Starting new 36" build

[Amac]

Re: Starting new 36" build

Dennis

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.

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.

[deejayoh]

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.

[Amac]

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.

[brickie in oz]

Re: Starting new 36" build

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.

[Amac]

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.

[deejayoh]

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.

[deejayoh]

Re: Starting new 36" build

Steel is better heat conductor than brick, about 12 times as good. Thermal Conductivity of some common Materials and Gases

How about just cutting a taper on the brick edge that is against the oven floor? that would leave a void between most of the oven landing and floor.

I can see that a solid piece of steel would be an issue - but I was thinking that stainless tubing might effectively create an air gap.

A taper would work, but I worry about chipping. That firebrick is not the strongest stuff, and getting a stronger material at the transition was another plus of the stainless.

[Laku]

Re: Starting new 36" build

Steel is better heat conductor than brick, about 12 times as good. Thermal Conductivity of some common Materials and Gases

How about just cutting a taper on the brick edge that is against the oven floor? that would leave a void between most of the oven landing and floor.

[deejayoh]

Re: Starting new 36" build

Looking good, Dennis! Yet another oven-builder who will soon me by while I twiddle my thumbs.

John

Thanks John. Not as pretty as yours, but I think it'll do.

I'm planning to incorporate/steal a couple of your ideas forthe 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)

I started building the vent arch last night, and since there is a heat break - it's basically a free-standing structure on the insulation. I'm a bit worried about stability. You have any thoughts on this? I will post a shot of the way that I am building it when I get home so you can see what I am talking about.


Edit: Scratch that comment about stealing the soapstone idea from you! I just went back and looked at your thread and saw that you'd dropped that and gone to stainless. I was thinking about putting a piece of rectangular stainless tubing (like this) between the firebrick oven floor and the soapstone vent floor. I figured that should stop much of the heat transfer into the vent. Should I reconsider? I'm not looking for perfect - but definitely want to have something that works.

[GianniFocaccia]

Re: Starting new 36" build

Looking good, Dennis! Yet another oven-builder who will soon me by while I twiddle my thumbs.

John

[Laku]

Re: Starting new 36" build

You can but when you install them you need to let them dry too or risk cracking the outer arch/vent chamber.

I think you can start curing 2 weeks after last brick was mortared. But take it really slow.
One week after laying of last brick I think you can put in a lamp or small heater. I'm sure the Pro's will correct me if that's not right.

[deejayoh]

Re: Starting new 36" build

I need some advice on curing the oven. How long do you need to wait for the mortar to dry out before you start heating the dome?

Can I start the curing process before I get the outer arch and vent installed?

[Amac]

Re: Starting new 36" build

That looks like the last "spike" joining the Union Pacific and Central Pacific railroads Dennis - it should be gold and have a ceremony with many dignitaries. Failing that it deserves at least a few beers - so cheers and congrats .
http://www.sfmuseum.org/hist11/spike.jpg

[deejayoh]

Re: Starting new 36" build

Whoohoo!

The dome is closed in! I am giddy with excitement! the interior shot is not a great one, but best I could get just putting my arm in there.

Now I am hoping to get some serious traction on the outer arch and flue with the rest of the day.

It's really coming together. I kind of can't believe it.

[Amac]

Re: Starting new 36" build

Looking good Dennis nice work - great to see it move in towards the plug. The dome shaped oven has kind of hypnotic attraction I can't imagine in the vaulted ovens.

I stood on mine and have some photos but I waited a judicious period before I would chance it, and not before I had closed the dome. With my lack of planning when I had finished the dome I was at a loss how to continue for a week or two ( I'm now at another crossoads where I can't decide which way to go)