On Thu, 06 Jul 2006 16:41:41 GMT, cavelamb
wrote:

I think the wet ice is acting as insulation to protect the food stuff from
the dry ice - depending on how close stuff is to the dry stuff.

That is exactly the right way to use dry ice in a cooler box.
Otherwise there are problems with things that come in direct contact
such as beverage cans which will quick freeze and rupture.

The one exception is carboard juice boxes which don't seem to mind
being frozen, and actually make an excellent ice substitute if frozen
in advance.

Brian Whatcott wrote:
On Sun, 02 Jul 2006 08:31:58 -0400, Jeff wrote:

Chris wrote:
For water, the heat of fusion is about 80 times specific heat...
So -70 C ice would have almost twice the cooling effect
as the same amount of barely frozen ice, right?
Wrong. You should look up the Heat Capacity of cold ice. Its only
0.5 BTU/lb-degree at freezing, but it goes down so that by -50 F its
only 0.4.

Since the Heat of Fusion is 144 BTU/lb, sub-cooling even 100 degrees
only adds a small amount of cooling capacity.

Hardly 'hadly effective'.
No, its hardly effective.

This is in error: using old CGS units
heat for fusion of ice is 80 cal/gm specific heat cap near 0degC is
1 cal/gm

Supercool to -40 deg C and its worth roughly another 40% of cooling
power cf. ice at freezing.

Brian Whatcott Altus OK

Sorry about the late reply - I've been out sailing.

You're making a common mistake. Although the specific heat of water
is 1 calorie per g-deg C, for ice its only about half that, or .5 cal
per g-deg C, or as I stated .5 BTU/lb-deg F. Thus, super cooling ice
add little cooling capacity.

GBM wrote:
"Jeff" wrote in message
Sorry about the late reply - I've been out sailing.

You're making a common mistake. Although the specific heat of water
is 1 calorie per g-deg C, for ice its only about half that, or .5 cal
per g-deg C, or as I stated .5 BTU/lb-deg F. Thus, super cooling ice
add little cooling capacity.

So what happens as the ice heats up from -100F to 32F? The heat capacity
changes as the temperature changes.

I agree though, that there is not much benefit.

It may be better to freeze and cool something else. For example, eutectic
solutions (plates) -see - http://www.epsltd.co.uk/eutecticmain.htm.

We have refrigeration in our icebox, but find that it hardly comes on for
the first day out, because we put frozen freezer packs in the box as well as
a supply of ice cubes (in a container) and frozen drinking water in bottles.

Properties of ice in SI units! :

http://www.engineeringtoolbox.com/ic...ies-d_576.html


Cold plates are certainly convenient, but the cooling capacity is not
as good as plain old ice. The ice packs made of water are generally
the best. You'll notice that the formulation with the best numbers
happens to be very similar to h2o.

http://www.epsltd.co.uk/PlusICE%20Table%20Sept-05.pdf

Of course, if your goal is to keep food frozen, you need something
different.

On Sat, 08 Jul 2006 20:06:58 -0400, Jeff wrote:



This is in error: using old CGS units
heat for fusion of ice is 80 cal/gm specific heat cap near 0degC is
1 cal/gm

Supercool to -40 deg C and its worth roughly another 40% of cooling
power cf. ice at freezing.

Brian Whatcott Altus OK

Sorry about the late reply - I've been out sailing.

You're making a common mistake. Although the specific heat of water
is 1 calorie per g-deg C, for ice its only about half that, or .5 cal
per g-deg C, or as I stated .5 BTU/lb-deg F. Thus, super cooling ice
add little cooling capacity.


This time I checked a little more carefully among the welter of old
American Customary, CGS old scientific and SI units people have been
applying.
First, agree that ice has half the specific heat capacity of water
and agree that fusion of ice takes 80 cal/gm or 80 X 4.2 J/gm or
80 x 4.2 x 1000 J/kg
so -40 degC to melting point provides 20 cal or 20 X 4.2 J/gm or
20 X 4.2 X 1000 J/kg
and
ice to water provides 80 cal/gm or 80 X 4.2 J/gm or 80 X 4.2 X 1000
J/kg

The ratio in question is ( 80 + 20 ) / 80 = 125%
(NOT 140%)

The extra 25% of cooling effect may qualify as "little" or
"appreciable". You choose! :-)

Regards
Brian Whatcott Altus OK

Wayne.B wrote:
On Thu, 06 Jul 2006 16:41:41 GMT, cavelamb
wrote:


I think the wet ice is acting as insulation to protect the food stuff from
the dry ice - depending on how close stuff is to the dry stuff.


That is exactly the right way to use dry ice in a cooler box.
Otherwise there are problems with things that come in direct contact
such as beverage cans which will quick freeze and rupture.

The one exception is carboard juice boxes which don't seem to mind
being frozen, and actually make an excellent ice substitute if frozen
in advance.


doh!