dwell-time calculation: axial vs radial

dave t · 5th January 2007, 08:42

sorry Paul I must have misunderstood your reason for posting the excel sheet. I thought you were infering that because the dwell time was the same in the radial and axial cannisters the duration would be the same

sorry for the misunderstanding

Dave

silent running · 5th January 2007, 09:14

Quote: (Originally Posted by paulraymaekers)

hello dave, this type of non-uniform gas-speed distribution is not taken into account, but this does not influence the 'overall' dwell time, as the scrubber is not a 'gas stocking device'

I will not restart the debate about dwell-time, I made my point and I know what I know...

But as you all know, I never said that a radial can not be more efficiant than an axial, I think we all agree on that. But this is due to other factors, like thermal design (isolation), non-uniform gas-flow due to 'water-stocking' in the sorb, non-uniform sorb packing due to position of the cannister during diving etc etc etc

regards
paul

Hi Paul, granted the better insulating qualities of radials are part of increased efficeincy, but do you really think that the rest of the increased efficeincy is only do to such vaugeries like "water stocking"-I assume you mean the swelling of the used sorb-or non-uniform packing? And how do either of these things exist outside the realm of actual dwell time? Seems to me that water stocking is likely to force a tighter, more uniform pack as the sorb expands and would influnce the non-uniform packing and thus increase dwell time. But then again, why wouldn't that also be true for an axial?

Also, anything that decreases the chance of channeling would also increase efficeincy and there is much less hard surface on the outside of a radial canister and the gas is traveling at 90 degrees to the canister wall as opposed to running along parallel to the canister wall in an axial.

Then there's WOB mystery: If a radial has less resistance, it should have less dwell time and be less efficeint than an axial of the same weight, yet this is obviously not true. Judging by the use patterns of in-out radials the bulk of the scrubbing is being done on the exhale side. If the velocity is the same in both kinds of scrubber, how is this possible?

I'm also wondering how much time the sorb needs to bond with the CO2, that would be another clue about how the 2 designs differ and how much the tidal cycle plays in the performance of each design.

Don't get me wrong, I appreciate you putting up the data/model and I'm not trying to be contrary for the sake of it, I'm just not convinced of the relevence of the data/info to the actual real life performance of the designs and I think there are still more questions than answers at this point...

Hudzen10 · 5th January 2007, 11:36

Just to flaunt my general ignorance & naive understanding of the whole shebang, how well do figures of dwell time correlate to the efficiency of sorb usage?
Is there even any merit in considering a scrubber's efficiency over pure "my dwell time is bigger than yours" type comparisons?

dave t · 5th January 2007, 12:05

I guess there is a point at either end of the scale where dwell time makes a difference. too fast is no good too slow is no good either.

the only way to properly test a scrubber is to do a controlled breakthrough test and then to do a percentage used test. The breakthrough test will tell you under given conditions how long your scrubber will last and a percentage used test will tell you how efficient it is, just because it breaks through at say three hours does not mean all the sorb is used up. A redesign of the scrubber may make it more efficient so your breakthrough test may now strech to three and a half hours because (due to a better design) your scrubber is more efficient

paulraymaekers · 5th January 2007, 12:29

Quote: (Originally Posted by silent running)

Hi Paul, granted the better insulating qualities of radials are part of increased efficeincy, but do you really think that the rest of the increased efficeincy is only do to such vaugeries like "water stocking"-I assume you mean the swelling of the used sorb-or non-uniform packing? And how do either of these things exist outside the realm of actual dwell time? Seems to me that water stocking is likely to force a tighter, more uniform pack as the sorb expands and would influnce the non-uniform packing and thus increase dwell time. But then again, why wouldn't that also be true for an axial?
...

