The power of steam

By Marcus Wilson 18/01/2013

Over Christmas, we were staying with my wife’s sister and her family in Dunedin. Early one morning (sometime before I got up, anyway – that is my definition of early) a loud ‘bang’ came from the direction of the kitchen, followed by the sound of eight paws beating a hasty retreat. There are two young cats in that house, and my  first thought was ‘pesky felines – I wonder what they’ve done now’. And I went back to sleep.

As it turned out, the cats were entirely innocent. The culprit was the coffee pot (now ex-coffee pot). It was a stove-top machine – one where water goes in the bottom half, the coffee is placed in a holder and inserted on top of the water, and the top half of the pot (which will contain the finished product) is screwed on. Whether there was too much coffee in it, or it was packed in too tightly, or some other problem, we don’t know, but clearly the steam made in the bottom wasn’t finding its way to the top. Instead, as the water boiled, the pressure inside the bottom half just kept increasing, until there was a mechanical failure and the top half departed from the bottom half in a hurry.

The bang I heard was the top half hitting the underside of the range hood at high speed. The dent in the latter was sizable. There was also a trail of coffee up the wall above the stove, and onto the ceiling, where coffee grinds were pasted in place. And splatterings of coffee were in every cup, box, drawer, container and cupboard that had the misfortune to be in the kitchen at the time.

It was rather fortunate that no-one was near it at the time. Beware the ideal gas law: Pressure times volume is proportional to temperature. In this case, with a constant volume, high temperature implies high pressure, and there is only so much pressure a coffee-pot can take.



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