Dry Ice

Solid carbon dioxide (CO2) is some­times used for refrigeration. It is a white crystalline substance formed by allowing liquid carbon dioxide to escape into a snow chamber. The heat for vaporizing the liquid is drawn from the interior of the chamber so that very low temperature, -109 F. is formed, with the result that quantities of the carbon dioxide solidify. This solid is pressed into various shapes and sizes and sold for refrigeration purposes under such names as dry ice, zero ice, etc.

It remains at a temperature of -109 F. while in a solid state at atmospheric pressure, and sublimes, that is, it goes directly from the solid to the vapor state without be­coming a liquid. It has some desirable characteristics in that it does not wet the surfaces that it touches, and the gas given off is a preservative. The very low tem­perature maintained permits handling frozen foods without an expensive insulated container. It is used a great deal by ice cream vendors and delivery trucks.

The latent heat of sublimation is 248 Btu per pound. The heat absorbed by the vapor in passing from -109 F. to 32 F. is approximately 27 Btu per pound. This added to the latent heat of sublimation makes a total heat-absorbing capacity of 275 Btu per pound. This is a greater heat-absorbing value than for water ice. Dry ice is generally more expensive per pound than water ice.