Recommendation? DIY Heat Treat Ovens - Specific Design Questions

Guys,lets please be nice. We all have our unique experience and perspective to draw from. Often we are in agreement, but interpretation of the words chosen produce anger ..
Lets try to keep that in mind please?

as i wanted this thread to be about specific design decisions:

JT - i am really interested about your comment re loading on the coils. That is new to me and would really like to know how that might effect amount of coil needed for a furnace. Can you please clarify or supply a link to go to ti learn more?

Natlek - as one engineer to another: as a given that it is small pockets of trapped air that supplies the real insulation in materials - WHY does kaowool, which is essentially a nonwoven textile (not closed cell foam ... and is permeable to air) have a LOWER thermal conductivity per thickness than brick? Or is it actually closed cell? As a second question, do you really think that loss of heated air when opening door is that significant compared to radiative loss lo the environment through the open door (when you open the door you FEEL that radiation on your face...)
 
You can find some information about surface loads for kanthal here. Dan Comeau has a coil calculator spreadsheet on his site, you can easily add another column to it to compute surface load and then compare with the values given for your coil wire and support. I think the coils I made were well below the max recommended load, so I am hoping they will last a good while.
 
Thanks Hubert. After a little searching I finally got a handle on what this is: power output from the elements in Watts per square inch (or per square centimeter.). the lower the value, the longer the elements last, but the bulkier the elements. One more thing to worry about :-). Once you know what to look for, there is a lot out there - but unfortunately it varies quite a bit. The most logical statement I read is that the typical loadings have been found by trial and error. Two technical sites I found are here, and here . There are others ... but the ones about pottery kiln elements tend to recommend a much higher loading value. I like the chart from NiWire industries as giving examples of ranges found in practice:
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sounds like something like 1.25 W/cm^2 (8 W/in^2) at 1000C comes out as a pretty common limit (also seems consistent with the data from Kanthal. Good to know. Unfortunately they say very little about things like embedding the wire into the wall, or winding around a refractory rod.

I still kind of like the idea of notching out the top bricks (nice big notches with lots of room for radiation to happen) and supporting across the span with ceramic rods. Saves oven space, and minimizes the risk of contacting the elements when taking stuff in and out of the oven....
 
Cushing H. said:
Guys,lets please be nice.

Natlek - as one engineer to another: as a given that it is small pockets of trapped air that supplies the real insulation in materials - WHY does kaowool, which is essentially a nonwoven textile (not closed cell foam ... and is permeable to air) have a LOWER thermal conductivity per thickness than brick? Or is it actually closed cell? As a second question, do you really think that loss of heated air when opening door is that significant compared to radiative loss lo the environment through the open door (when you open the door you FEEL that radiation on your face...)
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Don t worry , it is just discussion about HT oven...:thumbsup:
This is quote from one manufacturer of insulating bricks...to answer your first question
The air trapped inside the firebrick is a fantastic insulator because of its thermal conductivity of 0.16 versus bricks which have 2.13 – 3.7 depending on mean temperature. Trapping the air in small chambers, like in foam, provides good insulation.
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About kaowool.......can I answer with question ?Why Styrofoam is that damn good insulator ?
It traps the air in its small pockets. Styrofoam has millions of small air bubbles trapped inside the foam. Since air is a bad conductor of heat Styrofoam efficiently prevents heat transfer. ... Styrofoam reduces conduction and convection
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What we will see if we look in kaowool blanket under microscope ? You will say that you can fly plane between that millions fiber .......... contact between individual fibers is low/ that s way I write weight for 1m3 of ceramic blanket I used in my oven/ so most transfer of heat is over the air and air is ..........air ?
 
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Well, yes, but I am used to seeing insulators that have small pockets of enclosed air (so called closed cell foam)... whereas kaowool has (I think) continuous paths of air through the structures ... I am not used to seeing that as a really good insulator

( one of the amazing examples of a good insulator are the heat shield tiles on the space shuttle. Extremely light closed cell aerofoam. You can heat those suckers up to thousands of degrees, pull them out of the oven, and hold them in your hand!
 
Natlek I want you to know I was not trying to be mean and hope I did not come across like that. We are all friends and nothing wrong with saying what’s on our mind between friends.

when I have done time I will post done data. I built a spreadsheet that calculates all this.
 
I just got back from that local refractory material supply place. Nice people - had a good long discussion with them. A couple of points that stick out in my mind:
1) For a 2000 degree oven, they were just fine with either fiber, or board, or bricks being the inside liner. they did agree, though, that for mechanical support at least one layer of brick on the inside would be a good idea, but were not incredibly concerned about the thickness of the brick
2) wool around the brick, including on the bottom is a GOOD IDEA. They were not really concerned about the compression of the wool underneath the bricks on the bottom.
3) they advised against board on the outside due to expense, and the good insulation given by the wool. Advised thin metal sides.
4) they were quite clear that in their experience, cracking of bricks is due to rate of heating, not absolute temperature. They recommended a 100 F degree per hour heating rate on the first fire (to burn off residual water and other chemicals). After that a heating rate of something like 300-400 degrees per hour should be enough to avoid cracks (but admitted most people would not be patient enough to do that.....).
5) they concurred that it is bests to avoid mortar of any sort, so that the bricks can more freely expand and contract with temperature changes....

