Here's another question for you. I feel that I sort of half-understand this, but not well enough to explain it clearly to another person, which is where I need to get to.

Why are the ventilation 'stacks' above mine shafts very tall and wider at the top than the bottom?

The same can be said of stacks/chimneys/vents above big underground tunnels, including modern ones like the Tyne Tunnel.

I'm pretty sure that it has something to do with temperature differences and relative pressure. I suspect that the air flow may not always be in the same direction.

Agricola in his 17th century work on mines shows square-section wooden stacks on mine shafts, clearly wider at the top than at the bottom, so this idea is not very modern.

Some people say that it is all to do with the 'Venturi effect', but this alone does not really explain it, plus the Wikipedia page (and every other page I've looked at on this) has a very bad explanation of the effect.

The sides of the stack stop air in the stack mixing with the open air, thus preserving differences in temperature, pressure, humidity etc. They also shade the air from the sun. The stack also increases the height of the column of air in the mineshaft, which increases its weight.

On sunny days, the air near the ground is warmed by the hot surface of the ground, so a stack might be useful to bridge past this air to much cooler air a little way further up.

If the air in the stack is cold, then the higher column of air will weigh more and so sink faster, pulling in more air behind it into the mine. Possibly the narrowing of the chimney in this direction serves to increase the pressure/density and further accelerate the air downwards.

If the air in the stack is (relatively) warm, then one argument is that the widening can lower the pressure of the rising air, thus drawing up more air from below. The trouble with that idea is that it would also draw air down from above for the same reason. If the rising air were still accelerating, then a tall stack would let it accelerate for longer, but that's a big IF.

Firstly, welcome

Secondly, are you the somewhat odd (in a good way) Lindybeige who does those rather good videos (Youtube, natch) ?

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"Why are the ventilation 'stacks' above mine shafts very tall and wider at the top than the bottom?"

That type of structure is usually associated with fan forced ventilation and is called an Evase.

Read down to the bottom.

https://cbpowerandindustrial.wordpress.com/2015/10/08/ventilation-design-considerations-for-underground-mines-part-2/ 

Divided Evase for twin fan installation.

🔗27280[linkphoto]27280[/linkphoto][/link]

Probably not. I do have a YouTube channel called 'Lindybeige', yes, but I'm not in any way odd.

Probably not. I do have a YouTube channel called 'Lindybeige', yes, but I'm not in any way odd.

Odd...?...Subjective.
Hugely informative and entertaining...? Definitely!

Welcome to AditNow :thumbup:

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Oh yeah:

In the old days T'Owd man would have been trying to pull air trough a mine in the addit along through the brattices and traps before the fire pulled the air up the vent shaft, most mines these days the air is pushed in - if you are lucky enough to get a trip around Boulby, after the lamproom you go through an airlock before getting to the pit head and giving your check to the liftman and heading down the manshaft, when you go over to the haulageshaft and see the skip coming up there's a mahooosive draft blowing in your face.

(interesting quirky videos- hmmmm my kinda stuff - you do owt on tanks 'n military stuff???????)

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Probably not. I do have a YouTube channel called 'Lindybeige', yes, but I'm not in any way odd.

Odd...?...Subjective.
Hugely informative and entertaining...? Definitely!

Welcome to AditNow :thumbup:

I would entirely agree, I've greatly enjoyed a large number of your videos - you manage to do the really hard trick of breaking down complicated subjects into understandable segments, and not overwhelming people whilst doing so.

I'll stop posting on this topic now, as the subject is not, in any way shape or form, an area I know anything about

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The design of a fan Evase has to do with maintaining efficiency of the fan. If you let the air go straight out at full pressure it creates turbulence which is inefficient, So the air is allowed to expand and reduce pressure before being released at lower pressure in a larger duct at reduced speed.

There is lots of maths involved which I have forgotten...

Are you Youtuber Lindebeige really looking for information or some lazy person trying to scam information for a thesis or book you plan to write; I suspect this as you have say you half the information (but are too idle to find the other half) :ban:

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Are you Youtuber Lindebeige really looking for information or some lazy person trying to scam information for a thesis or book you plan to write; I suspect this as you have say you half the information (but are too idle to find the other half)

Well, I am a YouTuber, and I don't want to talk about something unless I feel I really understand it.

That written, research is research, and why should a thesis writer ignore sources of information? I have spent many hours trying to answer this question, and have consulted many people and many websites and books, and I am yet to come across a clear answer.

If Agricola used a tall stack wider at the top in the 17th century, there was presumably a reason for it, especially since modern vent stacks are the same shape.

OK, you've had the answer for the Tyne Tunnel and modern vent fan Evase shape.

Could you post an image of the 'Agricola' structure?

Without checking Agricola (I don't have one sadly) I would guess that he was describing ventilation using a furnace. Therefore his outlet was basically a chimney, not an evase. Modern evase design is based on quite fast moving air and there would be little if any benefit when using fire ventilation. In any case, they would not have had the technological understanding of the physics at that time.

Are you Youtuber Lindebeige really looking for information or some lazy person trying to scam information for a thesis or book you plan to write; I suspect this as you have say you half the information (but are too idle to find the other half) :ban:

Lazy? Scam? Ban??! WOW! :surrender:

No better place than this to come looking for assistance and information for a thesis or book. People will soon stop responding if someone is pushing their luck.
Give him a chance eh...?

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https://www.flickr.com/photos/bookhistorian/33696587890/in/album-72157682587188056/ 

This picture from Agricola shows vertical shafts with brattices dividing them into separate parts. One, the nearer, has a structure designed to catch the wind from any direction and direct it downwards into one shaft. In the background, another shaft has a stack built over it, wider at the top, apparently out of wooden boards.

Old-fashioned clay chimney-pots are very often tall and wider at the top, and they are not dealing with motorised air.

I have been pondering this point over "laziness". This may be a bit philosophical, but there is nothing wrong with asking for information as a valid research method. Sure, there are books and papers and online resources that one can dig up and quote and use, but there is also a huge resource of information that resides unpublished with individuals, and as long as that information can then be checked out as also being reliable, it's no different from asking fellow researchers for a few pointers. Please can we all just be helpful and positive and not accuse each other of being lazy. Let's preserve Aditnow as a rare jewel of friendly uncontroversial social media.

I also don't think it's lazy to ask for information - all I do is ask people stuff - I've probably had far more useful info from people than from books or the internet. So if I can return the favour, even if only a little, so much the better.

Let's preserve Aditnow as a rare jewel of friendly uncontroversial social media.

Yes Peter, and as long as I ..........................

Aditnow is a huge educational resource for all. Thing is though, in a nice way, exchange of information is a two way process, if this is possible or the opportunity for feedback arises.
:flowers:

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Thanks for the image. Structure 'A' looks to be around waist height, not really 'a stack', it also appears to be mounted on a board that will fit any two of the three apertures. I would guess its purpose is to ensure the ventilation outlet is not accidentally obstructed by any carelessly position material, or the 'standing on board' at the other hand winding position.

There was an excellent book on coal mining ventilation/fan houses given out in the delegate packs at one of the NAMHO conferences. I'm sure I will still have my copy somewhere.

https://www.flickr.com/photos/bookhistorian/33696587890/in/album-72157682587188056/

This picture from Agricola shows vertical shafts with brattices dividing them into separate parts. One, the nearer, has a structure designed to catch the wind from any direction and direct it downwards into one shaft. In the background, another shaft has a stack built over it, wider at the top, apparently out of wooden boards.

The big question is what note does Agricola give against "A" or "B"?