×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Calculating time to heat samples in a rectangular chamber

Calculating time to heat samples in a rectangular chamber

Calculating time to heat samples in a rectangular chamber

(OP)
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.

RE: Calculating time to heat samples in a rectangular chamber

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

TTFN

FAQ731-376: Eng-Tips.com Forum Policies

RE: Calculating time to heat samples in a rectangular chamber

(OP)
I'd settle for +/- 50%.

RE: Calculating time to heat samples in a rectangular chamber

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: Eng-Tips.com Forum Policies

RE: Calculating time to heat samples in a rectangular chamber

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.

RE: Calculating time to heat samples in a rectangular chamber

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!!

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources