Why Is There Liquid Nitrogen On the Street Corner?

Any NYC hackers may have noticed something a bit odd this summer while taking a walk… Giant tanks of the Liquid Nitrogen have been popping up around the city.

There are hoses that go from the tanks to manholes. They’re releasing the liquid nitrogen somewhere… Are they freezing sewer alligators? Fighting the Teenage Mutant Ninja Turtles? Or perhaps, cooling our phone lines??

Luckily, we now have an answer. Popular Science writer [Rebecca Harrington] got to investigate it as part of her job. As it turns out, the liquid nitrogen is being used to pressurize the cables carrying our precious phone and internet service in NYC. The cables have a protective sheath covering them, but during construction and repairs, the steam build up in some of the sewers can be too much for them — so they use liquid nitrogen expanding into gas to supplement the pressurized cables in order to keep the them dry. As the liquid nitrogen boils away, it expands 175 times which helps keep moisture out of the cables.

Sounds expensive, but apparently liquid nitrogen was the cheapest option. Helium is way too precious, liquid oxygen is combustible, and carbon dioxide is also more expensive than nitrogen. Who would have thought.

As for the public’s reaction to these tanks?

While the warning signs on the tanks may seem alarming, [Bonomo] said they have never exploded or significantly leaked in the city, but they have tipped over on occasion when a car has backed into them, for instance. The cans are so sturdy that toppling hasn’t caused any leaks, [Diachok] said, and Verizon technicians are able to simply stand them back up.

Did we mention the tanks also randomly release pressure by blasting liquid nitrogen into the air? It’s like a little poof of snow on a hot summer’s day…

In the future when everything is run through fiber optics we won’t have to waste liquid nitrogen on silly things like this. Then we can get back to making our own liquid nitrogen at home in order to super-cool our Arduinos!

108 thoughts on “Why Is There Liquid Nitrogen On the Street Corner?

      1. nitrogen is basically a waste product of air liquification, since there is so much of it….and since there is demand for the oxygen and the noble gases, it ends up being very cheap, CO2 has to be made, capturing it out of the atmosphere would be ridiculously expensive…

          1. Thank you. I guess that finally explains (to me) why I never hear people b*tching about carbon emissions and the availability of dry ice in wal-mart/grocery stores.

  1. They are also frequently used to freeze water pipes. When they replace water meters going to larger buildings they often don’t have valves between main line and the meeter. Instead of shutting down water to a large area they freeze pipe before the meeter.

      1. Yet you wrote “Liquid Nitrogen have been popping up around the city.”… popping up… since 25Y. You make it sound like they are just recently added what is not the case.

      2. It’s more startling to find these parked in a field in the great beige beyond of the Midwest. Nitrogen-breathing alligators in the sewers of Manhattan I can understand since I used to live there, but high-pressure cornfields, not so much. To be fair, they usually use compressed-gas tanks rather than liquefied gas Dewars out here.

          1. See them all the time in the Clear Lake area. I’ve often wondered if people knew what dewars were worth, if they wouldn’t vanish like copper wire does.

    1. I only know NYC from TV shows and from videos, and oddly enough in neither of those I ever noticed them.

      Oddly too I don’t recall seeing them in other cities, and you’d think modern infrastructure would be similar. But perhaps they don’t just put them on the street like that in other places. I know you don’t want to do it in my area, punks will mess with it.

  2. ” it expands 175 times”. Nope. Nitrogen occupies about *650* times more volume as a gas at room temperature than it does as a liquid. Those Dewars are 200-250 litres in size; they contain enough nitrogen to completely fill a typical New York apartment, asphyxiating everyone inside. Don’t try that at home.

      1. It’s the 21% oxygen that’s the important part. Displace that with the nitrogen and you won’t even notice yourself dying. So don’t bring one of those shiny cans home to your cozy apartment. You might wake up dead.

        True, they normally vent only 2% of their contents per day, so in the absence of gross stupidity the likelihood of an oxygen-free apartment is low But who can resist the party trick of blasting LN2 on the floor and watching the dust bunnies in the fog? Or putting a hundred watts into a 1-watt LED submerged in the liquid (bright!) and blowing off billows of fog? Or cooling your beer in ten seconds?

