Wikipedia:Reference desk/Archives/Science/2017 October 1

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October 1

CO2 fire extinguisher vs. burning office cubicle

OK, so we know what happens in a CO2 fire extinguisher in terms of thermodynamics and chemistry:

1) When the valve is opened, the liquid CO2 (which is in equilibrium with the vapor inside the tank) is propelled through the valve by the pressure of its own vapor (thermodynamic changes are negligible in this process);

2) As the liquid CO2 passes the valve, it instantly flashes to gas because of the reduced pressure, and cools because of the latent heat of vaporization;

3) The gas undergoes free expansion to many times its original volume, and cools further because of the Joule-Thomson effect;

4) As the gaseous CO2 cools, it hits its sublimation point, and part of it turns into flakes of dry ice;

5) When the gaseous/solid CO2 mixture hits the fire, it instantly stops the combustion reaction by displacing oxygen, thus stopping the generation of heat by the fire (so now only the heat energy already present in the burning fuel must be dissipated); and

6) The cold CO2/dry ice mixture also absorbs some heat from the hot fuel by the sublimation of the dry ice and the heat exchange with the cold CO2 gas (however, this effect is much less than for water).

Which brings up a few questions:

A) Is the above sequence of events correct?

B) I've done some VERY rough calculations, and come up with a ballpark figure that 1 mol (44 grams) of CO2 from a fire extinguisher absorbs about 6.7 kJ of heat as its temperature goes up to 100 C, and that at 100 C the cloud of CO2 can extinguish fires within a radius of 0.625 m of its center (approximating the CO2 cloud as a sphere, and assuming that 25% of the available oxygen must be displaced to stop combustion) -- are these figures in the right ballpark?

C) Suppose you're spraying CO2 from a full fire extinguisher at a typical office cubicle which is on fire and equilibrated at cherry-red heat (about 800 C) (because everything else in the office space is on fire as well and equilibrated at the same extreme temperature) -- how much CO2 is needed to extinguish that cubicle and cool it down to 100 C (assuming that this happens so rapidly that heat transfer from the rest of the room is negligible), and is this even possible with a CO2 fire extinguisher? (I don't know how the firefighter can survive such temperatures -- maybe he's standing just outside the room and spraying through an open door, so he's not exposed to the full heat.)

2601:646:8E01:7E0B:D403:68F1:A297:C74A (talk) 02:49, 1 October 2017 (UTC)

• I don't think CO₂ extinguishers work that way, or at least not primarily. I think the primary mechanism is the separation of the heat from the fuel by displacement as the stream of gas from the extinguisher blows the heated combustion products away. In a self-sustaining fire heat gassifies the solid fuel and the gassified fuel burns, causing heat and completing the cycle. This is why you are instructed to "aim at the base of the fire" and " sweep from side to side" (quoting from the instructions on my extinguisher). -Arch dude (talk) 03:35, 1 October 2017 (UTC)

• See a comparison of carbon dioxide and Halon fire extinguishing systems. StuRat (talk) 04:14, 1 October 2017 (UTC)
  • That appears to be a comparison of a Halon flood system with a CO₂ flood system, not a hand-held extinguisher. A flood system (apparently) needs to raise the CO₂ concentration of the flooded volume to above about 34%, which reduces oxygen availability to the fire. This is a different mechanism. -Arch dude (talk) 21:38, 1 October 2017 (UTC)

• CO₂ extinguishers don't deliver either liquid CO₂ or dry ice, unless you hold them upside down. They work by a simple gas blanket excluding O₂. Andy Dingley (talk) 10:47, 1 October 2017 (UTC)

The primary efficiency of CO₂ extinguishers is to break the fire triangle thus removing atmospheric oxygen rather than heat. Heat radiates away pretty quickly, yet the CO₂ blanket needs to be maintained until the risk of further combustion has subsided due to radiation. This doesn't apply though to aluminum/magnesium etc., fueled fires because these metals being highly reactive can sequester the extinguishers oxygen (from the CO₂). Nitrogen gas can be used against these fires as it again denies the fuel it necessary oxidant. Thus, braking once more the triangle. Note also, if one is having to use that much CO² one is in danger of asphyxiation oneself. Aspro (talk) 14:42, 1 October 2017 (UTC)

I had thought that CO₂ extinguishers had a secondary cooling effect, but this article says: "Danger - This type of extinguisher does not cool the fire very well and you need to watch that the fire does not start up again". Alansplodge (talk) 15:06, 1 October 2017 (UTC)

