How does liquid Helium/Nitrogen cool?

My understanding is that when a liquified gas expands back into a gas it cools down because energy is need for the expansion and this energy is supplied by the available heat in the nearby area, thereby causing cooling.

If this is the the case then I would assume that a tightly enclosed container holding liquid helium will not be cold but rather at the ambient temperature. The helium may be exerting a lot of pressure on the wall of the container but so long as no helium escapes the container will not be cold.

If my assumptions so far are correct how does immersing something like the infrared detectors used on telescopes in liquid helium cause them to be cooled unless there is a deliberate venting of the helium?

Am I missing something or are my assumptions correct?


That’s correct - the liquefied gas in gas bottles is liquid because of the pressure, not the temperature.

I don’t know about this but perhaps they do vent it - I think liquid gases are a consumable for some kinds of equipment such as MRI machines.
Alternatively, they might be using it in some sort of refrigeration cycle.

Whenever I’ve used liquid nitrogen to cool stuff (in a chemistry lab) it has always been in an open vessel. Hence, it doesn’t last that long before evaporating and you need to replenish it*. If you wanted continuous cooling I imagine you’d have some sort of evaporation/condensation cycle, like in a refrigerator.

  • And also make sure you have good ventilation, otherwise the large volume of nitrogen gas produced can displace the nearby oxygen, causing you to asphyxiate, or at least wake up on the floor a few minutes later.

No, that’s not true.
If it were, you could store cryogenic gasses in pressure containers, which you can’t, they need to be stored in insulated containers. If my memory serves, these gases won’t liquify at any pressure at room temperature.
Anyway, it’s obviously a liquid, because you can pour it just like water, and it doesn’t vaporize instantly.

Oops. I think you’re right - for other heavier gases, keeping them liquid under pressure at room temperature is practical/possible, but for helium and possibly some of the others, it’s not.

I found articles that suggested helium could be liquefied under pressure, but they’re talking about using diamond anvils to compress tiny amounts of it.

According to this page - 4He remains liquid at zero temperature if the pressure is below 2.5 MPa (approximately 25 atmospheres). This implies to me that if I cooled Helium down to a few K and then put it in a closed superstrong container when the container heated up to room temperature the pressure inside would increase but the Helium would remain liquid because of the increased pressure. Why would the container need to be insulated?

P.S. I am assuming that the container does not leak and is able to resist any pressure that the gas inside is exerting on it.

It’s been 25+ years since my last chemistry class, but if I remember correctly, this won’t work. What you will have is a super-dense gas, not a liquid. Don’t ask me to explain the difference (wait for the chem guys to pop by).

You’re mis-reading that. That’s not talking about the transition from liquid to gas, but the trasition from liquid to solid. That is to say, if you have a pressure below 25 atmospheres, you’ll never get helium to freeze, no matter how close to absolute zero you get. In principle, you could liquify helium and then allow it to warm up and evaporate, but you’d need a very hefty container to handle that kind of pressure.

As for the difference between a liquid and a gas, it’s the relationship between pressure and density (called the equation of state). For an ideal liquid, the density is constant, regardless of the pressure (of course, real liquids aren’t ideal, but their density will change only a small amount with pressure). For an ideal gas, meanwhile, the density is proportional to the pressure divided by the temperature. So if you keep a gas at a constant temperature in a piston, and increase the pressure on the piston, the gas will become more dense and shrink. Note, incidentally, that there are many other possible equations of state beyond just rho = constant and rho = constant*P/T, which means that there are many possible fluid states other than just liquid and gas. Those two equations of state just happen to very often be good approximations, for conditions like we see on Earth at human scales.

When I cool something with liquid nitrogen, I pour liquid nitrogen into the container until it no longer evaporates very quickly. By doing this, I cool the reaction to -196˚C (or very close). I suspect filled with liquified helium work very similarly. During filling, the container will vent gas like crazy. I’ve seen this when the NMR guy does a helium fill. By the time the helium is no longer venting, the cooling tank will be nearly at the temperature of liquified helium. Over the course of a week the helium will evaporate until it needs to be filled again. So yes, the helium will be constantly vented, but ideally not too quickly.

Infrared satellites become useless once they have vented the last of their helium coolant: satellite “out of coolant” helium.

In case there is any confusion: The venting of the helium isn’t there for cooling. In fact, it’s indicative of an inefficiency in the helium dewar’s insulation. The liquid helium sits in a vessel surrounded by near vacuum surrounded by another vessel filled with liquid nitrogen surrounded by near vacuum surrounded by the outer shell of the container. It a double Thermossup[/sup]-type setup. The idea is to have as little heat flow as possible, and the venting is there just because there will be some evaporation, and you have to let that gas escape. But it isn’t part of the cooling process.

Sure it’s part of the cooling process. It’s what makes the liquid cool down in the first place once the pressure is removed and it’s what keeps it cold afterward, to the extent that the insulation is imperfect. The venting cools the liquid by carrying off the “heat of vaporization”.

Well, I guess I wouldn’t define that as “cooling the liquid” since its temperature doesn’t change. My main point, though, is directed toward this part of the OP:

This isn’t the case, but it’s because of insulation, not venting. Venting is a real-world artifact. (Expansion may or may not be involved in the original cooling process, but that’s a separate task from cooling a working device. In practice, liquid helium arrives as a liquid and stays a liquid thanks to insulation, except for unfortunate losses which result in venting (and which do indeed cause ice crystals to form on the exhaust valves, but that’s not why the helium is cold.))

Different gases have a different critical temperature above which they cannot be liquified by pressure alone- above that temperature you get either a high pressure gas or a “supercritical fluid”, which isn’t the same as a liquid. It appears the OP is making an analogy between liquid helium and nitrogen and gases which can be liquified under pressure at room temperature, like propane or freon. I’d have to look up the critical temperatures for helium and nitrogen but I’m sure they’re well below room temperature.

My question should maybe have been ‘If I fill up a container of liquid nitrogen and perfectly seal this container will the container stay cold - yes or no?’ I would think not but rather it would heat up to room temperature and the pressure inside the container would increase. Whether or not the Nitrogen inside is a gas, a liquid or something else is immaterial to the observer on the outside.

If I heat this container up to 1000C surely the Nitrogen inside will be at 1000C and not at -196C. This theoretical container is infinitely strong and perfectly sealed.

I think I may have made a mistake in mentioning Helium which I know has some strange properties.

Thanks for the answers so far though.

If you build an unbreakable container and fill it with gas that is cooled to a liquid, then seal the container, it will gradually return to room temperature. The contents might do strange things, but it will not remain cold (because ‘cold’ is the potential to absorb heat and when things absorb heat, they stop being cold).

Gotta be careful about throwing around phrases like that. There’s no such thing as infinitely strong or sealed, so now it’s out of context of our universe and makes no sense and can’t be answered in a truly satisfying way.

I’m no physicist, but I think if you let the container full of liquid helium warm up to room temp, the pressure will, of course, rise. Depending on it’s construction, you may have a bomb on your hands. Or at least a projectile. Nature always finds the path of least resistance, and in this case, instead of breaking one of the laws of thermal dynamics, it’s easier to make it’s container go boom.

Now, someone correct my assumptions! :smiley:

It must be theoretically possible to make a container capable of containing the helium, even if you have to make it the size of a planet and use gravity to counteract the pressure (I doubt this is actually necessary). The pressure is finite, isn’t it?

It’s just PV = nRt, same as other gases.

The ideal gas law is only valid under some conditions.