right - we're having CO2 problems at a dig face
(body sized tube, filled to roof with loose sand, virutually horizontal but dips down slightly so CO2 will sump)
Its not desperate - but after a couple of hours - we are getting a bit short of breath/panting/claustrophobic.

at the moment we've got 2 times 12v pc fans, inline, blowing down a rainwater drainpipe, that then reduces to plastic overflow pipe (inch ish) to go along to the dig face.

its kinda working, but at about 10 meters - its barely managing to push the air far enough.

Wondered if anyones got any ideas for 12v fans that would provide a bit more omph. beter still - powerful enough to suck the co2 out.

Suck is better than blow (so I've heard).

is it essential to use internal motors?
you could always stick with the pc fans and bodge in a 12v electric motor with more grunt, like the ones for radio controlled cars.

you could also try using a 12v cooling fan from an old car, but it may need some bodging as they tend to be 12" fans and would need a frame and traffic cone type arrangement to get all the air into the right place.

Cheers
Charlie

Try the heater fan from a car.
Something like a Series Land Rover or 300/400 LDV van has a nice centrifugal blower motor in a stand alone case (without being encumbered with heater matrix and all the other stuff).

Chris.

Anyone know what kind of ampage a car heater fan works at, the 12v nicads we are using are 3700 mAh.

(from componentshop.com or what ever its called (cant be arsed to look up the link).

about 80watts is standard. 80watts/12volts=6.6amps.
too much for those batteries to handle unless you have several sets in parallel, even then you will only get a runtime equal to 30 mins per set.

do you know what power your pc fans are? could be helpful to know.

Cheers
Charlie

Have you looked at shower and bathroom extractor fans? They're often 12v.

Charlie said:

do you know what power your pc fans are? could be helpful to know.

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I seem to recall calculating approx 15 hours run time off the 3700 mAh pack.

so does that work out at .4 A - which is .2A each.

seems to ring a bell.

so that gives you about 5watts total power.

80w could be overkill, as well as draining your batteries.

anyone out there know of 10-20w fans?

There are two things you need to know before embarking on a dig ventilation project: the first is that the amount of air you can shift with a fan is a function of how much power you can put into your fan blades, and the second is that 'normal' fans are designed to work with almost no pressure differential across them - i.e. they cannot suck or blow, they can only move air from one side to the other.

The result of the first fact is that providing more power to the fan blades is by far the most important thing you can do if you want to shift more air. The lesson from the second is that if you want to suck or blow air against any significant pressure differential (i.e. along a pipe) then the design of the fan itself is a critical factor in determining how efficient the process will be. Fans which are designed for use in free air cannot shift air efficiently down a small pipe.

Put these two factors together, and what you learn is that (1) you need a lot more power and (2) much of that power will be wasted if you don't use it to run a fan which is suited to your particular application.

I have several fans which are designed to work in high back pressure situations (i.e. for blowing air down a pipe). These have been acquired over the years with a view to using them for digging projects, but they are (currently) mains powered. The blades and volute design have more in common with a turbocharger than they do with a 'normal' cooling or ventilation fan.  I seem to recall that one of them is about 750 W and the other is about 250W - the 250W one could probably be converted to work off a 24V motor (also ex-stock Hucklow Digging Supplies) if you wanted to have a go at it, but you're still looking at having to find the same sort of battery power as you would for a drill. OTOH, you may be able to get away with only running it for one minute in 10 or something like that.

Hmmm, I think I can feel a dose of tinkering coming on...

Nick

How about this?

http://www.screwfix.com/prods/14750/Heating-Cooling/Extractor-Fans/Manrose-Axial-20W-Bathroom-Fan

It does say 240v at the bottom, but it says SELV in the description (Safety Extra Low Voltage) and it requires a transformer, so it's probably running on 12v after the transformer. There are some others on the site but not all say if they're SELV.

I'm interested Nick.

The PC fans, which I'm sure you're aware, are only designed for free air conditions do actually manage to push a bit of air down the pipe, it 'trickles' (for want of a better word) out the end.

You know for a fact I can do 24v.

I'm going to speak to Henry R tomorrow about what he's done in Western Highway.

you may be able to get away with only running it for one minute in 10 or something like that.

Click to expand...

that's a definite possibility

Like I say, its kinda working at the moment, we can get by, so I'm not desperate, and therefore not particularly into spending dosh. Factored with the fact that its a bit of an awkward, not to mention delicately decorated crawl to get there, so we are not into lugging stuff (batteries) back and forth every trip.

Im also interested in the 12v bathroom fans. Anyone find out what amps they run at. (quick google reveals only voltages).

SamT said:

Im also interested in the 12v bathroom fans. Anyone find out what amps they run at. (quick google reveals only voltages).

Click to expand...

I think there's one sitting in our pile of samples to get rid of right now. I'll check it out tomorrow for you.

Nick.

I checked the spare sample I have. Unfortunately, the fan is 230V ac, it's the integral light which is 12V.

Nick.

Would it be possible to use bellows like in the old days? :-\

How far from the entrance are you? Ex Fire service positive pressure ventilation fans will shift around vast quantities of air but run from an engine. They would also make you deaf without wearing hearing protection. Probably not much good actually

Far to far to consider that - and the level of the problem is so slight that it wouldn't warrant the effort.

I was just wondering if someone could suggest some alternative 12v fans to replace the ones we have.

We have a few leads.

Would something like this be any use? http://www.coolfansonline.com/servlet/Detail?no=2 Or is is too big and/or weak?

Im guessing its not going to be much more effective than our two PC cooling fans.

As pointed out by nick - both are designed to operate in 'free air' conditions, i.e. with no back pressure, eg pushing or sucking the air down a pipe.

current system is two pc fans taped back to back.

I've got some suggested improvements which include - separating the fans by about a foot with some drainpipe, then putting static fins at quarters down the pipe between the two fans - which stops the air spinning down the pipe, and therefore increases the effectiveness of the 2nd fan. Worth a go for the price of some cardboard, gaffa tape and 1 foot of drain pipe.

As I've stated, the current system works (just about) and allows us to dig, but I'm just after improving it.

It's better when the cross sectional area of the pipe just downstream of the fan is as large as possible. This creates a larger cavity => less increase in pressure to flow rate.

Or that could all just be wrong, i don't know for sure...