Ice is used in a cooler in order to cool its contents. Which of the following will speed up the process?
A) Wrap the ice in a metal foil
B) Drain the water from the cooler periodically
C) Put the ice as a single block
D) Crush the ice
Here are my arguments (maybe wrong, confused b/w the options):
For option A, wrapping with a metal foil would improve conduction.The air would get cooled by convection which I believe is slower.But even at the surface of the metal the heat exchange takes place through convection.
For option B, if the excess water is drained periodically, the ice would melt by heat transfer through air instead of the water cooling.But would this happen?Because the water and the ice are at the same temperature so no conduction?
For option C, I think its the worst idea among the given options.
Option D, crushing the ice will increase surface area but we would do work on the ice therefore partially melting it.
So what would be the most appropriate option?
Best Answer
You want large area of the ice to improve heat transfer. And you want good thermal contact between the ice and the contents of the box.
For this reason, crushing the ice and leaving the water in would be good. Gently stirring the contents would be the finishing touch (heat transfer by convection is much more efficient than by conduction, and the work done in stirring will be quite small compared to the benefits).
As for the other points: wrapping in foil limits convection -> bad. Single block of ice limits area -> bad. Draining the water means that the heat capacity of the convection medium (air instead of water) is lower -> less ability to carry heat from A to B, bad.
The point that crushing the ice does work is true but irrelevant. The question was about rate of cooling, not maximum final amount of heat extracted. Assuming that the contents of the box will end up at 0°C with some ice still solid, the work done will not materially change the outcome.