Hank Murrow on sun 10 feb 08
On Feb 10, 2008, at 10:23 AM, Robert wrote:
> I am currently designing the wood firing portion of my kiln. It
> will have two external bourrey boxes parallel to the axis of the arch.
> I am currently running the numbers for the outlet flue and intuitively
> am not settled with the numbers I'm getting. I have Steve Harrison's
> numbers from Laid Back Wood Firing, as well as numbers from a kiln
> published on sidestoke.com
> The kiln will be approx. 27 cu ft under the arch. Downdraft cube,
> i.e. arch in the cube. Another factor is that I will be building a
> modified double venturi flue box ala Nils Lou (mainly increasing the
> volume of the box after the outlet and before the chimney entry) and a
> fiber lined stainless wrapped chimney (12" steel for 10" interior
> chimney, 104 sq. in. )
Dear Robert;
You may want to line the pipe with 2" of fiber, which would yield an
8" diameter dcross-secttion and an area of 50 sq.ins. Alternatively,
you could enlarge the pipe to 14" and line with 2" of fiber to yield
your original area of 104 sq.ins. My recommendation of a 2" lining is
based on keeping the pipe from getting too hot. I could show you the
similar but bigger chimney I designed for the Anagama @ the U of Oregon.
> Depending on whether I include the flame throat
> area in my calculations and which set of numbers I use I get a range
> from 98 sq. in. to 151.2 sq. in. While I realize wood needs
> significantly more than gas these still seem huge to me. The last
> kiln
> I built was 85 cu. ft gas kilns and fired perfectly with 32.5 sq in
> outlets. Granted it had a 20' culvert chimney and faced a prevailing
> westerly so the draft kicked. But it seems to me even accounting for
> the additional volume of air required for wood, that a flue 3 times
> larger for a kiln three times smaller is unnecessary. Nils Lou
> references a 60 cu ft kiln he built in Alaska with an outlet port
> of 52
> sq. in. that was subsequently reduced to 33 sq. in. and an increase in
> chimney height, though this did not use bourrey fireboxes. At this
> point I'm inclined to build to his larger size but would really
> appreciate any comments or observations from those firing wood
> kilns of
> similar size or insights into the nature of the draft in a bourrey
> kiln
> as opposed to other types of wood kiln. TIA
Can't help there, but would advise fairly large 'tunnels' into your
chimney to accommodate a few wares where the ash is really
concentrated before going up the chimney.
Cheers, Hank
Robert on sun 10 feb 08
I am currently designing the wood firing portion of my kiln. It
will have two external bourrey boxes parallel to the axis of the arch.
I am currently running the numbers for the outlet flue and intuitively
am not settled with the numbers I'm getting. I have Steve Harrison's
numbers from Laid Back Wood Firing, as well as numbers from a kiln
published on sidestoke.com
The kiln will be approx. 27 cu ft under the arch. Downdraft cube,
i.e. arch in the cube. Another factor is that I will be building a
modified double venturi flue box ala Nils Lou (mainly increasing the
volume of the box after the outlet and before the chimney entry) and a
fiber lined stainless wrapped chimney (12" steel for 10" interior
chimney, 104 sq. in. ) Depending on whether I include the flame throat
area in my calculations and which set of numbers I use I get a range
from 98 sq. in. to 151.2 sq. in. While I realize wood needs
significantly more than gas these still seem huge to me. The last kiln
I built was 85 cu. ft gas kilns and fired perfectly with 32.5 sq in
outlets. Granted it had a 20' culvert chimney and faced a prevailing
westerly so the draft kicked. But it seems to me even accounting for
the additional volume of air required for wood, that a flue 3 times
larger for a kiln three times smaller is unnecessary. Nils Lou
references a 60 cu ft kiln he built in Alaska with an outlet port of 52
sq. in. that was subsequently reduced to 33 sq. in. and an increase in
chimney height, though this did not use bourrey fireboxes. At this
point I'm inclined to build to his larger size but would really
appreciate any comments or observations from those firing wood kilns of
similar size or insights into the nature of the draft in a bourrey kiln
as opposed to other types of wood kiln. TIA
Take care,
Robert
Paul Haigh on mon 11 feb 08
The entrance flue in a Bourry box kiln not only needs to be large for the same reason as a crossdraft wood kiln (requiring a lot more area than a gas kiln), but one of the common complaints for Bourry firers is that the throat gets choked up by the coal bed because the primary air doesn't pass through the coal bed. Put in plenty of mouse hole volume under the boxes to burn out the coal beds- you can keep them closed off if you don't need them, but you're screwed if you do and don't have them.
I have a single Bourry box 40 cu ft stacking space kiln with one fire box and the throat doesn't get clogged at all (fire 2 and a half times), which I attribute entirely to using a lot of white pine that leaves fewer coals. This creates its own issues with gaining temp, but the kiln fires great when you get to know it. Smashing down the burning logs off the hobs once in a while, and occasioanlly throwing a thin split of hardwood through the side cleanout door into the ash pit are the remedy, but I will mix in short (don't fit the hobs) cuts of oak the next firing to drop some coal builders in there semi-auto style.
By the way- I was surprised at the double Bourry design for a 24 cu ft kiln, as mine fires evenly now that I've learned to control it. On sidestoke.com there's a double Bourry kiln to even out the firing, but IIRC it was pretty big.
Paul Haigh
In frigid Londonderry, NH
Robert on tue 12 feb 08
Paul,
Thanks for checkin' my math. Was kinda spacin' on the area of a circle.
