Hank Murrow on thu 4 mar 04
Dear Chimney Group;
The difference in atmospheric pressure along the height of a chimney is
minuscule and has no measurable effect on the pull of the chimney. The
buoyancy of the hot gases in the chimney (which depends upon
displacement by the denser ambient air around the kiln) is the main
factor in developing draft. This can be done by decreasing the diameter
of the chimney progressively as Michael Wendt pointed out today. The
teardrop shape of a typical anagama kiln is an intuitive response to
this by earlier potters. Do my ping pong ball experiment to see it most
graphically.
Buoyancy may also be increased by retaining heat in the escaping gases
by utilising an insulated chimney so less heat is lost to the chimney.
A well-insulated chimney can be shorter, yet produce the same draft as
a taller, uninsulated one. I believe insulating the chimney is the more
effective way in most situations. A fine chimney may be fabricated
using aluminum sheet rolled at a sheet metal shop to the desired
diameter and lined with ceramic fibre pinned to the aluminum tube. And
no, the fibre will not 'get into the air' or cause lung cancer. Use
care when handling, of course. A chimney made this way is quite
inexpensive, light and easily movable, can use the cheapest grade of
fibre, and need be only 60% of the height of a brick one.
Thanks to all who have contributed to the discussion,
Hank in Eugene
Ivor and Olive Lewis on sat 6 mar 04
Dear Hank Murrow,
Thanks for those interesting thoughts.
From what you say, if there is even heat distribution throughout the
fabric of the kiln and the stack, then changing the height of the
stack will have no effect on the velocity of the effluent gases.
If this is the case a smoke stack is irrelevant to the behaviour of
the kiln and it will only serve to remove noxious gases from the
proximity of the kiln. Is this believable?
It might be a good idea for those who have kilns with stacks to do
some testing and time the passage of gas through their kilns. All that
is needed are some pieces of old cloth dipped in oil. To test, a piece
is tossed into one of the fire mouths or popped though a spy port and
the time, from that instant, to the time smoke emerges from the top of
the stack, recorded. In keeping with lab practice I suggest five or
ten repetitions and to calculate the average. With enough samples and
a broad variation in kiln stack height it becomes possible to plot the
graph and determine the Rule. If people wish to do that I will act as
coordinator for the results and publish the findings.
One important note. There must be no gap between the flue outlet from
the kiln and the base of the stack. If effluent from the kiln rises
freely into an open canopy which takes the smoke to a pipe and through
the roof, measure from vertical distance from the fire ports to the
kiln outlet. Disregard the stack outside.
The important measurements are: the Time Interval, in Seconds; the
distance between the Fire Ports and the Top of the Stack, in Metres;
Pyrometer temperature in Degrees Celsius; Atmospheric Pressure in Kilo
Pascals.
Stack interior area could prove useful in making the volumetric
calculations according to the Gas Law, as was done by Michael Wendt.
We can then cross check our efforts.
Note that there is no definition of Stack Elevation or height from
fire port to gas exit point in the calculations made by Michael. So he
proves that the velocity of the gas is solely due to temperature
difference and volumetric increase supporting the notion that stack
height is irrelevant. As I asked before, is this so ?.
Hank, methinks we are progressing into unknown territory.
Best regards,
Ivor Lewis. Redhill, South Australia
Hank wrote
> Dear Chimney Group;
>
> The difference in atmospheric pressure along the height of a chimney
is minuscule and has no measurable effect on the pull of the chimney.
The buoyancy of the hot gases in the chimney (which depends upon
> displacement by the denser ambient air around the kiln) is the main
> factor in developing draft. This can be done by decreasing the
diameter of the chimney progressively as Michael Wendt pointed out
today. The teardrop shape of a typical anagama kiln is an intuitive
response to this by earlier potters. Do my ping pong ball experiment
to see it most graphically.
> Buoyancy may also be increased by retaining heat in the escaping
gases by utilising an insulated chimney so less heat is lost to the
chimney.
> A well-insulated chimney can be shorter, yet produce the same draft
as a taller, uninsulated one. I believe insulating the chimney is the
more
> effective way in most situations. A fine chimney may be fabricated
> using aluminum sheet rolled at a sheet metal shop to the desired
> diameter and lined with ceramic fibre pinned to the aluminum tube. >
> Thanks to all who have contributed to the discussion,
>
> Hank in Eugene
Michael Wendt on sat 6 mar 04
Dear Ivor,
For my calciner research, I used a blower powered
burner so no stack was required. Before the invention
of blowers, chimneys performed the same function
using the power of the rising hot gasses to create draft.
Try this:
Build a stacked brick chimney 2 feet tall over a fire pit.
Try your oil soaked rag experiment. Then extend the
height to 3 feet and 4 feet and 5 feet and see how long
it takes to traverse each taller chimney.
If they are of uniform cross section, and the same
amount of hot gas enters the bottom, the same amount
must leave the top. The velocity of the gas does depend
upon the temperature and cross sectional area.
The ideal chimney would keep the exhaust gasses
as hot as possible for maximum draw with minimum
height. If it were also equipped with a dilution port
at the bottom, the cooler air introduced would allow
very fine draft control when needed.
I expect you should find a taller chimney will produce
more draw than a shorter one.
I do agree that other factors reduce the effectiveness
of a tall chimney and offset the greater pressure
differential that exists as a result of greater height.
Most notable are increased heat
loss, the fact that as we increase gas velocity, turbulence
can set in, that the power required to move a greater
volume of liquid or gas through a tube increases in an
exponential relationship to the amount moved and
increased drag due to length. These all serve to mask
the fact that a chimney that is twice as tall starts with
twice the pressure differential over one that is half
its height. Further pressure information below.
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, ID 83501
wendtpot@lewiston.com
www.wendtpottery.com
As to why I say the atmospheric pressure is greater at the bottom of a
chimney than at the top, consider a more familiar model we can sense. Enter
a swimming pool and dive to the bottom 8 feet down. The pressure on your
ears at 8 feet is 4 pounds per square inch higher than is was at the
surface. The pressure increases in a uniform, measurable and calculable way
as you go deeper. If you place a small spherical balloon in the water and
take it to the bottom, you exploit the fact that the pressure on the
underside of the balloon is greater that the upper side by some factor and
this pressure difference causes the balloon to rise. If you placed a long
sausage shaped balloon in the water whose axial diameter was the same as the
diameter of the spherical balloon but whose height was twice as tall, it
would clearly exert more lift than the shorter balloon. As you extend this
sausage shape longer and longer, the pressure differential gets bigger and
bigger and the resulting buoyancy increases. It can do more work just like
the delta P in the taller chimney. Like all things, there is a point of
diminishing returns with tall chimneys.
With very tall stacks, is it possible they extend high enough to engage the
venturii effect of higher speed winds found a few hundred feet above the
surface?
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