Why does putting a wet paper towel around a can in the freezer cool it so fast?



Why does putting a wet paper towel around a can in the freezer cool it so fast?

In: Physics

The water transfers cold into the can faster than air. Or, said more correctly, the water draws heat out of the can and into the surrounding (freezer) air faster than the aluminum is able to do it on it’s own.

Think about it like this. If you have a wet paper towel that’s been sitting out on the counter all day (so it’s room temperature), and you put your hand on it, does it feel colder, warmer or the same as putting your hand on the (dry) counter or a room temperature soda can? It feels colder because it can extract the heat from your body faster. Same reason a marble kitchen table or leather couch feels cold even though they’re the same temperature as everything else.

There are two types of heat transfer: conduction and convection.

Convection is when an air current goes past something of a different temperature; think of a cool breeze or a warm clothes dryer.

Conduction is when two masses are in direct contact with each other. Think of putting your hand in cold water or accidentally touching a hot pot on the stove. Conduction is MUCH more powerful.

Now air, by itself, is actually a pretty good insulator; it doesn’t absorb much energy (low thermal mass). A can in the freezer is limited in how much energy it can transfer to the air right next to it. (A blower would help, replacing air that has been slightly warmed by the can with air that hasn’t.). Wrap it it a wet towel, though, and suddenly the can is exchanging heat with something which can absorb a LOT more energy.

That’s enough to explain why a can gets cool quickly when immersed in icy cold water (and why the cans in the slushy bottom of the insulated bin are coldest, as the gaps between the ice cubes are cold water and not air) but doesn’t fill out the whole picture.

So, the next question is where the energy goes from the towel. That’s explained by evaporative cooling — when water evaporates, it carries off the energy it takes to transform water into water vapor.

So, you get conduction between the can and the towel, and a mixture of convection and evaporative cooling between the towel and the freezer air.

As /u/Alexis_J_M comment has already explained, it does cooling by evaporation.
There is one more factor: A fridge contains quite dry air, because water in the air would freeze out. The dryness makes the water from the towel to evaporate faster. Evaporation needs heat, and the heat is taken from the can.