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  #91  
Old 05-29-2012, 08:46 PM
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Default Re: Starting new 36" build

Here is a picture of my plan for the thermal break. I'm planning to use some 1/4" or 3/8" ceramic rope in the inside, and a larger piece on the back side.

I also to tie the arch into the dome at the top, under the duravent anchor plate - using a piece of IFB. I feel like the arch needs to be anchored to the dome somewhere in order to provide stability. Since I have a relatively shallow landing, the back side of the bricks on which the chimney anchor plate sits will have to be over the inner arch no matter what. I thought by using IFB for the top center mount, where there is very little mass to transmit heat, I could have an effective continual thermal break.

but all that could be hooey, or maybe I need a picture to 'splain it.
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Starting new 36" build-wp_000034.jpg   Starting new 36" build-wp_000035.jpg  
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  #92  
Old 05-30-2012, 05:57 AM
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Default Re: Starting new 36" build

I used a piece of steel over vermiculite - like Giannis exept he put it on insulating brick. Still not sure how effective it is. I take Lakus point about the conductivity of steel, I guess if it is less than 1/12th the thickness of the brick it should still be somewaht effective. I would not use the hollow steel recangle. I think it might be more effective it you stuffed it with loose vermiculite since air circulating would also convect heat.

Last edited by Amac; 05-31-2012 at 03:33 PM. Reason: should be 1/12th.. and "convect instead of convey
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  #93  
Old 05-31-2012, 12:38 AM
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Default Re: Starting new 36" build

Quote:
Originally Posted by Amac View Post
I think it might be more effective it you stuffed it with loose vermiculite since air circulating would also convey heat.
The transfer rate of heat through steel is very rapid, I doubt stuffing it with anything will retard the heat transfer.
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  #94  
Old 05-31-2012, 03:33 AM
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Default Re: Starting new 36" build

[QUOTE]The transfer rate of heat through steel is very rapid, I doubt stuffing it with anything will retard the heat transfer.[/QUOTE

Not the conduction through the steel obviously but it should prevent the added air convection. According to Laku the conductivity is 12 times that of brick so with my 3" bricks I would need steel of 1/4" thickness to get the conductivity of one brick. I don't have a micrometer but the steel thickness is definitely less than 1 mm (it feels about as thick as a credit card) so that is about 1/100th of a brick. Why do I have a feeling I am ignoring something obvious here

It does seem to be somewhat effective - hard to say since I never had anything to compare it with - because the bricks at the outside of the metal strip are never really hot. I have taken some more measurements with the IR thermometer, the next time I fire it up I wll note them and maybe we can get some comparisons from others on here.
I made the strip exactly a brick width so I could replace it with bricks if need be. If I do replace it I will use 1" thick firebricks - laid on top of vermicrete or vermiculite. I got a few when I visited the refratory place and saw a stack of them. I asked him if I could take a couple and he gave me them gratis since I was buying stuff.

You could slice a few of your normal bricks to an inch thick or less and use them - but if the figures above are anyway right, the steel should still be very much more effective.

Pic shows a test piece the engineering guy folded for me.
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Last edited by Amac; 05-31-2012 at 06:30 AM.
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  #95  
Old 05-31-2012, 02:27 PM
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Default Re: Starting new 36" build

Thanks Amac.

I think I am missing something scientific with the heat transfer rate of steel thing. One can't treat a piece of tubing like it is a solid piece of stainless steel. The 12x metric cannot be the right one. It's essentially a double walled insulating barrier with air in between. I have a double walled stainless steel coffee mug sitting on my desk and it transfers almost zero heat from the inside to the outside. A single walled stainless cup would burn my hand.

The potential heat transfer should be the sum of what passes through the two planes + the transfer through the "solid" surface of a steel tube which would be the thickness of the top and bottom of the tube.

For the side planes, The same site Laku linked had another page that gave a formulas for "Heat Transfer through Plane Walls In Series" that is probably applicable here. And if I was better at Math, I would figure it out - but just looking at that formula compared to the one for a solid surface it looks like the resulting heat transfer through two planes of material is exponentially less than through the same solid material. I'm betting it is far less heat transfer than firebrick.

On the solid surface - 16g steel is 0.06 inches thick so that would be less than an eight of an inch of material. I can get a 2" x 1" piece of 16g steel tubing - which at 12x conductivity (actually it's 11.4x to be precise) would be equivalent to a 1.4" face of firebrick.

Taken together, I think the tubing offers some insulation value and what it lacks vs. alternatives (IFB?, angle cuts) sounds like a reasonable trade off for the benefits of having an attractive, durable transition between my vent and oven.

As an alternative I have looked around to see if I can find some stainless steel "C" channel that I could fit over a piece of IFB - but can't seem to find any in retail quantities. So I'm thinking I am going to go with the stainless tubing set over the top of an IFB split. I guess I could try cutting the tubing in half, but that sounds like a real pain.

