Emile
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