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RE: [PSUBS-MAILIST] Scrubber CFM
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
-----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 12:15 AM
To: personal_submersibles@psubs.org
Subject: Re: [PSUBS-MAILIST] Snoopy life support test
OK - thanks. I had another look at that thread, and the only other
thing I would add is that you want uniform flow through the scrubber
bed, so in the case of a radial flow scrubber where flow is from outer
to inner diameter, while you need the fan to generate the pressure head
& air flow consistently in the presence of resistance, you want the air
flow at the fan exit to be as diffuse as possible, such that the flow
across the scrubber bed is predominately the result of the pressure
difference, as opposed to the air velocity at the fan exit. For
example, a fan with a small outlet on one side only of the canister with
no additional mechanical diffusion may unevenly consume scrubber media.
Avoiding this ensures that you get the same air flow over the whole
length and therefore meet the design dwell time within the bed, or more
importantly, that the scrubber media is exhausted evenly throughout the
bed. A good design check would be to use the purple-indicating media,
and checking at the end of the dive if the extent of colour change is
consistent from top to bottom of the canister. In any case, for a given
design volume flow rate, you want to minimize the velocity through the
scrubber bed for maximum dwell time.
-Sean
On 09/08/2011 8:08 PM, Jon Wallace wrote:
>
> Date was 3/14/2011
> Subject was "open source scrubber design"
>
>
> On 8/9/2011 10:49 PM, Sean T. Stevenson wrote:
>> Alec - what was the subject and/or date of the message you quote?
>> I'm trying
>> to find it so I can put it in context and perhaps comment further,
>> but I'm
>> drawing a blank.
>>
>> -Sean
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