Sunday, July 25, 2010

Why asking people to turn up the heat is a bad motivational analogy

With boiling water comes steam, and bad motivational analogies. After two of them it perhaps is time to contact the Analogy Police, for a warning that next time they will take away your poetic license.

On December 30, 2009 I blogged about how the inspirational book 212 Degrees: The Extra Degree was where inspirational vapor clashes with reality. That book confused temperature and the amount of heat required to boil water. At 2:54 in their accompanying video there also is a call to action which says that:

“You are responsible for your results. And it’s time to turn up the heat.”

That statement may seem obvious if your experience with heat transfer is li
mited to watching pots or teakettles boiling on your kitchen stove. But, it is not how the boiling heat transfer process always works. Chemical engineers, mechanical engineers, and metallurgists all know better.

When you look at the section on boiling heat transfer in the Wikipedia entry under heat transfer you will see that there are two very different types of boiling. The more familiar one is called nucleate boiling, but there also is another one called film boiling.

If you keep turning up the heat in an industrial boiler you eventually change over to film boiling. During film boiling there is a blanket of vapor covering the surface which insulates it from the liquid and greatly reduces heat transfer. You can see film boiling on your stove at home if you put a drop of water on a hot pancake griddle. When it is hot enough for the drop to dance around without evaporating quickly, then you are watching film boiling. Film boiling also is discussed in the Wikipedia entry under the Leidenfrost Effect. A more technical discussion appears in one section of the Wolverine Tube Handbook. So, once again inspirational vapor clashes with reality.

My father taught chemical engineering for twenty years. When I was a teenager he told me a story about how film boiling could seem completely counterintuitive. Back in the 1930s he was hired by a glue company who wanted to cut costs and increase production. He looked at their process, and told them to turn DOWN the heat. They were skeptical until he told them why that would hel
p. After they did their glue production soared. Their warehouse began to fill up until their salesmen managed to get more customers.

Asking people to turn up the heat sounds pretty silly when peak daytime temperatures currently are hitting above 100 degrees. Maybe it is time for a big glass of iced tea instead.

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