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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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