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