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Blackdamp is the condition where the breathable oxygen is removed from the air underground. Causing asphyxiation in humans and humanoids. Normally seen in modern day mines.

The base area of the dwarven fortress is almost mined out and the dwarves are looking to other places to expand their empire, the idea is to go down, but they started to struggle with breathing so the dwarves goes to the drawing board.

  • How could the dwarves achieve the airflow needed with no better (but preferred worse) technology than Mr. Leonardo Da Vinchi could achieve?

ps. Changing the dwarven physiology to being able to breathe in low oxygen conditions is not a viable solution since later on human dungeon crawlers should have the possibility to investigate the maybe dwarven ruins.

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    $\begingroup$ Dwarves is just the means of a goal, that question is about how a dwarven society could work, they even mention developing CO2 tolerance and lack of air. My question is maybe more about dungeon design, but with the technology possessed of a dungeon (see the note about investigation about dwarven ruins later), if i would just talk about dungeon design in general it would leave it too broad for my purpose, but i could just as well write a post about how to pump air down in to a mine, it would serve the same purpose - leaving the dwarves out (but again it would leave it to broad, $\endgroup$ – Magic-Mouse Oct 13 '15 at 11:21
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    $\begingroup$ since i didn't want suggestions about how goblins could pump with bellows or beholders could make air magically appear). Beside this question covers Black-damp not just the absence of air, black-damp includes the dilemma of Methane (highly explosive gasses who appears underground) carbon oxide and hydrogen sulfide. Those subjects aren't covered in the linked question. $\endgroup$ – Magic-Mouse Oct 13 '15 at 11:21
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    $\begingroup$ You may want to elaborate on your question to specifically mention the things you have discussed in the comments. $\endgroup$ – James Oct 13 '15 at 14:53
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    $\begingroup$ It's spelled "da Vinci". No H. Nobody's going to use that tag if it's misspelled. :) $\endgroup$ – Roddy of the Frozen Peas Oct 13 '15 at 19:02
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    $\begingroup$ @JanDvorak do we need another way for them to die? $\endgroup$ – user487 Oct 14 '15 at 15:43
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If you think about traditional fantasy dwarves - and you should if you insist calling them dwarves - the most distinctive thing is not really that they live underground or that they are short. The distinctive thing about dwarves since the Norse sagas is their single minded focus and supernatural ability in metalworking and artifice.

So every dwarf worth the name knows something about metalworking and every dwarven habitation is built around its forges. Presumably those forges and the fires that burn in them would have deep religious meaning to the dwarves as well.

How is this relevant to the question?

Every forge (or other fireplace) needs a chimney to get rid of the smoke and create air flow to feed the fire. Basically the air heated by the fire rises up the chimney taking the smoke with it and this creates an under-pressure that sucks fresh air to feed the fire. With a long already hot chimney this can be quite powerful. Just the thing dwarves will want for their religiously significant forges.

But how is this relevant to the actual question, you ask.

The air sucked by the fire places will come from the dwarven habitation built around it. This creates an under-pressure inside the entire complex. Which means that if you have a ventilation shaft to the surface the fresh air will be sucked in. So as long as those fires keep burning it is enough to open new ventilation shafts when you expand the complex. Which is pretty good reason for the dwarves to consider those forges important. And place them near the center or, more precisely, for the complex to grow around them.

Also the number of forges and fires will naturally scale up as the dwarven population - and need for ventilation - goes up. And the sufficiency of available ventilation can be seen simply by observing how well the fires burn. Something that any dwarven smith will pay attention to.

Note that if the vertical shafts of the chimneys are heated by sun you'll get some of the energy for free resulting in better ventilation and hotter fires without using extra coal or wood. The easiest method is to use a cliff facing towards the equator to place the shaft. But simply keeping the fires always burning should be enough to keep the chimneys warm.

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    $\begingroup$ +1 for recognising that the forge chimneys will act as air columns. I didn't even think of that when I wrote my answer! $\endgroup$ – Joe Bloggs Oct 13 '15 at 12:09
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    $\begingroup$ While this would work for habitable areas (and is neat and clever and dwarfish) I'm not seeing how it would help in the deep areas of mines which is what the question is asking about. You're not going to build a forge while you're working a new vein. $\endgroup$ – Dan Smolinske Oct 13 '15 at 14:18
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    $\begingroup$ @Dan Smolinske : That's precisely what we used to do in coal mines, only with fires instead of neatly contained forges. I'm sure the dwarves can make a portable stove to move from one outflow column to the next. $\endgroup$ – Joe Bloggs Oct 13 '15 at 16:14
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    $\begingroup$ Sentences of the form "you're not going to <do something that stops you dying> while you're <doing something important>" are often on shaky ground ;-) $\endgroup$ – Steve Jessop Oct 13 '15 at 19:00
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    $\begingroup$ I'm actually intrigued at a dwarven city which worked its way deeper into a mountain, using the fires from its forges to pull oxygen in from the surface, but abandoned the old forges as the city moved deeper. You could get some really neat political or mythological intrigue around a city whose fires went out, never to be relit again. It might even work into how they define geographic politics! $\endgroup$ – Cort Ammon Oct 15 '15 at 1:20
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Have airflow tunnels to the surface!

