, given indefinite amount of time?

 

[Hootenanny]

EDIT: What if I would http://www.teegardenmotorsports.com/thermo.htm the stainless steel pipe. Would the pipe eventually reach the underhood temperatures (by convection) or would the heat wrap prevent it from reaching such a high temperature, given indefinite amount of time?

The heat wrap certainly would significantly reduce the heat flow, I think you could say conduction would effectively be stopped. I don't think it would completely stop thermal transfer though, if you left the system running for long enough.

~H

 

[whereami]

So the pipe wouldn't get as hot (as it would without the heat wrap), no matter how much time it is exposed to the hot air for, if I understand correctly?

 

[Hootenanny]

So the pipe wouldn't get as hot (as it would without the heat wrap), no matter how much time it is exposed to the hot air for, if I understand correctly?

Yes, that's correct, if they both (wrapped and un-wrapped) started at the same temperature and were exposed to the same conditions for the same time period, the un-wrapped one would always be the hottest. The important point to note here is that the heat transfer mechanics here is conduction not convection as you said.

~H