[PSUBS-MAILIST] Co2 scrubbers on eBay

Sun Apr 23 13:11:43 EDT 2017

To elaborate a bit on Alec's comments, a few years back I did some work the
scrubber for my one-man boat and came away with some conclusions.  The
first was that a radial design was better than an axial design for air flow
throws the absorbent and the second was that the goldilocks rule applies
for fan/blower associated with the scrubber.  Engineers make a distinction
on equipment used to compress air.  They define a parameter known as the
specific ratio which is defined as the discharge pressure divided by the
supply pressure where each pressure is in terms of absolute pressure rather
than gage pressure.  If the device has a specific ratio less than 1.1, they
call it fan, if it has a specific ratio greater than 1.2 they call it
compressor and if it has a specific ration between 1.11 and 1.2, it is a
blower.  What I found from my testing on the scrubber was that fans like
you would typically see on PC are axial flow and these are designed for
high flow rates but low head. When you try and use them to push air through
the CO2 absorbent, they just don't have enough head and the resulting
flowrate is very low.  In this case they are not operating anywhere near
their best efficiency point (BEP).  What I found worked better were
squirrel cage blowers. These are designed for lower flow rates than PC
axial fans but with more head.  I am sure there are many models of squirrel
blade blowers that would work but the model I use is from Papst, model
RL90-18/24.  This blower operates off 24VDC and has a power rating of 7.5 W
which translates to 0.31 amps.  If you look on ebay, these blowers come up
all the time.  Back to the goldilocks rule;  to meet ABS rules, you have to
demonstrate that your life support system will operate through the the
emergency time period which is 72 hours on the backup battery.  The current
during this period is known as the “Hotel Load” for obvious reasons.  When
I tested axial PC fans, they were great on battery endurance because they
pull a very low current but they did not work well because they did not
have enough head to overcome the pressure drop through the CO2 absorbent
material , SodaSorb HP in my case.   This showed up as having erratic CO2
levels in the boat and not being able to sustain concentrations less than
ABS required maximum of 5000 ppm (1/2%).  When I tried larger axial fans
like you would use for a bilge fan, the unit would keep the CO2 level below
the 5000 ppm limit but they pulled way much current and  would not last
anywhere near the 80 hours.  The Papst, model RL90-18/24 squirrel cage
blower turned out to be  perfect with enough head to circulated  the cabin
air  to keep the CO2 level typically below 2000 ppm but also because they
only pull 0.31 amps.  This blower did not let me meet the 72 ABS endurance
limit but got me close.  Below is a graph of hotel load current through my
backup battery and the voltage across the backup battery as a function of
time on a life support test in my boat.  You can see from the graph at
about 69 hours into the test the backup battery was exhausted.  Also the
hotel load started at about 1.6 amps but slowly climbed to 1.7 amps over
the 69 hours.  This hotel load was a little higher than the 1.5 amps that I
had designed around.  I need to go back and look at the contributors to
this hotel load and see if I can reduce.  I am happy with the 69 hours
because during a real emergency like be stranded on the bottom due to
entanglement, I could utilize at least some of the main battery.  For
reference, the backup battery consist of two AGM  100 Ah battery.  If you
divide the capacity by the hotel load you get the expected endurance of
100Ahr/1.65A is 61 hours so my 69 hours did better than expected.
.

[image: Inline image 2]

Cliff

On Sat, Apr 22, 2017 at 6:07 PM, Alec Smyth via Personal_Submersibles <
personal_submersibles at psubs.org> wrote:

> Hi Brian,
>
> If by "straight flow fan" you mean the geometry you would see on a
> computer cooling fan for instance, they are way less efficient for this
> purpose. I believe the reason is they move good volumes of air but develop
> very little pressure. I've tested both kinds, and the sort I'm using now
> has much better performance. Cliff has done similar tests and had the same
> results.
>
> Best,
>
> Alec
>
> On Sat, Apr 22, 2017 at 6:08 PM, Brian Hughes via Personal_Submersibles <
> personal_submersibles at psubs.org> wrote:
>
>> Alec,
>>
>> Just ordered a tank holder that has two bungies about an inch apart, used
>> to strap tanks down on a boat. I'm thinking I can hang this scrubber from
>> the roof using the aft most reinforcing ring, holding it up in the middle.
>> If it works, straight flow fan.
>>
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>> http://www.psubs.org/mailman/listinfo.cgi/personal_submersibles
>>
>>
>
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