Ventilation fans
- Thread starter SamT
- Start date
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tomfoord
Guest
I may completely wrong here, but my feeling is that you'd be considerably better off sucking than blowing.
I'll try and explain myself, and to make this easier I'll pretend that your CO2 is in fact water. So your dig face is at a low point, which contains a pool or sump of water.
(I realise that this is far from a perfect analogy since CO2, being gaseous, will mix with air when there is any kind of turbulence, whereas water will clearly not! But still, there may be something in this.)
Ok, so imagine that you are blowing fresh air down your pipe into the bottom of that pool of water. All that happens is your fresh air rises up through the water and disappears back up the passage. Your nasty pool of water (or CO2) stays exactly where it was.
But what happens if you reverse the flow? The end of your pipe, being at the low point, will suck up only the dense water and carry it away, and the only possible result is that it will be replaced by fresh air flowing inwards along the passage. Hey presto, you remove your water (or CO2) and replace it with nice fresh air.
The real problem is that you need enough power to raise that dense water up the height difference required to get it out of the dig.
But this has got me thinking... in a situation where the 'water' was in fact real water you might be able to set a syphon going. Now how good would that be if you could do the same with CO2?!?
I'll try and explain myself, and to make this easier I'll pretend that your CO2 is in fact water. So your dig face is at a low point, which contains a pool or sump of water.
(I realise that this is far from a perfect analogy since CO2, being gaseous, will mix with air when there is any kind of turbulence, whereas water will clearly not! But still, there may be something in this.)
Ok, so imagine that you are blowing fresh air down your pipe into the bottom of that pool of water. All that happens is your fresh air rises up through the water and disappears back up the passage. Your nasty pool of water (or CO2) stays exactly where it was.
But what happens if you reverse the flow? The end of your pipe, being at the low point, will suck up only the dense water and carry it away, and the only possible result is that it will be replaced by fresh air flowing inwards along the passage. Hey presto, you remove your water (or CO2) and replace it with nice fresh air.
The real problem is that you need enough power to raise that dense water up the height difference required to get it out of the dig.
But this has got me thinking... in a situation where the 'water' was in fact real water you might be able to set a syphon going. Now how good would that be if you could do the same with CO2?!?
CO2 is a compressible gas so unfortunately no syphon is possible. you can't syphon compressible fluids since compression of the fluid will occur in the pipe rather than flow. Also the gases will just mix. 33 feet suction head of water causes it to shear (cavitation) and break the syphon. Different fluids have different density and viscosity so will shear at different suction heads. A gas will effectively shear with no suction head which means no syphon. I agree with the vacuum idea though.
James
James
Peter Burgess
New member
braveduck said:
Swat I said n days ago.
Les W
Active member
I seem to recall that the BEC used to use a domestic vacuum cleaner on the surface with a long hose, powered by a generator to remove CO2. I think they took photos and sent them to the manufacturer who sent them a n ew vacuum for free.
Don't suppose that will help you though.
Don't suppose that will help you though.
M
MSD
Guest
I agree you want to remove the CO2, but isn't it best to put the fan at the lowest point and blow the CO2 out? i.e the inlet hose should be short and the outlet hose long.
Mark
Mark
Peter Burgess
New member
Probably. The important principle is to concentrate on removing the CO2, not by blowing in fresh air which will behave a bit like bubbles blown into water, and will rise straight back out of the dig again, leaving most of the heavier gas behind.MSD said:
Peter Burgess
New member
Fine, as long as there is enough oxygen there, otherwise you would be wasting your time.Les W said:
nickwilliams
Well-known member
I've never been able to satisfactorily resolve the suck out/blow in conundrum. I don't find the idea of sucking bad air out very convincing - I've always thought that it is better to have a known good pool of air at the end of the pipe rather than an indeterminate extent of air of unknown quality which it seems to me is all you are likely to end up with if you try to suck poor air out.
The mine applications I have seen generally appeared to be blowing good air in but on further examination were actually configured to provide a circulating draft throughout the working tunnels. It makes sense that the ideal solution is probably a combination of both blow in and suck out.
I don't think I buy that analogy. It might be true for a passage which has no significant air disturbance for a long period and where there is a high concentration of CO2, but that's never going to be the situation in a passage which is actively being dug. The movement of the diggers in the passage will stir the air up, and the thermal effects resulting from the presence of warm bodies (and their breathing) would overwhelm the gravitationally generated effect of the higher density of the CO2.
The problem is maintaining reasonable air quality through the duration of a digging session so it's a case of having to dilute the CO2 generated to a comfortable level, hence the need to put good air in rather than pull bad air out.
Nick.
The mine applications I have seen generally appeared to be blowing good air in but on further examination were actually configured to provide a circulating draft throughout the working tunnels. It makes sense that the ideal solution is probably a combination of both blow in and suck out.
Peter Burgess said:
I don't think I buy that analogy. It might be true for a passage which has no significant air disturbance for a long period and where there is a high concentration of CO2, but that's never going to be the situation in a passage which is actively being dug. The movement of the diggers in the passage will stir the air up, and the thermal effects resulting from the presence of warm bodies (and their breathing) would overwhelm the gravitationally generated effect of the higher density of the CO2.
The problem is maintaining reasonable air quality through the duration of a digging session so it's a case of having to dilute the CO2 generated to a comfortable level, hence the need to put good air in rather than pull bad air out.
Nick.
Peter Burgess
New member
I think the idea of pumping carbon dioxide out rather than fresh air in is more relevant when considering a vertical blind shaft. Well-diggers faced with bad air at the bottom of wells used to use the well bucket to remove the bad air, I believe. They must have looked totally bonkers winding up empty buckets and carefully emptying them onto the ground.
