So what makes a good scrubber design.....

Drmike · 18th February 2006, 17:35

Quote: (Originally Posted by paulraymaekers)

hello mike, can't you see that the overall cross sectional area (the sum of the sections between all the fins, at any diameter of the scrubber, and mesured perpendicular towards the fins) has decreased, and that that is the reason for the increased speed?l

Paul, dont forget the flow is also split equally between each fin section. So the sum of the flows are the same and the sum of the cross sectional areas are the same as the original*

*Paul, if you are suggesting that the fins themselves take up some volume then yes the addition of the fins must result in a small reduction in the internal volume of the scrubber and therefore must result in an overall increase in gas velocity - yes you are quite right.

However if you look at the likely magnitude of this increase in velocity it will be very small. So you are right the gas will have a slightly higher velocity - but the negative impact of that on the overall scrubbing will likely be insignificant compared to the positive impact of the increased pathlength.

Drmike · 18th February 2006, 17:40

Quote: (Originally Posted by nigelh)

To increase the path through a fixed volume you have to decrease the cross-section.

Or force the gas to take a longer non direct path through the fixed volume

= Fins

Drmike · 18th February 2006, 17:50

Quote: (Originally Posted by nigelh)

You can't beat the fact that for a given gas flow rate the dwell time depends only on volume.

Nope it also depends on the path (ie path length) the gas takes through that volume. ...pant pant pant

.

The key thing is - this is not like a rectangular scrubber or axial one where subdivisons would result in a increase in mean velocity due to the fact all the gas goes through a singular or series of restrictions that are in series This is a circular scrubber subdivided radially ie the restrictions are in parallel hence no mean increase in velocity - I think this is where the confusion is.

Genesis · 18th February 2006, 18:46

Quote: (Originally Posted by Drmike)

Nope it also depends on the path (ie path length) the gas takes through that volume. ...pant pant pant

.

The key thing is - this is not like a rectangular scrubber or axial one where subdivisons would result in a increase in mean velocity due to the fact all the gas goes through a singular or series of restrictions that are in series This is a circular scrubber subdivided radially ie the restrictions are in parallel hence no mean increase in velocity - I think this is where the confusion is.

Doesn't matter Mike.

If the volume of the scrubber is fixed, assuming that there are not "dead spaces" in the scrubber volume (that is, its all being used) the dwell time is related only to the volume of the scrubber.

The velocity of gas flow changes with scrubber configuration (axial, radial, "screw-shaped" internally, etc) as does the dwell time for a single molecule of gas on any particular granule of media, but the overall dwell time of the gas in the can does not for a fived scrubber volume.

paulraymaekers · 18th February 2006, 19:08

Quote: (Originally Posted by Drmike)

Paul, dont forget the flow is also split equally between each fin section. So the sum of the flows are the same and the sum of the cross sectional areas are the same as the original*

*Paul, if you are suggesting that the fins themselves take up some volume then yes the addition of the fins must result in a small reduction in the internal volume of the scrubber and therefore must result in an overall increase in gas velocity - yes you are quite right.

However if you look at the likely magnitude of this increase in velocity it will be very small. So you are right the gas will have a slightly higher velocity - but the negative impact of that on the overall scrubbing will likely be insignificant compared to the positive impact of the increased pathlength.

hello mike, the error in your explanation is that you think the cross section is equal, but it is not!!
with the longer gas path, you have to take the cross-section perpendicular towards the gasflow, and that between each fin, then you add all these sections and you'l see that the total is now less than without the fins, where the cross section is simply a circle concentric with the scrubber

you can easaly demonstrate this with your drawing: suppose you exxagerate and you make the spiral very long, one complete tour around the scrubber: the gass-path will become very long, but the scetion between each finn where the gas hass to pas will become very narrow

so cross-section decreases, in the same way as the gasspeed increases, and therefore equal dwell-time

but you can still explain it with the volume between the sorb that can be taken by the gas
regards
paul

***schford*** · 18th February 2006, 19:30

God I read this like 3 times trying to get my head around it....

Can someone tell me if this is correct...

