My back-of-the envelope calculation supports Cal's intuition. In other words, yes.
If my calculation is correct, your 40' tall 7' diameter flue would draw like a 20' tall 8.3" diameter one.
Approximately.
author="Prosecond" date="1293085420"]Thanks for the replies. Is that calculation for real? I like the answer but would like to understand how you got it. Can you teach me?
Yes, that calculation is for real, and is an approximation, and I believe is accurate enough to confirm our intuition--that a 40' tall 7' diameter flue would draw like a 20' tall 8.3" diameter one.
As for teaching you, sure, if you have the patience. I've been happily geeking around for an hour, checking my work, and trying to figure an easy way to explain. I'm going to post all this in a Wiki article, so others can use it. I'll blather on, and you can tell me if you find it useful.
I think some general understanding first might be helpful, for I find it aids intuition, which is very useful in understanding this sort of thing. So I'll do the general stuff in this post, and the equation next if you're still interested, lol.
The insight that allowed me to get a feel for how chimneys work is:
Chimneys are driven by gravity, and the weight of the air above them.
Air has weight, and the denser air is, the more a given volume weighs. The air in a chimney is hotter and therefore less dense and therefore
weighs less than the air around it--so it wants to rise and float up. Just like a boat or hot air balloon is lighter than the volume it displaces, so it wants to float upward.
The key to getting a feel for this is to realize that the more the air above a given point weighs, the more it pushes down. So what holds it up? Air pressure! Air getting squished by gravity pushes back just like a compressed spring, and its pressure rises until it is exactly enough to support the weight on it. So the air pressure on the ground is exactly the amount required to hold up the weight of the entire column of air above it.
Less dense air? Less weight in that column of air rising up to space, less pressure required to hold it up, lower barometric pressure, and viola!, a low pressure system!
Here's a thought experiment that helps me to understand. Imagine yourself riding an elevator that goes all the way up into space. Remember, at each height in the elevator, the air pressure is exactly the amount required to support the column of air
above you. So at ground level your barometric pressure gauge shows some starting value, and as you rise into space, it gradually lowers as the less and less air remaining above you weighs less and less, until finally you are in a vacuum, with no air above you, and no air pressure. (choke)
This understanding, that
air pressure is determined by the weight of the air above you, and therefore decreases for every foot rise in height, is the central insight you need to understand chimneys.
With that insight in mind, take a look at this stuff I copied from Wikipedia, and included in another post.
First look at this illustration of the chimney effect--keeping in mind my long-winded (har) discussion of air pressure being determined by the weight of the air above it--as you look at the air-pressure gauges in the illustration:
The stack effect in chimneys: the gauges represent absolute air pressure and the airflow is indicated with light grey arrows. The gauge dials move clockwise with increasing pressure.
http://upload.wikimedia.org/wikiped...imney_effect.svg/220px-Chimney_effect.svg.png
Now it's easy to understand why the pressure gauges read higher at ground level--they have a greater weight of air above them. And why the gauge in the chimney reads lower than one at the same height outside the chimney--
the hot air in the chimney is less dense, so the column of air inside the chimney weighs less than the air outside the chimney, so it pushes down with less pressure, and less pressure is required to hold it up
The difference in pressure between the air inside the chimney bottom and the outside air next to it is what creates draft.
Gravity is the force that drives draft, by pushing down harder on the air outside of the chimney, and pushing it inside.
Now chimney performance is easy to understand:
Why does a hotter chimney produce more draft? Bigger weight difference from less air in the chimney.
Why is draft worse on a warm day? Less temperature difference between stove and outside means less weight difference between inside and outside the chimney.
Why does a higher chimney draft better? a
taller column of light air creates a bigger difference in weight relative to a column of outside air the same height.
Why does a vertical run of pipe immediately after a stove start drawing faster than an horizontal pipe? The
taller a column of hot air, the greater its weight difference relative to a column of unheated air the same height. So a chimney creates draft only when rising sections are hot, and a hot
horizontal section creates no draft.
Why does it take a while for draft to start? The chimney is only lighter by the amount of hot air in it--it won't draft best until it is filled with hot air.
Why do downdrafts mess up draft so much? The backflow fills the chimney with cold air and chills it, instantly killing draft until the chimney can refill itself with warm air.
Why do uninsulated, oversized and/or masonry chimneys perform worse? Now you can answer that one yourself.
I've run out this post's character allotment, so the calculation part will have to wait, if you're still interested.
If this general explanation makes sense, then understanding and using the equation should be fairly easy. %-P
