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Alec,
The comment about placing the fan near my feet was more about just getting
the noise away from my head especially if the acoustics of the canopy turn out
to be like a sound chamber. I'm expecting that the inlet for the scrubber
would be somewhere near my head. I like the concept of having the fan
downstream in the configuration to draw air through the scrubber(s) instead of
pushing it through. I also want to consider electrical interference with
other instruments and components in placing the fan.
Since air exhaled during respiration contains moisture and the air
in a sub is often humid to begin with, I'm hoping there would be some
method of having the air as dry as possible prior to entering the scrubber, but
I have no specific concept or design in mind for accomplishing that goal.
Depending on the temperature and moisture content of the air leaving the
scrubber system, it might be useful to duct it toward viewports to prevent
fogging.
I probably should drop out of discussions until after the April 15th tax
deadline, so if you don't hear from me for a bit, that's why. I'll catch
up later.
Cheers,
Jim
In a message dated 3/15/2011 8:42:29 A.M. Central Daylight Time,
Alec.Smyth@compuware.com writes:
From prior scrubber
experiments, the agent gets soggy and drips. There would be condensation on
the scrubber insides for sure. But water would still not run uphill, so if the
fan is mounted at the highest point I don't see a need for a water trap. Your
comment about putting the fan at your feet sounds like the fan might be under
the scrubber. Is that correct? In this present configuration, the scrubber is
a vertical cylinder (actually two concentric cylinders) with the fan on top,
drawing air through the center. The bottom end of the cylinder can hold
water.
BTW here's last night's
update. My new computer fans didn't arrive yesterday but I poked around
and found I had a squirrel cage fan on hand, just like the ones Cliff had
referred to only a bit bigger. Cliff's references were 75mm by 75mm. The one I
had was 120mm by 120mm, 23 cfm, 50db. I adapted it to the scrubber and
tested with and without a Sofnolime load. BTW the scrubber capacity
turned out to be a hair under 10 lbs of Sofnolime. I was very surprised
by the result, compared to my earlier tests with computer fans. Cliff, in a
word you were right! Although I was not measuring anything, seat-of-the-pants
there is no noticeable decrease in airflow when the scrubber is
loaded. The fan didn't even notice the resistance caused by drawing air
through the Sofnolime, and was putting was putting out a gale. Probably too
much so, I can't imagine I would need quite that much airflow. So last night I
ordered a squirrel cage fan with about half the airflow for another test. The
50db noise level is tolerable but tiresome, and my goal is to
find a fan that has sufficient airflow yet no more than what is needed, in
order to minimize the noise.
To be
continued...
Alec
Alec,
I'm tending toward having the fans draw the air through the scrubbers
rather than push it. I think that might make it easier to design for
even airflow through the material. It also makes it easier for
the fans to be located nearer my feet instead of near my head.
I would think the scrubbers could be subject to degradation from moisture
intrusion or some other causes I'm not aware of, so I'm planning
redundancy there, too. The question I have is if it would be
prudent/practical to install a filter/water trap upstream. Hopefully that
could be done without significantly impeding airflow. This brings up the
related topic of humidity control in the sub in general.
Comments?
Thanks,
Jim
In a message dated 3/14/2011 4:53:04 P.M. Central Daylight Time,
Alec.Smyth@compuware.com writes:
Excellent points gentlemen,
thanks. I had initially mounted the fans to
blow into the center
cylinder, but will now reverse them based on Sean's
suggestion. Note I
say "fans" because I'm mounting two of them in
series, for redundancy,
although only one would normally be used. The
scrubber itself has no
moving parts, so I figure the fan is all that can
fail.
Cliff, I
agree squirrel cage fans would be a better choice from a
performance
perspective. However, I'm so tight on space that I'm at
least giving
axial a try before discarding the idea. Also, Deep Worker
uses axial.
Note I'm not shooting for 72 hours, and because Snoopy has
12V main
propulsion, the whole boat runs off one big battery bank so
current draw
should not be an issue. I'll have to see about the pressure
and noise.
I've convinced my daughter to join me in Snoopy for some
hours, so we can
make it a two person test. However, I am still awaiting
delivery of the
fans, which should be here any day.
Jon, the photo you linked to
looks functionally identical to the eBay
filters, except the end pieces
are plastic while these are aluminum. The
thickness of scrubbing agent is
about 2". The one I'm using is eBay item
#400187881680.
I'll
report back post-test.
Alec
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From:
owner-personal_submersibles@psubs.org
[mailto:owner-personal_submersibles@psubs.org]
On Behalf Of Sean T.
Stevenson
Sent: Monday, March 14, 2011 4:01
PM
To: personal_submersibles@psubs.org
Subject: Re: [PSUBS-MAILIST]
open source scrubber design
The scrubber is a radial flow design,
with the flow direction from the
outer diameter to the inner one.
This makes most efficient use of the
scrubber material, as the flow area
is greatest at the outer diameter
when the gas contains more CO2, and
reduces as you approach the inner
tube, as CO2 is removed. Also,
the inlet flow area of the scrubber
(outer diameter x pi x
length) is
huge in comparison to the minimum scrubber inlet diameter,
making the
velocity (assuming even pressure distribution) almost nil
through the
absorbent. One thing to be wary of when comparing
submersible
scrubbers to rebreather scrubbers, is that the gas flow
through the
rebreather device is cyclic - there is a very small pressure
differential
between the gas in the exhale counterlung and the gas in
the inhale
counterlung, which leads to slow gas movement through the
scrubber - the
"dwell time" of gas inside the scrubber is only
interrupted when the
inhale counterlung collapses and gas must be drawn
through.
Submersibles, on the other hand, must use steady flow and so
the gas
velocity through the material must be accounted for - of course,
you can
make up for higher speeds with more passes, but the least noise
and power
consumption solution is a slow flow through a large
device.
-Sean
On Monday 14 March 2011 12:12:54 you
wrote:
> Alec's test results will be very interesting to follow.
These
> canister units, if I understand the way Alec intends to use
it, are a
> radial design. One of the potential problems I see
is that there is
> only 1-1.5 inches of material in the void between
the canister walls
> which makes me question whether that will
provide adequate "dwell
> time" for the CO2 to pass over the material
and be absorbed
> efficiently. It will also be interesting to
hear from Alec how
> difficult the units are to pack with
material.
>
> Alec, depending upon your results I would also
look a modifying the
> can somewhat by adding a center tube, maybe 1
inch in diameter through
> the axial center of the can (like this
> http://www.flickr.com/photos/billreals/2966584447/ ). Add
sodasorb
> into the interior (and between the canister walls if you
want) and
> feed the air flow through the center tube via a
centrifugal fan. This
> would allow more dwell time through
more material.
>
>
Jon
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