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why balloons float/tall chimneys/ reduction

Michael Wendt on fri 5 mar 04

Sorry if this seems pedantic.
I think we really need to understand the mechanisms we use in our work.
Helium filled balloons do float upward. Why? Certainly, they are less dense
than air and that is a precondition. What MAKES them move upward? It takes
force to move things, right? What is the force? No, not the star wars kind.
:-)
If I had one balloon and it could lift a single paper clip and I wanted to
move 100 paper clips upward, I could get 100 balloons and each would lift
one paper clip and I could then move 100 paper clips.
Imagine your chimney filled with a column of balloons. If the chimney is 50
balloons high, it can only do half the work of one 100 balloons high.
As to whether a tall chimney affects reduction, ask yourself, how can
increased pull or suction on the outlet affect the amount of air that gets
in?
Snail's answer was right on. If you want reduction and have too tall a
chimney, you have to restrict the cross section of the primary and secondary
air ports in some way or follow Dave F's recommendation to install some kind
of dilution or passive damper into the chimney to reduce the draw.
Balloon physics below only if you care.
Regards,
Michael Wendt
Wendt Pottery
2729 Clearwater Ave
Lewiston, ID 83501
wendtpot@lewiston.com
www.wendtpottery.com
Why balloons float:
draw a circle on a piece of paper.
draw 8 equally spaced arrows around the outside with their points touching
the circle and eight similar arrows meeting them from the inside.
This represents a cross section of a balloon.
Since the pressure represented by the arrows is equal, the arrows should all
be the same length. Now, lengthen the one arrow at the bottom of the balloon
slightly to represent the increased atmospheric pressure that always exists
and depends on how high you are above sea level. This is the free body
diagram of the forces acting on the balloon. No others.
Since the balloon is free to move and the force at the top is minutely less
than the force at the bottom, the balloon moves upward, trying to balance
the force. It will continue to rise until it finds a location where the
forces are all equal.
The atmospheric pressure differential per foot is truly so small it hardly
seems worth considering, yet it is real and it is almost twice as much at 2
feet as it is at 1 foot and it is nearly10 times greater at 10 feet than it
is at one foot.