Really? The only way the heat dissipates in this scenario is by radiation, and the cold CO2 has negligible effect on it? Then I guess in my scenario the cubicle wouldn't cool at all (because everything else around it is also on fire and glowing red-hot, so the heat would radiate into the cubicle as fast as it radiates away) -- so the firefighter would have to keep spraying it constantly just to keep it from reigniting, and as soon as the CO2 runs out it would catch on fire again! 2601:646:8E01:7E0B:D403:68F1:A297:C74A (talk) 01:15, 2 October 2017 (UTC)

Yes, CO₂ extinguishers are designed for use in the early stages of a fire, not for when the whole building is ablaze. Dbfirs 13:25, 2 October 2017 (UTC)

Mars in 2024

An article in WSJ Sept 30-Oct 1 pg B1 titled: "Musk's Mars Shot: To Red Planet by 2024." It means 7 years from now! Briefly, he proposes building a fleet of capsules larger than superjumbo airliners and sending them to Mars, perhaps even in 2022! He claims he figured how to pay for the project but it is not my concern. Does he think about radiation protection? This NASA website[1] is very vague about the current state of technology. No numbers are offered. At the end of the article they mention hydrogen-rich materials. It is water! So, will they fly to Mars in an aquarium? This Wikipedia article[2] gives a radiation comparison chart. The major point I want to make is what to do on Mars? It is a dead planet with no energy anywhere. The Sun is too far away, no tectonics, no wind energy. If they get there they need a nuclear power plant to bring with them. They need to generate heat since the place is so cold, sometimes below $100^{o}$ Centigrade[3]. OK, they got there and what is next? --AboutFace 22 (talk) 22:23, 1 October 2017 (UTC)

You would need to take a lot of water with you, in any case, both to consume directly (because recycling is never 100% and there's very little water to be had on Mars), and also it could be split to create hydrogen and oxygen. So, putting the water tanks on the outside of the crew compartment for additional radiation shielding might make sense. Of course, there would be far more fuel, at least most of the trip, so surrounding the crew quarters with that might make sense, too. This sounds extremely dangerous, in general, but in this case, if the fuel tanks bursts they are going to die anyway.

I believe solar power could still work on Mars. Sure, there's less sunlight at that distance, but there's also few clouds (except maybe an occasional dust storm) and no trees to block the light. Power production would be modest, of course. StuRat (talk) 22:40, 1 October 2017 (UTC)

Storing the water and other water-containing supplies around the outside of the living quarters as radiation shielding has indeed been a standard design feature in manned Mars mission vehicles for years. {The poster formerly known as 87.81.230.195} 2.217.210.199 (talk) 18:27, 2 October 2017 (UTC)

The history of rovers, Opportunity and Curiosity clearly showed that the energy that can be had from the Sun is infinitesimal. They are able to crawl a mile in a year or so and go into hibernation in the winter. --AboutFace 22 (talk) 00:07, 2 October 2017 (UTC)

Mars is between 1.4 and 1.7 times as far from the Sun as Earth is, so the solar insolation is between 1/2 and 1/3 that of Earth. I wouldn't call that "infinitesimal". It means a solar array has to be 2-3 times the area of one on Earth to generate the same power, although as StuRat says, the lack of clouds and the much thinner atmosphere may reduce that somewhat. The rovers needed to carry their solar arrays with them, limiting the possible size of the array. Even on Earth, solar powered cars are just barely possible with current technology, but stationary solar arrays can generate significant power. Martian dust storms shouldn't be discounted though; a dust storm can last for months and greatly reduce insolation. CodeTalker (talk) 00:41, 2 October 2017 (UTC)

The linked article says that a thinner atmosphere increases solar insolation - also a lack of clouds would be beneficial to a solar array, not detrimental. I think StuRat made that last point. 81.147.142.152 (talk) 09:15, 2 October 2017 (UTC)

Ah, I think that the poster is saying that the size of the array can be reduced in these circumstances. 81.147.142.152 (talk) 09:17, 2 October 2017 (UTC)

See The Martian - it is fiction, but I believe it has a high degree of technical accuracy. The protagonist (Watney) uses a combination of a RTG for heat and big solar panels + rechargeable batteries for electrical power. Gandalf61 (talk) 10:56, 2 October 2017 (UTC)

I want to know what they are going to do on Mars once they arrived. Any long term plans? --AboutFace 22 (talk) 10:49, 2 October 2017 (UTC)

There are several possibilities:

0) Since most plans I've seen involve pre-landing supplies with robotic ships, they would then need to go and collect those supplies. Supplies would need to continue to arrive afterwards, as a truly self-sustaining Mars colony would not be possible initially, and they would need to go collect those, too.

