REFRIGERATION. In refrigerating machines there is a transference of heat from the substance which is to be re frigerated to the cooling agent, which is evaporating fluid, expanding gas, or a material which promotes evaporation of the liquid to be cooled. If 80.025 pound Centigrade units of heat be withdrawn from a pound of water at 0° C. it will become a pound of ice of the same tem perature. If this heat be withdrawn from the water by an evaporating liquid there are two conditions which must be fulfilled; the evaporating liquid must evaporate very rapidly, and the latent heat of evaporation (s. e., the heat ab sorbed from outside during evaporation) must be as great as possible. Ether boils at 35.5° C. F.), and has at 0° C. (32° F.) a vapor-pressure of 18.4 cm. (7.36 inches) of mercury; at C. it requires 94-pound-Centigrade units of heat to evaporate a pound of it; and at that temperature its evaporation ought accordingly to be able, if the whole of the heat required for evaporation were withdrawn from water, to free 94 80.025 times its weight of water at 0° C., so that a ton of ice (2,240 pounds) would be produced by the evaporation at 0° C. of a minimum of 1,907 pounds of ether. Liquid ammonia boils at C. ( F.), and has at 0° C. a vapor-pressure of 318 cm. (127.2 inches), or more than four atmospheres; it is thus extremely rapidly volatilized at 0° C.; and, as its latent heat of evap oration is as much as 294, the produc tion of a ton of ice would thus only de mand the evaporation of a minimum of 610 pounds of liquid ammonia. Ma
chines for using ether have been con structed by Siebe, Duvallon, Lloyd, Miihl and others. The ether is caused to evap orate rapidly by an air pump or pumps worked by steam; it cools brine or a solution of calcium chloride, and this cools the water to be frozen or the air to be refrigerated; the ether vapor is condensed by pressure and cold and used over again. Ammonia was first used by Carre in 1860; ammonia gas driven off by heat from its solution in water is condensed in a cooled vessel under its own pressure; the original ammonia ves sel is now cooled, and the liquid ammonia rapidly evaporates (its vapor being ab sorbed), chilling its surroundings. An hydrous liquid ammonia has been used by Reece and others.
The Bell-Coleman apparatus, greatly employed for producing cold dry air for use in the refrigerating chambers of dead-meat-carrying steamers, the prin ciple is that compressed and cooled air will, when allowed to expand against an external resistance, so that it does me chanical work during expansion, lose heat equivalent to the energy which it has expended.
Porous jars, used to keep water cool, are among the simplest kinds of refrig erating apparatus; the evaporation at the outer surface of the jar of the water passing through the porous earthenware taking latent heat from the water.