(This is probably a simple question, but I've been staring at it so long I'm not sure I know how to do it any more. I have an Excel spreadsheet with like 3 different ways to calculate this, and they all give different results, so I know I've screwed up somewhere.)
I would like to build a do-it-yourself 1700-degree-Celsius (though will settle for 3000F) electric muffle furnace because I like electricity and playing with fire. I am trying to get an idea of the cost, which means I need to know how thick the insulation need to be.
Unfortunately, materials rated for those kinds of temperatures tend to both be expensive and have relatively high thermal conductivity values, so I plan to use enough high-temperature insulation to reduce the chamber temperature to about 1300C/2400F, surrounded by cheaper, lower-conductivity insulation to bring the exterior temperature down to the target. I want the inside of the furnace as large as I can affordably make it, so instead of a rule of thumb I'd like some real equations I can play with the parameters of.
- The furnace will be of a front-loading box design.
- The furnace will run on 120V AC, with a target power dissipation of 1200W.
- For convection calculations, I am assuming a 10 W/m^2K heat transfer coefficient as this will be free-standing. I also assume room temperature is 21C and I'd like to target a maximum of 43C on the exterior surface.
- My current calculations use a 12" x 12" x 8" furnace chamber, though I'll probably settle for whatever it turns out I can afford.
I am considering the following refractory materials as insulation:
- Kast-O-Lite 30 LI Insulating Castable: Max 3000F/1650C, max ~0.75 W/mK
- Alumina Fiberboard: Max 3100F/1700C, max ~0.45 W/mK
- NC-28 Insulating Fire Brick: Max 2800F/1540C, max ~0.44 W/mK
- NC-26 Insulating Fire Brick: Max 2600F/1430C, max ~0.32 W/mK
- NC-23 Insulating Fire Brick: Max 2300F/1260C, max ~0.27 W/mK
- 8# Ceramic Fiber Wool: Max 2000F/1100C, max ~0.3 W/mK
(I'm thinking about using Satanite, ITC-100HT, and ITC-296A as coating materials, but I can't find data about how much that would improve the conductivity ratings.)
I'm also pretty new to this, so any advice at all is appreciated. (Hell, maybe I'm overthinking this, the ratings mean nothing, and I should just put "a bunch" of insulation in an see what happens.) I plan to use MoSi2 elements and a nitrogen atmosphere, and will be sintering various powdered metals, including steel. I'll be slapping a PID controller on it for controlled ramp-soak phases.