To ask an engineering question please visit our forum site at Eng-Tips.

Hey I am designing a furnace for a creep testing machine capable of max Temp of up to 800degC (for testing tough metals such as titanium).

I have a good idea of the the dimensions and shape ie: chamber volume and the outer body volume, a good idea of the heating element that'll be used, an adequate range of possible insulation materials.

Now my real question is, How do I work out the thickness of insulation needed to maintain T= 800degC in the chamber for a lengthy period of time - days, using a 1.6kW (or so) heating element, and T= 20degC (~ ambient room temp) on the outer surface of the furnace body? - by how, I am preferably looking for a suitable energy equation that'll get there directly.

The furnace chamber should be completely sealed (with ceramic fibre insulation) and there shouldn't be a significant pressure build up during operation due to the nature of the heating process (will be relatively slow,~ +10degC/min). Therefore, the air in the chamber should remain at atmospheric pressure during the course of the test.

-I have tried equations such as:
electrical Work in = [mCp(T2-T1)]air +[mCp(T2-T1)]ins.
and from that tried to find the mass of insulation needed and from that and the dimensions, tried to find the thickness.
-I have also tried using the resistance equations Q = (T1-T2)/Rt

In both cases i'm getting absurd final values for thickness.

I am only in the first stages of design development and I appreciate any help you can give me.
Thanks a lot for your time.

Noah

Also, after finding the thickness of insulation, how can I calculate the time it takes for the chamber to heat up to a stable 800degC? (after the system comes to eqm)

Going back to your original post - what values of insulation are you getting?

Have you confirmed that you have compatible units? This is one of the biggest errors in engineering equations.

Niel Leon
engineering.com

Well i used the thermal resistance eqn again and i think i've found realistic answers for the thickness. The problem was the units as you've just mentioned. so for a 1.6kW heating element i'm getting a thickness of around 22mm, sound good?

I used Kaowool as an example for insulation which has a k of 22W/mK @800degC.

Now i'm still trying to figure out the time to heat up to max working temp. You see, standard lab bench top furnaces take a huge amount of time to heat up to a working temp of 800+, sometimes hours. I'm not sure exactly what's going on there, do they use some sort of temperature stepper that increases the temp by say 10degC a minute so as to minimise pressure build up?