#11  
Old 12-13-2013, 06:18 PM
mirassou's Avatar
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Default Re: How to get brick oven HOT?

If you have wood that isn't cured, it'll seem like it takes forever. Kindling+cured wood gets my oven up to temps within an hour.
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  #12  
Old 12-13-2013, 06:50 PM
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Default Re: How to get brick oven HOT?

Quote:
Originally Posted by wotavidone View Post
It needs to be turned to go up the flue, and it won't do a sharp right angled turn.

If smoke does not travel up the throat into the flue even when cold, then in most cases, the throat is incorrectly sized.

The thermal dynamics in an oven, even a cold one at the beginning of a fire, force the gas and smoke up into the throat and flue. As another example, masonry heaters have multiple right angles built in them, in series...and they still draw when they are cold.


The flue is a compromise, we try to make it fairly big, but we don't want to have to far to reach into the oven.

You can't really over flue an outdoor oven, unless you have a very deep, cavernous vent area.

......... wide across the oven, and not very deep........we ask the smoke to spread laterally.

That is where most throats go wrong...they are not wide enough along the width of the oven opening. See below....


I reckon the layout on the right would work better because it would leave more length for the smoke to make the turn, but it would mean we have to reach a long way to get into the oven.
\
That would work but the problem, as noted, is that it creates a large vent area. And that will create new problems such as less visibility, limited accessibility, and draw issues even with the large throat like in your drawing on the right, if the flue is undersized.


I don't see the issue of smoke roll-out being cased by the throat width from front to back, but with the throat width across the oven opening - left to right. From the measurements of my current and past builds, the 60%-64% ratio for opening height seems to apply to the throat width (across the opening) too. My current oven has a 17.5" opening and my throat is 10.75" across...close to 62%

I don't see any reason to be conservative when building the opening for the throat, because any heat which exits the oven opening arch is lost, and has no benefit outside of assisting draw.
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  #13  
Old 12-13-2013, 07:26 PM
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Default Re: How to get brick oven HOT?

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Originally Posted by stonecutter View Post
I don't see the issue of smoke roll-out being cased by the throat width from front to back, but with the throat width across the oven opening - left to right.
We've hijacked this thread a bit. We aren't now talking about "how to get a brick oven HOT."

I should have made it clear I am thinking in terms of semicircular arch type oven openings.
I think, if you are talking about an oven opening that is rectangular, or one of those openings that has straight, buttressed sides and a shallow arch across the top, then yes, a wide opening is best.
However, for an oven with a semicircular arch type opening, like I have, it isn't IMO.

My point is the combustion gases are flowing at some speed as they come out of the oven and the semicircular arch channels the gases into a fairly concentrated stream in the centre which is moving fairly quickly. For that sort of flow, openings that are wide (from left to right) and narrow (from front to back) are not the best way to go, from a gas-flow point of view.

From a purely gas flow point of view, you need adequate width of course, but depth from front to back matters.
The fast moving, channelled gas flow has mass and momentum and needs a little distance to turn. Of course, the deeper you make it, the deeper your whole set-up is, and the further you have to reach in.
Hence my statement that it's a compromise, we have to make it big enough to do the job, small enough we can still reach everything easily. I meant to make that statement about the throat, by the way, not the flue.

Anyway, there is a top limit in flue size for natural (unassisted) drafting.
I don't think anyone's found that top limit yet, it's probably a lot bigger than any size that is practical to build.

I do agree that any heat that has come out of an oven has no other purpose than to help the flue gases rise up the flue.

BTW isn't this different to a masonry heater? I always thought that the protruding masonry bits in the chimney that make the hot gases turn on their way up the chimney, I don't know what they are called, were there to ensure that the hot gases contacted as much masonry as possible, so that less of your heat from your fuel is lost up the chimney?
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  #14  
Old 12-13-2013, 09:31 PM
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Default Re: How to get brick oven HOT?

Quote:
Originally Posted by wotavidone View Post
We've hijacked this thread a bit. We aren't now talking about "how to get a brick oven HOT."Well you are right this is a hijacking . Hopefully the OP doesn't mind a bit of extra material,after all, it does have to do with oven building.

I should have made it clear I am thinking in terms of semicircular arch type oven openings.
I think, if you are talking about an oven opening that is rectangular, or one of those openings that has straight, buttressed sides and a shallow arch across the top, then yes, a wide opening is best.
However, for an oven with a semicircular arch type opening, like I have, it isn't IMO.


No, actually, I am talking about all types of opening arches. In fact most of the freestanding ovens and beehives I built had either semi circular or a modified equilateral opening. There were only two which had segmental arches.

All things being equal, I don't see how a slight variance in an arch radius would cause any marked difference in the amount of exhaust leaving the oven. A properly sized throat and flue draw like a vacuum, even before the oven gas heats the flue. Seeing all of them in action ( along with building many,many fireplaces) is how my opinion was formed on this matter ( venting ).



My point is the combustion gases are flowing at some speed as they come out of the oven and the semicircular arch channels the gases into a fairly concentrated stream in the centre which is moving fairly quickly. For that sort of flow, openings that are wide (from left to right) and narrow (from front to back) are not the best way to go, from a gas-flow point of view.

From a purely gas flow point of view, you need adequate width of course, but depth from front to back matters.
The fast moving, channelled gas flow has mass and momentum and needs a little distance to turn. Of course, the deeper you make it, the deeper your whole set-up is, and the further you have to reach in.

The vents that I have seen that can't contain smoke usually have a throat with a short width span relative to the width front to back. My point is that you want to scale the width to the depth, while considering the oven dimension. I get what you are saying about a narrow opening and it is logical.


The reason I am not agreeing with a large boxy throat design is because it is not necessary to successfully vent all the exhaust into the flue....which makes it impractical to build. It wouldn't hurt anything by doing it, if it made a builder feel more secure about containing the exhaust, so that it doesn't stain the front of the oven.


I do agree that any heat that has come out of an oven has no other purpose than to help the flue gases rise up the flue.

BTW isn't this different to a masonry heater? I always thought that the protruding masonry bits in the chimney that make the hot gases turn on their way up the chimney, I don't know what they are called, were there to ensure that the hot gases contacted as much masonry as possible, so that less of your heat from your fuel is lost up the chimney?


The point of bringing up a masonry heater was to illustrate that smoke and combustion gases can and will vent out of properly built right angle throats and flues.
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  #15  
Old 12-14-2013, 12:29 PM
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Default Re: How to get brick oven HOT?

In my kiln I pull the flame not just sideways, but down.It travels up over two feet then back down again to exit the flue. Downdraft kilns are the most efficient and produce the most consistent temps in the chamber.

My view on the entry for a WFO is that it is of great benefit to make it as shallow as possible reducing the reach into the oven proper. To prevent smoke exiting the front and staining the arch you can compensate for this by making the flue larger. The tricky part is then designing an entry that is only as deep as the diameter of the flue pipe. This is far easier if the entry gallery is cast. It's a bit like a vacuum cleaner head that is wide but not deep to feed into a pipe.
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