[PSUBS-MAILIST] Co2 scrubbers on eBay

Sun Apr 23 13:22:51 EDT 2017

Cliff, I have to wonder if extending the battery duration is a simple matter of reducing the resistance on the blower, by either increasing the length of the scrubber or both the internal and external diameters?

On April 23, 2017 11:11:43 AM MDT, Cliff Redus via Personal_Submersibles <personal_submersibles at psubs.org> wrote:
>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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>>> Personal_Submersibles at psubs.org
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>>>
>>>
>>
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>>
>>
>
>
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