Colder Ice

[Jim Koepke]

Just a quick post to share an idea.

We fill old milk jugs with water and freeze them to use in our ice chest. If you do this, do not fill the jug all the way since the water expands when it freezes. Also leave the cap loose to allow air to escape as the water freezes.

We just recently finished a 5 liter jug of olive oil. The jug has about the same footprint as a gallon jug and fits in the ice chest standing up.

Ice tends to form on the top and works downward. This can distort a bottle even if the cap is left off. So my thought was to just put about two inches of water in the jug at a time.

Another thought came to mind. Why not add salt since salt water freezes at a lower temperature than fresh water?

So salt was added and the jug was shaken. Each time more water was added, more salt was added.

It got its first real test yesterday when we were at the farmer's market. It kept our other ice from melting throughout the day.

We also will put a couple inches of water in a plastic bottle and let it freeze and then fill with water to take with us for driving trips.

The next big test will be to see how "Mr. Salty" does on one of our shopping trips with ice cream.

jtk

[David Weaver]

I'm not sure I follow on the salt. If the freezer is 0 degrees, the salted ice will freeze at a lower temperature, but all ice in the freezer will approach the freezer temperature once it's been in there for a while.

The salt would cause the ice to then melt at a lower temperature once it's out of the freezer, which would allow you to have water lower than 32 degrees F (if your goal was to have something surrounded by the coldest liquid possible).

[Tom Stenzel]

I cribbed the numbers from Wikipedia:

It takes 1 BTU to raise a pound of water or ice 1 degree F.

To take a pound of ice from 0 degrees F to 32 degrees F would take about 32 BTUs.

To melt a pound of 32 degree ice to 32 degree water (phase change) takes about 144 BTUs. The majority of ice's cooling power is in the phase change.

Jim is shifting the melting point to a lower temperature although to total amount of BTUs absorbed in the melting process will be the same. Or maybe less with some of the water replaced by salt.

If I had to haul ice cream long distances in summer heat dry ice would be my first choice. But frozen brine will work too.

Jim, there will be a slightly greater temp differential between the inside of the cooler and the outside air. Make sure you have enough "freeze power" to keep the ice cream safe!

-Tom Stenzel

[Matt Schroeder]

Engineers, help me out here, but I think Jim may be on to something (although the effect may be small)...

Since the heat capacity of water (1 btu/lb) is greater than that of ice (about 0.43 btu/lb) by having water at a lower temperature is Jim's cooler going to have more heat absorbing capacity (cooling capacity) than one with an equal amount of pure water ice? Furthermore, since the phase change from ice to water will occur at a lower temperature, will the cooler stay colder longer?

Tom, I agree that the majority of the cooling power is in the phase change, so by shifting this phase change to a lower temperature will the heating of the items plateau at (for example) 20 F instead of 32 F? It will then start heating again from the 20 F after all the water has melted, but now we have a medium with a higher heat capacity (liquid water) for the run from 20 to 32 F than if we had pure water ice heating from 20 to 32 F before it melted. The heat capacities of liquid and solid water are different.

[Steve Jenkins]

Sort of off topic but an interesting fact about water. As it cools it gets denser until it gets to 38degrees F., where it is at it's most dense,then it starts to expand until it freezes. That is for fresh water.

[Jamie Schmitz]

Try adding some rubbing alcohol to the water.

[David Weaver]

Wow...it really does look like ice at -10 degrees celsius has 1/2 the heat capacity of liquid water at 0 degrees celsius.

I never knew that.

Presumably if you can keep water as a liquid, it will have better specific capacity? If that's true, based on the table on wikipedia, the heat capacity as a liquid is twice as good, so is it really as simple as saying brine would be able to provide twice the cooling between -10 and 0 celsius as ice if it were liquid the whole time?

Learn something new every day, there's always another variable!

[Steve Meliza]

Great idea Jim. I'm sure we all know to salt the ice in our ice cream makers, but applying that principle in new ways is pretty clever. Later this month I can test it out while camping.

[Brian Elfert]

The Mythbusters did a myth about the fastest way to cool down beer quite a while back. I think adding salt to make ice was something that worked well.

I'll have to try the salt water trick at some point. I don't like buying dry ice because it costs so much. To keep a small cooler of ice cream frozen for half a day I spent $15 on dry ice. I'm pretty sure the dry ice cost more than the ice cream.

[Larry Edgerton]

I worked at an ice house in Montana when I was a kid and we always added salt to the large blocks that went in the crusher. Freezes faster.

