Did a search but hope more info on ice coolers

[Ross]

Just a little science for your ice box.

Water whether liquid or frozen has a specific heat of 1 BTU per pound. That means that 1 BTU will raise the temperature of one pound of water one degree fahrenheit. Ice has a latent heat of fusion of 140 BTU's per pound. This means that melting one pound of ice will cool 10 pounds of water 14 degrees fahrenheit. However cooling ice to zero degrees will only add 32 BTU's of cooling potentail to a pound of ice.The "R" rating for insulation is the resistance that the insulation has to the passage of thermal energy. This is expressed as BTU's per square foot per degree fahrenheit per hour. This means that if your Ice box has a surface area six square feet and the outside air is 90 degrees and the inside is forty degrees and the box is made of aluminum (zero insulation) you will have a 50 degree differential and six square feet so you gain 300 BTU's per hour. Since ice melts at a rate of 140 BTU's you will melt more than 2 pounds of ice per hour. Except of course that as the ice melts the inside temperature goes down so the differential increases and the ice melts faster.

 

[Jerry]

Slight correction

As I recall, the heat required to change the temperature of ice (or steam for that matter)is about 0.5 BTU/#/degree F. while to change the temperature of water takes about 1 BTU/#/degree F. This means that is takes only 16 BTUs to raise the temperature of one pound of ice from 0 to 32 degrees F.The big change occurs during a change state - ice to water or water to steam. Here, as Ross stated, it takes 144 BTUs/# to melt ice. These numbers are for controled laboratory conditions. The "real" world is a bit messy. But still, lowering the temperature of ice in theory is of limited value.The best thing you can do to slow the rate of thawing, as many of the previous posts have stated, is to increase insulation, open the cooler only when needed and eliminate leaks to prevent outside air entering the cooler.One last thing. Adding salt to ice will reduce the temperature to about 28 degrees F. I don't know how long this will stand up to outside temps as compared to plain ice but it might be worth a little researh or experiment.

 

[Jerry]

One more last thing

Also, to reduce thawing you should pre-cool your food or drink when possible. This will reduce the heat load on the ice.

 

[Ross]

Thanks Jerry, I thought that I remembered

that zero fahrenheit was the lowest temperature that could be gotten with salt and ice. As in a home ice cream freezer. I know that below some winter temperature salt won't melt ice on the road.