Brian Whatcott wrote:
On Sat, 29 Sep 2007 12:59:39 -0700, Keith Hughes
wrote:


My, my: "it's just plain wrong": he said a column of 32 ft,
Uhmmm, no, he said a "32' column of water". Can you see the difference?

Hmmm..Priestley certainly could. His water barometer had a water
column round 32 or 33 ft high.
How 'bout that! :-)

Quite true, which probably engenders the confusion. However, the
barometer *starts* with an evacuated column. That's how the atmospheric
pressure can push the water 32' up the column - you have 14.7 PSIA to
work with to lift the water. Same for a mercury barometer, or a McCleod
gauge, etc.

Different story than using the water column to *generate* the vacuum.


Gee, I didn't know you were using 'smart' molecules that travel *only*
in the direction you want them too.
....
Keith Hughes



Ho, hum: if half of them go in the wrong direction
until their first collision, it must take them a really, really,
REALLY long time to diffuse through the water vapor/rarified air mix!


Half? More likely 99.99999++% of them will not be traveling parallel to
the axis of the column. Half of those that *are*, are going in the wrong
direction.

How difficult this type of mass transport actually is can be seen by
looking at flow rates for water vapor from lyophilizer chambers to the
condensers (yes, I have done this a lot). Putting a 90° bend in the tube
connecting the chamber (where the ice sits on heated shelves) and the
condenser roughly (very roughly, given the variability of other design
factors) cuts the flow rate in half. That's in a 48" ID tube too! And
pulling vacuum through the condenser to maintain 50-100 microbar
pressure - i.e. maintaining a significant delta-P from chamber to
condenser. And with condenser coils maintained at -65°C.

It may seem counterintuitive, but the molecular motion you referenced
just *causes* the pressure, while providing little impetus for mass
transfer from point A to point B. And once the pressure reaches
equilibrium throughout the system, you have to rely virtually entirely
on diffusion, which is much, much slower.

Keith Hughes