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Linde Air Liquefaction
Snoopywv (Wikimedia Commons) · CC BY-SA 3.0
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20. សីហា 2026DK
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Linde Air Liquefaction

How to make air cold enough to pour. Air becomes a liquid at about -195 °C, and no ordinary refrigerator gets anywhere near that. Carl von Linde's answer in 1895 was to use the gas's own expansion to cool itself, and then to use that cooled gas to pre-cool the gas arriving behind it — a feedback loop that walks the temperature down in steps until the air condenses. Compress, cool, expand through a valve, and send the chilled gas back around the incoming stream. William Hampson in England patented essentially the same cycle the same year, which is why it is often called the Hampson-Linde cycle. Liquid air made industrial oxygen and nitrogen cheap, and with them cheap welding, steelmaking and eventually the ammonia industry. This is documented plant, not a bench build: the pressures and temperatures are far outside what a workshop can hold.
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1 hour

ការណែនាំ

1

Feel the effect you are about to industrialise

The Joule-Thomson effect is not exotic. You can feel it in a minute.

  1. Pump a bicycle tyre and touch the pump barrel — compression warms the gas.
  2. Let air out of a full tyre across your hand — expansion cools it.
This is the entire physical basis of the plant. Note it is a small effect: a single expansion drops the temperature by only a few degrees. Getting from room temperature to -195 °C by this route needs the trick in the next step.

Materials for this step:

Instant-Read ThermometerInstant-Read Thermometer1 ដុំ
2

Regenerative counter-current cooling

The cooled gas is not thrown away. It is used to chill the gas coming in behind it.

  1. Compressed air passes down through a heat exchanger.
  2. It expands through a throttle valve and cools.
  3. The cold expanded gas flows back up around the incoming pipe, chilling it.
  4. The next portion therefore starts colder, and expands to colder still.
The cycle bootstraps. Each pass lowers the starting temperature, so a few degrees per expansion compounds down to liquefaction. Linde's real contribution was this counter-current arrangement, not the discovery of the cooling effect itself.
3

The inversion temperature — and why hydrogen surprised everyone

Throttling only cools a gas if it starts below its inversion temperature.

  1. Above that temperature, expansion warms the gas instead.
  2. Air is comfortably below its inversion temperature at room temperature, so the cycle works directly.
  3. Hydrogen and helium are not — they must be pre-cooled first or the machine heats them up.
This caught early experimenters out. James Dewar had to pre-cool hydrogen with liquid air before he could liquefy it in 1898, and helium waited until Kamerlingh Onnes in 1908.
4

Separating the components

Liquid air is a mixture, and the parts boil at different temperatures.

  1. Nitrogen boils at -196 °C.
  2. Argon at -186 °C.
  3. Oxygen at -183 °C.

Fractional distillation of the liquid separates them.

Nitrogen boils off first, so it comes out of the top of the column and oxygen collects below. That 13-degree gap between nitrogen and oxygen is the entire basis of the industrial gas industry.
5

History and context

Carl von Linde was a professor of engineering in Munich who had already built a successful refrigeration business on ammonia compression before turning to air. His air-liquefaction patent dates from 1895. William Hampson, working independently in England, filed for a very similar regenerative cycle in the same year — the two are close enough that the cycle is commonly credited to both.

What it unlocked: cheap oxygen. Oxy-acetylene welding and cutting, the oxygen used to blow steel, and the nitrogen used as an inert blanket in industry all begin here. Liquid nitrogen became a routine laboratory coolant. When Haber and Bosch needed enormous quantities of pure nitrogen for ammonia synthesis, air separation is where it came from.

Georges Claude improved the cycle a few years later by expanding the gas through an engine that does external work, which cools it much more effectively than a plain throttle. Modern plants use expansion turbines on the same principle.

Why there is no build here. Reaching -195 °C requires sustained high-pressure compression and a heat exchanger with very low losses; a leak of high-pressure gas or a cold-embrittled fitting is genuinely dangerous, and liquid oxygen in contact with oil or grease is an explosion hazard. The Joule-Thomson step in this blueprint is real and safe to feel; the plant that multiplies it is not something to attempt.

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