dwell-time calculation: axial vs radial

paulraymaekers · 1st September 2006, 10:10

hello, here you find an excel calculation of the dwell-time in a radial and an axial scrubber of exactly the same shape: only for the axial the gas flows in the length, in the radial gas goes from in to out

regards
paul

paulraymaekers · 1st September 2006, 10:12

Quote: (Originally Posted by paulraymaekers)

hello, here you find an excel calculation of the dwell-time in a radial and an axial scrubber of exactly the same shape: only for the axial the gas flows in the length, in the radial gas goes from in to out

regards
paul

sorry, seems excel is not supported...
any suggestions?

***jradomski*** · 1st September 2006, 10:20

Quote: (Originally Posted by paulraymaekers)

sorry, seems excel is not supported...
any suggestions?

zip the file..

paulraymaekers · 1st September 2006, 12:21

Quote: (Originally Posted by jradomski)

zip the file..

thanks joe! :-)

here it is

regards
paul

**Scott** · 5th January 2007, 01:46

So for the *same dimension* scrubber, the dwell time is the same.. implying that there is..... no difference in efficiency between the two... at least in this proportion.... correct?

**Steve** · 5th January 2007, 02:47

Quote: (Originally Posted by Scott)

So for the *same dimension* scrubber, the dwell time is the same.. implying that there is..... no difference in efficiency between the two... at least in this proportion.... correct?

Yes, That is Paul's view on it. Many of us disagree however, and feel he doesn't understand the roles gas path length and tidal movement play and is focused on static gas modelling and as a result the only answer he can come up with is volume is the determining factor. One has to ignore how a rebreather works to arrive at this theory. He isn't wrong as such but rather has opted for an over simplified model that results in a part answer.

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.

**Scott** · 5th January 2007, 03:20

Steve,

I am not an engineerer.. but..
I can see logic.. in both explainations..

By the flowing along the radius, the Radial scrubber is going either from a larger surface area, to a smaller.. or conversely from a smaller surface area, to a larger.

When passing through the larger surface area, it should have a lower velocity, and while passing through the smaller surface area, should have a higher velocity.. which is shown in theory in Paul's spreadsheet..

As you suggested that his explaination uses static flow, versus more human tidal flow.... I would think, would affect both designs similarly?

On the Axial design, it seems to me, that the dwell time would be Uniform for the entire scrubber, ... where as on the Radial the dwell time would be smallest at the inner diameter, and highest on the outer diameter.

I can see how this could be a negative, resulting in rapidly exhausting the scrubber in the outer diameter volume, and not allowing sufficent dwell time to utilize the inner diameter volume...

Maybe you could explain to me.. in regards to axial scrubbers, why it is more common to have tall, smaller diameter design versus a short, large diameter design.

In my mind, given the same volume, an nearly equivalent amount of gas passing through whether tidal or continual, (difference due possibly due to WOB).. It seems, that the larger diameter cannister would reduce the velocity of the gas such that the dwell time remains the same. In the smaller diameter cannister, the velocity would be higher.

My head hurts =(

Thank you for any education I will soon likely receive =)

-Scott

silent running · 5th January 2007, 05:34

Quote: (Originally Posted by Scott)

Steve,

I am not an engineerer.. but..
I can see logic.. in both explainations..

By the flowing along the radius, the Radial scrubber is going either from a larger surface area, to a smaller.. or conversely from a smaller surface area, to a larger.

When passing through the larger surface area, it should have a lower velocity, and while passing through the smaller surface area, should have a higher velocity.. which is shown in theory in Paul's spreadsheet..

As you suggested that his explaination uses static flow, versus more human tidal flow.... I would think, would affect both designs similarly?

On the Axial design, it seems to me, that the dwell time would be Uniform for the entire scrubber, ... where as on the Radial the dwell time would be smallest at the inner diameter, and highest on the outer diameter.

I can see how this could be a negative, resulting in rapidly exhausting the scrubber in the outer diameter volume, and not allowing sufficent dwell time to utilize the inner diameter volume...

Maybe you could explain to me.. in regards to axial scrubbers, why it is more common to have tall, smaller diameter design versus a short, large diameter design.

In my mind, given the same volume, an nearly equivalent amount of gas passing through whether tidal or continual, (difference due possibly due to WOB).. It seems, that the larger diameter cannister would reduce the velocity of the gas such that the dwell time remains the same. In the smaller diameter cannister, the velocity would be higher.

My head hurts =(

Thank you for any education I will soon likely receive =)

-Scott

Hello Scott, first off if you look at a spent radial scrubber in-out flow, you will notice the spent sorb clustering around the inner tube and less color change on the outside edge, showing that most of the radial dwell takes place on the exhale side.

I think Steve is right that the tidal cycle is the missing factor in the comparison, it's the only explanation for why a radial works at all as without it and if the gas was flowing at a constant rate, it would fill up the scrubber and just keep flowing out at whatever rate the flow and restrictions dictate and would in fact be no different than an axial of the same weight.

