Heat Transfer through multiple walls

[Joey Dela Cruz]

Hi guys,

I am newbie on thread and currently working on a project
I need some help on the Heat flow on multiple walls.

Here is the setup (see figure below)
The first wall is a Steel material which has a 350C temperature(1kwatts) and the an Air Gap is present then followed by a Plastic wall which is required to have around 60C (30watts)

350C(1kwatts)--> |(Steel)|--- |Air Gap| ----|(Plastic)|--->60C(30watts)

The goal for me is to know or get thickness/distance of the Air gap between the Steel and Plastic so to reduce the Heat or Temperature leaving the Plastic wall at 60C
I was trying to use and figure out the solution using the formula
dQ/dT = AΔT/(L1/k1+L2/k2+L3/k3) but was stuck
Any advise here?

 

[Mech_Engineer]

I would advise you to try setting up a "thermal-resistive circuit" analysis for this problem. Similar to the classic V=I*R electrical circuits equation, there is a heat transfer analysis method which uses a "T=q*Rt" equation, where T (temperature, K) is equivalent to voltage, q (heat flow, W) is equivalent to current, and Rt (thermal resistance, K/W) is equivalent to resistance. You can find equations for thermal resistance for each of your wall's layers, and then analyze just as you would any other electrical circuit (series thermal resistances are added, parallel thermal resistances use the inverse relationship).

MIT has an online summary of the analysis method I'm describing, you can find it here: http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node118.html

 

[Joey Dela Cruz]

I would advise you to try setting up a "thermal-resistive circuit" analysis for this problem. Similar to the classic V=I*R electrical circuits equation, there is a heat transfer analysis method which uses a "T=q*Rt" equation, where T (temperature, K) is equivalent to voltage, q (heat flow, W) is equivalent to current, and Rt (thermal resistance, K/W) is equivalent to resistance. You can find equations for thermal resistance for each of your wall's layers, and then analyze just as you would any other electrical circuit (series thermal resistances are added, parallel thermal resistances use the inverse relationship).

MIT has an online summary of the analysis method I'm describing, you can find it here: http://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node118.html

Thanks Dude...will start from here!

 

[Baluncore]

@ Joey Dela Cruz.
Will you keep the top and bottom of the air-gap open, so air can rise in the gap and continuously cool the walls by convection ?