In article HfDBi.13364$Yg.1832@trnddc02, M Q wrote:
Actually, the conductance of the ice is irrelevant.

I'm not sure I agree...

What is important
for removing heat is the phase change happening at the surface of the ice.

... because in order for that phase change to occur, you must first transfer
heat from the beer to the ice.

If you could keep the ice in perfect contact all around with the beer can,
you would not need the mel****er (and there would be no room for it).
As the ice is melting and changing shape, you cannot keep it in contact.

Certainly no disagreement there.

So now the question is: is it better to fill that gap with air or mel****er?

Water, clearly. Air (still air, at least) is an excellent thermal insulator.

The advantage of water: it is a better conductor of heat than air (although I
suspect that the greater effect is due to convention rather than
conductance).

Nope, it's due mostly to conduction. Air is a remarkably effective convector.
However, before any heat can be removed from the beer can by convection
(whether in water or air), it must be transferred from the can to the water or
air by conduction. This transfer is far more efficient with water.

A disadvantage of water might occur if you are trying to lower the temperature
below 32F using a salt. In that case you would be using some of your cooling
power to lower the temperature of earlier mel****er.

No, the mel****er would simply be at the lower temperature from the beginning.
I'm guessing you haven't made any more ice cream at home than Richard has. g

Also, the mel****er
would also better conduct heat from the walls of the vessel holding the mix.

So insulate the walls of the vessel.

Best to just drain the excess mel****er.

Define "excess". g The heat transfer will be most rapid in a bath of
icewater. If you don't have enough water to keep both the beer and the ice in
a water bath, then you don't have any excess.

--
Regards,
Doug Miller (alphageek at milmac dot com)

It's time to throw all their damned tea in the harbor again.