SamT
Moderator
All good stuff, we are fully aware of the suck/blow conundrum [phnar]
Like I stated, the problem really is very minor, we where able to dig prior to the fan, it just bit tiring after about 2 hours which is usually pub-o-clock anyhow. The current PC fans have improved matters slightly.
Considering making the improvements and reversing things to suck. More on the grounds that I feel the fans may be able to shift more air sucking up from smaller tube into increasingly larger tubes, rather than trying to force air down a series of increasingly smaller pipes, rather than on any air quality grounds.
Its a point worth making that in the suck blow argument, if your sucking co2 out, just as with water, do you not have to make sure it doesn't just run back in and that it has a place to run away too.
This dig is practically horizontal, if sucking, we would be pulling from a huge volume of good air. and the bad co2 expelled from the fans would sink into a the appropriately positioned blind pot over which its placed.
Like I stated, the problem really is very minor, we where able to dig prior to the fan, it just bit tiring after about 2 hours which is usually pub-o-clock anyhow. The current PC fans have improved matters slightly.
Considering making the improvements and reversing things to suck. More on the grounds that I feel the fans may be able to shift more air sucking up from smaller tube into increasingly larger tubes, rather than trying to force air down a series of increasingly smaller pipes, rather than on any air quality grounds.
Its a point worth making that in the suck blow argument, if your sucking co2 out, just as with water, do you not have to make sure it doesn't just run back in and that it has a place to run away too.
This dig is practically horizontal, if sucking, we would be pulling from a huge volume of good air. and the bad co2 expelled from the fans would sink into a the appropriately positioned blind pot over which its placed.
B
Bouncing Bom
Guest
Vacumn cleaner sounds a good idea but anything with a compressor is what you are looking for, if you want to force air into or out of pipes. Simple ideas for the kind of scale you are looking at include the portable compressors used to inflate car tyres or airbeds etc. Miminmising loses of pressure head is also important and most of your losses will come from changes of pipe size unlike the simple fan idea where fins may be of some use limiting rotation of flow (not normally an issue unless using fans in series ) fins in the use of a compressive system will only add to frictional losses.
Compressive systems are more vulnerable to leakage at joint and losses from constrictions and tight bends too.
The issue of whether you "suck" or blow (obviously best to do both and set up circulation) is more one of keeping your equipment outside the cave and what type of equipment you have, (vacumn and compressor on separate tubes best). If only one tube available in general you want to blow fresh air down and dilute the air. In most caves the air is not completely stationary but draughts, water movement, even movement of people within the cave will do a good job of mixing the gases up. The second issue to think of is if you just "suck" air out, where is the air which replaces the air displace going to come from? In most cases it will air from other places in the cave, (especially on draughting faces or when a long way from the entrance). this air you air now mixing with the remaining "old" air is likely to not be as fresh (ie higher CO2 lesso2 etc) than if you were mixing it with the nice clean air fom the surface so you may have made less difference to the air quality at the face, even if over all the air throughout the cave may be that little bit nice as apposed to blowing where you are blowing bad stuff throughout the cave.
I hope someone understands what I am trying to say as I may have even confused myself here :-\
Compressive systems are more vulnerable to leakage at joint and losses from constrictions and tight bends too.
The issue of whether you "suck" or blow (obviously best to do both and set up circulation) is more one of keeping your equipment outside the cave and what type of equipment you have, (vacumn and compressor on separate tubes best). If only one tube available in general you want to blow fresh air down and dilute the air. In most caves the air is not completely stationary but draughts, water movement, even movement of people within the cave will do a good job of mixing the gases up. The second issue to think of is if you just "suck" air out, where is the air which replaces the air displace going to come from? In most cases it will air from other places in the cave, (especially on draughting faces or when a long way from the entrance). this air you air now mixing with the remaining "old" air is likely to not be as fresh (ie higher CO2 lesso2 etc) than if you were mixing it with the nice clean air fom the surface so you may have made less difference to the air quality at the face, even if over all the air throughout the cave may be that little bit nice as apposed to blowing where you are blowing bad stuff throughout the cave.
I hope someone understands what I am trying to say as I may have even confused myself here :-\
How about a DIY traditional bellows unit made from a couple of bits of ply, a flap valve, and some reasonably tight weave material (old oversuit material). Bolt one face of the unit to a side wall some where out of the way. Try about 5 to 10 litres capacity per stroke , rear man could give a few pumps every few minutes assuming he gets some spare time. You possibly wouldn't need rigid pipe for the delivery (if you can arrange not to lie on it) as the pressure would be quite reasonable. (e.g. 2" lay flat) Simple technologies sometimes work well.
JAM
New member
Have you sorted this problem Sam ?
I was thinking that you could strip down an old Hoover and use the fan from that.
If you where to attach a power drill of (X, volts) to the spindle you would be able to give the fan some serious revs to do the business of either blowing air in or sucking it out, depending on which way you mount the fan. You would probably have a cordless power drill in your gear already.
Now if only someone could come up with some way of squeezing the trigger remotely and on a timer, you could have a pulsed automated system.
Just a wee thought.
Rich
I was thinking that you could strip down an old Hoover and use the fan from that.
If you where to attach a power drill of (X, volts) to the spindle you would be able to give the fan some serious revs to do the business of either blowing air in or sucking it out, depending on which way you mount the fan. You would probably have a cordless power drill in your gear already.
Now if only someone could come up with some way of squeezing the trigger remotely and on a timer, you could have a pulsed automated system.
Just a wee thought.
Rich