.... well, things are always more complex to explain...

conc water stocking: just one example, imagine an axial scrubber in horisontal position during diving (like CK, Inspi, Meg....)

exxes water coming from the reaction will tend to move frome inlet to outlet, more water at the outlet, because the gasflow is transporting humidity towards the outlet side of the scrubber, but also to one side of the scrubber, namely towards the divers back, because gravity does play its role also: meaning one side of the scrubber will be more charged and 'compacted' than the other, provocing higher gas-flow trough the less compacted part of the scrubber... giving less 'overal' efficiency of the scrubber

this phenomena is not there where the gasflow in the scrubber is vertical during diving, (like MK or rEvo) or less where the hight versus width ratio is low (like boris)

even in the rEvo the 'compacting' effect is reduced strongly by the removal of exxess water between the 2 scrubbers (the condensating area of the coverplate)

just one other thing that plays a role, among others...

regards
paul

dave t · 5th January 2007, 13:14

Paul

I have to say that is not something I have seen in my inspiration, the clumping of the sofnolime is ussually centred around the central spine of the scrubber, the spread being fairly even with the looser stuff around the edges, not clumping to one side as you suggest. The clumping around the spine is due mainly to the fact that that is where the majority of the action takes place and due to the design of the scrubber that is where the pack is at its tightest

do you have any pics of the Revo scrubber after a lengthy dive? I would very much like to see the state of the pack

best

Dave

paulraymaekers · 5th January 2007, 13:38

Quote: (Originally Posted by dave t)

Paul

I have to say that is not something I have seen in my inspiration, the clumping of the sofnolime is ussually centred around the central spine of the scrubber, the spread being fairly even with the looser stuff around the edges, not clumping to one side as you suggest. The clumping around the spine is due mainly to the fact that that is where the majority of the action takes place and due to the design of the scrubber that is where the pack is at its tightest

do you have any pics of the Revo scrubber after a lengthy dive? I would very much like to see the state of the pack

best

Dave

hallo dave, compacting will play a much higher role when you have conditions of very high and constant workload, at low temperature: typpically in the conditions where scrubbers are compared: the CE test: 1.6l/min CO2, at 5 bar and at 4° celcius!
very high waterproduction, and strong condensation due to low temp!

under those conditions you see diff between axial and radial, beween better or worse isolation etc

in normal use, this plays lesser role, but anyway, scrubbers are not compared under 'normal-use' conditions :-(

at low workload and high temp, the diff between scrubbers is far less significant, and depends mostly of.. the amount of sorb in the scrubber

paul

rEvo scrubber after use... you don't notice that much difference: mostly the sorb is still 'granular' when you trow it away

pabusk · 5th January 2007, 13:50

Quote: (Originally Posted by dave t)

the only way to properly test a scrubber is to do a controlled breakthrough test and then to do a percentage used test. The breakthrough test will tell you under given conditions how long your scrubber will last and a percentage used test will tell you how efficient it is, just because it breaks through at say three hours does not mean all the sorb is used up. A redesign of the scrubber may make it more efficient so your breakthrough test may now strech to three and a half hours because (due to a better design) your scrubber is more efficient

Good point ... makes me wonder how this would relate to the temp stck in the Vision. If the temp sensors are from the bottom of the stack to the top, having just one bar left might put you closer to a breakthrough than indicated. Cheap solution is off course to only measure, say 4/5th of the total length (which might be implemented anyway).

Sorry for dropping it in here ... just needed to vent a thought

Paul

**nigelh** · 5th January 2007, 15:07

Quote: (Originally Posted by Steve)

For the record, it is well understood that gas moves through a rebreather in steps (breaths) and dwell time is directly related to the size of these steps and the distance from one side of the scrubber to the other. In short (and generally speaking) axials, due to their longer bed length have a longer dwell time. The trade off is a higher WOB.

That sounded good when I first read it but the more I think about it the more I don't believe it. Certain portions of gas will be in the scrubber longer and correspondingly others will be in it for less time. The total flow must average out unless the pressure in the loop is seriously different in different places.

N kilos of sorb has X litres of space within it. If gas moves round on whatever tidal profile the quantity of gas within the scrubber is constant so the fraction of the loop in the scrubber is constant so how can it be there 'longer' dependant on geometry? The only thing you can do with the geometry is make it worse by providing dead spots that never get used.

The idea that gas moves faster on the walls is interesting. I'm just trying to see how this can be more than a film effect and the flow will adhere to the wall and probably be slower.

paulraymaekers · 5th January 2007, 15:23

Quote: (Originally Posted by nigelh)

That sounded good when I first read it but the more I think about it the more I don't believe it. Certain portions of gas will be in the scrubber longer and correspondingly others will be in it for less time. The total flow must average out unless the pressure in the loop is seriously different in different places.

