PHP Warning: Use of undefined constant VBA_SCRIPT - assumed 'VBA_SCRIPT' (this will throw an Error in a future version of PHP) in ..../includes/functions_navigation.php(802) : eval()'d code on line 1 Ice vs Dry Ice - Technically which is better
So if you add 23% salt by weight into water, the freezing point drops to 21 below 0. That is cold.
334 kj/kg is the latent heat of fusion of fresh water, and 199 kj/kg is the heat of sublimation of dry ice so it takes more energy to melt (water) ice than dry ice (even allowing for the greater density of dry ice (1562 kg/m3).
So ice would seem to be a better source of cooling than dry ice (assuming that the specific heats of water and CO2 are not significant compared to the latent heats).
But everyone will say that dry ice lasts longer than block ice.
There is something wrong here, but I don't know what it is. I'll ask some of the engineers at work, but I would welcome any other comments.
I have heard many times about the effects of salt on fresh water ice, and I do salt my ice quite heavily. The result in temperature drop is very significant and a can of drink can damn near freeze in 20 mins. I usually buy a 25 kg bag of pool salt from Woolies for about $3.00. Is pool salt ok for this or should I be using cooking salt? What, if any, are the notable differences?
Sorry if I'm cutting in on this thread Whichway, this is a very interesting topic and one which really effects all fisho's/campers. Thanks for the facts and info.
The 23% salt by weight - would that be the ratio against the weight of the ice without water??
Whichway,
Yes the latent and sublimation heats of water and dry ice you have are correct.
If you take 2 exactly same insulated boxes and place the same size (volume) blocks of water ice and dry ice in them and keep them in exactly same conditions the dry ice will last longer.
So why a block of dry ice keeps longer?
Starting with the correctly noticed densities:
Same size (volume) block of dry ice would be need only about 20% more energy to sublimate as a same size block of ice (not considering the specific heats of liquid water and CO2 gas).
Important in the whole thinking is to remember that there is no actual "cold" provided but only absorption of heat happens.
And to absorb heat it has to be transferred.
1. The heat transfer coeficient (K factor) would be much higher when heat is transferred from liquid to solid (wet melting water ice block) than gas solid situation when dry ice is involved.
2. Total heat transfer would be also to a great degree by temeparature difference between to bodies as well (a colder body would absorb more heat than a warmer one if exposed to the same ambient conditions).
3. And the next thing to remember is that when dry ice sublimates to becomes gas (very high volume/weight ratio) so as the insulated box is not hereticaly seales (as it could go BUUUUM) there is a positive pressure inside and gas is escaping preventing warm outside air getting in.
When a block of water ice is melting the volume actually reduces by a bit and thus outside air is sucked insde bringing with it the heat.
Overall the most imortant factor is mentioned in point 1 above.
Reel Nauti,
As I have written on the other thread with 23% salt solution and freeezing point of -21C it would be very difficult to freeze the solution using home appliances. Solutions with a freezing point of -10C (or -15C if you have a good freezer) are much easier to handle.
And the ratio is as a % of the total. That is x% of salt + 100-X% of water.
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