Chimney height

Devil505 wrote:Here's my "layman's" understanding of how a chimney works:

Warm air rises
As the warm air rises it expands
Trapped within a chimney's walls, this air expansion is "squeezed" & since it wants to expand (but is prevented from doing so) it increases it's velocity upward to escape it's bounds
This increase in upward velocity creates a vacuum below the upward rushing air.
Since nature abhors a vacuum, it (nature) fills it (the vacuum) by sucking the exhaust out of the fireplace/stove/furnace
The higher the chimney, the greater the upward air velocity & vacuum

Thus, we have Draft!


(bows)



Any engineers out there wanna critique my analysis?





So the question is: Is there a minimum height necessary to achieve this chimney effect, & if so.......how does one formulate it?

Yes warm air rises because its lighter but actually a chimney should work without the warm air rising thing. The reason being is that it works not because of air temperature, but air pressure. At the bottom of a 30' tall chimney, let's say the air pressure is 14.7 PPSI. Now if you climb the chimney, you may find that the air pressure only produces about 14.4 PPSI Because there is 30' less air squeezing it down. The draft is created by a long cylinder seeking to equalize pressure through out its length. So any long length of pipe stood up should create a draft of sorts even without a fire. It's Bernoulli's law!

It's the engineer in me.

cArNaGe wrote:Would a 6x6 flue of 45 feet work? What the calculations to figure it out?

The only thing you need to figure out is how big of a baro that beast needs.

Near the bottom there is a sizing chart. It looks like 7-8" but it is hard to tell with that height chimney, I would call Field Controls Tech Support as they have a very helpful staff.

http://www.fieldcontrols.com/draftcontrol.php

coaledsweat wrote:es warm air rises because its lighter but actually a chimney should work without the warm air rising thing. The reason being is that it works not because of air temperature, but air pressure.

Interesting! I never realized that...Thanks for the info.

coaledsweat wrote:Yes warm air rises because its lighter but actually a chimney should work without the warm air rising thing. The reason being is that it works not because of air temperature, but air pressure. At the bottom of a 30' tall chimney, let's say the air pressure is 14.7 PPSI. Now if you climb the chimney, you may find that the air pressure only produces about 14.4 PPSI Because there is 30' less air squeezing it down. The draft is created by a long cylinder seeking to equalize pressure through out its length. So any long length of pipe stood up should create a draft of sorts even without a fire. It's Bernoulli's law!

It's the engineer in me.

And as the chemistry of combustion takes place the fuel + air react with the individual elements in each to produce gases which have different pressures. All hopefully ends up in the chimney producing what we call draft. The nitrogen in the air is usually undesirable because it plays no part in the combustion chemistry equation and just conducts heat out the flue.

For an exact answer on chimney draft you need to evaluate the chemistry equations for combustion of Anthracite coal in air. That a look at this spreadsheet link:

http://www.repp.org/discussiongroups/re ... 7-2003.xls

It's a calculation for the combustion of wood in a cooking stove. Note how the author has carefully calculated the moles of gases produced by burning the wood, added excess air and then determined chimney draft. You would need to do the same for Anthracite coal, empirical formula: C240H90O4NS

I'm brand new to the coal and alternative heat scene, and I've been trying to research chimney designs, and came across these links in the above mentioned wood stove site.

It seems as though having a chimney as hot or hotter than the exterior air is the key to proper drafting. But like I said, I'm new to all of this.

http://www.woodheat.org/chimneys/trichim.htm
http://www.woodheat.org/chimneys/evilchim.htm

I guess if you have the stove running all the time, this Neutral Pressure Plane idea is not an issue.

frankg7 wrote:It seems as though having a chimney as hot or hotter than the exterior air is the key to proper drafting. But like I said, I'm new to all of this.

Very true & welcome Frank!

What you said above really become critical when it's relatively warm outside & you are trying to keep a cool fire burning in your appliance. The greater the temp difference between the air inside the chimney & the air outside, the better the draft....& vice versa!

frankg7 wrote:I'm brand new to the coal and alternative heat scene, and I've been trying to research chimney designs, and came across these links in the above mentioned wood stove site.
http://www.woodheat.org/chimneys/evilchim.htm

Thanks Frank. Some good reading at that link. Chimney height has been a bother to my plans. I'm set up for a chimney on the outside end of my ranch house. The vent hole through the foundation is below grade and there are a few course of block outside bringing it about 30 inches above grade then capped off. It is located very near the lowest part of a steep pitched roof (outside corner of the house). I figure I would need to build a chimney very high to have enough clearance for a good draft. I'm considering a hand fired and I think I'll look into a power vent on my chimney base.