Liquid Nitrogen Isn’t Suitable For Steam Engines

Liquid nitrogen is fun stuff to play with, as long as you’re careful and avoid freezing your own fingers off and shattering them on the workbench. As the liquid turns to gaseous nitrogen at around -196 C, [The Action Lab] figured that it could be used to propel a simple steam engine at room temperature. Testing this out had amusing results.

The device under test is a Hero’s Engine, otherwise known as an aeolipile. This consists of a hollow sphere filled with water, fitted with a series of nozzles that shoot out steam when the vessel is heated. Via the rocket principle, this causes the device to rotate about its axis.

When filled with water and heated with a candle, the aeolipile spun at up to 2520 RPM. [The Action Lab] next tested it filled with water in a vacuum chamber, with the low pressure causing the water to boil at room temperature. The effect was less impressive however, with the engine spinning at a much slower rate.

The best result was with liquid nitrogen inside the engine. With the nitrogen quickly boiling at room temperature, the aeolipile quickly spun up to a great speed. The engine stand had to be steadied to avoid it tipping over, before the seal at the top of the engine blew off from overpressure.

We’d love to see the same experiment done with a piston-type steam engine, too. Video after the break.

29 thoughts on “Liquid Nitrogen Isn’t Suitable For Steam Engines

  1. When filled with water and heated with a candle, the aeolipile spun at up to 2520 RPM
    With the nitrogen quickly boiling at room temperature, the aeolipile quickly spun up to a great speed.

    *facepalm

  2. This is a classic highschool experiment, but normally you use a ping-pong ball with holes poked in it at an angle. Heat it up over a bunsen burner before putting it into the liquid nitrogen and it’ll pull LN2 into the ball, then spin as it boils off.

      1. I think it’s a rather sophisticated troll. Any kind of ping pong ball will melt or deflagrate quickly in the flame of a bunsen burner.
        Sadly, there are also very boring ABS ping pong balls on the market that just smoke and stink when you burn them.

          1. You can boil water in a plastic cup on open flames without melting the cup. Water inside will regulate the temperature to a maximum of 100ºC and plastic melts at a higher temperatura

  3. I once saw this demonstrated at the Royal Institution with a piston steam engine, and a “boiler” filled with liquid nitrogen being “stoked” with ice cubes in water. This was back in the 1960s with Bill Coates demonstrating.

  4. Use less liquid nitrogen. This is the equivalent of using water and heating it too much. Since it doesn’t make sense to cool the room, just feed it LN at a slower rate to stop overpressurizing. This is actually very dangerous because if the seal doesn’t fail in a controlled way, you’re creating a bomb.

  5. fun stuff i just wish PPL would stop propagating Hollywood myths about LN2…. “Liquid nitrogen is fun stuff to play with, as long as you’re careful and avoid freezing your own fingers off and shattering them on the workbench.” freezing yes shattering of limbs and extremities not so much except if you REALLY are trying to…

    1. The real danger with liquid nitrogen is if there’s not enough ventilation. Depending on the amounts you use the size of the room and the ventilation, it’s possible to build up enough nitrogen content in the air to suffocate yourself. The scariest part of this is you won’t feel like you’re suffocating so it’s probable you will not realize that it’s becoming a dangerous environment. You’ll suffer the symptoms of oxygen deprivation which includes impaired judgment hence why this can be quite dangerous.

      1. i fully agree although dizziness is usually also a symptom and a dead giveaway that your ventilation might be inadequate. i was just referencing to the myth that LN2 will instantly freeze everything it touches and that everything frozen by LN2 will become so brittle that they can be shattered easily both of which is not true.

  6. I’m old, so when I was a kid classroom science demos were more fun. Our school district had a sort of center for them, and the guy who ran it did indeed run a toy piston steam engine by filling the boiler with liquid nitrogen. He also used to throw it at the audience.

    He also had some demo with mercury on a (covered) watch glass reacting with something… violently enough to make it bounce around. I have no idea what that produced…

    1. Interesting question. I believe the answer is “no”, but I can’t produce a very concise explanation for it. It’s just that material entering an area of lower pressure doesn’t cause the kind of reaction you get when material exits an area of higher pressure. The material entering is coming from all directions, and immediately replaced by material behind it, whereas the material exiting is all focused in one direction. Also, you can have lots of pressure greater than atmospheric, whereas you can’t have less than 0 pressure (below atmospheric).

      1. My gut feel is yes, if you could get rid of (enough) friction. My vacuum cleaner hose pulls quite strongly as it approaches a surface. I can’t imaging that force is zero when away from a surface but rather is just very small.

    2. Martin Gardner spent a lot of time talking about this in one of his books, although in his case it was a related version: what happens if you put a spinning sprinkler in a swimming pool and draw a vacuum on the hose? Does it spin? How fast?
      Much of the answer has to do with the vector of the mass movement: when it’s expelling water, the water exits at high velocity in one direction, but when it’s admitting water, the mass reacts against the inside of the duct, so if it’s not shaped fairly carefully you don’t get the same vector, and as a result, less force applied.

  7. Well, there is some serious lab wisdom that says never pull a vacuum on LN2. It will evaporate so quickly that it will freeze solid starting at the surface. Depending on the container, it can plug the top followed by an explosion. You can wreck a lot of stuff.

  8. What you described is pretty much the ‘Dearman Engine’. The Dearman engine is a bit more efficient and runs longer before freezing by also injecting warm water.

  9. “ We’d love to see the same experiment done with a piston-type steam engine, too.”
    Oh god, someone’s going to get maimed. Action Lab is lucky his didn’t go off like a grenade (saved by the full port!). There’s a reason LN2 is moved around in open containers despite the risks if dropped… pressure reliefs have a nasty habit of freezing up with atmospheric ice. Depending on how full the container is, the nitrogen will reach thousands of atmospheres (about 3000atm if full) if required in order to break the container. A steam boiler is good at heat transfer and not designed for de-icing. Also piston steam engines are much better at blocking the flow than aeliopile jets.
    Don’t fill your Mamod with LN2

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