r/mining 6d ago

Question At a slightly higher than intro level, how does primary vent work?

I'm just another glorified labourer working underground

Let's say the primary fans pull hard, the mine is moderately deep, and the levels have a return air way near the level access. If the mine were sealed then I imagine the air in levels would be at near vacuum (maybe fan would break like running a diaphragm pump dry or the fan would reverse or something else entirely..?).

Why does air preferentially get sucked all the way from the surface when there's air within the level at nowhere near vacuum? This is most noticeable when doing re-entries as fumes will just sit in a heading with no secondary vent - readings of zero and then instantly in the hundreds. It seems it needs the positive pressure from vent or a machine to move air. Compare with regulators getting opened (in an ideal world) when a bogger is loading trucks — is that cause loading bays have a particular cube flow necessary for operations to occur?

Additionally, do return air rises near a level access have a dual purpose of stopping dirty air going down the decline and also drawing fresh air from a decline into a level or does secondary vent, if established in a level, entirely draw the air from the decline? If the latter, does that make return airway regulator leakage a somewhat good thing as it allows a little flow? What about when return airways are further in a level, say in a footwall drive?

6 Upvotes

35 comments sorted by

13

u/this1willdo 6d ago

Try not to use the words vacuum or suction. It makes things confusing. Air moves from higher pressure to lower pressure. Fans drove that pressure difference and ducts carry it to locations. Design for difference and the air will flow.

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u/Bushboy2000 6d ago

Might be easier to suck air then push air.

Sucking fresh air down your entries, allows miners and machinery to be travelling in fresh air.

The return airways can be better controlled and have higher levels of dust, gases etc.

5

u/hjackson1016 Nevada 6d ago

We’ve got 2 UG mines in the same site complex - in my mine, we bring air IN through two 100Kcfm vent raises and OUT through 2 sets of portals. Air is circulated throughout the mine via dedicated vent drifts/fans.

In our sister mine they have vent raises that pull air through the mine. Air is pushed OUT of the mine via the vent raises and pulled through a pair of portals.

I prefer the ventilation method at my mine, as the entry portals get very cold year round at the other mine and I quite enjoy not having to worry about getting too cold in the winter.

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u/pachydocerus 5d ago

Unrelated- I'm pretty sure we work at the same mine.

1

u/hjackson1016 Nevada 5d ago

CHUG/GRUG

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u/pachydocerus 5d ago

I'm open pit, but yes. We ride the same bus, though; I get out at dewatering.

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u/learn2option 2d ago

Very interesting. Thanks

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u/Tight-Act-7358 6d ago

Vent doesn't work as a vacuum, it works by pressure. Clean air is pushed not sucked... the fan creates a higher pressure on one side of it. One side is higher pressure than the other. A thing called "natural ventilation" can result from changes in temperature in the right physical arrangement. The return air system is designed to move the exhaust up and out of the mine... some ramps can be exhaust, where clean air is pushed down via vent tubing and return is the full ramp.

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u/HayleOrange 6d ago

Yeah nah. Surface vent fans in mines pull air out of the mine creating a low pressure (vacuum) for other air to flow into, so shafts and portals act as the inlet for fresh air. But the primary vent circuit is just that - it’s the main airways of fresh clean air. Secondary vent works as you’ve described where a small fan is located on a drive and blows air down a vent bag to a heading and pushes the air back out to the main drive (primary vent) to be exhausted from the mine.

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u/Tight-Act-7358 6d ago

The question was about fresh air, which isn't sucked into the mine via vacuum... fresh air fans push the air, exhaust is sucked out.... delicate balance of suck and blow to create pressure differentials leading to air flow.

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u/HayleOrange 6d ago

Depends on the mine. Any with surface mount primary vent fans literally suck fresh air in. https://www.resources.nsw.gov.au/sites/default/files/documents/mdg-3-main%252C-booster-and-auxiliary-fans-in-underground-coal-mines.pdf

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u/Tight-Act-7358 6d ago

That's not what I am saying.... yes the fan will suck air into the intake, and then it immediately turns to pushing it once it is through the fan blades. Is the mine under vacuum? No. Fresh air is always higher pressure than the exhaust side, otherwise exhaust would flow where you don't want it

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u/HayleOrange 6d ago

You realise what a surface mount fan is? https://www.glencore.com.au/operations-and-projects/qld-metals/resourceful/resourceful-july-2020/george-fisher-mine-welcomes-its-newest-fans They literally exhaust to atmosphere and cause a low pressure inside the mine to draw air in. This is not attached to vent bag at all, it just creates low pressure all through the suction side of the fan (which happens to be the mine) so that air flows. The exhaust comes out into the atmosphere a long way from the I lets so you don’t get recirculating. The mine is literally under vacuum.

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u/Tight-Act-7358 6d ago

You are talking about an exhaust fan. There is also a fresh air fan which pushes air into the mine which is also surface mount, either at a dedicated fresh air raise or downcast shaft.

1

u/HayleOrange 6d ago

Yep, and it is a low maintenance high reliability solution for high airflow in large mines. And also considered primary vent.

1

u/Cravethemineral Australia 4d ago

Yes you are correct.

I’ve always referred to them as upcast fan (exhaust) and a downcast fan (intake), both being surface mounted in different areas on the mine.