Boris with fins = more run time from scrubber

Stuarts simple reasoning is that Fins make the CO2 Molecules hit more Sofnalime CO2 grabbing molelcules cos the paht is longer - there for the scubber becomes more efficient so it last longer.

??

Genesis · 18th February 2006, 19:33

In all probability the fins in the scrubber have as their main "active ingredient" getting rid of dead spots in the flow, thereby insuring a more even consumption of the pack.

This would give you greater runtime, but not for the reason you'd think - its simply a matter of using up more of the pack before it breaks through, because there are fewer "inactive" granules.

SimonK · 18th February 2006, 20:00

Boris scrubber has six fins so each segment has a sixth of the scrubber material. The more curved the fins the longer the path through the segment but its volume remains the same so the the cross section area must have reduced and so the gas velocity increases.

I would go with Genesis theory that the increased efficiency comes from better control of the gas flow. Perhaps the higher velocity gives a wider active band and less possibility of a sudden breakthrough towards the end of the scrubbers life.

Simon

***caveseeker7*** · 18th February 2006, 20:55

Quote: (Originally Posted by Genesis)

In all probability the fins in the scrubber have as their main "active ingredient" getting rid of dead spots in the flow, thereby insuring a more even consumption of the pack.

This would give you greater runtime, but not for the reason you'd think - its simply a matter of using up more of the pack before it breaks through, because there are fewer "inactive" granules.

Karl may be on to something here. Have a look at ccrscuba's scrubber picture from the library, after 6 hours of use.

The shape of the fins is repeated by the color change of the absorbant between them. But it is not entirely even radiading out from the center. I don't know where the darker side was pointing at, but I asume towards the opening to the inhalation bag (shortest path) or maybe the top of the unit during the normal diving position. Anyway, it could well be that all the fins do (and supposed to do) is reduce that effect and use the scrubber more evenly.

paulraymaekers · 18th February 2006, 21:17

hello, this is what I wanted to see: as the resistance to get trough the sorb is less (big surface and short path), the slightest 'non-equal) packing results in a faster breaktrough trough part of the scrubber where the resistance is less (gas tries to find the easiest way to pas the scrubber)

by increasing the resistance (making the path longer), the gasflow is distributed more even over the whole scrubber