Paul may believe that the pressure drop btw the inner and outer side of a radial is inconsequential, but I don't, especially when you factor in the tidal/step cycle that the breathing gas follows in a CCR powered by a set of human lungs. The flow of the gas is nowhere near constant, it ebbs and flows and I think this is where the radial shines as it's asymetrical gas path interacts more favorably with the tidal cycle. During an inhale or an exhale, you have lower velocity at the begining and end and the gas will slow and actually stop before being drawn through the inhale side of the loop. And any pressure drop at all is going to affect the dwell as that exhaled gas runs out of velocity, IMHO. -Andy

dave t · 5th January 2007, 08:06

What is also not taken into consideration is the gas will travel at different speeds in different parts of the scrubber, for instance gas will travel faster (less dwell time) on the edges where the granules meet the smooth face of the cannister. I think this is less likely in a radial can which may contribute to the supposed better performance of the radial scrubber, at least in terms of duration.

Dave

paulraymaekers · 5th January 2007, 08:20

Quote: (Originally Posted by dave t)

What is also not taken into consideration is the gas will travel at different speeds in different parts of the scrubber, for instance gas will travel faster (less dwell time) on the edges where the granules meet the smooth face of the cannister. I think this is less likely in a radial can which may contribute to the supposed better performance of the radial scrubber, at least in terms of duration.

Dave

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

1st September 2006, 10:10	#1 (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: 1,345	dwell-time calculation: axial vs radial hello, here you find an excel calculation of the dwell-time in a radial and an axial scrubber of exactly the same shape: only for the axial the gas flows in the length, in the radial gas goes from in to out regards paul __________________ www.rEvo-rebreathers.com .... the earth is flat, Elvis is alive, and radial scrubbers give longer dwell time than axials...
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1st September 2006, 10:12	#2 (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: 1,345	Re: dwell-time calculation: axial vs radial Quote: (Originally Posted by paulraymaekers) hello, here you find an excel calculation of the dwell-time in a radial and an axial scrubber of exactly the same shape: only for the axial the gas flows in the length, in the radial gas goes from in to out regards paul sorry, seems excel is not supported... any suggestions? __________________ www.rEvo-rebreathers.com .... the earth is flat, Elvis is alive, and radial scrubbers give longer dwell time than axials...
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1st September 2006, 10:20	#3 (permalink)
*jradomski* Moderator Current Rebreather/s: Inspiration Classic Sport Kiss Optima rEvo Other CCR Home Build Other Rebreather/s: Inspiration Vision Evolution Megalodon Classic Kiss rEvo Other CCR Home Build Join Date: Mar 2005 Location: "Da" Bronx Posts: 2,796	Re: dwell-time calculation: axial vs radial Quote: (Originally Posted by paulraymaekers) sorry, seems excel is not supported... any suggestions? zip the file.. __________________ Joe Radomski CCR Trimix Instructor Trainer ANDI Instructor Trainer Director #10 All posts are personal opinions and DO NOT reflect any affiliated agency unless specifically stated.
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5th January 2007, 01:46	#5 (permalink)
Scott Supporting Member Current Rebreather/s: Other CCR Dolphin Ray Other Rebreather/s: Megalodon Other CCR Dolphin Ray Join Date: Mar 2006 Location: Mendota, Virginia Posts: 149	Re: dwell-time calculation: axial vs radial So for the same dimension scrubber, the dwell time is the same.. implying that there is..... no difference in efficiency between the two... at least in this proportion.... correct?
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5th January 2007, 02:47	#6 (permalink)
Steve Ladies bring a plate Current Rebreather/s: Megalodon Home Build Other Rebreather/s: Not Bought Yet Inspiration Classic Sport Kiss Classic Kiss MK 15.X Home Build Join Date: Feb 2005 Location: Perth - Australia Posts: 1,041	Re: dwell-time calculation: axial vs radial Quote: (Originally Posted by Scott) So for the same dimension scrubber, the dwell time is the same.. implying that there is..... no difference in efficiency between the two... at least in this proportion.... correct? Yes, That is Paul's view on it. Many of us disagree however, and feel he doesn't understand the roles gas path length and tidal movement play and is focused on static gas modelling and as a result the only answer he can come up with is volume is the determining factor. One has to ignore how a rebreather works to arrive at this theory. He isn't wrong as such but rather has opted for an over simplified model that results in a part answer. 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. __________________ WARNING: I contain occasional coarse language, extreme sexual references, nudity, and adult themes, which may offend some people - Usually churchy types. Last edited by Steve : 5th January 2007 at 02:49.
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5th January 2007, 03:20	#7 (permalink)
Scott Supporting Member Current Rebreather/s: Other CCR Dolphin Ray Other Rebreather/s: Megalodon Other CCR Dolphin Ray Join Date: Mar 2006 Location: Mendota, Virginia Posts: 149	Re: dwell-time calculation: axial vs radial Steve, I am not an engineerer.. but.. I can see logic.. in both explainations.. By the flowing along the radius, the Radial scrubber is going either from a larger surface area, to a smaller.. or conversely from a smaller surface area, to a larger. When passing through the larger surface area, it should have a lower velocity, and while passing through the smaller surface area, should have a higher velocity.. which is shown in theory in Paul's spreadsheet.. As you suggested that his explaination uses static flow, versus more human tidal flow.... I would think, would affect both designs similarly? On the Axial design, it seems to me, that the dwell time would be Uniform for the entire scrubber, ... where as on the Radial the dwell time would be smallest at the inner diameter, and highest on the outer diameter. I can see how this could be a negative, resulting in rapidly exhausting the scrubber in the outer diameter volume, and not allowing sufficent dwell time to utilize the inner diameter volume... Maybe you could explain to me.. in regards to axial scrubbers, why it is more common to have tall, smaller diameter design versus a short, large diameter design. In my mind, given the same volume, an nearly equivalent amount of gas passing through whether tidal or continual, (difference due possibly due to WOB).. It seems, that the larger diameter cannister would reduce the velocity of the gas such that the dwell time remains the same. In the smaller diameter cannister, the velocity would be higher. My head hurts =( Thank you for any education I will soon likely receive =) -Scott
(Offline)