1) Dig an underground bunker, to protect from radiation (the thin atmosphere won't stop it all). Perhaps the lander can then be flown into the bunker and covered up.

2) Deploy a greenhouse to grow food.

3) Deploy a communications array to talk with Earth.

4) Deploy a solar panel array.

5) Keep a daily e-journal and send it home. This would be important to monitor their mental health, among other reasons.

6) They would need to regularly do medical self-exams. Hopefully, they could deal with some minor problems, like excising skin cancer, setting a broken bone, or pulling an infected tooth.

One thing I wouldn't think they would need to do is exploration, since unmanned rovers can do that, and they probably can't travel far to investigate anything interesting the rovers find, and such travel would be dangerous (exposure to radiation, space suit tears far from the base, etc.). Something else they wouldn't need is a latrine, since human waste would have water extracted from it then used as fertilizer. Also note that many tasks would take far longer than on Earth, if they involve donning a space suit first. StuRat (talk) 13:21, 2 October 2017 (UTC)

Its definitively doable. The odds are actually a lottery win in cosmic comparison. I am just an amateur fanboy for "space science" but, far as i have read and no matter the hunt for Exoplanets has only just recently started with Kepler (spacecraft) in 2009, it's already very clear by now, that there are not so many systems with 2 habitable Planets, not to speak of one of them being a paradise or 1000 trillion $ lottery win in sense of habitability. So its only a question of time till we send first settlers there. Maybe a Billionair wants his name in the history books, maybe politics starts a new race to space, maybe it takes another 100 years.. whatever.. it will happen. --Kharon (talk) 15:34, 2 October 2017 (UTC)

It's not all that difficult to keep a Martian base comfortably warm using only thermal energy from the environment. You need a heat pump and the heat pump can itself be powered by temperature differences in the environment, e.g. between the surface and below the surface. With only two heat reservoirs , you need an external power source to drive the heat pump, with 3 heat reservoirs at 3 different temperatures (the Martian base, the outside environment and the environment at some depth below the surface), that's not needed. Count Iblis (talk) 20:21, 2 October 2017 (UTC)

Or you could just put a bunch of mirrors on the hills and trust that neither wind nor Martians will steal them away. Wnt (talk) 22:25, 2 October 2017 (UTC)

Most of what @Kharon attributes to the future Martians can be accomplished here on Earth. In catacombs under the city of Rome or even in New York there is plenty of room to enjoy underground life. It is my understanding that there is no underground heat on Mars, there is no tectonics thus there is no heat. --AboutFace 22 (talk) 00:17, 3 October 2017 (UTC)

The sources of underground heat on a planet are:

1) Residual heat from the formation of the planet. Since Mars is thought to be as old as the solar system and is somewhat smaller than Earth, nearly all that heat would have dissipated to space by now.

2) Tidal heating. Since Mars has only a couple tiny moons, that won't be much there.

3) Radioactive decay. This should be the major source of heat on Mars, I suspect. Should have lots of heavy elements there to decay, mainly in the core.

4) Heat from sunlight that seeps underground. Sunlight will be less significant on Mars than Earth. StuRat (talk) 00:28, 3 October 2017 (UTC)

I notice that our Space-based solar power article says "A considerable fraction of incoming solar energy (55–60%) is lost on its way through the Earth's atmosphere by the effects of reflection and absorption". If accurate, and assuming that negligible solar energy is lost passing through Mars' thin atmosphere, that means that the available solar energy on Mars is practically the same as that on Earth's surface. It's actually slightly higher than on Earth when Mars is at perihelion (10% - 25% higher than on Earth) and slightly lower when Mars is at aphelion (15% - 25% lower). CodeTalker (talk) 03:21, 3 October 2017 (UTC)

True if talking about solar panels, but not if talking about heating Mars, because you have to take night into account, too. Earth's thick atmosphere acts as a blanket at night, especially when cloudy, while heat would radiated unencumbered into space at night on Mars. You can somewhat see an indication of this on Earth deserts, where the dry air doesn't hold in heat as well as elsewhere on Earth, and temps drop more dramatically at night. StuRat (talk) 05:47, 3 October 2017 (UTC)