regards
paul

18th February 2006, 17:35	#21 (permalink)
Drmike RBW Member Current Rebreather/s: MK 15.X Ouroboros Other CCR Home Build Other Rebreather/s: Inspiration Classic Other CCR Home Build Join Date: Feb 2005 Posts: 3,333	Re: Balance Quote: (Originally Posted by paulraymaekers) hello mike, can't you see that the overall cross sectional area (the sum of the sections between all the fins, at any diameter of the scrubber, and mesured perpendicular towards the fins) has decreased, and that that is the reason for the increased speed?l Paul, dont forget the flow is also split equally between each fin section. So the sum of the flows are the same and the sum of the cross sectional areas are the same as the original* Paul, if you are suggesting that the fins themselves take up some volume then yes the addition of the fins must result in a small reduction in the internal volume of the scrubber and therefore must result in an overall increase in gas velocity - yes you are quite right. However if you look at the likely magnitude of this increase in velocity it will be very small. So you are right the gas will have a slightly* higher velocity - but the negative impact of that on the overall scrubbing will likely be insignificant compared to the positive impact of the increased pathlength. __________________ Cave diving is a sport Wreck diving is a sport Diving in general is a sport 'Rebreather diving' is not a sport its the delusional obsession with a highly dangerous and often inappropriate piece of equipment
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18th February 2006, 17:40	#22 (permalink)
Drmike RBW Member Current Rebreather/s: MK 15.X Ouroboros Other CCR Home Build Other Rebreather/s: Inspiration Classic Other CCR Home Build Join Date: Feb 2005 Posts: 3,333	Re: Balance Quote: (Originally Posted by nigelh) To increase the path through a fixed volume you have to decrease the cross-section. Or force the gas to take a longer non direct path through the fixed volume = Fins __________________ Cave diving is a sport Wreck diving is a sport Diving in general is a sport 'Rebreather diving' is not a sport its the delusional obsession with a highly dangerous and often inappropriate piece of equipment
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18th February 2006, 17:50	#23 (permalink)
Drmike RBW Member Current Rebreather/s: MK 15.X Ouroboros Other CCR Home Build Other Rebreather/s: Inspiration Classic Other CCR Home Build Join Date: Feb 2005 Posts: 3,333	Re: Balance Quote: (Originally Posted by nigelh) You can't beat the fact that for a given gas flow rate the dwell time depends only on volume. Nope it also depends on the path (ie path length) the gas takes through that volume. ...pant pant pant . The key thing is - this is not like a rectangular scrubber or axial one where subdivisons would result in a increase in mean velocity due to the fact all the gas goes through a singular or series of restrictions that are in series This is a circular scrubber subdivided radially ie the restrictions are in parallel hence no mean increase in velocity - I think this is where the confusion is. __________________ Cave diving is a sport Wreck diving is a sport Diving in general is a sport 'Rebreather diving' is not a sport its the delusional obsession with a highly dangerous and often inappropriate piece of equipment Last edited by Drmike : 18th February 2006 at 18:16.
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18th February 2006, 18:46	#24 (permalink)
Genesis Bubbless Box of Death Current Rebreather/s: Home Build Other Rebreather/s: Home Build Join Date: Oct 2005 Location: Sunny Florida Posts: 1,451	Re: Balance Quote: (Originally Posted by Drmike) Nope it also depends on the path (ie path length) the gas takes through that volume. ...pant pant pant . The key thing is - this is not like a rectangular scrubber or axial one where subdivisons would result in a increase in mean velocity due to the fact all the gas goes through a singular or series of restrictions that are in series This is a circular scrubber subdivided radially ie the restrictions are in parallel hence no mean increase in velocity - I think this is where the confusion is. Doesn't matter Mike. If the volume of the scrubber is fixed, assuming that there are not "dead spaces" in the scrubber volume (that is, its all being used) the dwell time is related only to the volume of the scrubber. The velocity of gas flow changes with scrubber configuration (axial, radial, "screw-shaped" internally, etc) as does the dwell time for a single molecule of gas on any particular granule of media, but the overall dwell time of the gas in the can does not for a fived scrubber volume. __________________ "A venturesome minority will always be eager to get off on their own, and no obstacles should be placed in their path; let them take risks for Godsake, let them get lost, sunburnt, stranded, drowned, eaten by bears, buried alive under avalanches - that is the right and privilege of any free American." http://www.denninger.net http://www.diversunion.org/liability.htm - Fix the Diving Cert racket
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18th February 2006, 19:08	#25 (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: 1,578	Re: Balance Quote: (Originally Posted by Drmike) Paul, dont forget the flow is also split equally between each fin section. So the sum of the flows are the same and the sum of the cross sectional areas are the same as the original* Paul, if you are suggesting that the fins themselves take up some volume then yes the addition of the fins must result in a small reduction in the internal volume of the scrubber and therefore must result in an overall increase in gas velocity - yes you are quite right. However if you look at the likely magnitude of this increase in velocity it will be very small. So you are right the gas will have a slightly* higher velocity - but the negative impact of that on the overall scrubbing will likely be insignificant compared to the positive impact of the increased pathlength. hello