Keeping things cold

[damauney]

Jibes (Dave)

One of the things I see is you are approaching it from strictly a physics standpoint. Understand with you as a ME. However from the chemistry (physical chemistry) standpoint which us mechanicals don't get a lot of exposure to energy of phase change is an important energy balance consideration. From the chemistry side the latent heat of fusion of melting ice has to be considered.

For me I think I will take two small glasses put an ice cube in each and sprinkle salt in one and watch both with an IR thermometer.

Goodwinds
DaveM

 

[tkanzler]

In an enclosed system, with perfect insulation, the energy content is fixed. Adding some warm salt adds a negligible bit of energy (due to the heat content of the additional mass at room temp), but the freezing point of the now salt water solution is lower, so some of the ice melts. The energy for the phase change has to come from the ice, both as sensible heat, and as latent heat. There is nowhere else for that energy to come from, so the temperature drops until equilibrium is reestablished.

It doesn't take much energy to change the temperature of a block of ice, compared to melting that ice, so the little bit of ice melting can make a big change in the temperature of the mass in that closed system.

Of course, in the real world heat is leaking in all the time, so it's a continuous process, with the rate of ice melting governed by the rate of heat transfer into the formerly closed system. The colder the interior of the space, the faster the heat will transfer into the space from outside the system, as temperature difference drives heat transfer, all other things being held constant.

So in that sense, making the interior colder will accelerate the rate of ice melting since the delta T is greater.

 

[jibes138]

Error of my ways

I stand corrected on the system temperature thing. No question at all the interior of the cooler will get colder at the expense of some of the ice melting with the rock salt addition. If the goal is to cool down a six pack of beer then this rock salt will be a wonderful addition.
The OP had requested how to maximize the length of time before the ice completely melts. Adding rock salt to cause the ice to melt is a bad thing. The way to meet the goals of the problem is to reduce the rate of heat transfer into the cooler (heat will always flow from high temperature to low temperature). Thus a higher "R" factor is what will satisfy the problem. So adding insulation to the outside of the cooler will help the ice last longer.
How about this idea to maximize the ice melting time. Put a cooler filled with ice and food inside another bigger cooler and fill the space between the two with ice. The lower delta T between the walls of the inner cooler will slow the heat transfer rate and keep the ice in the inner cooler longer? The ice in the bigger cooler will continue to melt at a constant 32 degrees until fully melted so the delta T can be maintained really low for a couple days. When that ice is fully melted you drain off the water and now have a dead air space between the two coolers giving a high R value to help slow heat transfer. So in a sense sacrifice the outer ice to keep the inner ice longer. Who thinks this will work? Might take a lot of space though.
My apologies for my error on the temperature situation to all the posters.

 

[Ross]

Re: Error of my ways

Jibes, Keeping ice from melting is as you say a matter of keeping the heat out of the icebox. I have 4 inches of extruded foam and it would keep cold with forty ponds of ice for a week but I got a lot of condensate. After I sealed the air leaks the condensation was reduced by about 90% and the same forty pounds of ice will keep cold for 10 days.
I have wrapped a frozen turkey in winter coats to allow it to thaw slowly and it was too effective it didn't thaw at all in four days.

 

[David W]

In order for ice to melt, heat has to be added to the system, with the system being the blob of ice and the space inside the cooler (and the contents). If the insulation was perfect, and no heat could sneak through it, the ice would absorb heat from the surroundings (air and cooler contents) and melt until equilibrium was reached. The ice would be at 32F, and so would the contents, and no more ice would melt.

Let some heat sneak into the system through the walls of the cooler, or by opening it and letting some warm air in, or by adding a warm object, and more ice melts, absorbing the heat that was added, and eventually restoring equilibrium at 32F (including the melt water, also at 32F).

If you add ice, the freezing point of the water is lowered, at least where the salt is (since salt water has a lower freezing point), and some more ice will melt with the heat to do that coming from the surroundings AND the ice blob itself, until equilibrium is restored. The drop in temperature comes from the phase change of the ice to water, and adding salt just changes the freezing point to a lower temperature.

