precaud said:
...a narrower flue draws air at a slower rate for a given temperature...
George, I think it is not a fair comparison to hold temperature constant. I think the constant should be the volume of gasses passing through the chimney. With that, I think you'll find the 5" chimney maintains a higher temperature, which at least partially makes up for the area difference.
It's on now!
Okay, the rest of you can ignore this if you like, for it is time to split some technical hairs! :cheese:
Interesting, Precaud
(BTW, what is the origin of your handle, anyway? Canadian Academy of Urban Design? I'm pretty sure it's not Congenital Anterior Urethral Diverticulum ).
Okay, perhaps I should have been even more geeky, and clarified by saying "a narrower flue's inherent, unthrottled flow rate is lower for a given temperature of exhaust gasses". Better?
Let me see if I follow your logic. For a given, intermediate burn, the added drag of a narrower flue will be compensated by a larger damper opening, causing identical flue *flow* rates for identical stoves burning at the same rate into different diameter flues, but higher gas *velocity* in the narrower flue. Correct?
Or perhaps your reasoning is simply that the narrower flue's reduced surface area leads to less heat loss and higher exhaust gas temps? And your reasoning is that this higher average gas temperature helps compensate for the reduced cross sectional area?
I take your point, and appreciate your insight, though draft is only proportional to the
square root of temperature difference (and chimney height), but
directly proportional to chimney area. This gives the counter-intuitive insight that changes in chimney diameter affect draft more than changes in height or temperature.
Granted, the flue gas temperature used in that particular model is an average one, which does not take into account the surface area effect I think you're mentioning. But that is a second-order effect.
If you're interested in the equation I'm using, you can follow this link to the Wikipedia article on chimneys, and scroll down to the "Chimney draught or draft" section, you'll find the model I'm using, which I find very useful--the illustration of chimney draft is particularly good for intuition:
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
http://en.wikipedia.org/wiki/Chimney#Chimney_draught_or_draft
As a "first guess" approximation, the following equation can be used to estimate the natural draught/draft flow rate by assuming that the molecular mass (i.e., molecular weight) of the flue gas and the external air are equal and that the frictional pressure and heat losses are negligible:
Q = C A(sqrt(2gH(Ti - Te)/Te))
where:
Q = chimney draught/draft flow rate, m³/s
A = cross-sectional area of chimney, m² (assuming it has a constant cross-section)
C = discharge coefficient (usually taken to be from 0.65 to 0.70)
g = gravitational acceleration, 9.807 m/s²
H = height of chimney, m
Ti = average temperature inside the chimney, K
Te = external air temperature, K.
Thoughts? Thanks for an interesting point!
I now return you to your thread, which is already in progress.