So…the physics building in which I work was shut down for the day due to a frozen helium dewar in one of the basement labs, turning it into a potential explosion hazard. I felt compelled to tell you (and by “you”, I mean anyone that gives a crap enough to keep reading, especially through this parenthetical statement) why a frozen dewar sucks pretty bad.
First off, what is a dewar? It’s basically a super Thermos that is used for storing cryogenic liquids, such as liquid nitrogen, oxygen, or helium, for use in low-temperature experiments. Here’s a helium dewar in my lab; we’re using it to cool down a cryostat so that we can perform superconductivity experiments.
The basic construction is an inner and outer reservoir, separated by vacuum. The thing is, even though there is vacuum inside and the reservoirs don’t physically touch, heat can still convect through the gap. This is why coffee in a Thermos…
…wait, I’m not drinking coffee right now. BRB…
Ok…crisis abated. Anyway, this is why coffee in a Thermos doesn’t stay warm forever. Convection of heat is not nearly as effiecent as conduction of heat (grab some hot metal LOL), but given time, it will transfer the heat from the coffee to the outside world. In this case, heat is transferred from the outside world into the cryogenic liquid in the dewar. Of course, when you heat up a liquid beyond the boiling point, it phase changes into a gas. In your coffee thermos, this isn’t really an issue, because even if you put cold liquid inside, the temperature outside is far below the boiling point of water (hopefully).
The problem with liquid helium is that it has a boiling point of 4.2K. That’s cold. Very cold. 4.2K is about -450 °F. Clouds of gas in interstellar space are 5 times warmer than this. Saying it’s cold doesn’t really get the point across. I’ve had the experience of pumping liquid helium into a device (see the above picture and check out the top of the cryostat) and the effect of being near it is basically the opposite of the experience of pre-heating the oven to 400, opening the door, and putting your face in. This poses several issues.
First, the ambient temperature of the room the dewar is in can easily supply enough heat through convection to boil the helium. So, unlike a Thermos, a dewar requires safety valves that can vent the gas that builds up inside. The other problem is that helium is so cold it will freeze the air (think about that for a minute) and build up ice in the above mentioned valves, blocking them.
That’s what happened in the physics building yesterday. A dewar of 100L of helium had been delivered to a lab on Wednesday. When they went to insert the pumping apparatus into the top of the dewar, ice had completely clogged the intake. They opened the safety valves to vent the helium gas that was certainly building up, but they were also iced up. The dewar was basically a giant dry-ice bomb waiting to explode. If you’ve ever made a dry-ice bomb and seen the devestation created there-by, imagine doing that with 100L of helium in a giant steel cylinder that is about 3 feet in diameter and 6 feet tall. It would be bad. In fact, don’t imagine it, allow me to describe it!
How Bad Could It Be?
There are several things at play here. We have a gas in a container that is building up pressure. When that pressure reaches some critical value, the vessel will burst, releasing 100L of liquid helium. Needless to say, it won’t stay liquid and will immediately expand into gas, releasing energy in the process. The question is, how much energy?
First, we have to think about latent heat. Latent heat is the energy it takes to make a substance complete a change of phase. For instance, if you have a block of ice at, say, -20 °C, you can add energy to warm it up to 0 °C, the melting point of water (under normal conditions). However, as you add more heat and melt the ice, the water/ice mixture will stay at 0 ºC until you melt all of the ice. Only then will the water, if you continue to add heat, increase in temperature until you reach the boiling point. The energy you added served to change the solid ice into liquid water and the amount required to do this is called the latent heat of fusion. Consequently, you would have to remove that energy to freeze water back into ice; the nifty science word for freezing is fusion, btw. In similar fashion, and assuming you could contain it, as you boil the water at 100 °C and create steam, the water/steam mixture will remain at that temperature until all of the water has been vaporized; this energy is called the latent heat of vaporization.
As you may imagine, the vaporization energy is much higher than the fusion energy. The heat of fusion for water is 334 kJ/kg whereas the heat of vaporization is 2260 kJ/kg, almost 8 times more! This is why steam sucks way more to be burned by; when it hits your skin and condenses, it puts all of that energy into you. Also, ice “feels” cold because, when you hold it, the energy of fusion is pulled out of your skin to melt the ice. SCIENCE!!
A bit of a side note…what is a kJ? The metric unit of energy used in science is the Joule, which has the symbol J. So, kJ is a kilojoule, or 1000 joules. But that probably doesn’t help, because most people are familiar with calories. A calorie is just over 4J. But (and this is really stupid), the calorie you’re probably familiar with is the one listed on food. You’ve probably never noticed, but the word “calorie” is always capitolaized. This is because “food calories” are actually kilocalories, not just calories. So, our 2000 Cal/day diet is actually 2,000,000 calories/day, which is about 8,200,000 J/day.
Back to the helium dewar. The latent heat of liquid helium and normal pressure is 20.3 kJ/kg and it turns out that 100L of liquid helium has a mass of about 12.5 kg. So, the process of turning 100L of liquid helium into gas would require about 254 kJ of energy. This is about the same as the number of calories in a single Oreo cookie.
The problem is not the heat, though you have to realize that that energy would be drawn out of the room into the helium; that’s what makes it feel so cold. The problem is that gaseous helium has a much greater volume than liquid helium. In fact, it will expand by a factor of 748 times. So, the 100L, which is 0.1 m³, will expand to fill a volume of about 75 m³ when released. Over a long period, this doesn’t matter. However, if the dewar ruptured, the helium would boiled off almost instantly and expand very quickly. Turns out, the energy released by the gas expanding from 0.1 m³ to 75 m³ is about 7,820,000 J. This is approximately the amount of energy released when detonating 4 sticks of dynamite.
That would be very bad. To make things worse, this doesn’t really take into account the energy released in actually rupturing the dewar, which would increase the above energy by about a factor of 10. If that dewar exploded in the basement, it would be akin to detonating 40 sticks of dynamite. That makes for a very bad day for all parties involved, particular the poor bastards in the room with it. Even if they managed to survive the blast, all of the air would be replaced with helium and they would asphyxiate. That’s why hazmat was called and the building was evacuated…and we were happy to leave. Not to mention the damage to the lab; if you need liquid helium, you can assume that what you’re putting into probably cost at least a million dollars.
That’s why cryogen dewars are kind of a big deal when they freeze up. As an added bonus, you now know that the energy intake you need everyday to run your body is equivalent to detonating 4 sticks of dynamite. Science rules!