Since the insulation is not perfect, there is a constant stream of heat leaking into the system, and the ice is constantly melting which absorbs that heat, and the system remains at equilibrium (dynamic), with the ice changing phase at whatever rated matches the heat input to the system. The only difference is the temperature of the ice where the salt has reduced the freezing point.

Would it make sense to make your ice from salt water as saltwater freezes at a lower temp so should stay frozen longer? Or have I missed the key points of the preceding thermodynamics lessons?

 

[Ross]

No because even seawater ice becomes fresh water when it freezes. Given just a few hours the salt will separate from the ice. One of the laws of thermodynamics requires just as much energy input to make ice as you get back when it melts.

 

[David W]

No because even seawater ice becomes fresh water when it freezes. Given just a few hours the salt will separate from the ice. One of the laws of thermodynamics requires just as much energy input to make ice as you get back when it melts.

Does this mean I can freeze salt water and once frozen, somehow remove the separated salt and the ice block will be fresh water when it thaws?

 

[justsomeguy]

Does this mean I can freeze salt water and once frozen, somehow remove the separated salt and the ice block will be fresh water when it thaws?

We're saved!

Yep, I'm wondering, too.

 

[tkanzler]

Put a cooler filled with ice and food inside another bigger cooler and fill the space between the two with ice. The lower delta T between the walls of the inner cooler will slow the heat transfer rate and keep the ice in the inner cooler longer? The ice in the bigger cooler will continue to melt at a constant 32 degrees until fully melted so the delta T can be maintained really low for a couple days. When that ice is fully melted you drain off the water and now have a dead air space between the two coolers giving a high R value to help slow heat transfer. So in a sense sacrifice the outer ice to keep the inner ice longer. Who thinks this will work? Might take a lot of space though.

That certainly would work to extend the life of the ice in the inner cooler, but putting the outer ice inside the inner cooler along with the ice already in there would work even better, as the total insulation R-value would be higher for all of the ice. The total time at the target temp (for the perishable contents) would be longest, but at the expense of space. Adding insulation of an efficient type to the outer cooler would probably be the best approach, so you could load more ice.

Sealing against humid air ingress should also make a noticeable difference, as another poster mentioned. It takes energy to condense the moisture out of that humid air, and that can only come from the ice (or rather, the ice absorbs the latent heat of the moist air, which results in more melted ice).

Latent heat is the reason an air conditioning system, when first turned on when it's really humid, will run for hours and not drop the air temperature in any significant way. It's condensing the moisture out of the air, which takes a LOT more energy than simply dropping the temperature of the air, the room's contents, and the interior structure (sensible heat).

 

[dawg2]

woodster,
Not looking for a war but I am struggling with what you are suggesting here. It is impossible for the addition of room temperature salt to make ice colder assuming no other input to the system. If I am interpreting what you are saying correctly this is the equivalent of a perpetual motion machine and defies the laws of thermodynamics. for the ice to get colder you must transfer heat from the ice. You can do this two ways, the first is put the ice in an environment colder than the ice already is, the second requires inputting energy to the system to force the transfer of heat opposite the direction it will transfer naturally, this is what a refridgerator, freezer, or air conditioner accomplishes. Heat will always transfer from hot to cold unless you input energy. What your rock salt probably does is melt the ice allowing the beer, ice cream, etc. to be surrounded by ice cold liquid that will transfer heat from the warm object much faster than will the air that would otherwise be surrounding the object where ice is not in direct contact.

In the "old days" they used rock salt and sawdust. The salt lowers the melting point.

http://antoine.frostburg.edu/chem/senese/101/solutions/faq/why-salt-cools-icewater.shtml

But to answer the OP I would buy the Coleman 5 day coolers. Use a coupole of them. Pack them more or less in the order you will use the items. This will keep you from constantly opening a cooler you won't need for for a day or two. Try to keep water drained off.