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 Furnace design (Heat Transfer) Last Post 18 Sep 2012 01:09 PM by tnoah91. 3 Replies. Sort: Oldest First Most Recent First
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tnoah91

New Member
Posts:3

 05 Sep 2012 05:42 PM 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
tnoah91

New Member
Posts:3

 11 Sep 2012 02:29 PM 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)
Nathaniel

Basic Member
Posts:193

 11 Sep 2012 03:36 PM 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
tnoah91

New Member
Posts:3

 18 Sep 2012 01:09 PM 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?
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