BTW - on the idea of stuffing the tube - Air looks to be less heat conductive than most of the insulation materials the list Laku linked.
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Last edited by deejayoh; 05-31-2012 at 02:46 PM.
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  #96  
Old 05-31-2012, 03:02 PM
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Default Re: Starting new 36" build

Dennis
Quote:
I think I am missing something scientific with the heat transfer rate of steel thing. One can't treat a piece of tubing like it is a solid piece of stainless steel. The 12x metric cannot be the right one. It's essentially a double walled insulating barrier with air in between. I have a double walled stainless steel coffee mug sitting on my desk and it transfers almost zero heat from the inside to the outside. A single walled stainless cup would burn my hand.
I suspect your double walled mug is enclosing a vacuum (like a thermos flask).
If it is air I would say that like in doubleglazing the distance between the walls is critical. If it is greater than 3/4" then there will be problems of heat transfer by convection.
Insulated glazing - Wikipedia, the free encyclopedia

That is why I suggested stuffing the hollow tube with insulating material. I guess you could experiment with this somehow before actually laying it on the floor.

Quote:
On the solid surface - 16g steel is 0.06 inches thick so that would be less than an eight of an inch of material. I can get a 2" x 1" piece of 16g steel tubing - which at 12x conductivity (actually it's 11.4x to be precise) would be equivalent to a 1.4" face of firebrick.
That is basically the point I was trying to make so no argument there but without some kind of insulation you will get heat convection in the hollow space.
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  #97  
Old 05-31-2012, 03:15 PM
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Default Re: Starting new 36" build

Amac - Yeah, I understand there is a vacuum - which has zero thermal conductivity. What I was thinking is that on that list, Air is only 1/60th as thermally conductive as fire brick. So even if it is not the best solution, I thought it should still pretty radically reduce the heat transfer.

But I guess what I am missing is that I add another material into the tube - I am essentially adding yet another plane through which the heat has to transfer which will be even more effective. I have some leftover bits of FB board that I can trim down and feed into the pipe (and probably put under it). That should work pretty well.
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  #98  
Old 05-31-2012, 05:09 PM
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Default Re: Starting new 36" build

Quote:
I'm planning to incorporate/steal a couple of your ideas for the entry: a soapstone floor (sourcing a scrap now); A heat break around the inner arch; and a metal divider between the vent floor and oven floor (still looking for this)
Ha! The term (long-standing among musicians) is borrow. Since imitation is the sincerest form of flattery, I'm flattered. Problem is, the more I learn about materials and conductivity, the more I become paralyzed in figuring out my entry plan.

First off, I sourced a piece of 316 stainless plate from a supplier in my territory and was subsequently told that heat will eventually cause this to warp. I'm not convinced of this because the flow of air at the entry floor goes into the oven, not out, and because of the sheer number of mobile ovens with this feature.

So after learning this, I am strongly considering going back to the soapstone entryway, primarily because of the complaints of the staining of firebricks.

Secondly, rather than cut a piece of square stainless tube lengthwise, you can buy a piece of channel, which is three sides of a square tube. I too considered this, but believe the optimal bridge is a full tube separating two masonry materials with beveled edges. The bottom wall of a square tube has no contact thus the thermal transfer is limited to only the top-most points of contact, which, despite it's 12x thermal conductivity, is basically the thickness of the tube, 1/8" or so.

Regarding a heat break and the strength of a standalone entry arch, I plan to move ahead with my original plan, which is similar to yours. I will incorporate some kind of buttressing, including suspending my flue so that it too, is separate from the entryway and doesn't weigh on the arch itself.

Anticipating my eventual need for a stainless vendor for a door, six months ago I approached a stainless steel fabricating company in my territory (I'm in outside sales) who last week called me in and may become a client. I think they have stock stainless channel as well as tube, plate and bar steel of all kinds and sizes. You can find them here:

Phillips Steel Company - Established 1915 - Long Beach, CA

Hope this helps,

John
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  #99  
Old 05-31-2012, 06:05 PM
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Default Re: Starting new 36" build

John - thanks for weighing in. I know you have been thinking a lot about this!

Borrowing, yeah - that's it!

I agree with you on the use of the channel. Seems like the way to go - I just haven't been able to find stainless channel - and stainless tubing looked better than carbon steel channel in terms of heat conductivity. But thanks for the lead on Phillips. I will give them a call.

As for the heat break - did my comment about using the IFB to tie the arch to the dome make sense? I think I need to do a sketchup, but my skills are failing me there. I'll try to post something later

Dennis
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  #100  
Old 06-01-2012, 05:57 AM
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Default Re: Starting new 36" build

Quote:
Air is only 1/60th as thermally conductive as fire brick. So even if it is not the best solution, I thought it should still pretty radically reduce the heat transfer.
Yes if you were only talking about conduction of heat - but you mustn't forget convection which air is good at if allowed the space to move about.

To quote wikipedia - the fount of all knowledge (my emphasis)

Quote:
Air and other gases are generally good insulators, in the absence of convection. Therefore, many insulating materials function simply by having a large number of gas-filled pockets which prevent large-scale convection.
It might not make a huge difference (I can't find comparitive figures for convectivity) but any improvement is welcome - loose vermiculite or perlite would be perfect and easy to do if there is any lying around.
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