#2 heat treat oven build

Stacy E. Apelt - Bladesmith said:
Just a comment about the statement that "Heat Rises".

Heat does not rise … heat radiates equally oin all directions.
Hot Air rises because of lower density (actually, the heavier cool air falls).
In an enclosed chamber, the convection and "rising" of the hot air is minimal compared to an open environment like a large room.
The reason one end of a heated chamber (whether vertical or horizontal) may be slightly higher/lower temperature than another is primarily the distance from the heat source.
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all I know is in my 3 tempering ovens forced air was a must to get an even temp graduation from top to bottom. The newest oven I bough was not forced air and thy claimed super even temps. The issue was to get this evenness the oven had to preheat for 4-5hrs. Once I popped in the fan and shroud it turned the preheat into 15-20 min. So with this experience I was worried about the same graduation going vertical in a high temp oven.
 
Is there a small fan that could be bought or made to circulate the high temp air very slowly to even out the air temps in the kiln.
 
JTknives said:
all I know is in my 3 tempering ovens forced air was a must to get an even temp graduation from top to bottom. The newest oven I bough was not forced air and thy claimed super even temps. The issue was to get this evenness the oven had to preheat for 4-5hrs. Once I popped in the fan and shroud it turned the preheat into 15-20 min. So with this experience I was worried about the same graduation going vertical in a high temp oven.
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I can try to clarify this. You are both right. Radiation heat transfer (which does not “rise - it goes in straight lines) increases at temperature raised to the fourth power. So at HOT temperatures you dont have to worry about anything “rising”. At lower temperatures (like tempering) the heat transfer is done by contact with heated air (“convection”) radiation is very secondary ... and you DO need to worry about both keeping the hot air circulated and evenly distributed, as well as the fact that hot air rises.
 
I have been studying these builds, and have a comment re. what you are referring to as "wool in the door". Do you know whether, for example Scaniaman's door, has thin firebrick FACED with wool on the inside, or whether it is solid kaowool throughout the door??
 
I would think it’s all wool. I believe wool has a better insulation value then brick.
 
JTknives said:
I would think it’s all wool. I believe wool has a better insulation value then brick.
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I tried looking up the data - but it was ambiguous because thicknesses were not clearly labeled. I’ll try again tomorrow when I have more time
 
Fire brick is pretty similar to the lightweight concrete we use to build insulated walls over here. Concrete with tiny tiny air bubbles in it. 2" of the mineral wool stuff we use to insulate walls is about 2.5x as good an insulator compared to 2" of the lightweight concrete. So I'd guess wool is substantially better an insulator than fire brick. Then there are other variables, such as the degree of compression of the wool, and how much heat escapes through a door made only out of fire brick etc etc.
 
The "firebrick" we are talking about isn't the hard firebrick (light weight concrete) used for fireplaces and stoves. It is soft firebrick rated at 2300°F or 2600°F. It is sold as K-23 and K-26.
The wool (Ins-wool) is slightly more insulating, but they are close. The wool comes in 2" thickness, which would be good for the door. It can be rigidized to be less fluffy.
 
Well.................

The historical insulation solution for 1300°C (2372°F) heat-treatment furnaces has been to use hard refractory firebricks and lightweight insulating firebricks with high heat storage, high wall heat losses and long heating time to stability. In modern heat-treatment furnaces, these three problems are reduced with ceramic fiber modules lining the walls, roof, doors, flues and stack, using less thickness than insulating firebricks.

