Calculating time to heat samples in a rectangular chamber

[skuntz]

I am designing a chamber that will be used to heat solid slabs of various composition to a specified temperature. I am planning to use silicone rubber heating blankets and a layer of insulation around the chamber. I would like to know how to model this in order to understand the relationship between the power delivered by the heating blankets and the time required to heat the samples. The samples are simply sitting on a rack in the chamber. I am guessing that this is not a steady state process, so just dividing the heat absorbed by the power is not going to give an accurate answer. Could someone provide me with guidance here? Thank You.

 

[IRstuff]

How accurate an answer do you think you can get? Transient analyses can be absurdly complex.

TTFN

FAQ731-376

 

[skuntz]

I'd settle for +/- 50%.

 

[IRstuff]

Seems to me that you could treat this as a convection problem, assume something like 1 W/m^2-K, figure out how hot you can allow the air to get, figure out the Joule-heat of the samples, and go from there.

TTFN

FAQ731-376

 

[imcjoek]

You may wish to beg borrow or steal yourself a heat transfer book (I have had good luck with Incropera) and look at the chapter on "Lumped Capacity". This is a method to estimate the heating rate of a sample at X temperature when thrown into a medium of another temperature.

You will need to do a conduction/convection analysis on your heating blanking/insulation device to determine what that medium temperature is. If you physically have samples of these parts you will probably get best results if you perform experiments to determine your convection coefficients. They are a bear to arrive at numerically, and are not known for being terribly accurate when done that way.

 

[IRstuff]

downloadable HT text:

http://web.mit.edu/lienhard/

http://www/ahtt.html

TTFN

FAQ731-376

 

[chicopee]

One of my past projects involved in determining curing temperature time on epoxy coated diagphrams (these are steel cross braces between girders that would be used on bridge). The curing chamber was heated by forced warm air.
I modeled the following way:
1)a transient heat transfer analysis thru chamber walls and roof to determine appx how long it would get me to maintain a steady state temp level(somewhat above the curing temperature)in the chamber.
2) determine initial temperature and mass of uncoated diagphrams
3)assumed the diagphrams to be thin objects, anotherword my Bi=Hc*Ls/Kst<.1; therefore)no internal resistance in the steel diagphrams meaning no temperature gradient.
4) forced air temperature to be constant before introduction of diagphrams
5) a value for convection heat transfer coefficient which was selected based on the air velocity around the diagphrams
6) an initial temperature for the diagphrams
7)determine thru differential analysis a time required to get a final steel temperature which in itself is the curing tempurature of the coating.
8) tweek the program with reality.
The program was done with GW-BASICS. How I love this programming tool!!