I learned, 6/17/16

This edition answers a question that likely bothers no one but me: Why are soda dispensers in restaurants so damn big?

Soda machines are, by volume, mostly ice bins. The soda syrup and carbonated water are either tucked in back or below the counter; the bulk of what you are looking at is a giant hopper for storing and dispensing ice.

So why store all that ice? Why can’t it be made and dispensed in real time?

Being science nerdy, I ran the numbers. The answer is physics, specifically the giant amount of energy that it takes to freeze water. Water is famous for having both a huge heat capacity – a large amount of energy is required to heat or cool it. And it has an even larger “heat of fusion” – the energy that goes into organizing the water molecules into little ice crystals.

If you wanted to make ice “on demand”, you have to supply a *spectacular* amount of power. To fill up a 20 oz cup from “on demand” ice as fast as you can from the hopper would require about 100 kilowatts of energy. That’s big time.

How much is 100 kilowatts? More than your house draws. With that kind of power you could run 100 hair dryers simultaneously. You could charge your iPhone 6 in 200 milliseconds. You could open your own Tesla Supercharger station.

Note that the number is power (“how fast”), not energy (“how much”). It’s another way of saying that the slow step in making ice cubes is not the cubing part. It’s getting the electrons into the ice maker.

In practice, when you pour all of the power from a conventional 120V, 20A circuit into making ice, it will blast out cubes at a maximum rate of about 1 cube every five seconds. Since most machines don’t draw the maximum power avaiable, it’s more like 1 cube every 10-20 seconds. To get around the power issue, restaurants generate ice throughout the day and night, and store it in the giant hopper behind the soda dispenser. Soda machines are big because if you tried to make them smaller, you’d melt all the wiring in the restaurant.

I’m probably alone in this, but I think that’s kind of cool. Something to think about at lunch tomorrow.

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