The basic idea is that the airflow tunnels are paired off, one of them being cooler than the average air in the mine, one being hotter. You can achieve this with furnaces, geothermal springwater (if you're lucky) or concentrating sunlight down the 'hot' shaft. You can also put the 'cool' shaft near to a water source or river, or possibly even just higher up in the mountains depending on climate and mine building ability. Then the air in the hot shaft rises, the air in the cool shaft sinks, and the whole thing drags fresh air through your dwarven kingdom without needing to have people man the air pumps. These tunnels also offer handy egress passages in case of catastrophic collapse (not that a dwarf would ever make a mine that might collapse)!

One thing to note with this method is that you have to be careful about which doors can be opened when to prevent parts of the mine from becoming cut off from the airflow. Your dwarven miners might have to build in a series of airlocks between different airflow sections in order to avoid one tunnel getting all the fresh air (which also gives you handy 'this is the end of this section, take a short break' markers).

Also worth pointing out that the airflow tunnels need capping to avoid rain (or goblins) getting into the mine!

And a nice link with a copypasta section below:

Ventilation became a serious problem as miners went deeper and deeper underground. The earliest solution was digging a down-shaft and an up-shaft. At the bottom of the up-shaft a fire was set ablaze, which sent hot air up the shaft. This in turn sucked fresh air into the down-shaft. To make sure that the fresh air reached all parts of the mine, trapdoors were put in all the galleries of mines, which were opened and closed as the coal trucks passed through. This ensured that there was a constant supply of air throughout the mine. The trapdoors were opened by small boys (trappers), whom sat in total darkness listening for the sound of the corves (coal trucks/sleds).

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    $\begingroup$ A bit like how Termite mounds do it... $\endgroup$ – Scott Downey Oct 13 '15 at 9:07
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    $\begingroup$ just like how termite mounds do it, but a bit larger in scale and with soot covered northern blokes instead of insects. $\endgroup$ – Joe Bloggs Oct 13 '15 at 9:36
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    $\begingroup$ Don't forget that dynamic pressure of moving air is lower than that of non-moving air. Use this to advantage by building a "chimney" for the outflow. Air passing over the top will apply a pressure gradient and help pull up the stagnant air. $\endgroup$ – Jim2B Oct 13 '15 at 17:32
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Another possible idea that could compliment the idea of having massive furnace systems for improved air flow is...biology!

The dwarves through their knowledge of earthly magic and minerals could have figured out how to grow biologically engineered plants that not only survive in low light conditions, but will also absorb massive amounts of CO2.

Imagine a massive, twisted vine-like structure growing out of some of the cave walls. For absorption of CO, perhaps a plant similar to chlorophytum comosum or the "spider plant" would be used.

Also, the dwarves could have used these plants to their advantage to search for aquifers by following the roots. Of course, those plants would have also been modified so that they grow within relatively short time intervals (perhaps like bamboo).

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  • $\begingroup$ The amount of CO2 a plant can absorb is limited by the amount of light it can get. At the bottom of the mine there is no sunlight whatsoever, and a fire would create more CO2 than what a plant could reabsorb with its light. $\endgroup$ – Davidmh Oct 14 '15 at 7:56
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    $\begingroup$ Plants? In the mines? We are dwarves, not like those tree-huggers of elves that live above... $\endgroup$ – STT LCU Oct 14 '15 at 8:20
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    $\begingroup$ It could be a part of a part of a solution, while you might think of plants as flowers, in this specific world there is a fluorescent fungus that climbs the walls, while not enough to solve the problem it can be a minor contribution. $\endgroup$ – Magic-Mouse Oct 14 '15 at 13:59
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You could just pump air down.

Harness some oxen up to a really big bellows and constantly push fresh clean air down through specially cut channels, or even through tubes made of wire and oil cloth.

Edit:
The bellows could be as big as you'd like, and if you had several large bellows working in series you could keep the air flowing constantly.

