#11  
Old 03-13-2010, 11:27 AM
Apprentice
 
Join Date: Jul 2008
Location: Norway
Posts: 118
Default Re: Vent dimensions. Please Help.

if you want to make a correlation which make sense you need to include the chimney height. A reasonably good correlation should be obtained by including flue opening size (which is normally proportional to oven size), flue opening/chimney size ratio (area) and chimney height. This is based on the assumption that the flue opening to chimney is made as a gradual transition which is the recommended method. If there is a lot of variation in dimensions in this transition this parameter need to go in as well.
Maybe we should put a spreadsheet on the forum and let everyone plot in their characteristic design (the three parameters) and state their experience (good, fair, bad) and we may see a correlation as data is collected?

karl
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  #12  
Old 03-13-2010, 04:02 PM
Peasant
 
Join Date: Oct 2008
Location: Tanja
Posts: 25
Default Re: Vent dimensions. Please Help.

I think Karl makes some good points.
With all the previous discussions regarding dimensions of flue outlets no one has mentioned oven volumes or chimney heights. Certainly, these are major considerations.
Surely someone, sinch the days of Pompeii, has come up with some formula for calculating the measurements related to venting a wood fired oven.
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  #13  
Old 03-13-2010, 10:00 PM
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Location: Townsville, Nth Queensland,Australia
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Default Re: Vent dimensions. Please Help.

The larger the chamber the more efficient it seems to be, so a small chamber will need to have a larger flue relative to its size. I once made a very small kiln for firing ceramic jewellery to high temps (1200C) I had to redesign the downdraft flue a couple of times and it was never particularly efficient. It had a large flue for its relative size. Another analogy is the fuel consumption of a small car relative to that of a truck. The car with a 1000 cc motor will use way more than a 1/5th of the fuel of a 5000 cc truck.
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Old 03-13-2010, 10:02 PM
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Default Re: Vent dimensions. Please Help.

My 21" oven works well with a 5" flue, but I don't think you would need to go as big as 10" for a 42"
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Old 03-13-2010, 11:29 PM
Peasant
 
Join Date: Oct 2008
Location: Tanja
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Default Re: Vent dimensions. Please Help.

I agree with you David. My experience with kilns and ovens suggests the same.
However there is a maximum that is often quoted by kiln builders - "It's always easier to block a hole than open one up".
Isn't there some record of someone formulating the relative sizes of openings to volume out there. There is in the kiln 'Bible' and they vary greatly!
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  #16  
Old 03-14-2010, 03:43 AM
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Default Re: Vent dimensions. Please Help.

I'll check out my Daniel Rhodes "Kilns" and get back to you.
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  #17  
Old 03-14-2010, 04:38 AM
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Default Re: Vent dimensions. Please Help.

Rhodes states "It is better to over engineer" The info I've got relates to kilns which are designed to reach their optimum efficiency at double our temps. but for example "a 10 cu ft downdraft kiln will need a flue hole of about 40 square inches and kilns up to 40 cu ft are usually given a 9"x9" chimney. The chimney height may vary from 6 or 8 feet to 30 ft. A 3 ft diam oven is about 7 cu ft (better check my maths) All this is not really much help because he is talking about down draft kilns which need a strong draw to bend the flame down. We only need enough draw to remove the smoke. The Ancient Romans knew what they were doing just follow what's been done before.
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  #18  
Old 06-23-2010, 03:38 PM
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Join Date: Apr 2010
Location: Toronto
Posts: 22
Default Re: Vent dimensions. Please Help.

When wlively refers to an optimal angle for the flue transition I assume he means the top angle when looking from the side?
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  #19  
Old 06-23-2010, 06:22 PM
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Default Re: Vent dimensions. Please Help.

Mine is 4 inches by 10 inches leading up to a 7 inch diameter metal vent. Draws just fine for the cooking phase.

Don't worry too much with this. I am of the opinion that most vents and chimney are far larger than needed.
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Old 06-23-2010, 06:59 PM
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Default Re: Vent dimensions. Please Help.

Technically they are not needed at all. The standard design will work fine exhausting smoke out of the front of the oven. If you do not want smoke out of the front, the standard rule of thumb for fireplaces seems to apply: 10% of the area of the opening for the flue.
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