Page:Popular Science Monthly Volume 39.djvu/30

20 containing water is kept wet on the outside with ether, the evaporation will chill the water and eventually freeze it. This is essentially the process by which the carafes frappées of French restaurants are produced. The decanters filled with fresh water are set in shallow tanks containing brine, which remains liquid below the temperature at which fresh water freezes. In contact with these tanks are receivers, which can be kept charged with newly formed ether vapor. The chilling vapor cools the brine, and this in turn takes heat from the water in the decanters, which soon freezes.

In making ice on the large scale, either ammonia or sulphurous oxide is used instead of ether, because these substances are cheaper and are not inflammable. Ammonia is a gas or vapor at ordinary temperatures. What is commonly called ammonia, or, more properly, ammonia water, is water with several hundred times its volume of this gas dissolved in it. For ice-making, anhydrous ammonia—that is, ammonia perfectly free from water—is used. The first thing to do is to get the ammonia into the liquid form. There are two ways of condensing a vapor to a liquid—by cold and by pressure. Practically it can be done easiest by combining the two. The ammonia gas is subjected to pressure, and forced through a coil of pipe called a condenser, where it is cooled by water from any convenient supply running down over the pipes. By this means the latent heat in the gas is pressed out, and is taken up and carried away by the water. After being liquefied in the condenser the ammonia is forced into pipes larger than the liquid can fill, where it immediately expands into a vapor and exerts its chilling effect.

Two methods of making ice, which differ, however, in only one step of the process, are now in use. In a factory established last year in New York city, which the writer has been permitted to go through, the "compression system" is used, with anhydrous ammonia as the cooling agent. The machinery employed consists of a powerful pump driven by steam, with which is connected the necessary condensers, piping, etc. Liquid ammonia is supplied by the makers of ice machines in strong iron drums. The ammonia is run into a cylindrical iron tank, from which it is allowed to pass through a small orifice into the coils of pipe in the freezing tank. In this factory the freezing tanks are of iron, about twenty by fifty feet in size, and four feet deep. Over them is a floor, which is cut up into rows and lines of rectangular covers. Each of these lifts up, showing a can under it, twenty-two by eleven inches in size, and forty-four inches deep. The tank contains a brine of regulated strength, and the cans when filled with the water to be frozen float in this brine, coming within an inch or two of the bottom of the tank. Back and forth across the