A combination cooler & freezer suitable for camping purposes can be constructed using the salt-water eutectic, which melts at about -6F. The eutectic composition is 20% salt by weight which is 1 cup table salt 5 cups water by volume.
The basic scheme is to place a container of frozen eutetic brine into an insulated box. This insulated box is placed into a somewhat larger insulated box.
The temperature of the inner box will stay close to -6F until all the eutectic brine is melted. The temperature of the outer box will stay at a temperature somewhere between -6F and the ambient temperature.
Let's consider a case where the ambient temperature is 90F and we want to keep the space between the boxes at 45F while the freezer box is at -6F.
Heat flow thru the walls of both boxes must be equal:
KoutAout(90-45)/tout = KinAin(45+6)/tin (approximately)
If the boxes are the same shape with characteristic dimension L and have the same K;
L2out/L2in=51tout/45tin= 1.13(tout/tin)
Specific example:
Say we want the inner box to have a characteristic length of one foot while the outer box has a length of two feet. Then:
tout/tin= 4/1.13 = 3.5 i.e. The inner box wall must be 3.5 times thinner the outside box wall. This sounds like a reasonable value. Better modeling will result in a better feeling for the practicality of the design.
Dmartin@newarts.com 2004 07 17