#21  
Old 07-07-2008, 01:47 PM
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Default Re: chimney flow rate calculator

Dave, maybe that's the trick.

I still have little or no soot on the arch bricks. I made a lower arch in front and also cut 45 degree angles on the bricks as they leave the oven and go up the chimney to give a smooth transistion.

Sometimes you get lucky...

Christo
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  #22  
Old 07-07-2008, 05:10 PM
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Default Re: chimney flow rate calculator

My guess is that the chimney will suck up a set volume of smoke or air ( per the calulcation). It will also pull from the nearest source. So if your arch is higher in the front, it will pull air from the front as well as "make up air" from the oven. Make up air being the remaining volume needed to fill the capacity of the chimney.
I am planing to have a lower arch in the front. This should create a "smoke chamber" directly under the chimney. It should function like a surge drum or "smoke capacitor", so if the smoke volume exceeds the draw of the chimney, it can hang out for a moment and not leak out the front.
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  #23  
Old 07-07-2008, 07:56 PM
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Default Re: chimney flow rate calculator

My barrel oven has a 6"x6" square tile, chimney is a touch over 4' (51.5") and my door opening is 11.5" tall 17.125" wide. My flow comes to 668 and I only get alittle smoke out of the front on initial startup after that... zilch. Found the chimney to be one of the most intriguing things to watch work.
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  #24  
Old 07-07-2008, 10:26 PM
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Thumbs up Re: chimney flow rate calculator

I laid in bed last night thinking about this calculator, (yeh, alright, I know I need to get a life, but I'm on holidays), and wondered what the figures would be at different times of the oven firing. You can doble check my calcs if you like as my chimney is 8" diasmeter (20cm) and 3metres tall The void is also large and catches all the smoke and directs it up the flue.
So with an outside temperature set at say a very pleasant 20˚C and the oven inside temps varying, the following results are:
at 50˚C 514.1
at 100˚C 781.3
at 150˚C 935.2
at 200˚C 1040.4
at 250˚C 1118.7
at 300˚C 1179.2
at 350˚C 1227.8
at 400˚C 1267.6
at 450˚C 1301.0
at 500˚C 1329.3
Put that into a graph and the chimney really sucks. The hotter the oven gets (and for that matter, the chimney also( then the better it draws.
If the oven is too hot, then either the chimney will suck off my toppings or they will burn off.
I guess the morel of this exercise is to not get my oven too hot!

Food for thought.

Neill
Attached Thumbnails
chimney flow rate calculator-neill-s-pompeii-4-vent-flue   chimney flow rate calculator-neill-s-spark-arrester-chimney-arrester  
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Last edited by nissanneill; 07-07-2008 at 10:34 PM.
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  #25  
Old 07-15-2008, 05:58 AM
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Default Re: chimney flow rate calculator

I've been meaning to do some research on what is important in chimney design, so this spreadsheet was really helpful to get me started.

However I am really unsure about the building principles of how I join a stainless steel round flue to a rectangular brick vent base. The plans don't go into much detail and I don't have a Simpson Duratech installation guide (maybe I'll get one when I buy the flue) ?
Does the flue just rest on a flat square(ish) base at the top of the vent ?
Or is it cemented in place or ... ?
Can I (should I) surround the flue with a brick tower or is this not a good idea ?

Rather than answer these ?s here, if anyone can point me at a thread where this is discussed it would be much appreciated - the search throws up so many threads that mention chimneys that I don't know where to start...

TIA Roger
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  #26  
Old 07-15-2008, 12:37 PM
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Default Re: chimney flow rate calculator

Quote:
Originally Posted by BrritSki View Post
However I am really unsure about the building principles of how I join a stainless steel round flue to a rectangular brick vent base.

TIA Roger

Here is one of a few ways. I made a plug out of left over pink foam board and then cast my vent transition around it. I wanted to keep the airflow smooth and efficient. You can see a picture here;
http://www.fornobravo.com/forum/8/ca...nt-1692-3.html (Casting vent)

Others have made some very nice brick transitions, look at some of the complete Pompeii postings.

Wade
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  #27  
Old 07-15-2008, 05:25 PM
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Thumbs up Re: chimney flow rate calculator

Hi TIA Roger
I have attached ascan of ther drawing I made in getting the dimensions for Hendo's oven chimney which I am building for his widow.
The flue sections have a reduced end which slides neatly into the sleeve ring which is welded to the stainless void body. It can be sealed with silicon or if a neat fit, no sealing is needed. You could screw or rivet it but I have found that if made to close tolerance, then it will fit and stay in place. My 3 sections totalling 10 feet are unsecured and self supporting.

Neill
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chimney flow rate calculator-hendos-chimney-.jpg  
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  #28  
Old 08-15-2009, 01:28 PM
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Default Re: chimney flow rate calculator

Handy tool for comparing the relative draft between different ovens.

But something didn't look right about the figure for my oven, which came out to 935 cubic meters of air per second. That's roughly the entire volume of air in your house per second! Hmmm. Looks like the cm figures in the spread sheet didn't get converted to meters. Well the nice thing about the metric system is that you just need to move the decimal place to fix this.

The formula also assumes negligible losses to friction or temperature & well balanced flow. So the assumption is a well insulated chimney, very smooth vent transition & a door height/dome height ratio of ~ 63%.

Anyway, the tool is still very useful for comparing the draft between ovens. But just realize that the actual numbers it generates are off by four decimal places. My oven doesn't draft 935 m≥/s. It's more like 0.0935 m≥/s or about 3.3 cubic feet per second. Unless there is a tornado overhead.

.
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  #29  
Old 08-15-2009, 03:49 PM
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Default Re: chimney flow rate calculator

Darn!!! I thought I fixed that. I'll get it repaired and repost with the more realistic numbers.
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  #30  
Old 08-21-2009, 06:04 PM
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Default Re: chimney flow rate calculator

Hey Pacoast
Thanks for catching those errors. I generated that spreadsheet in about 10 minutes and continue to fine tune as people spot problems. I only intended it as a quick tool for forum members to figure out if they had enough draw.
Anyone or anything that can improve it is great. Please feel free to update, correct etc to make it better and then post so we can all benefit. I don't think there is any protection of the spreadsheet so have at it. If there is protection, drop me a line and I will send you a new copy.

Bruce
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