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RE: [PSUBS-MAILIST] Scrubber CFM
I agree, but the way to circulate that cabin air best would be with a
separate circulation fan- not the scrubber itself.
Taking this to extreme, imagine a guy alone in a big sub hooking up a leaf
blower to the scrubber!
So, we can agree that 3 things are relevant;
(1) Amount of CO2 produced by occupants.
(2) A Device that can absorb more than that.
(3) Good circulation in the environment to alleviate dead spots.
Greg
-----Original Message-----
From: owner-personal_submersibles@psubs.org
[mailto:owner-personal_submersibles@psubs.org] On Behalf Of Sean T.
Stevenson
Sent: Thursday, August 11, 2011 1:34 PM
To: personal_submersibles@psubs.org
Subject: Re: [PSUBS-MAILIST] Scrubber CFM
True, but if you assume that the waste CO2 is evenly distributed in the
cabin air, you need to move enough of that air through the scrubber such
that the entire cabin volume is processed within a reasonable amount of
time. It doesn't matter how efficient your scrubber is - if by
adjusting flow rate you achieve better than 95% removal through a unit
that only processes 5% of your cabin volume per hour - even if the
amount of media is sufficient for the CO2 load, you're not putting
enough air in contact with it to rely on the theoretical scrubbing capacity.
-Sean
On 11/08/2011 10:15 AM, Greg wrote:
> I would say that air flow through the scrubber should only be based on the
> amount needed to achieve the most efficient chemical reaction.
>
> A large cabin volume will not benefit from an air velocity that is too
high
> and lowers scrubber function.
>
>
> Greg
>
>
>
> -----Original Message-----
> From: owner-personal_submersibles@psubs.org
> [mailto:owner-personal_submersibles@psubs.org] On Behalf Of Sean T.
> Stevenson
> Sent: Thursday, August 11, 2011 12:37 PM
> To: personal_submersibles@psubs.org
> Subject: Re: [PSUBS-MAILIST] Scrubber CFM
>
> Be wary of confusing input variables with intermediate or output
> variables. Changing volume flow rate through your scrubber to achieve a
> target flow velocity will affect the turnover rate of your cabin air.
> In the design stage, it makes sense to develop a scrubber design in a
> logical order:
>
> 1) Determine the required amount of scrubber media on the basis of
> expected CO2 production (i.e. production per occupant x dive duration or
> interval between media changes).
> 2) Determine the required volume flow rate through the scrubber on the
> basis of overall cabin volume and desired number of complete air changes
> per hour.
> 3) Adjust scrubber design to achieve target volume flow rate at a flow
> velocity through scrubber bed which supports the scrubbing rate used to
> determine media quantity in step 1.
>
> Just my $0.02.
>
> -Sean
>
>
> On 11/08/2011 9:08 AM, Greg wrote:
>> Hi Jon,
>>
>> The only way to know the true airflow over the surface is to measure the
>> flow entering or exiting the unit. The true CFM is not only a function of
>> the fan rating but also restrictions to flow in a particular scrubber
> design
>> and type of fan used (radial/axial).
>>
>> Where a flow meter when you need one?!
>>
>>
>> Greg C
>>
>>
>> -----Original Message-----
>> From: owner-personal_submersibles@psubs.org
>> [mailto:owner-personal_submersibles@psubs.org] On Behalf Of Jon Wallace
>> Sent: Thursday, August 11, 2011 11:40 AM
>> To: personal_submersibles@psubs.org
>> Subject: Re: [PSUBS-MAILIST] Scrubber CFM
>>
>>
>> Just another variable that needs to be considered as part of the testing
>> process. Too large airflow and the air will be flowing too quickly for
>> good CO2 transfer efficiency across the chemical. The fan I used to
>> convert the ExtendAir canister may be way too big since the lowest
>> setting is 20 cfm. Might be able to turn it down with a rheostat.
>>
>> Jon
>>
>>
>> On 8/11/2011 11:07 AM, Greg wrote:
>>> I was talking with Tom Daley over at Micropore this morning about proper
>>> airflow through their "Extendair" cartridges (specifically the
"SR0101C",
>>> which is a popular size used in a rebreathers) He did some quick
>>> calculations and came up with a value of 125 LPM (assuming 70 %
> efficiency
>>> of the cartridge)to meet the needs of the standard man at rest. I
thought
>>> that this sounded surprisingly low- but Tom reminded me that calcium
> based
>>> scrubbers can easily dry out with too much airflow, specialty if the
>> ambient
>>> humidity is low.
>>>
>>> I then referred back to Phil Nuytten's White paper and noted that he
also
>>> specified a fan with a flow rate of only 160 LPM (page 5). 160 LPM
>>> translates to less than 6 CFM- a VERY small fan indeed. Most small CPU
>> fans
>>> flow 30 CFM or more. Apparently Phil chose this size carefully (why am I
>> not
>>> surprised?)
>>>
>>> Of course, the good news is that a 5 CFM fan also consumes very little
>>> power, which comes in handy in a long duration dive or emergency. Tom
> also
>>> suggested that a lung powered option might be best in an emergency, not
>> only
>>> to save power but also because breathing into the canister supplies
>>> additional humidity. Of course, the optimum humidity range for Extendair
>>> cartridges may be different than the range needed using granules.
>>>
>>> Greg C
>>>
>>
>>
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