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Thermal modelling

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pop

New Member
I'm building a heating controller using PIC (16F877) and heat trace cables (cables that essentially act as resistors and heat up when current is passed through them).
Something like what you would see in the oil industry, but that is designed for household applications (e.g. roof/driveway deicing (very useful here in Canada :D ), floor heating, pipe freeze protection etc.)

Now, I need to implement different control schemes for the controller (PID, bang-bang and maybe fuzzy).

To create a digital PID controller I need to model floors, driveway and water pipes with transient heat transfer equations.

I'm familiar with steady state modelling (natural and forced convection, conduction and radiation) but I don't have a clue about transient modelling.

Does anybody have any experience with transient heat transfer modelling?
In other words does anybody know any sites where I could find differential equations or any hints on modelling for these heat transfer processes?

Any info on this would be appreciated seeing how I'm lost.
 

Russlk

New Member
Transistor makers deal with this problem all the time. If you look for transistor thermal calculations, there should be some info. I think an analog for transient thermal problems would be an R - C circuit where the resistance is analogous to the thermal impedance, the voltage is analagous to the temperatue, and the capacitance is analagous to the mass of the material. Since, in most cases, you will be dealing with a distributed circuit, the system may become complex.
 

stevez

Active Member
Look in ASHRAE handbooks. American Society of Heating, Refrigeration and Air conditioning Engineers. The handbooks deal with transients as it relates to building construction. If the handbooks don't have what you need check the references at the end of the chapter. Other sources - Carrier Corp publications. They (among others) have developed software to model the heating/cooling performance of buildings and building systems - which requires some level of transient simulation.

I do recall ASHRAE funded studies on temperature profiles thru soil and basement walls - in an attempt to understand the benefit (or not) of various foundation insulation systems.

Safe to say that for the things you listed that heat transfer is relatively slow.

Have fun.
 
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