Other Compression Systems.— Water, sul phuric ether, sulphur dioxide, carbonic acid, methylic ether and certain petroleum products may be used in compression systems in the same way as ammonia. In the case of water temper atures colder than 32° F. require a suction pres sure of within .08 pound per square inch or 0.16 of mercury of a perfect vacuum. The dimen sions of a compressor cylinder of such a plant is about 150 times that for ammonia. The use of water as a working fluid is, therefore, impracti cable except for the production of ice as de scribed under section Sulphuric ether was the first substance practically applied to refrigeration by the compression system. The volume of the compressing cylinder is, however, about 17 times that required for ammonia, and its use has, therefore, been abandoned. Sulphur dioxide machines produce a temperature of about 5° F. in the cooler with a suction of about three pounds below, and a condenser pressure about 35 pounds per square inch above the atmosphere. Their economy is about the same as with ammonia, but the compressing cylinder must be about three times as large as for ammonia. The Pictet system uses sulphur dioxide, and in some cases what is called the Pictet fluid, which is sulphur dioxide mingled with about 3 per cent of carbonic acid. Car bonic acid gives a temperature of F. in the cooler with a suction-pressure of about 330 pounds per square inch above the atmosphere and a condenser pressure of about 900 pounds per square inch. The fuel required is more than with ammonia, but the size of the com pressor cylinder is about one-fourth that re quired for ammonia. Carbonic acid is used only to a limited extent; but the compactness of the machine employing it, together with the fact that any bursting of a pipe or leakage will not lead to the same disastrous results as with am monia, renders it more suitable for service on ships and also in some other special lines of work Nothing definite is known of the economy of the few machines which have used methylic ether and petroleum products. From the physical properties of these substances it would appear that they would give about the same economy as ammonia or sulphur dioxide.
Ice.Making.— The production of artificial ice. The heat is usually abstracted from the water to be frozen by evaporating a volatile liquid, such as ammonia, sulphur dioxide or ether, which in evaporation absorbs heat. An hydrous ammonia is most commonly employed. This boils at 27° F. below zero at atmospheric pressure. If the anhydrous liquid ammonia is introduced in a coil of pipe which is placed in a tank of brine it will absorb heat from the brine during the process of boiling equal to the latent heat of evaporation. If a can of water is immersed in the brine it will in turn be cooled and then frozen, and this process consti tutes the elements of the Can System of ice making. If on the other hand, the ammonia in the coil is made to cool a flat plate forming the vertical side of a water reservoir ice will be formed directly on the plate, and this consti tutes the elements of the Plate System of ice making. The vapor produced when the liquid ammonia evaporates in the coil is, in all cases, again converted to the liquid state so that it may be used over again. In abstracting heat from the refrigerating coil the vapor is at a low pressure, say 15 pounds per square inch above the atmosphere at which the boiling point of the liquid is 0° F. To convert it to a liquid it must be raised in pressure, to say 140 pounds per square inch above the atmosphere, at which the boiling point of the liquid is 80° F. As the temperature at the boiling point of a vapor is the same as that at the condensing point, the vapor at 140 pounds pressure can be condensed by cooling it to 80° F., and after such conden sation it may be returned to the refrigerating coils and used over again for abstracting heat from the brine. To raise the pressure of the as from 15 pounds to 140 pounds per square inch we may employ either of two methods— compress it by means of an engine, or we may absorb the vapor in cold water and then heat the aqueous solution of ammonia in a still, and so drive the gas off at high pressure. These two methods of compression constitute the distinguishing features of what is called the compression and absorption systems of ammo nia refrigerating machines.
In the can system of ice-making, distilled water which has been freed of air is ordinarily used for freezing and this produces a cake of transparent and clear ice with the exception of a thin feather of air bubbles in the centre of the cake and a portion of the top of the cake which is white where the water expands over that first frozen. The distilled water from which ice is made is obtained by con densing the exhaust steam from the engine of the refrigerating machine and by condensing live steam. In an economical plant the steam required to run the engine and for other pur poses is less than the amount of ice produced, and to make up the difference some live steam is admitted directly to the condenser in order to supply the necessary amount of distilled water. in an ice-making plant where the water is distilled by evaporating it in an ordinary boiler, the weight of ice made per pound of coal is, therefore, limited by the evaporation of the boiler and the amount of ice produced is less than the total amount of steam generated by what is lost through the drips from the steam pipes and other causes. It is possible, therefore, to make more ice per pound of coal by the plate system of ice-maldng where the water is not distilled than it is by a can system as ordinarily operated. In an economical com pression plant where the ice is made by the can system a single cylinder engine working with out a vacuum will require less steam to run it than the weight of ice which is produced, and if distilled water is used for freezing it will not increase the efficiency to employ a more eco nomical engine. In a plate system, however, a compound engine may be used with a corre sponding gain in the efficiency.
In order to make it possible in a can system to make a greater amount of distilled water ice than the weight of water evaporated by the boiler it has been proposed to use multiple effect evaporators for distilling the water. This was done in a plant at Newark, N. J., but the system had to be abandoned on account of the evaporator becoming coated with a hard scale which was deposited from the water to such an extent that the evaporator soon fell off in its capacity and efficiency. Where the steam from the engine operating the refrigerating machine is condensed in order to obtain distilled water for making the ice, the oil which this water contains must be eliminated. This is accom plished by reboiling the water which comes from the condenser in an open tank by means of a jet of live steam; then cooling and run rfing it through an oil separator and finally through charcoal filters. Reboiling the dis tilled water removes the air which it may con tain. It is possible to so thoroughly remove the oil that there will be no traces of it in the ice, whereas, if this is done imperfectly, the centre of the cake, when freshly broken open, will have a slight smell resembling that of tar. The presence of a trace of oil can be detected by the odor when it cannot be detected by taste. In the plate system the ice is formed on the flat cooling surface to the depth of about six inches in two days, nine inches in four days and 12 inches in seven days. The plates are usually allowed to build up to about 12 inches thick before harvesting. They are then melted free from the cooling plate by passing hot ammonia gas under pressure into the cooling coil which by condensation supplies the necessary heat. After this the blocks of ice are cut to the size required by sawing or by a parting tool heated by steam. In some plate systems cold brine instead of ammonia is circulated through the cooling coils. In the plate system the water in contact with the freezing plates is agitated by means of com pressed air which is admitted from a perforated pipe placed near the bottom of the tank As the ice freezes from one side only it forces the impurities and the air contained in the water to its surface, and the circulation produced by the air bubbles floating upward through the water assists in removing the impurities and air bubbles from the surface. A clear transparent ice is thus produced.