        1. You don’t have to replace much of that 21% of oxygen to cause problems, a couple percent in either direction will do it.

          As I once discovered, it’s possible to hyperventilate without realising it and getting more oxygen then you’re supposed to. Your body starts doing weird things, and it hurts!

          The cure is to breathe into a paper bag for a while.

          1. It’s impossible to get “too much oxygen” by hyperventilating ordinary air. What happens when you hyperventilate is you remove carbon dioxide from your body. CO2 regulates blood flow: too much CO2 causes vasodilation and increased blood flow (to remove the excess CO2). Too little CO2 in your bloodstream causes vasoconstriction and reduced blood flow, particularly in the brain. It’s trivial to change the blood flow in the brain by a factor of two just by changing your breathing pattern (I have published journal articles in “Stroke” and “Medicine & Science in Sports & Exercise” characterizing the effect). So, paradoxically, hyperventilation can cause you to pass out due to too little oxygen delivery to the brain.

            And “a couple of percent in either direction” is barely perceptible. Unless you are exercising you won’t notice until the partial pressure of oxygen approaches 15% of atmospheric. There is no real O2 fraction upper limit at atmospheric pressure, though there may be some long-term toxicity effects (hours-days).

          2. Paul: That is absolutely amazing. I’ve always avoided learning much about the human body as it just reminds me of how squishy and feeble we are, but that is really interesting.

          3. It is no health problem to breath 100% oxygen as long it is NOT under increased pressure or for very prolonged periods. Somewhere between a partial pressure of 1,4bar and 2,8 bar the problems start, depending on exposure time.

          1. I used to work for a home-health company that did LOX fills (they all do) curbside for people that used LOX instead of O2 concentrators or compressed cylinders. Preventing fire was a big deal for us.

            Interesting note, we were required to report any unintended discharge of LOX. If you spilled it on asphalt you were supposed to wait around for half an hour to make sure nobody provided an ignition source. Asphalt is a great fuel, it turns out.

            Also, we had to warn people against petroleum jelly based products. Sucks to have your face burn.

  3. Just plastic tie-wrapped to a light pole? This seems very …very temporary at best (I’m being polite). A permanent installation would involve a right-of-way cabinet with a AC power, a compressor and air dryer. Since in NYC every square foot of sidewalk is precious, build the compressor into a box that forms the base of a lamp post. Thinking this through… the air is so dirty next to the road that the filtering would require constant maintenance, replacement or cleaning. If we accept that the phone company has lots of experience and has tried every which way to deal with this… here we are with Nitrogen tanks.

  4. This is interesting. I’m wondering why the cables are not pressurised all the time from the CO. Here in France, the “ILEC” (France Telecom, now Orange) has huge compressors in every CO to pressurize the cables, since a long time.

  5. Liquid nitrogen in bulk is something like 6 cents a liter. It’s not like it’s hard to find nitrogen, and you just need some processing to separate out the other gasses and to liquify it. Electricity and air in, liquid nitrogen out.

    What is /actually/ expensive in this process is the flasks themselves and handling. Refills for the big dewars are more like $1-$2/liter, and the dewars themselves are a few thousand dollars each and you have to have them to get the $1-2 rate, so you have to be doing something worth the equipment cost of the dewar to work with it cheap. Maintaining the pumps to pressurize for the occasional case really probably /would/ cost a lot more.

        1. “all the LN2 you want for free” is kind of like owning a cow to get all the milk you want for free… You still need to buy it, store it and feed it and dispose of it when it’s worn out. In the end, unless you’re selling the product it’s cheaper to just buy it.

          1. But you can’t buy raw milk in many places in the US, if that’s what you are after. Raw milk is more regulated than most things you can kill yourself with.

          2. No, because you don’t need to buy a cow to get LN2. Nitrogen is all around you for the taking. All you need is the equipment, as nitrogen does not need to be produced by anything. Milk, on the other hand, requires a cow to be produced, or more often, many cows.

