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Q & A: Why does a dryer dry wet cloth?

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Most recent answer: 05/18/2013
Q:
what causes a wet cloth become dry in the drier of a washing machine?
- priyanka (age 17)
chandigarh,India
A:

You know that water evaporates faster when it's hot. And you kinow that pulls energy out from somewhere, because you can feel your skin cool off when sweat evaporates.

Almost all dryers work by heating up the wet clothes. When they're hot there's more energy around to help water molecules break away from the liquid and go into the vapor. Then the vapor-filled air is usually carried away  out a vent, and replaced with drier air from outside

Usually before the dryer a washing machine will spin the clothes very fast, so that a lot of the water will be pulled out  of the clothes by centriifugal force. That takes less energy than drying by heating, but it doesn't get things dry enough because some of the water sticks to the cloth.

A new type of dryer is very efficient. It uses a heat pump, like the ones in refrigerators. Heat is pumped from some coils into the dryer. One joule of electrical energy can be used to pump several joules of heat energy. Then on the way out the vapor goes by the coils, where much of it condenses and then drains away. When it condenses, it returns much of the energy that it took to evaporate. That increases the efficiency even more.

Mike W.   posted without checking by Lee, until his return


(published on 05/18/2013)

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