"Gymmie Bob" wrote in message
...
Your logic on pressure due to column height is just plan nonsense also.
Column height of air does not change pressure significantly inside a house
or oven. A ceiling fan changes the pressure of air much greater than the
difference of the top to bottom of a 30 foot column of air.
The updraught on the chimney you speak of is from two factors. The heated
air being lighter than the outside air causing convection (as you spoke
of)
and the venturi action of a cross wind passing the opening of the flue.
This is all basic grade 9 physics where I was raised.
Maybe you shouldn't have stopped at grade 8 then. You seem to have a warped
idea of what 'change pressure significantly' means. Trouble with using such
inexact terms as 'significantly' is you have to put them in context. In the
context of ventilation in a house, the 11 Pascal pressure difference I
calculated *is* 'significant'. If we were discussing the air pressure in
all four tires of your car, then an 11 Pascal pressure difference would not
be 'significant'.
Here, try some physics...
ideal gas law, gives us the density of 1 ft^3 of dry air at a given
temperature....
At 70 F...
density = 14.696*144 / ((70+460)*53.3) = 0.074913 lbm/ft^3
At 0F...
density = 14.696*144 / ((0+460)*53.3) = 0.086313 lbm/ft^3
The difference in the pressure exerted by gravity on two 20 ft columns of
air at these densities is approximately....
delta pressure = 20 ft*( 0.086313 - 0.074913) lb/ft^3 = 0.228 lbf / ft^2.
0.228 lbf/ft^2 is about 11 Pascals
So if the pressure at the top of the house is the same inside the building
envelope as it is outside, then the pressure at ground floor level inside is
0.228 lbf/ft^2 *less* than the pressure outside at the same elevation. Cold
air pushes in down low, near the foundation and pushes warm air out near the
roof.
Now, do you have *any* idea how much air flow that 11 Pascal pressure
difference can develop if it is pushing air through a combination of cracks
and crevices that add up to a total cross-section of about 0.4 ft^2 (similar
to a crack about 1/8 inch wide totaling 40 ft in length)?????
Here, try this if the math is too much for you....
http://chuck-wright.com/calculators/stack_effect.html
Just enter 0.4 for area, 20 for height difference, 70 for indoor temperature
and 0 for outdoor temperature. Voila!
ASHRAE's stack effect formula is for a deliberate opening with something
near circular shape ( Cd of 0.65). A very long and narrow crack would not
provide this much flow because of its very different hydraulic diameter.
Can you compensate for the difference in hydraulic diameter from an
unobstructed circular opening and a very long and narrow rectangular
opening? Here's a hint, for openings where the length width, just use
the length for the hydraulic diameter, and for circular openings, use the
diameter of the opening. So the correction factor for the different shape
would be 0.714 / 40. Now, if you just knew how to apply it....
You don't need a *changing* temperature to develop this pressure difference,
you just need gravity, a fluid that is a different density at different
temperatures (like air) in the two columns, and a vertical distance over
which the density difference acts.
Regarding ovens, most gas ovens rely on this same effect to move the
combustion products from where the gas is burned, out of the oven (no
'venturi action' inside the kitchen). The developed pressure difference is
less, but since the openings are deliberately made to encourage this flow,
it works just fine. Of course there is a trade off, because you want the
combustion products to be vented out effectively, but too much flow and the
oven cools rapidly between burner cycles, wasting energy and heating up the
kitchen.
daestrom
P.S. A chimney that relies on it being a windy day in order to draw
properly is a waste of stove pipe.