          3. Yes, because a cow is a system that makes milk.

            Nitrogen liquifier = black box that takes in air and produces liquid nitrogen.

            Cow = brown sphere that takes in feed and produces liquid milk.

            Both require lots of energy. Both require capital investment and a place to store them. Both need to be disposed of at the end of their useful lives.

            The only difference is that one (with a little help) can make more of itself.

    1. I ran an MRI lab ~15 years ago. We bought LN2 and LHe in 500L Dewars. We called nitrogen being the same price as beer, and helium being the same as wine. (At the time, and in Canada, this was $1-2 per litre and $10-20 per litre respectively, depending on quantity and phase of the moon)

    2. I pay about 40 cents a liter when I fill my 50L dewar and this is actually quite a discount for around here (portland, OR). First places I called wanted well over $80 fill it.

  6. This has been a practice for 30+ years. Typically it’s used here to dry out the mega pair cables that have paper wrapped over the insulated wires and underneath the PVC jackets to keep the moisture out. I always though how much fun I could have taking one of those Dewers home….

  7. It’s used to keep the subterranean superconducting maglev transit system for the 1% running smoothly. You wouldn’t want to slow down their commute from the Hamptons to Wall St, now, would you?

  8. “As the liquid nitrogen boils away, it expands 175 times which helps keep moisture out of the cables.”
    HAD, more misinformation than you can handle. As a broadcast engineer we also use nitrogen to purge moisture from our sealed transmission lines to the antenna. Its not that a gas can expand as all compressed gasses can and do expand. Its because when expanded and warmed, nitrogen ( think low cost gas ) has room for the moisture in between the gas molecules. As the moisture is absorbed we purge the gas and the absorbed moisture from the antenna end through a pressure relief valve. Same thing for the phone cables.

  9. These dewars are actually a fairly standard size, they aren’t ‘huge’. They are composed of an inner tank and outer insulated shield. Also, they don’t release ‘liquid nitrogen’, they release gaseous nitrogen to relieve pressure. Releasing liquid for pressure release would be wasteful and dangerous.

    1. These are 180L dewars, so yes, they are a pretty common size. They have multiple tap valves for vapor or liquid. There is typically only one “dip tube” valve for liquid, and one to three valves for vapor (depending on dewar manufacturer).

  10. Is it just me? This looks so unsafe. If a car accident ruptured a cylinder then even if people didn’t have limbs frozen off then they would die from oxygen deprivation as the air is displaced by gaseous nitrogen.

    1. Cars? Have you ever been in Manhattan? The cab drivers are some of the best drivers I have ever seen, and most of the time traffic moves too slow to rupture one of these. The insulation layer acts as a shock absorber.
      And, if you will notice, they are attached behind a lamp post to protect them.

      1. Lol never been there. To me a risk assessment is just as much about calculating out the risk outcomes as it is about analysing mitigating factors.

        How much of a risk does a cylinder (or two) pose to human breathing by air displacement if ruptured?

        I have seen what happens when someone thinks “Hey, wouldn’t it be cool to put liquid nitrogen into the swimming pool” – the people in the pool pass out and if you don’t pull them away from the pool very quickly they will drown.

        1. Asphyxiation is a very small probability in this particular case. The tall building tend to funnel wind and the air would be replaced in less than a minute.

          It is like saying Times Square is a breathing hazard due to lack of oxygen on New Years eve because of the number of people concentrated in one area.

        2. Even if all the dewars ruptured near a subway entrance / exit, the forced air system in the subway would turn over the air mass fast enough to prevent problems to the average person.

    2. The cylinders are surprisingly tough. Side ruptures are unlikely. The danger is more from knocking the top off. It would vent all in one direction, and evaporate quickly.

      Not sure if it would become a rocket like compressed cylinders do. I doubt it. Though?

  11. Pretty sure that if a tank contains liquid nitrogen, it has to be marked as LIQUID NITROGEN and not simple NITROGEN. These tanks are filled with ordinary compressed nitrogen gas, not liquid nitrogen.

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