Dust on...Dust-off?
May 7, 2015 5:03 PM   Subscribe

I just broke the nozzle on a can of Dust-off compressed gas. It is now stuck "on." Is this dangerous?

With summer now upon us in the Northeast (finally), it was time to bring the fans out of the basement and knock the dust off them. This involved a can of Dust-off, naturally, to get to some places a duster couldn't reach.

While sticking the long straw in, I broke the nozzle, leaving it stuck on "spray" mode, hissing at various speeds and volumes. The can got really cold... then I put it down on the deck and walked slowly away.

Now it seems less cold and has gone very quiet (though still a bit hissy). Can I assume this will eventually just run out and become inert/safe, at which point we can just throw it away?

Or should I worry that it will blow up?

Sorry, Scientificans of AskMe - I'm have no chemistry-physics-science brain of my own, so I have to rely on yours!

For reference, this has been going on for the length of a quick google search and then this post.
posted by kythuen to Home & Garden (11 answers total) 2 users marked this as a favorite
Not at all dangerous especially as it sounds like you've put it outside. Frostbite from the cold can would be the biggest risk.
posted by FishBike at 5:08 PM on May 7, 2015 [2 favorites]

Yeah, just put it in a well ventilated space (sounds like you have it outside?) and let it tire itself out. Much like you'd do with a toddler.

Then yes, you can just pitch it.
posted by phunniemee at 5:09 PM on May 7, 2015 [6 favorites]

Nah, it's fine. I'll post about the science in a bit, but it won't blow up.
posted by spelunkingplato at 5:10 PM on May 7, 2015 [1 favorite]

Response by poster: MANY THANKS, everyone! Fear comes from ignorance, etc! I feel lots better.

I'll leave it outside until it stops doing things on its own. And I'd love to hear about the science, spelunkingplato! What the hell is this stuff? How does it work?
posted by kythuen at 5:13 PM on May 7, 2015

Best answer: In general terms, it can be explained using something called the Ideal gas law:

PV = nRT

P is pressure, V is volume, n is a measure of how much gas you have, R is a special constant called the universal gas constant, and T is temperature.

Roughly speaking, your can is the size it is, so the volume stays the same. The amount of gas total you have stays the same, too (you're not adding any gas to the container--more on letting it out later). R is defined to stay the same. So since V, n, and R aren't going to change on us, let's think about what happens to this equation whenever you spray the gas.

When some gas leaves the container, the pressure inside the container goes down. We said that all of the other numbers stay the same. If the left side of the equation goes down (because P got smaller), the right side of the equation has to go down too, to keep the equation equal. So T has to go down: the temperature decreases, and your can gets colder.

Another way of thinking about it is like this: the gas inside the container has a certain amount of energy. The molecules of gas inside are all bumping around into each other and exchanging that energy back and forth. When you let some of the gas out, some of the energy escapes with it (it doesn't go away, it just goes into the room or the outdoors). So the rest of the gas still inside the container has a decreased amount of energy compared to what it did before. This is reflected in its temperature.

The ideal gas law is pretty cool and the relationships between pressure and volume and temperature in particular can be used in all kinds of interesting ways. Along with a lot of principles in thermodynamics, it's responsible for making things like combustion engines and refrigerators work.

Hope that helps! I'm sure someone else probably has more sciencing for you.
posted by spelunkingplato at 5:30 PM on May 7, 2015 [17 favorites]

Response by poster: That is a flat out awesome answer and helps a lot, spelunkingplato! Thanks for explaining it in such clear terms - amazingly I feel I kind of understand it now! :)
posted by kythuen at 5:40 PM on May 7, 2015

Best answer: Oh, and you also asked what the stuff was! It'll say on the can, but it's usually a refrigerant like R-134a, probably chosen for economical reasons. It's usually a substance which boils (that is, turns from liquid to gas) at a fairly low temperature and is convenient for the application. If you shake a full can you can sometimes hear the stuff swishing around, and if you turn the can upside down and spray it, you can sometimes get a droplet of liquid to come out which will quickly turn into gas. (That is, it'll absorb heat from its environment and get so much energy it'll change phases into vapor--same thing as when you boil water on the stove, only the heat just from the room is enough.) You might also notice the liquid droplets skating around: that's the Leidenfrost effect, which most people know from using water droplets to test if a pan is hot enough to make pancakes. :)

I'm glad you found the explanation helpful! It helps me to think about these things, and especially to practice explaining them to people (which reminds me how much I don't understand them, which drives me to understand them better). So, thank you!
posted by spelunkingplato at 6:01 PM on May 7, 2015 [2 favorites]

Best answer: spelunkingplato's explanation is spot on for a gaseous system, but not entirely correct for your can of compressed air.

Inside the can is actually a liquid, not a gas. As you release some of the pressure as described above, some of the liquid is converted into gas. It's this process - the conversion from liquid to gas (evaporation) that is most responsible for the can rapidly cooling. The existence of liquid in the can means the air pressure in the can remains relatively constant through the process - any gas which is lost is rapidly replaced through evaporation keeping the ideal gas law happy.

If the contents of the can were just air (and not something that would liquify under moderate pressure) the pressure required would be huge...making your little can of air very dangerous. Using a molecule that will liquify makes things much safer and allows for a single can to produce a lot of gas...a small amount of liquid is converted to a huge volume of gas...
posted by NoDef at 6:06 PM on May 7, 2015 [6 favorites]

Best answer: Ooh, that's a good point, NoDef. I think I got too caught up in the 'simple explanation' of the relationship between P and T. I knew some extra sciencing would be necessary! That is a more correct and nuanced explanation.
posted by spelunkingplato at 6:14 PM on May 7, 2015 [1 favorite]

Not sure I follow the explanation. Open the valve, some gas comes out. So how can n stay the same?

Seems like n goes down, which causes P to go down (V, R, and T constant). Some cooling effect of the gas happens from the Joule-Thomson effect, but that's in the expelled gas, not the can. When P goes down, more of the liquid evaporates, removing latent heat of vaporization from the rest of the liquid, cooling it, which makes the can cold from contact.

I very well could be wrong. I know bleeding off high pressure air through a throttle valve causes the pipe to frost up, and there's no liquid in there.
posted by ctmf at 7:27 PM on May 7, 2015 [1 favorite]

I've made a little lookup table for R134a that give you density at various temperatures and pressures.

I used Ref Prop, it's very cool and I can generate more graphs and data if that helps. I can also do fluids, air, water, etc...
posted by Confess, Fletch at 6:29 AM on May 8, 2015

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