[PSUBS-MAILIST] Co2 scrubbers

Sun Apr 23 21:11:53 EDT 2017

Yes I like it,  Psub Baristas with one small difference, if you get your
DIY coffee machine wrong, you get a bad cup of Joe; if you get your DIY CO2
scrubber wrong you die!

Cliff

On Sun, Apr 23, 2017 at 5:37 PM, Alan via Personal_Submersibles <
personal_submersibles at psubs.org> wrote:

> Some great thoughts thanks.
> This all seems very much like the art of coffee making!
> You have the type of bean ( absorbent choice). There is the grind (
> granule size)
> Moisture has an effect on coffee extraction.  There is the amount of
> coffee grind
> in the shot! If you have differing volumes of absorbant in the scrubber
> this would
> have an effect. There is the tamping of the grind that effects the speed
> water flows through the ground coffee, which is equivalent to how the
> absorbent is compacted. There is a set perfect time that it should take for
> the water to flow through the grind
> to get the maximum desired extraction, this relates to the previous
> factors & the pressure of the pump. With a scrubber the pump is replaced by
> the fan & the flow
> rate is what is required to keep CO2 levels below required levels & this
> is dictated
> by the size of the hull & number of passengers.
> Baristas can spend ages fine tuning their coffee machines every day &
> during
> the day.
> Not saying we should be this fussy; but there are a lot of factors to
> balance if
> you want it perfect!
> Alan
>
>
>
>
> Sent from my iPad
>
> On 24/04/2017, at 9:33 AM, james cottrell via Personal_Submersibles <
> personal_submersibles at psubs.org> wrote:
>
> Hi Cliff,
> Thanks for the excellent info. Great research. If I could add a couple of
> things they would be-
>
> Pushing air through absorbent vs pulling air has different effects too.
> Pushing air through with a blower has the benefit of slightly higher
> pressure in the media which can yield more efficiency compared to drawing
> air through which lowers pressure.
> However, too much airflow can cause the the media to dry out and lose
> effectiveness. Humidity may have to be controlled within limits.
> Pushing air through can also cause uneven distribution and poor
> performance depending on scrubber design.
>
> So the challenges are to keep humidity within limits, air flow sufficient
> with even distribution all without exceeding available power over duration
> of dive (especially in an emergency).
>
> For these reasons, lung powered devices are great as a back up. The lungs
> provide the airflow and the humidity. If the unit is made from clear
> acrylic, it is possible to use color changing media.
> One last thought- Absorbent cartridges (like micro-pore re-breather
> cartridges) can make it easier to change used up media in the sub during an
> emergency than trying to empty and replace granules. Just pull out the old
> cartridge, insert the new one and continue breathing.
>
> Greg C
> ------------------------------
> *From:* Cliff Redus via Personal_Submersibles <
> personal_submersibles at psubs.org>
> *To:* Personal Submersibles General Discussion <
> personal_submersibles at psubs.org>
> *Sent:* Sunday, April 23, 2017 1:17 PM
> *Subject:* Re: [PSUBS-MAILIST] Co2 scrubbers on eBay
>
> 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.png>
>
>
> 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 <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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