[Gasification] Water Disscociation Is Pressure Dependant?

[Daniel Chisholm]

The pressure changes of interest here are quite small (a few inches of wc,
above or below atmospheric pressure).  Since sea level atmospheric pressure
is about 400 inches of water column, a pressure that two or three of four
inches of water column higher or lower, is still within a percent or so of
ambient pressure.

On Tue, Mar 20, 2012 at 07:59, Greg Manning <a31ford at gmail.com> wrote:

> Hi Andy, Stephen, list and all.
>
> The proof is in the pudding,  if a ship or submarine can produce steam
> in water that is less than 50f (cavitation), and we can boil water at
> room temperature, the entire scale must slide when dealing with
> temperatures in less than ATM pressures, two different ways of
> dropping the pressure, end result is the same.
>

A table of water's vapour pressure versus temperature tells the tale (for
example, vapour pressure is 14.7psia/101325Pa at a temp of 212F/100C - this
is the relevant point for a tea kettle).  It is instructive to look up the
pressure at which water boils at 50F (the propeller cavitation example) or
72F (the room temperature boiling example).  I won't do the lookup but I
will insert a spoiler - these pressure are much lower than can be produced
with a suction fan.

Look again at the pressures involved in that video that showed water
boiling and then freezing in a vacuum jar (hint: don't mix up "gauge" vs
"absolute" pressure).  Another interesting thing from that video is that
they didn't produce the vacuum by running a pump, they opened up a valve to
a vacuum tank (not shown, but I'll bet it was impressively big).

Oh, BTW, speed of flow (velocity) and pressure (or the lack of) go
> hand in hand, don't they ? I'm under the understanding, this is how a
> wing works on a plane........ (someone correct me if I'm wrong).
>

Yes, but the numbers are important - there may or may not be an effect that
is interesting/useful.

Look this up: How much pressure does it take to produce a (say)
20mph(~10m/s) air flow? (equivalently, a 20mph airflow could be
theoretically captured to produce that amount of pressure increase).  If
this sort of air stream speed is of relevance to gasifier gas flows, it is
a good idea to know  the amount of pressure involved in dealing with these
sorts flows

In aeronautical considerations, air flows slower than 400mph are usually
considered "incompressible".  It's not because 200mph/300mph air over a
wing doesn't change density as a result of its speed changes (it does),
it's just that for many purposes the density is close enough to unchanging
that it is useful (and accurate-enough) to make that approximation.  So if
you are looking for a useful density change, don't bother looking for it in
flows that are moving slower than 400mph.

For example the hot rodder in us can imagine designing a "ram air scoop"
for a car travelling 60mph or 100mph to increase the density of air in
engine's intake manifold, but when you do the math of how much pressure
boost is available it shows just how much potential is there (not much!).




-- 
- Daniel
Fredericton, NB  Canada
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