Block Ice
Submitted: Sunday, Mar 20, 2005 at 22:11
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Nick R
Gudday travellers,
going to do some block ice for cooling the essentials, beer and meat. Mrs JohnR says she adds salt to the water so it is more difficult to melt and keeps stuff cooler.
Is there foundation to this theory?
I know that salt water freezes at a cooler temperature but I'm not sure that is has any more thermal mass.
If the theory has foundation what is the amount of salt suggested to use?
Regards,
NickR
Reply By: Nudenut - Monday, Mar 21, 2005 at 08:10
Monday, Mar 21, 2005 at 08:10
adding salt does not give any more thermal mass
fresh water ice melts at 0C
salt water ice melts at a temp lower than 0C...the temp dependant on amount of salt.
Water or solution in esky increases thermal gains to ice thus melting it quicker...
So if you want ice to last longer use fesh water and keep draining the water out
AnswerID:
103247
Reply By: Pedro14 - Monday, Mar 21, 2005 at 12:27
Monday, Mar 21, 2005 at 12:27
G'day Nick R,
A professional fisherman I know uses
rock salt on top of the ice which is then covered with a wet towel or two.
This is to retain ice in a separate esky pending later use in his main esky.
He says the ice lasts longer this way.
What I do even at
home is to attach a piece of garden hose to a hose attachment the male click on part (13m) and insert that in the plug hole of esky.
By placing a paver under esky thhis drains the water progressively, which in turn reduces the melting process , as mentioned by nudenut and others.
Pedro
AnswerID:
103274
Reply By: basecamp15 - Monday, Mar 21, 2005 at 21:16
Monday, Mar 21, 2005 at 21:16
Freezing salt water will definitely give you a colder esky. Salted ice melts at a lower temperature. What I did once was prepare a layer of block ice for the bottom out of saltwater. On top of this I put normal feshwater ice. As the warmer freshwater ice melted, it dribbled on the saltwater blocks and refroze. Take it from me, I'm a chemist.
Watch out though. Saltwater in your car is not a good thing, is it really worth it?
I did my trick on a fishing trip so no worries about leakage or whatever but I wouldn't do it if the esky was in a car. A mate of
mine checked out a ute for sale that had rust in one corner of the tray from this.
Hope this helps.
Cheers, Mark
PS as for how much, experiment. Dissolve as much as you can in normal temp water and see if your
home freezer can freeze it. Sometimes it takes several days to freeze. If it does, you're on a winner.
AnswerID:
103368
Follow Up By: shaggy - Tuesday, Mar 22, 2005 at 10:04
Tuesday, Mar 22, 2005 at 10:04
As a chemist, you should know that all the salt in the water does is move the triple point on the phase diagram. This refreezing of fresh water occurs at the expense of adding thermal energy to the frozen salt water, and raising the temp of the frozen salt water.
The temperature of saltwater at which ice forms can be lowered down to approx -21 deg C by adding up to approx 18% w/w NaCl.
This does not mean that the ice will last longer then fresh water ice, just that it will keep that temperature until all the ice is gone. The energy required to melt freshwater ice, compared to saltwater ice, at equivalent ice temps of say -30 deg C is greater, because you have not got 20 % of your ice mass as sodium chloride crystals....
Anyway, chilling your beer down to -21 deg C is enough to freeze it, assuming that you have enough saltwater ice. Not ideal...
cheers
Shaggy
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Follow Up By: basecamp15 - Wednesday, Mar 23, 2005 at 01:29
Wednesday, Mar 23, 2005 at 01:29
If you read my post carefully i point out that a layer of frozen saltwater will give you a colder esky. What you say is partly true regarding the thermal energy of the fresh water freezing on the salt block. However, the salt ice temp will not raise, it will melt. Ice cannot go above it's melting point (which as you say veries depending on the salinity).
By having this layer (and not the whole esky) you preserve the fresh water ice for much longer by having a colder esky and so on. Although you may lose the saltwater blocks the fresh water blocks which take up the rest of the esky (greater volume - several layers) last much longer. I wouldn't use it to chill beer either, just for storing meat etc.
FollowupID:
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Follow Up By: shaggy - Wednesday, Mar 23, 2005 at 18:46
Wednesday, Mar 23, 2005 at 18:46
All true, except for one thing. Salt ice can raise temperature. If you read my post carefully, I made the assumption that your salt ice is at -30 deg C, and that the coldest temp that salt ice can melt at is -21 deg C. Therefore the salt ice CAN raise in temperature, from -30 deg C up
to -21 deg C, as you point out.
cheers
FollowupID:
361131
Reply By: Nick R - Monday, Mar 21, 2005 at 23:10
Monday, Mar 21, 2005 at 23:10
Interesting point there,
There is a bit to esky management, got a mate who rafted the colorado river, in the
grand canyon, on day 30 they still had a little ice, I think they did get 1 delivery, not sure. they emptied them 1 at a time and always kept transferring what ice they had left to where it could be useful. the water would have been drained off and when they set up
camp they would put them in the shade with wet towells and stuff over them, so it can be done.
NickR
AnswerID:
103388
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