mike, the error in your explanation is that you think the cross section is equal, but it is not!! with the longer gas path, you have to take the cross-section perpendicular towards the gasflow, and that between each fin, then you add all these sections and you'l see that the total is now less than without the fins, where the cross section is simply a circle concentric with the scrubber you can easaly demonstrate this with your drawing: suppose you exxagerate and you make the spiral very long, one complete tour around the scrubber: the gass-path will become very long, but the scetion between each finn where the gas hass to pas will become very narrow so cross-section decreases, in the same way as the gasspeed increases, and therefore equal dwell-time but you can still explain it with the volume between the sorb that can be taken by the gas regards paul __________________ www.rEvo-rebreathers.com .... the earth is flat, Elvis is alive, and radial scrubbers give longer dwell time than axials... Last edited by paulraymaekers : 18th February 2006 at 19:14.
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18th February 2006, 19:30	#26 (permalink)
*schford* RBW Founder Current Rebreather/s: Megalodon Sport Kiss Other Rebreather/s: Join Date: Jan 2005 Location: UK Posts: 3,564	Re: So what makes a good scrubber design..... God I read this like 3 times trying to get my head around it.... Can someone tell me if this is correct... Boris with fins = more run time from scrubber Stuarts simple reasoning is that Fins make the CO2 Molecules hit more Sofnalime CO2 grabbing molelcules cos the paht is longer - there for the scubber becomes more efficient so it last longer. ?? __________________ Bailout and Kit Management account for Murphy's Law The only bad question is one you did not ask and later wish you had. Use of RBW is subject to the Rebreather World Terms & Conditions of Use
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18th February 2006, 19:33	#27 (permalink)
Genesis Bubbless Box of Death Current Rebreather/s: Home Build Other Rebreather/s: Home Build Join Date: Oct 2005 Location: Sunny Florida Posts: 1,451	Re: So what makes a good scrubber design..... In all probability the fins in the scrubber have as their main "active ingredient" getting rid of dead spots in the flow, thereby insuring a more even consumption of the pack. This would give you greater runtime, but not for the reason you'd think - its simply a matter of using up more of the pack before it breaks through, because there are fewer "inactive" granules. __________________ "A venturesome minority will always be eager to get off on their own, and no obstacles should be placed in their path; let them take risks for Godsake, let them get lost, sunburnt, stranded, drowned, eaten by bears, buried alive under avalanches - that is the right and privilege of any free American." http://www.denninger.net http://www.diversunion.org/liability.htm - Fix the Diving Cert racket
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18th February 2006, 20:00	#28 (permalink)
SimonK Custom Title Allowed! Current Rebreather/s: Inspiration Classic Other Rebreather/s: Join Date: Oct 2005 Location: Cumbria UK Posts: 231	Re: So what makes a good scrubber design..... Boris scrubber has six fins so each segment has a sixth of the scrubber material. The more curved the fins the longer the path through the segment but its volume remains the same so the the cross section area must have reduced and so the gas velocity increases. I would go with Genesis theory that the increased efficiency comes from better control of the gas flow. Perhaps the higher velocity gives a wider active band and less possibility of a sudden breakthrough towards the end of the scrubbers life. Simon
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18th February 2006, 20:55	#29 (permalink)
*caveseeker7* Who loves ya, baby Current Rebreather/s: Sport Kiss Other Rebreather/s: Prism Topaz Join Date: Jan 2005 Location: Too far from Neverland Posts: 5,526	Re: So what makes a good scrubber design..... Quote: (Originally Posted by Genesis) In all probability the fins in the scrubber have as their main "active ingredient" getting rid of dead spots in the flow, thereby insuring a more even consumption of the pack. This would give you greater runtime, but not for the reason you'd think - its simply a matter of using up more of the pack before it breaks through, because there are fewer "inactive" granules. Karl may be on to something here. Have a look at ccrscuba's scrubber picture from the library, after 6 hours of use. The shape of the fins is repeated by the color change of the absorbant between them. But it is not entirely even radiading out from the center. I don't know where the darker side was pointing at, but I asume towards the opening to the inhalation bag (shortest path) or maybe the top of the unit during the normal diving position. Anyway, it could well be that all the fins do (and supposed to do) is reduce that effect and use the scrubber more evenly. __________________ Cheers Stefan "Political Correctness is a doctrine, fostered by a delusional, illogical minority, and rabidly promoted by an unscrupulous mainstream media, which holds forth the proposition that it is entirely possible to pick up a turd by the clean end.!" Last edited by caveseeker7 : 18th February 2006 at 20:58.
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18th February 2006, 21:17	#30 (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: 1,578	Re: So what makes a good scrubber design..... hello, this is what I wanted to see: as the resistance to get trough the sorb is less (big surface and short path), the slightest 'non-equal) packing results in a faster breaktrough trough part of the scrubber where the resistance is less (gas tries to find the easiest way to pas the scrubber) by increasing the resistance (making the path longer), the gasflow is distributed more even over the whole scrubber 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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