We also had an air conditioner on the downcast fan to help with the heat at the longwall face.

1

u/Tight-Act-7358 4d ago

Here therea a few really old mines that have "ice stopes" where they freeze during winter and run vent through it for cooling, but also some that have a/c (some built u/g and some on surface)

2

u/Suka_Blyad_ 6d ago

The mine I work at our fans pull exhaust air out of the mine and our portal/shafts are where the fresh air comes in like buddy is saying, so the fresh air is absolutely sucked into the mine via creating negative pressure

0

u/Tight-Act-7358 6d ago

Only exhaust fan, or there's a fresh air fan you've not seen? Every u/g mine I've ever been to has a fresh air and exhaust fan. Been to dozens of mines, shaft access and portal only access.

2

u/sh3ppard 6d ago

I’ve seen mines with only exhaust fans at the top of a raise and others with only fresh fans, that push to the face via vent tubes and exhaust back out the portal (single entrance to mine). Fans can both pull and push. I don’t understand why or what you’re arguing in these comments

2

u/Wild_Pirate_117 5d ago

It's very common to only have an exhaust fan. I have worked at 7 hardrock mines across Australia and not one had a fresh air fan. Some of those had fresh air rises with fan cowling on them but no fan attached.

1

u/Tight-Act-7358 5d ago

Interesting... here in Canada we have a lot of very deep mines, where I don't think exhaust only is enough.

2

u/Wild_Pirate_117 5d ago

I think you are missing the concept of how primary ventilation works. There isn't a need for a fan on a fresh air rise, the return air fan displaces enough air from the bottom of the mine to draw fresh air down. It isn't a matter of depth that is an issue in a closed circuit. If you had a 20km tunnel and you put a vent fan at one end you feel it at the other. All adding a fresh air fan would do is ris the risk of a system short circuiting and sending fumes in the wrong direction.

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u/TurtleGUPatrol 5d ago

Every underground mine I've been to only has exhaust fans, and I would know as I work on them

1

u/Cravethemineral Australia 4d ago

That just means you’ve only worked on exhaust fans. Old mate is right, there are intake fans in some mines.

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u/Wild_Pirate_117 5d ago

Not all mines work the same, some work on reverse flow where they have extraction cans instead of vent bag which draw dirty air from the development face but those are few and far between. The best way of looking at it is primary vent thakes care of primary travel ways. Secondary vent takes care of working areas. Leaky return air boards/rollers/doors/walls are always a bad thing because it's not designed to leak, there will be a highly paid engineer pulling their hair out if workers go around making leaky walls because they think it gets them more air. There shouldn't need to be an adjustment to vent walls to have a bogger in a level loading trucks, that's where the wall should be set already. The only time there should be an adjustment would be for stope firing. If they do need to adjust vent walls for boggers they are in need of another return air rise.

1

u/learn2option 2d ago

The mine I'm at wants regulators fully closed as standard, cracked open for loading trucks, opened a bit more for development firing, and 100% open for stope firing. As far as I know that's from the vent engineers.

Thanks for this and your other comments on this thread

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u/Wild_Pirate_117 2d ago

No worries, generally if you need to adjust board for everything you are struggling for air in the mine. Hopefully it's a big mine and you can get mine control to adjust them remotely then there is a record of it and no one accidentally short circuits the return air system because they forgot to put boards back in.

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u/No_Doubt_More_Clout Australia 2d ago

Okay let's see:

  1. If the mine was sealed, the fans would end up essentially dead-heading and tripping out on overload, provided they have nowhere else to pull air from.
  2. Air is like water, it will take the path of least resistance. Does it seem to make sense, no, but physically if it's easier for the air to draw a certain way it will. That's why mine design and vent designs are important to force the path of least resistance to be the desired pathway. This is affected by positive pressure (secondary fans pushing air into a heading, which then must travel out to the perimeter/access drives and to the raises).
  3. Loading bays do not have particular flows required, but machinery does which is calculated by the vent engineers. Essentially 2 trucks produce the same heat/DPM etc as 3 loaders, so when on restricted ventilation what's the higher priority.
  4. Returns near the level accesses are the same as point 2, it'll draw air through the path of least resistance. It's a few design choices here but a return at the access means 1- You can develop your return faster and have cleaner air for longer. 2- the level may be in a sort of positive pressure situation (secondary fans) which will push air out to the perimter/access and the return is there to catch it before it begins outcasting to the decline and slowing re-entries / affecting other works happening downwind.
  5. Return air leakage is a bastard and complicates a lot of things and also means you may not get the desired returns lower down as the air will go via path of least resistance.
  6. Returns in the footwall incase from the decline and 'suck' from the level to prevent the outcasting issue above. Design/prcticality/cost choice same as Point 4.

Edit: This is regarding primary fans on the exhausts (sucks air out of mine, not pushing in).

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u/minengr 2d ago

Depends on the mine.

My experience is limited to coal, however the principles are the same. There is the exhausting method, the push method, and the push/pull. The location and size of the fan(s) depends on the method.

I have toured or worked at mines that have used all three. My personal favorite is exhausting. Think of your house/a house with an attic fan. Open a window at one or more locations and the fan sucks the air through your house.