Larry

[Bob Coleman]

The mythbusters method worked not necessarily because of the salt, but I thought more so due to the fact that air is a really bad thermal conductor. So adding the salt allowed water in the bath to be at a lower temperature (the ice melted at a lower temp) so more of the can was in contact with water --> larger surface area for heat transfer --> colder. (This is a BIG deal in designing and operating boilers)

I don't recall if they investigated this aspect (ie, by continuously draining the water)

Did they try a fresh water bath also, or just a bucket of ice?

[David Weaver]

Bob, that's my understanding of cooling drinks, is regardless of the temperature, you want maximum cold surface touching the cans or bottles of whatever. We always add cold water to the ice if the ice hasn't already melted some. Of course, proper planning would allow us to not rush, but we had trouble with the proper planning part.

Same as above, if we were making ice cream, the way to do it was always to add salt to the ice.

The real key here, though, is the heat capacity of brine at negative celsius vs. ice at the same temperature. I'd never known that, but I never had to take natural sciences courses in college as a math student. (Always felt like I got away with something not having to go through the weed-out chem, bio and physics labs).

[Myk Rian]

I'm wondering if that jug was glass.
Bad idea.

[Jim Koepke]

Jim, there will be a slightly greater temp differential between the inside of the cooler and the outside air. Make sure you have enough "freeze power" to keep the ice cream safe!

My favorite way to keep ice cream safe involves the judicious use of a spoon.

I am good at saving ice cream from freezer burn.

jtk

[Larry Klaaren]

This was the principle of the old rotary ice cream freezer. You put salt in the ice water and it froze the ice cream but stayed liquid in the freezer bucket. You put the container in the middle and agitated it with a beater, the rotary handle also turned the conainer in the salt water. If you got tap water on the metal ice cream container (like if you rinsed it because the ice from the ice house was dirty), it would freeze on the side very quickly.

[Van Huskey]

In the end I think all the science can be simplified. Given two equal masses of water which have been turned to ice the one at the lower temperature will always absorb more heat energy before the closed system (in this case semi-closed if you will) reaches entropy. So if you produce ice with a high contaminant level so it freezes at a lower temperature it can asorb more heat and keep the contents of the cooler stabilized at a lower temperature until all the ice melts and the entropy of the system stabilizes, since the system is not truely closed once "entropy" is reached it will continue to rise in temparature until equal with the larger system it is in.

Bottom line mass for mass salt water ice will last longer than fresh water ice in a cooler.

[Kevin W Johnson]

The mythbusters method worked not necessarily because of the salt, but I thought more so due to the fact that air is a really bad thermal conductor. So adding the salt allowed water in the bath to be at a lower temperature (the ice melted at a lower temp) so more of the can was in contact with water --> larger surface area for heat transfer --> colder. (This is a BIG deal in designing and operating boilers)

I don't recall if they investigated this aspect (ie, by continuously draining the water)

Did they try a fresh water bath also, or just a bucket of ice?

It was ice water mixed with salt, not just ice with salt.

[Brian Elfert]

Does anyone know ice made with salt water will keep ice cream products frozen? I know dry ice is the real answer, but dry ice is not cheap and ice cream really gets rock hard from the low temps.

[Mark Patoka]

Interesting discussion. I'm going to give this a try with some of the milk jug ice blocks we make. How much salt do I need to add to 1 gal of water for maximum effectiveness?

[Jim Koepke]

How much salt do I need to add to 1 gal of water for maximum effectiveness?

No idea, I just added some salt each time water was added to the jug.

Most likely for a real answer we need someone schooled in thermal sciences.

jtk

[daniel lane]

You guys are killing me - I just spent an hour going through my reference texts. I have to get some work done, so I'm abandoning the search for now, but I'll mention that the most salt you can add is about 27% by weight, although this is temperature dependent. Also, thermal conductivity changes with salt concentration and temperature, but neither linearly. Here's a contour plot, you can see that around 160F is the max:

236505

And for fun, here's the contour for density:

236506

I know I'll keep playing just to see what I can figure out... *mumblegrumble*



daniel

[Bob Coleman]

Without actually looking anything up, and without doing anything crazy like making a supersaturated solution (which would be a fun experiment for a 5th grader) the best you can do is a brine (saturated salt solution), which freezes at 0F (which also makes a good way to calibrate your freezer dial)

I'd be surprised if the thermal conductivity change of adding more salt would make up for lowering the temperature.

[Belinda Barfield]

Without actually looking anything up, and without doing anything crazy like making a supersaturated solution (which would be a fun experiment for a 5th grader) the best you can do is a brine (saturated salt solution), which freezes at 0F (which also makes a good way to calibrate your freezer dial)

I'd be surprised if the thermal conductivity change of adding more salt would make up for lowering the temperature.

And in this instance I think we should all listen to a guy named Coleman. :D