Flue Science

[EatenByLimestone]

Many people could gain better performance from their woodstoves if their chimney was a few feet higher. (Something I want to work on this summer.)

Would it be possible to trick a stove into thinking the chimney was taller than it really is? Could a venturi installed at the top create a vacuum that literally sucks air into the stove and up the chimney?

Matt

 

[keyman512us]

Good question...Maybe Corie could go into some detail?

 

[Corie]

Shhhh



That's a secret!

 

[keyman512us]

Corie:

Shhhh

That’s a secret!

Is that a secret we can work on?? lol

 

[elkimmeg]

think of it this way Why does the same caliper bullet leave a rifle barrow with more velocity than a pistol barrow?

 

[GVA]

coreolis? ;-P
Hee hee hee.....

 

[Corey]

I don't think a simple venturi (ie just restricting the flue) is going to gain you anything. True, flue gas speed will increase and pressure will decrease in the venturi, but volume will also decrease. The overall mass of gas passing through the opening will be the same. If you had some outside source of energy (blast of compressed air) then you could actually increase the draft (old time steam engines used this principle by forcing exhaust steam up the stack to create more draft and a hotter fire). Another way to think about it is if a simple venturi did increase the draft, could you stack 10 or 12 in series and vacuum your carpet with the resulting draft? The most reasonable answer is -no.

Now if you had a way to channel the prevailing wind through a venturi....

Corey

 

[begreen]

Hmm, Elk's rifle barrel proposition is interesting. Corie, do you know if a rifled smoke pipe would draft stronger or more efficiently than a smooth pipe? Are there any studies on this?

And does the bullet really leave the barrel with more velocity for an equal powder charge? I always thought that rifling gave the bullet spin which made the bullet more accurate over distance, but didn't know it increased velocity too.

http://en.wikipedia.org/wiki/Rifling
The grooves most commonly used in modern rifling have fairly sharp edges. More recently, polygonal rifling, a throwback to the earliest types of rifling, has become popular, especially in handguns. Polygonal barrels tend to have longer service lives because the reduction of the sharp edges of the land reduces erosion of the barrel. Supporters of polygonal rifling also claim higher velocities and greater accuracy. Polygonal rifling is currently seen on pistols from Heckler & Koch, Glock and Kahr Arms.

This is out of my realm of experience, but from what I can tell, a rifle has higher velocity because it can support a larger weight bullet which requires a longer barrel to develop the velocity. Is that in error?

 

[Gunner]

think of it this way Why does the same caliper bullet leave a rifle barrow with more velocity than a pistol barrow?

uh...more gun powder.

Begreen you are right, the rifling does not increase the velocity greatly, 2-3 times more gunpowder does.

How about flexible dryer vents, do they speed up exhaust?

 

[elkimmeg]

Check out a 22 calliper bullet one in a pistol same in a rife which leaves the barrel with more velocity same charge Yes it has a bit to do with rotational forces as well
what you think smoke does not spiral?

 

[Gunner]

the problem with the 22 pistol and rifle example is the length on the barrel elk. I should have said not only the amount of gunpowder but also amount of time (length of barrel) the bullet is confined and therefor forced to accelerate.

If the smooth liner was 10ft and the corrugated was 40ft of course the taller is going to draft better. Apples to oranges

 

[begreen]

Check out a 22 calliper bullet one in a pistol same in a rife which leaves the barrel with more velocity same charge Yes it has a bit to do with rotational forces as well
what you think smoke does not spiral?

We all know smoke spirals, the question is whether that could be improved by rifling the smoke pipe?
Forgive my ignorance on the rifle analogy, not my field, though I actually learned something today.

(broken link removed to http://www.riflebarrels.com/articles/bullets_ballastics/long_barrel_velocity.htm)

 

[GVA]

Well at the very least by rifling your chimney liners they are more accurate than a smooth one.... Therefore you could then direct your smoke either towards the people at BI or away from them... :cheese:

 

[keyman512us]

Hmmm...seems like Limestones' original concept has turned into a comparison between "apples and bowling balls"...lol

I'm no expert at flue design...but I can imagine over the years quite alot of concepts have been thought of and tried out.