I posed the question of how hot the outside would be with 2 inches of brick and 4 inches of wool, and they were very happy to run the heat transfer calculations to estimate that ... but they had a new piece of software that was not behaving well. They will send me those results when they get them.... the person I spoke with said that in his experience something like 200F is cool enough that if you put your fingers on the outside, you will have time to pull those fingers away without risk of getting burned.

Nice folks - I had fun (they spoke a lot of people coming in having watched FIF, and asking them to do some really scary things in terms of large BTU input and low insulation value ......
 
JTknives said:
Natlek I want you to know I was not trying to be mean and hope I did not come across like that. We are all friends and nothing wrong with saying what’s on our mind between friends.

when I have done time I will post done data. I built a spreadsheet that calculates all this.
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I know that , do not worry :thumbsup:
 
Cushing H. said:
Well, yes, but I am used to seeing insulators that have small pockets of enclosed air (so called closed cell foam)... whereas kaowool has (I think) continuous paths of air through the structures ... I am not used to seeing that as a really good insulator

( one of the amazing examples of a good insulator are the heat shield tiles on the space shuttle. Extremely light closed cell aerofoam. You can heat those suckers up to thousands of degrees, pull them out of the oven, and hold them in your hand!
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Well my friend, I have very ,extremely close relative/scientist who work in NASA laboratory in USA.....:rolleyes: Maybe I should ask him about ............:D
 
I received word back from the shop on their thermal calculations (what they call "Heat Flows" (accurate enough). I asked them to estimate outer temps given 1.5" of brick, and 4" of blanket and a 2000F inside temperature. they estimate an outer temperature of 279F. I am not sure what JT's insulation is ... but I keep firmly in mind that these are estimates - it is particularly challenging to accurately model the heat loss from the outer surface. If you look closely, they assumed only radiation from the outer surface, which is not correct, as convection (heat loss to air moving over the outer surface) will be really important in lowering the outside temperature. the 279F would then best be considered an upper limit on the likely temperature. Their output is below. I think based on this I will end up using 2" bricks on the sides (notched out to give a 6" wide chamber), and 4" of blanket insulation around sides, bottom, back, and top. I would like to try for 4" of blanket (only) on the door ... but need to think how I would construct that.

Oh - one of the nice outputs from the calculation is the total heat flux (488 BTU/(hr ft^2). With the dimensions of the chamber (16L x 5.5H x 6"W), that gives a total inside area of 434 in^2, which then translates into 430W of heat lost through the wall - very well within my planned ~1500 W input power (the extra insulation is helping a lot with that relatively low heat loss) (I know a lot of the design criteria out there talk about chamber volume ... but what really matters is the wall surface area - the volume being closely related to surface area). I think this design will work - and give a nice efficient oven - though the outer dimensions are larger than I had imagined.....
upload_2020-2-29_11-13-51.png
 
It looks like, with my considered design, I am going to need to do some sheet metal cutting and fabbing (like making a small "box" to contain the insulation in the door). Locally it looks like Menards/Home Depot carry galvanized steel. QUESTION: there has been some discussion in the past re. galvanized steel giving off nasty fumes when overly heated. Do you think there would be a problem is using galvanized steel on the outside of the oven?
 
Cushing H. said:
It looks like, with my considered design, I am going to need to do some sheet metal cutting and fabbing (like making a small "box" to contain the insulation in the door). Locally it looks like Menards/Home Depot carry galvanized steel. QUESTION: there has been some discussion in the past re. galvanized steel giving off nasty fumes when overly heated. Do you think there would be a problem is using galvanized steel on the outside of the oven?
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NOOOOOOOOOOOOOOOO, don’t dredge up this argument again lol. If your shell is getting hot enough to smoke zinc you have other problems to wory about. I personally would not use it but not because it’s zinc. I would find a metal place and thy will cut/shear everything to size. Tell them exzactly what you want and how much and the go pick it up.
 
JTknives said:
NOOOOOOOOOOOOOOOO, don’t dredge up this argument again lol
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Dont worry :-) I sincerely doubted the temperature would be a problem - but feel it is always better to ask to double check! I would rather it all just be mild steel so I can 1) weld it, 2) paint it. It now looks like Lowes near me has 1/32 plain cold rolled steel. that should work, and also be thin enough to bend into a box. I have never worked with any metal places near me - if they can cut and bend I might just go that route.
 
Cushing H. said:
sides, bottom, back, and top. I would like to try for 4" of blanket (only) on the door ... but need to think how I would construct that.
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This is door from one of my ovens... to give you idea ;)
D6KfkN6.jpg

19FrUDC.jpg
 
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so - I finally had a chance to go through the dimensions and material count.

For overall out side dimensions I get 20 inches wide, 22 inches high, and 22 inches deep. Is that an unreasonable size for a 6"x5 1/2" chamber?? (natlek - yours seems to be about that - my apologies for the non-metric dimensions.....)
 
I think something like this would work..............you know what is what on drawing , right?
I will let bricks to protruding slightly so kaowool on door can seal on bricks tight
rh3LtC8.png
 
Natlek said:
This is door from one of my ovens... to give you idea ;)
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that is actually pretty close to what I was thinking - except you seem to have less framing on the inside than I was thinking (which is 1 inch, because I can easily buy 1 inch angle iron....
 
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