[PSUBS-MAILIST] Co2 scrubbers

Sun Apr 23 17:33:26 EDT 2017

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-manboat and came away with some conclusions. The first was that a radial design was better than an axial design forair flow throws the absorbent and the second was that the goldilocks rule appliesfor fan/blower associated with the scrubber. Engineers make a distinction on equipment used to compress air.  They define a parameter known as the specificratio which is defined as the discharge pressure divided by the supply pressurewhere each pressure is in terms of absolute pressure rather than gagepressure.  If the device has a specificratio less than 1.1, they call it fan, if it has a specific ratio greater than1.2 they call it compressor and if it has a specific ration between 1.11 and1.2, it is a blower.  What I found from mytesting on the scrubber was that fans like you would typically see on PC areaxial flow and these are designed for high flow rates but low head. When youtry and use them to push air through the CO2 absorbent, they just don't haveenough head and the resulting flowrate is very low.  In this case they are not operating anywherenear their best efficiency point (BEP). What I found worked better were squirrel cage blowers. These aredesigned for lower flow rates than PC axial fans but with more head.  I am sure there are many models of squirrelblade blowers that would work but the model I use is from Papst, modelRL90-18/24.  This blower operates off24VDC and has a power rating of 7.5 W which translates to 0.31 amps.  If you look on ebay, these blowers come upall the time.  Back to the goldilocksrule;  to meet ABS rules, you have to demonstratethat your life support system will operate through the the emergency timeperiod which is 72 hours on the backup battery. The current during this period is known as the “Hotel Load” for obviousreasons.  When I tested axial PC fans,they were great on battery endurance because they pull a very low current butthey did not work well because they did not have enough head to overcome thepressure drop through the CO2 absorbent material , SodaSorb HP in my case.   This showed up as having erratic CO2 levelsin the boat and not being able to sustain concentrations less than ABS requiredmaximum of 5000 ppm (1/2%).  When I triedlarger axial fans like you would use for a bilge fan, the unit would keep theCO2 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 cageblower turned out to be  perfect withenough head to circulated  the cabinair  to keep the CO2 level typicallybelow 2000 ppm but also because they only pull 0.31 amps.  This blower did not let me meet the 72 ABSendurance limit but got me close.  Belowis a graph of hotel load current through my backup battery and the voltageacross the backup battery as a function of time on a life support test in myboat.  You can see from the graph atabout 69 hours into the test the backup battery was exhausted.  Also the hotel load started at about 1.6 ampsbut slowly climbed to 1.7 amps over the 69 hours.  This hotel load was a little higher than the1.5 amps that I had designed around.  Ineed to go back and look at the contributors to this hotel load and see if Ican reduce.  I am happy with the 69 hoursbecause 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 oftwo AGM  100 Ah battery.  If you divide the capacity by the hotel loadyou get the expected endurance of 100Ahr/1.65A is 61 hours so my 69 hours did betterthan expected.  .

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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