How Does a Vacuum Flask Keep Drinks Hot? Blocking Heat Three Ways at Once
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Heat leaves a cup of coffee in three ways, and a vacuum flask blocks each of them in turn. It conducts through the cup's walls into the hand; it convects, carried away by the air and steam rising from the surface; and it radiates, as infrared light that every warm object emits. A flask is a bottle inside a bottle with nothing between them, not even air, which stops the first two, and the walls are silvered like a mirror, which stops the third. The device was invented in 1892 by a Scottish chemist for keeping liquid gases cold, and he never patented it, which is why two German glassblowers made the fortune and gave it the name on every lunchbox.
Three ways heat moves
Conduction is heat passing through a material by the jostling of its atoms, fast in metal and slow in wood, plastic or still air. Convection is heat carried by a moving fluid, the warm air over a radiator rising and drawing cool air in, or the coffee's own vapour lifting off its surface. Radiation is heat travelling as electromagnetic waves, mostly infrared, which needs no material at all and is how the Sun warms the Earth across empty space. An ordinary mug loses heat by all three, and an insulated mug with a foam jacket slows conduction and convection but not radiation, which is why it works less well than a flask.
The vacuum and the silver
The flask is two vessels, one inside the other, joined only at the neck, with the air pumped out of the space between them. A vacuum cannot conduct heat, since there are no atoms to pass it along, and it cannot convect, since there is nothing to move; the only path left for conduction is the thin neck where the two walls join, which is made as narrow and as poorly conducting as possible. Radiation crosses a vacuum without difficulty, so the facing surfaces of the two walls are coated with a reflective layer, silver in the original glass flasks and polished steel or copper in modern ones, which reflects the infrared back toward the liquid instead of absorbing and re-emitting it. Each measure attacks one route:
- •Vacuum between the walls: blocks conduction and convection
- •Reflective coating on the walls: blocks radiation, reflecting up to 95 percent of it
- •Narrow neck of thin, low-conductivity material: minimises the one solid path for conduction
- •Stopper of cork or hollow plastic: blocks conduction and convection at the opening, which is where most of the remaining heat escapes
Who invented it
James Dewar, a Scottish chemist at the Royal Institution in London, built the first vacuum-jacketed vessel in 1892 to store the liquid oxygen and, later, the liquid hydrogen he was producing at temperatures near minus 250 degrees, which boiled away in minutes in any ordinary container; the flask kept them for days, and the laboratory version is still called a Dewar. He did not patent it. Two German glassblowers, Reinhold Burger and Albert Aschenbrenner, who had made flasks for him, saw the domestic possibility, patented a version with a protective metal case in 1903, and sold the rights to a company that held a competition to name it; the winner was Thermos, from the Greek for hot, and the trade mark became so general that the American courts declared it a common word in 1963. Dewar sued and lost, and the man who invented the vacuum flask is remembered by physicists and the men who cased it by everyone else.
How well it works
A good steel flask keeps a litre of coffee above 60 degrees for about twelve hours and cold water cold for a day, and the loss is mostly through the stopper and the neck, which is why a full flask keeps better than a half-empty one, since a smaller share of the heat is near the top, and why preheating the flask with boiling water before filling it, so that the inner wall does not steal heat from the drink, makes a measurable difference. Glass flasks insulate slightly better than steel and break; steel ones survive a dropped rucksack. The same physics runs the tanks that carry liquefied natural gas across oceans, the vessels that hold the liquid helium cooling the magnets of an MRI scanner, and the cryogenic storage of sperm, embryos and vaccines, which are Dewars scaled up and down, and the flask in a coat pocket on a cold morning is the laboratory apparatus of 1892 keeping tea warm.
The takeaway
A vacuum flask is a double-walled vessel with the air removed from between the walls, so that heat cannot conduct or convect across the gap, and with the walls silvered so that the infrared the liquid radiates is reflected back; the neck and stopper are made narrow and poorly conducting to close the last routes. James Dewar built it in 1892 for storing liquefied gases and did not patent it, the German glassblowers who cased it named it Thermos, and the same design holds liquid helium, natural gas and frozen embryos.