5th January 2007, 05:34	#8 (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: 1,743	Re: dwell-time calculation: axial vs radial Quote: (Originally Posted by Scott) Steve, I am not an engineerer.. but.. I can see logic.. in both explainations.. By the flowing along the radius, the Radial scrubber is going either from a larger surface area, to a smaller.. or conversely from a smaller surface area, to a larger. When passing through the larger surface area, it should have a lower velocity, and while passing through the smaller surface area, should have a higher velocity.. which is shown in theory in Paul's spreadsheet.. As you suggested that his explaination uses static flow, versus more human tidal flow.... I would think, would affect both designs similarly? On the Axial design, it seems to me, that the dwell time would be Uniform for the entire scrubber, ... where as on the Radial the dwell time would be smallest at the inner diameter, and highest on the outer diameter. I can see how this could be a negative, resulting in rapidly exhausting the scrubber in the outer diameter volume, and not allowing sufficent dwell time to utilize the inner diameter volume... Maybe you could explain to me.. in regards to axial scrubbers, why it is more common to have tall, smaller diameter design versus a short, large diameter design. In my mind, given the same volume, an nearly equivalent amount of gas passing through whether tidal or continual, (difference due possibly due to WOB).. It seems, that the larger diameter cannister would reduce the velocity of the gas such that the dwell time remains the same. In the smaller diameter cannister, the velocity would be higher. My head hurts =( Thank you for any education I will soon likely receive =) -Scott Hello Scott, first off if you look at a spent radial scrubber in-out flow, you will notice the spent sorb clustering around the inner tube and less color change on the outside edge, showing that most of the radial dwell takes place on the exhale side. I think Steve is right that the tidal cycle is the missing factor in the comparison, it's the only explanation for why a radial works at all as without it and if the gas was flowing at a constant rate, it would fill up the scrubber and just keep flowing out at whatever rate the flow and restrictions dictate and would in fact be no different than an axial of the same weight. Paul may believe that the pressure drop btw the inner and outer side of a radial is inconsequential, but I don't, especially when you factor in the tidal/step cycle that the breathing gas follows in a CCR powered by a set of human lungs. The flow of the gas is nowhere near constant, it ebbs and flows and I think this is where the radial shines as it's asymetrical gas path interacts more favorably with the tidal cycle. During an inhale or an exhale, you have lower velocity at the begining and end and the gas will slow and actually stop before being drawn through the inhale side of the loop. And any pressure drop at all is going to affect the dwell as that exhaled gas runs out of velocity, IMHO. -Andy
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5th January 2007, 08:06	#9 (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: 610	Re: dwell-time calculation: axial vs radial What is also not taken into consideration is the gas will travel at different speeds in different parts of the scrubber, for instance gas will travel faster (less dwell time) on the edges where the granules meet the smooth face of the cannister. I think this is less likely in a radial can which may contribute to the supposed better performance of the radial scrubber, at least in terms of duration. Dave
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5th January 2007, 08:20	#10 (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: 1,345	Re: dwell-time calculation: axial vs radial Quote: (Originally Posted by dave t) What is also not taken into consideration is the gas will travel at different speeds in different parts of the scrubber, for instance gas will travel faster (less dwell time) on the edges where the granules meet the smooth face of the cannister. I think this is less likely in a radial can which may contribute to the supposed better performance of the radial scrubber, at least in terms of duration. Dave 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 __________________ www.rEvo-rebreathers.com .... the earth is flat, Elvis is alive, and radial scrubbers give longer dwell time than axials...
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