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Does wrapping a beer in a wet paper towel increase the rate of cooling

by axiom1
Tags: beer, cooling, increase, paper, rate, towel, wrapping
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axiom1
#1
Feb13-13, 12:37 AM
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It is commonly said (based on a quick Google search) that wrapping your beer in a wet paper towel and placing it in the freezer will allow it to cool faster than placing it in the freezer without said paper towel.

Newton's Law of Cooling suggests otherwise. The fact that water releases energy when it freezes (the paper towel tends to freeze) suggests otherwise (although this effect is probably minimal). Specific heat capacity doesn't seem to be relevant here because the water is initially above the temperature of the air in the freezer (so it is likely detrimental). Perhaps it could be that water has better "thermal conductivity" (is that even a thing?) but then its still just an intermediate layer between the air and the glass, so we'd still be bound by the rate at which the air cools the water.

Anyway, I'd love to hear a definitive answer to I can put an end to this (likely) urban myth.

Thanks.
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NascentOxygen
#2
Feb13-13, 01:57 AM
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Hi axiom1.

I'd be pleased to give you my thoughts on this, but I'm hesitant to do so as it sounds suspiciously like a science question that you are supposed to think about yourself.
axiom1
#3
Feb13-13, 02:11 AM
P: 10
Ummm, this isn't a homework problem if that's what you're implying.

The physics of cooling beer just happen to be of great interest to me.

I guess I went overboard in trying to make the question sound professional.

Dadface
#4
Feb13-13, 02:15 AM
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Does wrapping a beer in a wet paper towel increase the rate of cooling

Why not try it out as an experiment first? You can drink the results.
axiom1
#5
Feb13-13, 02:20 AM
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As a mathematician I've always preferred theory to experiment. But regardless, the experiment is underway, but I'd rather just hear a good theoretical argument.
willem2
#6
Feb13-13, 02:38 AM
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I think it's supposed to work by evaporation. The air in the freezer is probably 100% saturated at -18C, but the water (or ice) will rapidly evaporate while it's still at a temperature above this. The temperature of the outside of the bottle will probably rapidly decrease to 0C, and this will take longer without a towel, because the air around the bottle doesn't conduct heat well.
Freezing of the water produces heat, but evaporation uses up about 8 times the amount of heat that freezing produces.
axiom1
#7
Feb13-13, 03:25 PM
P: 10
So you're saying the cooling you get through evaporation is enough to account for the fact that the water is initially warmer than the surrounding air (which decreases initial rate of cooling) and the release of latent heat when some of the water on the towel freezes (which tends to happen from my own experience).

And what do you mean by "air doesn't conduct heat well"? What is it about air that makes it a worse conductor?
Drakkith
#8
Feb13-13, 03:31 PM
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Quote Quote by axiom1 View Post
And what do you mean by "air doesn't conduct heat well"? What is it about air that makes it a worse conductor?
Air is a gas and as such isn't very dense. Compared to many solids and liquids it simply doesn't conduct heat as well.

From here: http://en.wikipedia.org/wiki/Heat_conduction

Fluids (and especially gases) are less conductive. This is due to the large distance between atoms in a gas: fewer collisions between atoms means less conduction. Conductivity of gases increases with temperature. Conductivity increases with increasing pressure from vacuum up to a critical point that the density of the gas is such that molecules of the gas may be expected to collide with each other before they transfer heat from one surface to another. After this point conductivity increases only slightly with increasing pressure and density.
axiom1
#9
Feb13-13, 03:44 PM
P: 10
Ok. So even if the air in the freezer doesn't conduct heat as well as water then that doesn't make the towel better, because the towel still has to be cooled by the surrounding air. In other words, the air is the still the weakest link and is the limiting factor here.

The only thing that seems to make sense here is evaporative cooling and I'm not convinced its significant enough to make a noticeable difference. There's not much water on the towel, and even if it gets cooled very fast by evaporation it will eventually reach a steady state (no more evaporative cooling) and we are again bound by the conductivity of the air.
Drakkith
#10
Feb13-13, 03:52 PM
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Quote Quote by axiom1 View Post
Ok. So even if the air in the freezer doesn't conduct heat as well as water then that doesn't make the towel better, because the towel still has to be cooled by the surrounding air. In other words, the air is the still the weakest link and is the limiting factor here.
Not true. The towel will have more surface area than the can does. (I think it does at least) This allows more air to be in contact with it and increases the total amount of heat transferred into the air over time.
axiom1
#11
Feb13-13, 03:57 PM
P: 10
How much surface area are we talking here?

The paper towel adds some small amount of radius to this cylinder-like structure, so 2*pi*r*L changes insignificantly (but this is assuming the surface is smooth).

Is the surface of the paper towel full of little "folds" (crevasses, ridges, etc) while the surface of the glass is comparatively smooth? If so, that would make sense.
Drakkith
#12
Feb13-13, 04:02 PM
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Quote Quote by axiom1 View Post
How much surface area are we talking here?

The paper towel adds some small amount of radius to this cylinder-like structure, so 2*pi*r*L changes insignificantly (but this is assuming the surface is smooth).

Is the surface of the paper towel full of little "folds" (crevasses, ridges, etc) while the surface of the glass is comparatively smooth? If so, that would make sense.
That is how I am seeing it.
ModusPwnd
#13
Feb13-13, 04:13 PM
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I just got a vision of an invention. An express beer cooler that is a cylindrical heat sink with a can/bottle shape in the center to slide your can/bottle into. And then fins sticking out.

The ambient air (in the fridge) will be cooler than the heat sink initially. But since the heat sink has higher thermal conductivity there will be some temperature greater than ambient which cools the can/bottle quicker than ambient. Just like the wet rag.

Better still, precool the heat sink and you will get the fastest cooling.

Cool thread! Experiments are certainly in order. It would probably be ok to open one and shove a thermometer into the can's center.
axiom1
#14
Feb13-13, 04:16 PM
P: 10
Just to clarify some of the theory.

If the increase in surface area was negligible, would the towel still cool faster?

My guess is no, because we are still limited by how quickly the air cools the towel (which in turn cools the bottle).

And another question, what about a dry towel? Would it have good thermal conductivity? I'm assuming air pockets would form underneath it, defeating the purpose of it.
davenn
#15
Feb13-13, 04:36 PM
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Quote Quote by axiom1 View Post
...........
And another question, what about a dry towel? Would it have good thermal conductivity? I'm assuming air pockets would form underneath it, defeating the purpose of it.
being dry... its full of tiny airpockets producing a good insulator and I would expect it to be worse than if the can was in the freezer uncovered

think of the big bulky jackets used by skiers, mountain climbers or those working in the polar regions. They are poor in heat transfer because they are full of air between the threads and filling of the internal fabric

Dave
axiom1
#16
Feb13-13, 04:52 PM
P: 10
What about a hypothetical thin layer of water surrounding the bottle (that stays put).

The surface area isn't changed considerably.

Would it be better than having no such layer? It seems that the effect of the water "cancels out" because we are still limited by the conductivity of air (which has to cool the water).
axiom1
#17
Feb14-13, 11:04 AM
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Could someone address my last question?

In general can you improve performance by inserting an intermediate layer with better conductivity assuming surface area is unchanged?
Algr
#18
Feb14-13, 04:17 PM
P: 152
But surely the metal of the can is a better conductor of heat then a towel. The additional service area of the wet towel would be defeated by the extra volume that needed to be cooled. Also, the conductivity of the air is irrelevant if the air is being blown around by the freezer fan. I'd always figured that the best thing to do is to have the can in as much contact with cold surfaces as possible.


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