N kilos of sorb has X litres of space within it. If gas moves round on whatever tidal profile the quantity of gas within the scrubber is constant so the fraction of the loop in the scrubber is constant so how can it be there 'longer' dependant on geometry? The only thing you can do with the geometry is make it worse by providing dead spots that never get used.

The idea that gas moves faster on the walls is interesting. I'm just trying to see how this can be more than a film effect and the flow will adhere to the wall and probably be slower.

aahhhh, another one has seen the light :-)

btw, why would gas travel faster on walls???
regards
paul

5th January 2007, 08:42	#11 (permalink)
dave t Custom Title Allowed! Current Rebreather/s: Inspiration Classic Inspiration Vision Evolution Sport Kiss Classic Kiss Dolphin Ray Azimuth Home Build Other Rebreather/s: Dolphin Ray Azimuth Home Build Join Date: May 2005 Location: UK Posts: 654	Re: dwell-time calculation: axial vs radial sorry Paul I must have misunderstood your reason for posting the excel sheet. I thought you were infering that because the dwell time was the same in the radial and axial cannisters the duration would be the same sorry for the misunderstanding Dave
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5th January 2007, 09:14	#12 (permalink)
silent running Mature mouth breather Current Rebreather/s: Prism Topaz Other Rebreather/s: Join Date: Jun 2005 Location: U.S.A. Brooklyn, New York Posts: 1,832	Re: dwell-time calculation: axial vs radial Quote: (Originally Posted by paulraymaekers) hello dave, this type of non-uniform gas-speed distribution is not taken into account, but this does not influence the 'overall' dwell time, as the scrubber is not a 'gas stocking device' I will not restart the debate about dwell-time, I made my point and I know what I know... But as you all know, I never said that a radial can not be more efficiant than an axial, I think we all agree on that. But this is due to other factors, like thermal design (isolation), non-uniform gas-flow due to 'water-stocking' in the sorb, non-uniform sorb packing due to position of the cannister during diving etc etc etc regards paul Hi Paul, granted the better insulating qualities of radials are part of increased efficeincy, but do you really think that the rest of the increased efficeincy is only do to such vaugeries like "water stocking"-I assume you mean the swelling of the used sorb-or non-uniform packing? And how do either of these things exist outside the realm of actual dwell time? Seems to me that water stocking is likely to force a tighter, more uniform pack as the sorb expands and would influnce the non-uniform packing and thus increase dwell time. But then again, why wouldn't that also be true for an axial? Also, anything that decreases the chance of channeling would also increase efficeincy and there is much less hard surface on the outside of a radial canister and the gas is traveling at 90 degrees to the canister wall as opposed to running along parallel to the canister wall in an axial. Then there's WOB mystery: If a radial has less resistance, it should have less dwell time and be less efficeint than an axial of the same weight, yet this is obviously not true. Judging by the use patterns of in-out radials the bulk of the scrubbing is being done on the exhale side. If the velocity is the same in both kinds of scrubber, how is this possible? I'm also wondering how much time the sorb needs to bond with the CO2, that would be another clue about how the 2 designs differ and how much the tidal cycle plays in the performance of each design. Don't get me wrong, I appreciate you putting up the data/model and I'm not trying to be contrary for the sake of it, I'm just not convinced of the relevence of the data/info to the actual real life performance of the designs and I think there are still more questions than answers at this point...
(Offline)

5th January 2007, 11:36	#13 (permalink)
Hudzen10 New Member Current Rebreather/s: Ouroboros Other Rebreather/s: Join Date: Apr 2006 Location: Nottingham, England Posts: 2	Re: dwell-time calculation: axial vs radial Just to flaunt my general ignorance & naive understanding of the whole shebang, how well do figures of dwell time correlate to the efficiency of sorb usage? Is there even any merit in considering a scrubber's efficiency over pure "my dwell time is bigger than yours" type comparisons?
(Offline)

