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Hi James,
Couldn't get a detailed picture of that unit, but
it looked ideal for using in fire prone
rural Australia.
There was talk by a major Australian concrete water
tank manufacturer
of using their tanks as safe havens should
fire sweep through a property, so a simple product
like that may be of benefit there.
Alan
----- Original Message -----
Sent: Tuesday, March 15, 2011 6:22
AM
Subject: Re: [PSUBS-MAILIST] open source
scrubber design
Hi all,
Regards
James
On 14 March 2011 16:46, Alan James <alanjames@xtra.co.nz>
wrote:
Hi Alec,
I did a quick google on image search for
activated carbon filters & there are a
host of products that look like they could be
adapted. If you searched long enough you're
bound to find something cheap, off the shelf,
that could double for a scrubber.
I've been in pet shops & seen filter
systems for fish tanks that looked tempting to modify.
When I contacted my CO2 absorbent supplier,
they suggested a water trap at the bottom
of the scrubber.
Alan
----- Original Message -----
Sent: Tuesday, March 15, 2011 3:35
AM
Subject: RE: [PSUBS-MAILIST] open
source scrubber design
What
an excellent post, thanks Cliff. I wish I could write this response in a
couple of weeks, because I am getting ready to test a new scrubber in a
few days. But I wanted to give you a heads up regarding something I found
on eBay. Search "activated carbon filters". These are intended for
the ventilation systems in environments with odor problems. They consist
of two concentric perforated sheet metal cylinders with activated carbon
filling the space between them, and a fan (not included) blowing air into
the innermost cylinder. As you can imagine, they are quite easy to convert
to a pretty nice scrubber. All you really need to do is
line the cylinders with fine mesh, because the activated carbon granules
are much larger than the ones in CO2 absorbent. In my case I also
replaced the cylinder walls, because the ones provided were zinc and I
like aluminum or stainless.
Alec
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When I build my boat, I to did some investigation into
commercially available scrubber systems that would be applicable for
a one man psub. I did not find any that were economically
viable. While I agree with Leander that fundamentally the scrubber is
not complicated, to make an efficient and practical scrubber
takes some work. I went through several iterations on
mine. One of the key items is, as you mention, the fan. ABS
requires 72 hours of emergency life support after your primary mission
time and this has to be able to run off the emergency battery
bank. If you under size the fan to minimize the power requirement to
the fan, then the fan can not generate sufficient differential
pressure across the fan to circulate the contaminated air. This is
particularly true if you use an axial flow fan rather than a radial
centrifugal fan. Axial fans are commonly installed on most PC
computers. They are designed for high flow rate but low head.
I made the mistake of using an axial flow fan on my first iteration of my
scrubber. While it would last 80 hours ( the current load is
very small), the scrubber effectiveness was not great probably because the
blades very stalling. My initial CO2 level would start at about
350 ppm but would build to 5000 ppm (0.5 mole %) over 30
minutes. My second iteration replaced the axial fan with a small
radial centrifugal fan (squirrel cage) which developed four
times the head at about 1/4 of the flow rate. The
current demand was only slightly higher. This worked great while
still meeting the low current requirment.
The scrubber absorbent choice to me is a no brainier. Sodasorb
HP is what is used most often these days for psubs
and re-breathers for divers. It is easy to get and not very
expensive. From a design perspective, after sorting out the
fan, it comes down to how do want to handle the storage of the emergency
Sodasorb. There are three options, 1) cartridges for the primary and
emergency absorbent, 2) scrubber sized to hold the primary and emergency
absorbent 3) scrubber sized for the primary absorbent and spare
Sodasorb stored in bulk requiring refill when primary absorbent becomes
saturated. There are advantages and disadvantages to each
technique. While cartridges are easy to replace in an
emergency situation, the spare cartridges don't pack as
efficiently as bulk absorbent. For a small psub like mine,
this rules them out; I just did not have the room. I note that the
Deepworker uses the option 2) . For small one man psubs, I
think option 2 is probably the best way to go. The
disadvantage to this approach is that for non emergency conditions, you
end of tossing a lot of partially used Sodasorb after each
dive. For a one person sub, this is not an issue but for more
than one, this makes this option not as attractive. Also, because it
is inexpensive, this is not a big deal for a one person sub. This
option provides the least stress in a stressful emergency as the operator
does not have to worry about changing out the absorbent when it becomes
saturated. On my boat, I use option 3 primarily because of
the storage issue. On my next one person boat, I would go with
option 2. Replacing 30 lbs of Sodasorb (80 hours of
life support for one man) is not a big deal given that it simplifies
life.
While the manufactures of these absorbents add a chemical that
turns the absorbent a different color than white when it becomes
saturated, this is not very practical to use as lighting in subs make
it hard to detect the color change. A better detection of a
saturated absorbent is a good CO2 sensor with an audible
alarm.
So to me, it comes down to correctly specifying the fan and how
to package the spare absorbent.
Cliff
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