Ville Niemi's answer of having ventilation shafts letting cool air down and furnaces lifting hot air up is probably best for the established areas of the mine, but in new passages where ventilation hasn't been dug you'd want to have a way to push good air in reliably.

For very deep mines, you have to watch out for auto compression (air heats up when it is compressed, and dropping deep into a mine will compress it, causing you to get negative cooling from the air), meaning that airflow should be kept to just what is needed to remove dust and gasses in those cases. Just something to consider.

Edit 2:
So, your biggest problem is not going to be bad air. It is going to be cooling.
The temperature of a mine rises 25 °C per km of depth (1 °F per 70 feet of depth). Add to that the problem of auto compression, which you'll face in either a forced air system or naturally circulated system, and it is going to be hot once you go down a ways.

The bellows wouldn't have to be excessively big, because the more air you push down, the hotter it will get. Meaning you only want just enough to keep the bad air down and help manage the mining dust.

Refrigeration accounts for about half of the energy usage in modern day mines, and while there are some low power methods, like ice cooling, you have to have access to ice.

There might be other ways, depending on where your mine is.

For instance, if your mine is high in the mountains, then it could be really really deep, and still have drainage for an underground river, which could be used as a heat sink, and could allow for ventilation too.

Another way would be to have stages, with another set of bellows every half KM down, and another set of bellows where air is drawn back up with a reverse bellows; hook the air intake on the bellows to an air duct deep in the mine, and every time it blows to a higher stage it will draw air up from below.
The de-compression and low pressure caused by this would cool the air down below, and hopefully balance out the auto compression, while also helping pull heavier bad air out.

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  • $\begingroup$ I get your point here but could you expand your answer with some theory about how large the bellows would have to be in order to pump that magnitude of air down? $\endgroup$ – Magic-Mouse Oct 13 '15 at 19:20
  • $\begingroup$ I think this answer has potential, since dwarves know their way around bellows thanks to their forging skills. $\endgroup$ – Sanchises Oct 14 '15 at 15:28
  • $\begingroup$ @Magic-Mouse How big/deep is your mine? $\endgroup$ – AndyD273 Oct 14 '15 at 19:58
  • $\begingroup$ For the purpose of the story, infinite dungeon style. But since the layout is more the like the planning phase, the mine is so deep it is possible depending on the solution given. $\endgroup$ – Magic-Mouse Oct 14 '15 at 20:04
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    $\begingroup$ @Magic-Mouse Edited $\endgroup$ – AndyD273 Oct 14 '15 at 20:55
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Why not use fans? The first record of axial fans goes back to 180AD and a Chinese inventor named Ding (or Ting) Huan. Leonardo da Vinci doodled helicopters so there's no doubt that the notion of axial fan blades producing airflow would be understood at the level of technology you're talking about and given the usual depiction of Dwarves as master craftsmen they should be able to construct high quality, large fan blades.

The question is how to power them and here the most obvious answer would seem to be livestock combined with drive belts or perhaps operated alongside chain or rope driven mechanisms to lift material out of the mines

This approach could be combined with the excellent convection based ideas outlined by Ville Niemi and others to ensure that air is brought to every part of the mine or used on their own. I would think that you'd need both fans pushing air in and fans pulling air out for best flow

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  • $\begingroup$ In my case the fans isn't the problem, the problem is that you need an airflow large enough to feed a city, and a mine with air, and you will need some way to motor the fans - 200 dwarves on bicycle driving fans isn't really a romantic narrative. So fans is an idea, but in that case (like the similar answer above, i need to know the magnitude of the fans and how to propell it) $\endgroup$ – Magic-Mouse Oct 14 '15 at 14:59
  • $\begingroup$ For how to drive it: as I said: livestock - e.g. pit ponies or donkeys. A 1hp fan shifts something like 500 cubic feet per minute. I would assume that a da Vinci tech fan would be less efficient so call it 250 cubic feet per minute. How much air does a city need? $\endgroup$ – Jack Aidley Oct 14 '15 at 15:23
  • $\begingroup$ @Magic-Mouse Fans placed in hot exhaust gas pipes could do it. Think simple gas turbine engine (modern terminology) -- could also lead to real gas turbines being developed. $\endgroup$ – Demi Oct 14 '15 at 15:28
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Sadly, my suggestion is a real material. 'Tuff' or 'tufa' is a mineral formed by compacted volcanic ash. It is so light and porous that it can float on water. It also has the habit of absorbing methane. If your dungeons are carved from tuff, rainwater would tend to percolate down from the surface; you'd need drains to get rid of excess water. But the water would also draw fresh air from the surface and absorb methane trapped in the tuff. A complete article on tuff is found here.

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