I actually didn't calculate the chimney yet. That was
intuitive/available materials. 1" liner 12" pipe.
I would've liked a passive on my gas kiln. Strong draft from a 20' 12"
pipe. No liner. I may have to size it up for this one. 12" id. But
I also don't want to choke the box. I think I'm settled on that though.
Throat arches are two 11.5" x 9" per side. 27.4% cross sectional area
in relation to the area of the kiln floor.
Am still playing with box proportions. Your coal bed comments were
quite pertinent.
Appreciate the conversation.
Take care,
Robert
Paul Haigh wrote:
> Sorry- exit flue calc, not entrance flue. If I read crrectly, you have a 10" id pipe for a chimney (I assume that accounts for a 1" liner in a 12" pipe...). That isn't 104 cu in, that is 3.14x(5^2) = 78.5 cu in.
>
> I'm definitely no expert, but I do not think that the exit flue needs to be much larger in cross section than the chimney interior area (according to Olsen, IIRC). However- if you put in multiple exits in the back to even out the draw, it's really easy to just stuff some up or pull out a passive damper or 2 extra in the chimney to slow the draft. Again- I'm no consultant on building kilns (have built exactly one... but I like it)- but this seems a bit on the small side for chimney diemnsions once you get both boxes rockin. What is the area of the 2 entrance throats on the B boxes?
>
> Mostly conversing rather than criticizing or even giving real advice here- I hope to learn something as well.
>
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Paul Haigh on tue 12 feb 08
Sorry- exit flue calc, not entrance flue. If I read crrectly, you have a 10" id pipe for a chimney (I assume that accounts for a 1" liner in a 12" pipe...). That isn't 104 cu in, that is 3.14x(5^2) = 78.5 cu in.
I'm definitely no expert, but I do not think that the exit flue needs to be much larger in cross section than the chimney interior area (according to Olsen, IIRC). However- if you put in multiple exits in the back to even out the draw, it's really easy to just stuff some up or pull out a passive damper or 2 extra in the chimney to slow the draft. Again- I'm no consultant on building kilns (have built exactly one... but I like it)- but this seems a bit on the small side for chimney diemnsions once you get both boxes rockin. What is the area of the 2 entrance throats on the B boxes?
Mostly conversing rather than criticizing or even giving real advice here- I hope to learn something as well.
Duff bogen on tue 12 feb 08
Robert
In my copy of Pioneer Pottery I've found that some of the drawings a printed "to scale" so if you can find a 9" brick in a drawing and measure it with an architects scale you might be able to learning something.
Wood fire boxes are gas generaters. My line of attack would be going to the rules in Leach- I recall 1s.f. superficial grate area per 6s.f. of floor area. As that's for hard brick I'd derate it to 60% and call that the net area- basiccly the bottom of the wood on the hobs as if you lay in the ash pit and looked up. The gasses are expanding aggressively here so you don't want to be cheap on the throat- auch curves are better than angles. In general for flues between A and B I think a small crossection means faster gas flow and less heat transfer but the opposite holds here. I've seen kilns constipated by a tight throat such that the gasses can't get to the ware chamber.
Duff
Robert wrote:
I am currently designing the wood firing portion of my kiln. It
will have two external bourrey boxes parallel to the axis of the arch.
I am currently running the numbers for the outlet flue and intuitively
am not settled with the numbers I'm getting. I have Steve Harrison's
numbers from Laid Back Wood Firing, as well as numbers from a kiln
published on sidestoke.com
The kiln will be approx. 27 cu ft under the arch. Downdraft cube,
i.e. arch in the cube. Another factor is that I will be building a
modified double venturi flue box ala Nils Lou (mainly increasing the
volume of the box after the outlet and before the chimney entry) and a
fiber lined stainless wrapped chimney (12" steel for 10" interior
chimney, 104 sq. in. ) Depending on whether I include the flame throat
area in my calculations and which set of numbers I use I get a range
from 98 sq. in. to 151.2 sq. in. While I realize wood needs
significantly more than gas these still seem huge to me. The last kiln
I built was 85 cu. ft gas kilns and fired perfectly with 32.5 sq in
outlets. Granted it had a 20' culvert chimney and faced a prevailing
westerly so the draft kicked. But it seems to me even accounting for
the additional volume of air required for wood, that a flue 3 times
larger for a kiln three times smaller is unnecessary. Nils Lou
references a 60 cu ft kiln he built in Alaska with an outlet port of 52
sq. in. that was subsequently reduced to 33 sq. in. and an increase in
chimney height, though this did not use bourrey fireboxes. At this
point I'm inclined to build to his larger size but would really
appreciate any comments or observations from those firing wood kilns of
similar size or insights into the nature of the draft in a bourrey kiln
as opposed to other types of wood kiln. TIA
Take care,
Robert
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Robert on tue 12 feb 08
Paul,
I think I'm alright on the firebox side. It's the outlet flue I'm
grappling with. My calculations put it a bit larger than my intuition
tells me.
I'm sure I could use a single bourrey box but I'm configuring the kiln
as a downdraft rather than a crossdraft. I will be firing with other
fuels at times and would prefer a more balanced distribution of ash and
flame in the wood fires. I'm not so fond of "one sided" pots, for lack
of a better descriptor, as a crossdraft produces.
Stay warm,
Robert
Paul Haigh wrote:
> By the way- I was surprised at the double Bourry design for a 24 cu ft kiln
>
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