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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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