REFRIGERATION

Since refrigerating machines have made artificial cooling of rooms and of material possible, industries which were formerly only carried on in cold weather are now operated at all seasons. The manufacture of ice is also a large and increasing industry, and is apparently forcing the natural product from the market more and more each year.
The principle involved in a refrigerating machine is the rapid absorption of heat by the rapid evaporation of a volatile liquid.
The substances most used are liquefied ammonia, sulphur dioxide, carbon dioxide, and the very volatile liquids derived from petroleum) chiefly cymnogene and rhigolene. In this country at least by far the greatest number of machines employ liquid ammonia.
The gas is heavily compressed and then liquefied by passing it
into a coil over which a large amount of cold water flows; the liquid is then forced through a small opening into a large chamber or coil of pipe, from which the gas formed may be rapidly exhausted by a pump. The rapid expansion and conversion of the liquid to a vapor here absorbs much heat from the walls of the coil or chamber, whose temperature consequently falls considerably below the freezing point of pure water. In order to increase the external surface

of the expansion coils, cast-iron disks are placed at frequent intervals on the pipe perpendicular to its line of direction. Only a comparatively small amount of ammonia or other volatile liquid is necessary for the continuous working of the machine. Since the gas is returned to the compressor, it is only necessary to supply that lost by leakage.
It is often customary to surround the expansion coils with a brine or calcium chloride solution, which is then pumped through coils or pipes in rooms to be cooled. For making ice, galvanized iron boxes are filled with water and immersed in the cold brine. In the system shown in the diagram (Fig. 12) a certain amount of oil is injected into the compressor along with each charge of ammonia. This insures complete emptying of the compressor ateach stroke, lubricates the piston, prevents the gas escaping behind the piston, and absorbs part of the heat evolved in the cylinder by the compression of the gas. This oil is separated from the liquefied ammonia by gravity in separating tanks and returned to the compressor.
The machines above described are called "compression machines," because the volatile substance is compressed directly to be used again. Another class of refrigerating apparatus depends for the recovery of the volatile substance upon absorption of the vapors in some liquid from which they can again be set free. The cooling effect in this case is also produced by rapid evaporation of the liquefied gas, the difference in the two classes of machines being in the method of recovering the vaporized liquid for use again. In the Carré ammonia absorption apparatus very concentrated aqua ammonia is heated in a closed iron retort, connected with another iron vessel which is surrounded by cold water. The ammonia vapor driven out of solution by the heat passes into this vessel, where it is liquefied by its own pressure. The retort containing the water from which the ammonia has been expelled is then rapidly cooled by submerging in a tank of cold water. A reabsorption of the ammonia vapors by the water in the retort then takes place, and owing to the decrease of pressure the liquefied ammonia in the receiver is evaporated
rapidly; heat is absorbed from substances in contact with the receiver walls, and thus water may be frozen. Another style of absorption machine evaporates water in a vacuum apparatus, and absorbs the vapor in concentrated sulphuric acid. The dilute acid thus produced is concentrated in open pans by evaporating the water, and is used again.
The absorption machines are not readily adaptable to the cooling of salt solutions, and hence are not used for chilling rooms. They also require a larger quantity of cooling water, and are generally more complicated and expensive than the compression machines.
A third class of machines are those depending on the sudden expansion of highly compressed air or other gas, which does not liquefyat the temperature and pressure used. These machines are large and complicated and are not adapted to making ice, but find limited use for cooling and ventilating storage buildings, especially where any traces of ammonia or other vapor would injure the contents.