5th January 2007, 12:05	#14 (permalink)
dave t Custom Title Allowed! Current Rebreather/s: Inspiration Classic Inspiration Vision Evolution Sport Kiss Classic Kiss Dolphin Ray Azimuth Home Build Other Rebreather/s: Dolphin Ray Azimuth Home Build Join Date: May 2005 Location: UK Posts: 654	Re: dwell-time calculation: axial vs radial I guess there is a point at either end of the scale where dwell time makes a difference. too fast is no good too slow is no good either. the only way to properly test a scrubber is to do a controlled breakthrough test and then to do a percentage used test. The breakthrough test will tell you under given conditions how long your scrubber will last and a percentage used test will tell you how efficient it is, just because it breaks through at say three hours does not mean all the sorb is used up. A redesign of the scrubber may make it more efficient so your breakthrough test may now strech to three and a half hours because (due to a better design) your scrubber is more efficient
(Offline)

5th January 2007, 12:29	#15 (permalink)
paulraymaekers rEvo's daddy Current Rebreather/s: rEvo Other CCR Home Build Other Rebreather/s: rEvo Other CCR Home Build Join Date: May 2005 Location: belgium Posts: 1,501	Re: dwell-time calculation: axial vs radial Quote: (Originally Posted by silent running) Hi Paul, granted the better insulating qualities of radials are part of increased efficeincy, but do you really think that the rest of the increased efficeincy is only do to such vaugeries like "water stocking"-I assume you mean the swelling of the used sorb-or non-uniform packing? And how do either of these things exist outside the realm of actual dwell time? Seems to me that water stocking is likely to force a tighter, more uniform pack as the sorb expands and would influnce the non-uniform packing and thus increase dwell time. But then again, why wouldn't that also be true for an axial? ... .... well, things are always more complex to explain... conc water stocking: just one example, imagine an axial scrubber in horisontal position during diving (like CK, Inspi, Meg....) exxes water coming from the reaction will tend to move frome inlet to outlet, more water at the outlet, because the gasflow is transporting humidity towards the outlet side of the scrubber, but also to one side of the scrubber, namely towards the divers back, because gravity does play its role also: meaning one side of the scrubber will be more charged and 'compacted' than the other, provocing higher gas-flow trough the less compacted part of the scrubber... giving less 'overal' efficiency of the scrubber this phenomena is not there where the gasflow in the scrubber is vertical during diving, (like MK or rEvo) or less where the hight versus width ratio is low (like boris) even in the rEvo the 'compacting' effect is reduced strongly by the removal of exxess water between the 2 scrubbers (the condensating area of the coverplate) just one other thing that plays a role, among others... regards paul __________________ www.rEvo-rebreathers.com .... the earth is flat, Elvis is alive, and radial scrubbers give longer dwell time than axials...
(Online)

5th January 2007, 13:14	#16 (permalink)
dave t Custom Title Allowed! Current Rebreather/s: Inspiration Classic Inspiration Vision Evolution Sport Kiss Classic Kiss Dolphin Ray Azimuth Home Build Other Rebreather/s: Dolphin Ray Azimuth Home Build Join Date: May 2005 Location: UK Posts: 654	Re: dwell-time calculation: axial vs radial Paul I have to say that is not something I have seen in my inspiration, the clumping of the sofnolime is ussually centred around the central spine of the scrubber, the spread being fairly even with the looser stuff around the edges, not clumping to one side as you suggest. The clumping around the spine is due mainly to the fact that that is where the majority of the action takes place and due to the design of the scrubber that is where the pack is at its tightest do you have any pics of the Revo scrubber after a lengthy dive? I would very much like to see the state of the pack best Dave
(Offline)

