[PSUBS-MAILIST] Co2 scrubbers on eBay

Sun Apr 23 13:56:11 EDT 2017

Sean, for sure a redesign of scrubber could reduce head requirement  and
thus reduce current.  It's probably more work than I want to do given
that I am happy with  scrubber container I am using which is an OTS
charcoal filter that was cheap and readily available from my major
supplier, Ebay.  I am using the same unit Alec used in Snoopy and gave a
show and tell at the Vancouver PSub convention.  I think it might be easier
just to go to 120 Ah batteries with the same form factor if I can find
them. When I did the test, I had my comms on the whole time.  If I ran the
comms 75% of the time, I would meet the 72 hour constraint.

Cliff

On Sun, Apr 23, 2017 at 12:22 PM, Sean T. Stevenson via
Personal_Submersibles <personal_submersibles at psubs.org> wrote:

> 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 t! ested 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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>>>>
>>>>
>>>
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>>>
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