[PSUBS-MAILIST] Co2 scrubbers

Sun Apr 23 18:37:28 EDT 2017

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> 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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