5th January 2007, 13:38	#17 (permalink)
paulraymaekers rEvo's daddy Current Rebreather/s: rEvo Other CCR Home Build Other Rebreather/s: rEvo Other CCR Home Build Join Date: May 2005 Location: belgium Posts: 1,501	Re: dwell-time calculation: axial vs radial Quote: (Originally Posted by dave t) Paul I have to say that is not something I have seen in my inspiration, the clumping of the sofnolime is ussually centred around the central spine of the scrubber, the spread being fairly even with the looser stuff around the edges, not clumping to one side as you suggest. The clumping around the spine is due mainly to the fact that that is where the majority of the action takes place and due to the design of the scrubber that is where the pack is at its tightest do you have any pics of the Revo scrubber after a lengthy dive? I would very much like to see the state of the pack best Dave hallo dave, compacting will play a much higher role when you have conditions of very high and constant workload, at low temperature: typpically in the conditions where scrubbers are compared: the CE test: 1.6l/min CO2, at 5 bar and at 4° celcius! very high waterproduction, and strong condensation due to low temp! under those conditions you see diff between axial and radial, beween better or worse isolation etc in normal use, this plays lesser role, but anyway, scrubbers are not compared under 'normal-use' conditions :-( at low workload and high temp, the diff between scrubbers is far less significant, and depends mostly of.. the amount of sorb in the scrubber paul rEvo scrubber after use... you don't notice that much difference: mostly the sorb is still 'granular' when you trow it away __________________ www.rEvo-rebreathers.com .... the earth is flat, Elvis is alive, and radial scrubbers give longer dwell time than axials... Last edited by paulraymaekers : 5th January 2007 at 13:40.
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5th January 2007, 13:50	#18 (permalink)
pabusk Custom Title Allowed! Current Rebreather/s: Inspiration Vision Other Rebreather/s: Not Bought Yet Inspiration Classic Join Date: Dec 2005 Location: Holland Posts: 131	Re: dwell-time calculation: axial vs radial Quote: (Originally Posted by dave t) the only way to properly test a scrubber is to do a controlled breakthrough test and then to do a percentage used test. The breakthrough test will tell you under given conditions how long your scrubber will last and a percentage used test will tell you how efficient it is, just because it breaks through at say three hours does not mean all the sorb is used up. A redesign of the scrubber may make it more efficient so your breakthrough test may now strech to three and a half hours because (due to a better design) your scrubber is more efficient Good point ... makes me wonder how this would relate to the temp stck in the Vision. If the temp sensors are from the bottom of the stack to the top, having just one bar left might put you closer to a breakthrough than indicated. Cheap solution is off course to only measure, say 4/5th of the total length (which might be implemented anyway). Sorry for dropping it in here ... just needed to vent a thought Paul
(Online)

5th January 2007, 15:07	#19 (permalink)
nigelh New Member Current Rebreather/s: Inspiration Vision Other CCR Other Rebreather/s: Inspiration Classic Other CCR Join Date: Mar 2005 Location: Brighton, Sussex, UK Posts: 838	Re: dwell-time calculation: axial vs radial Quote: (Originally Posted by Steve) For the record, it is well understood that gas moves through a rebreather in steps (breaths) and dwell time is directly related to the size of these steps and the distance from one side of the scrubber to the other. In short (and generally speaking) axials, due to their longer bed length have a longer dwell time. The trade off is a higher WOB. That sounded good when I first read it but the more I think about it the more I don't believe it. Certain portions of gas will be in the scrubber longer and correspondingly others will be in it for less time. The total flow must average out unless the pressure in the loop is seriously different in different places. N kilos of sorb has X litres of space within it. If gas moves round on whatever tidal profile the quantity of gas within the scrubber is constant so the fraction of the loop in the scrubber is constant so how can it be there 'longer' dependant on geometry? The only thing you can do with the geometry is make it worse by providing dead spots that never get used. The idea that gas moves faster on the walls is interesting. I'm just trying to see how this can be more than a film effect and the flow will adhere to the wall and probably be slower. __________________ nigelh
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5th January 2007, 15:23	#20 (permalink)
paulraymaekers rEvo's daddy Current Rebreather/s: rEvo Other CCR Home Build Other Rebreather/s: rEvo Other CCR Home Build Join Date: May 2005 Location: belgium Posts: 1,501	Re: dwell-time calculation: axial vs radial Quote: (Originally Posted by nigelh) That sounded good when I first read it but the more I think about it the more I don't believe it. Certain portions of gas will be in the scrubber longer and correspondingly others will be in it for less time. The total flow must average out unless the pressure in the loop is seriously different in different places. N kilos of sorb has X litres of space within it. If gas moves round on whatever tidal profile the quantity of gas within the scrubber is constant so the fraction of the loop in the scrubber is constant so how can it be there 'longer' dependant on geometry? The only thing you can do with the geometry is make it worse by providing dead spots that never get used. The idea that gas moves faster on the walls is interesting. I'm just trying to see how this can be more than a film effect and the flow will adhere to the wall and probably be slower. aahhhh, another one has seen the light :-) btw, why would gas travel faster on walls??? regards paul __________________ www.rEvo-rebreathers.com .... the earth is flat, Elvis is alive, and radial scrubbers give longer dwell time than axials...
(Online)

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