If this were a "serious" discussion I would ask Matt a few questions first. Matt? What exactly do you want to do? How high is your current stack? What type of construction? How high do you want to make it? How good is the draft right now? Do you have ocassional "bad drafts"? Are there any roofs close by disrupting the draft? What are the "natural wind patterns around you"?? Does your stove produce "heavy smoke" becuase of flue performance?

I'm hoping this is a "serious" discussion and not just a "bait topic"....we'll see.

 

[EatenByLimestone]

Certainly not trying to bait. If I wanted to do that, I'd start a cat/no cat or is pine good to burn thread.

I have an external masonry chimney. I want to place 3 blocks on the top of it which will give me ~2 feet more. I get a small chimney smell when the stove isn't running. I believe this is because the house reverses the draft and pulls air down the chimney. This doesn't happen when the stove is running. Occasionally I have a back draft from the wind when it's gusting and coming in just the right direction. This summer, I'll invest the $40 or so needed to put a few more feet on the chimney.

I thought up the venturi thing when looking at a webpage for a water powered sump pump, then I started thinking about a variable venturi carb a friend used to have... then I started thinking about chimneys.... The cooling towers for nuclear plants look like they neck in then expand so I posed the question.

It still really hasn't been answered... The only explanation is a situation where a restriction in the barrel could be disasterous.

Matt

 

[EatenByLimestone]

Oh yeah, if you could increase the performance of your chimney (round metal tube would probably be easiest) with a bolt in or even a bolt on to the top, some marginal chimneys could be made to work much better.

I was just trying to help.

Matt

 

[begreen]

I think it's a good question. What are all the options for improving draft? There are cap that are supposed to help with wind related draft issues like Vacu-stack.

(broken link removed to http://www.chimneycaps.com/ICP_pages/installation.html)

 

[webbie]

My limited experience:
There are too many variables to compare with a rifle, etc.
That smoke is PUSHED, this is pulled.
Over the years, it has been claimed that a smaller top section (many chimney pots have this) would result in better draft - but I doubt it is proven.
Amount of make-up air is very important also.
Not much other than increased height or diameter will increase draft in NO WIND situations. However, many types of chimney caps will help out when there is any wind greater than the normal chimney velocity.
Yes, a smooth or round inner chamber can help.
A smaller inner chamber can help the speed of the smoke, but hurts the capacity.

And on and On.

 

[keyman512us]

I hear ya' Matt...

In the "somewhat near future"...this may be an issue I have to 'tackle' myself. My setup has a twenty foot 'stack'...only problem is I have to figure out how to put another twenty feet on it...lol

I might just have to "bite the bullet" and invest in a wood gasification boiler from say Tarm...and "re-vamp" the house. Not something I look forward to with a forty foot or so, old, un-lined chimeny...and the "loss of real estate" in the basement...arrggh!

My current set-up is somewhat "hick" at best...but it works with as close to zero% smoke as possible...thanks in large part to the current flue I designed and built. I've asked web and Corie their opinions...but I think they might consider it a little to "red-neck-tech"...lol

 

[Corie]

Not at all. I've been too dang busy to really thinking about it. Hopefully after this week from hell is over, I can really dedicate some time to all the ideas pulling at my sleeves.

 

[elkimmeg]

Corie one does not have to dig too far in the Mechanical codes it really is spelled out the relationhip to the fire place opening vs flue size / verticle run.

The code is setup in a scale similar to a parabolic curb. There is a verticle distance relationship for draft code used to use the 1/10 rule but verticle length was added to reflect the additional dfart with height

Basically the manufactures set a minium height that height has enough verticle rise to get minium draft results for the stove. To achieve optium results one would use a taller chimney
I wish I had the source but at one time it was generalized that for optium results the chinmeys should be taller than 20 feet. Like any parabolic curb once optium is achieved itstarts going down hill from there. some where at 36 to 40 feet the additional length the frictional drag starts taking effect I liken this to school grades 16' you pass witha C 20 feet a solid B.
Personally My parents were more impressed with B's