Due to the high density of refractory and insulating materials (from 131-191 pounds/foot3 in refractory bricks and 27.3-78.7 pounds/foot3 in insulating fire bricks), heat storage, wall heat losses and time required to reach equilibrium are fuel-consumption elements. Also, due to the material weight and thickness of this type of insulation, a heavy furnace structure is required.
With the use of ceramic fiber due to the lower density (12-14 pounds/foot3), heat storage is reduced, less time to equilibrium is required and wall losses can be reduced. Depending on the wall, door and roof thickness, thermal insulation with high-temperature ceramic fiber insulation enables a more lightweight furnace construction, resulting in many economic and ecological benefits. Another excellent advantage is faster operating-temperature recovery time after loading and closing the furnace door.
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vtTykHE.jpg
 
Natlek - hard firebrick is really heavy and hard ... my guess is that unless one is building a large, non-movable furnace, hard brick is not practical.

i get that kaowool in the door (where it is supported by a frame) is a good idea (reduces weight, makes a better seal with the front of the oven) ... but if, as some have commented, the outside of the soft firebricks on the main part of the oven dont get very warm ..
How much benefit is there to putting wool on the outside of the soft brick around the main part of the oven?
 
I would say it gives diminishing returns the thicker the brick gets. At 3” brick thickness maybe it’s has some viability but my current oven has 3” walls and I honestly don't know if and extra 1” of wool would even pay for its self in electricity savings. I just ran a temp gun oven the outside of my oven and average temp was 200-250° and it had bee at aebl temps for 3-4hrs by that time. And like I said those bricks are turned so that the walls are 3” thick. I don't even think turning the bricks to get 4.5” wall thickness would save enough power to cover the almost double the needed bricks for this new build. But I want fast a stable heats so money is not important.
 
makes sense. I ran the $$ on the build, and it looks like a small oven can be built for something like $ 200-$250? (unless I made a mistake). I might be tempted to try a build for simple stuff (the more complicated I will leave to you JT!!). right now though I do not know if I can access a 20 amp circuit in a reasonable place to run a welder off of!
 
JTknives said:
I would say it gives diminishing returns the thicker the brick gets. At 3” brick thickness maybe it’s has some viability but my current oven has 3” walls and I honestly don't know if and extra 1” of wool would even pay for its self in electricity savings. I just ran a temp gun oven the outside of my oven and average temp was 200-250° and it had bee at aebl temps for 3-4hrs by that time. And like I said those bricks are turned so that the walls are 3” thick. I don't even think turning the bricks to get 4.5” wall thickness would save enough power to cover the almost double the needed bricks for this new build. But I want fast a stable heats so money is not important.
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HT furnace is like mousetrap .If mouse get in trap and find way out...........More heat you trap in oven , it is more efficient and most important it will improve heat uniformity inside ....
look this chart on page 32
https://web.wpi.edu/Pubs/ETD/Available/etd-082208-114851/unrestricted/Purushothaman.pdf
 
Natlek said:
HT furnace is like mousetrap .If mouse get in trap and find way out...........More heat you trap in oven , it is more efficient and most important it will improve heat uniformity inside ....
look this chart on page 32
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Yes ... but at a cost of more weight, more financial cost, and increased size. you have to stop adding insulating bricks at some point.....
 
Cushing H. said:
Yes ... but at a cost of more weight, more financial cost, and increased size. you have to stop adding insulating bricks at some point.....
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adding bricks....................:) Look ,HT oven don t start and end up with only bricks, keeping thermal mass as low as it is possible is good thing...
 
Cushing H. said:
Yes ... but at a cost of more weight, more financial cost, and increased size. you have to stop adding insulating bricks at some point.....
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I have to disagree. I actually live in a super insulated house I designed years ago that has payed out huge actually dividends over the year I've owned and lived in it. You can never go wrong with more insulation. Be it quicker to heat or more stable heat after the fact.

I lucked out when building my homemade electric kiln. I salvaged a kiln that had added insulation around the standard fire brick. My homemade kiln gets to 1950 with 30 minutes and holds that temp within 2 degrees.
 
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LCoop said:
I have to disagree. I actually live in a super insulated house I designed years ago that has payed out actually dividends over the year I've owned and lived in it. You can never go wrong with more insulation. be it quicker to heat or more stable heat after the fact.
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I understand your point .... but I was mostly thinking about weight and size (in terms of picking the thing up or moving it some day). There has got to be a point where you say "no, I dont want it that heavy".....
 
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