mitch@cepu.UUCP (Bob Mitchell ) (05/08/85)
No metaphysical BS here!
The situation: A styrofoam ice chest containing beer and a large block of ice.
The problem: How to make the ice last for as long as possible.
The question: As the ice melts, should the water be drained from the cooler?
If the block of ice is allowed to float in the water, will it melt faster,
or will the volume of cold water keep the overall temperature lower, and
thus help the ice last longer?
I can see a few obvious things that would help, like starting out with
cold beer instead of warm, and it's clear that a solid block of ice is
better than cubes (smaller surface area?), but I can't decide about the
meltwater.
I'll be testing your theories in a few weeks, so no quick answers!
--
Bob Mitchell
UCLA Dept of Neurology
uucp: { {ihnp4, uiucdcs}!bradley, hao, trwrb}!cepu!mitch
ARPA: cepu!mitch@ucla-cslwall@sdcrdcf.UUCP (Larry Wall) (05/09/85)
In article <463@cepu.UUCP> mitch@cepu.UUCP (Bob Mitchell (ADM)) writes: > >The situation: A styrofoam ice chest containing beer and a large block of ice. > >The problem: How to make the ice last for as long as possible. > >The question: As the ice melts, should the water be drained from the cooler? First an assumption: that the ice, air and water in the ice chest are about the same temperature, so that it is ONLY heat transfer across the inner surface of the styrofoam that matters, as long as the ice chest remains closed. In favor of leaving the water in: You're removing a certain number of "negative btu" in the cold water, which must be replaced by warm air. A counterflow heat exchanger would be of minimal value, since a given volume of water contains so much more heat (cool?) than a corresponding volume of air, and the exchange of air and water would be equal in volume. In favor of taking the water out: Water picks up heat from a surface much more quickly than does air, at least in the case of 98.6F vs. 32F. (I.e. you'd last much longer standing naked in a snowstorm than swimming in the Arctic.) I suspect this also holds for a small temperature difference. So the ice box would lose more "cool" across the water-styrofoam interface than across the air-styrofoam interface. I'll leave the nitty-gritty physics of this to the physicists, but I suspect it has something to do with the fact that there's more specific heat within easy conduction distance at the water boundary. There is, of course, a compromise solution. Leave the water in, but don't let it touch the styrofoam. A heavy plastic bag might do nicely. Of course, there is some insulating value in the plastic bag, so the assumption stated above doesn't necessarily hold. But given the way the problem was stated, the correct solution to the problem is pointed at by our plastic bag. The problem was only to make the ice last as long as possible, but no mention was made that the beer had to be kept ice-cold. If you thicken up that plastic bag a little, and add a few "R-values" to it, the ice will last even longer, but the beer won't be as cold. The way to have the ice last longest is to put the most insulation around it, separating it from the beer, which of course defeats the purpose of putting the beer in the ice chest in the first place, but you didn't say you wanted cold beer. :-) Do I win the prize? Another way of insulating the ice might be to keep it up out of the water, since there is in fact some temperature difference at the ice-air or ice-water boundary, or the ice would never melt. Of course, putting the block of ice up on top of the beer makes the beer hard to get at, and the water beneath (and the beer) will be slightly warmer than ice-cold. >...and it's clear that a solid block of ice is >better than cubes (smaller surface area?)... Not entirely clear. It's only true if the extra surface area contributes to greater heat flow from the non-ice to the ice. If the ice cubes were in a plastic bag, convection of air or water through the "core" would be largely prevented, and the temperature of the air or water in the middle would remain very close to freezing. Larry Wall {allegra,burdvax,cbosgd,hplabs,ihnp4,sdcsvax}!sdcrdcf!lwall
hes@ecsvax.UUCP (Henry Schaffer) (05/13/85)
I agree with the people who have said there is a trade-off between more heat transfer across a water-styrofoam interface vs an air-styrofoam interface and the loss of cold if the water is drained off. But I don't think that adding insulation is a proper answer - since the question was about presence vs. absence of the water. I think the ice-cubes are worse if less mass of ice can be put in the chest (packing problems) than in a single cube. Otherwise I don't see any difference. --henry schaffer n c state univ