From what has been said, it will have been seen that, so far as the mere application is concerned, there is no difference whatever between the absorption and compression processes. The following considerations, therefore, which chiefly relate to the application of refrigerating apparatus, will be dealt with quite independent of either system. The application of refrigerating apparatus may roughly be divided into the following heads:
a. Ice making.
b. The cooling of liquids.
c. The cooling of stores and rooms.
Ice Making.—For this purpose two methods are employed, known as the can and cell systems respectively. In the former, moulds of tinned sheet copper or galvanized steel of the desired size are filled with the water to be frozen, and suspended in a tank through which brine cooled to a low temperature in the refrigerator is circulated. As soon as the water is completely frozen, the moulds are removed, and dipped for a long time into warm water, which loosens the blocks of ice and enables them to be turned out. The thickness of the blocks exercises an important influence upon the number of moulds required for a given output, as a block 9 in. thick will take four or five times as long to freeze solid as one of only 3 in. In the cell system a series of cellular walls of wrought or cast iron are placed in a tank, the distance between each pair of walls being from 12 to 16 in., according to the thickness of the block required. This space is filled with the water to be frozen. Cold brine circulates through the cells, and the ice forms on the outer surfaces, gradually increasing in thickness until the two opposite layers meet and join together. If thinner blocks are required, the freezing process may be stopped at any time and the ice removed. In order to detach the ice it is customary to cut off the supply of cold brine and circulate brine at a higher temperature through the cells. Ice frozen by either of the above described methods from ordinary water is more or less opaque, owing to the air liberated during the freezing process, little bubbles of which are caught in the ice as it forms, and in order to produce transparent ice it is necessary that the water should be agitated during the freezing process in such a way as to permit the air bubbles to escape. With the can system this is generally accomplished by means of arms having a vertical or horizontal movement. These arms are either withdrawn as the ice forms, leaving the block solid, or they are made to work backward and forward in the center of the moulds, dividing the block vertically into two pieces. With the cell system agitation is generally effected by making a communication between the bottom of each water space and a chamber below, in which a paddle or wood piston is caused to reciprocate. The movement thus given to the water in the chamber is communicated to that in the process of being frozen, and the small bubbles of air are in this way detached and set free. The ice which first forms on the sides of the moulds or cells is, as a rule, sufficiently transparent even without agitation. The opacity increases toward the center, where the opposing layers join, and it is, therefore, more necessary to agitate toward the end of the freezing process than at the commencement. As the capacity for holding air in solution decreases if the temperature of the water is raised, less agitation is needed in hot than in temperate climates. Experiments have been made from time to time with the view of producing transparent ice from distilled water, and so dispensing with agitation. In this case the cost of distilling the water will have to be added to the ordinary working expenses.
Cooling of Liquids.—In breweries, distilleries, butter factories, and other places where it is desired to have a supply of water or brine for cooling and other purposes at a comparatively low temperature, refrigerating machines may be advantageously applied. In this case the liquid is passed through the refrigerator and then utilized in any convenient manner.
Cooling of Rooms.—For this purpose the usual plan is to employ a circulation of cold brine through rows of iron piping, placed either on the ceiling or on the walls of the rooms to be cooled. In this, as in the other cases where brine is used, it is employed merely as a medium for taking up heat at one place and transferring it to the ammonia in the refrigerator, the ammonia in turn completing the operation by giving up the heat to the cooling water during liquefaction in the condenser. The brine pipes cool the adjacent air, which, in consequence of its greater specific gravity, descends, being replaced by warmer air, which in turn becomes cold, and so the process goes on. Assuming the air to be sufficiently saturated, which is generally the case, some of the moisture in it is condensed and frozen on the surface of the pipes; and if the air is renewed in whole or in part from the outside, or if the contents of the chamber are wet, the deposit of ice in the pipes will in time become so thick as to necessitate its being thawed off. This is accomplished by turning a current of warm brine through the pipes. Another method has been proposed, in which the brine pipes are placed in a separate compartment, air being circulated through this compartment to the rooms, and back again to the cooling pipes in a closed cycle by means of a fan. This plan was tried on a large scale by Mr. Chambers at the Victoria Docks, but for some reason or other was abandoned. One difficulty is the collection of ice from the moisture deposited from the air, which clogs up the spaces between the pipes, besides diminishing their cooling power. This, in some cases, can be partially obviated by using the same air over again, but in most instances special means would have to be provided for frequent thawing off, the pipes having, on account of economy of space and convenience, to be placed so close together, and to be so confined in surface, that they are much more liable to have their action interfered with than when placed on the roof or walls of the room.
In addition to the foregoing there are, of course, many other applications of ammonia refrigerating machines of a more or less special nature, of which time will not permit even a passing reference. Many of these are embraced in the second class, cold water or brine being used for the cooling of candles, the separation of paraffin, the crystallization of salts, and for many other purposes. In the same way cold brine has been used with great success for freezing quicksand in the sinking of shafts, the excavation being carried out and the watertight tubing or lining put in while the material is in a solid state. In a paper such as this it would be quite impracticable to enter into details of construction, and the author has therefore confined himself chiefly to principles of working. In conclusion, however, it may be added that in ammonia machines, whether on the absorption or compression systems, no copper or alloy of copper can be used in parts subjected to the action of the ammonia. Cast or wrought iron and steel may, however, be used, provided the quality is good, but special care must be taken in the construction of those parts of absorption machines which are subjected to a high temperature. In both classes of apparatus first-class materials and workmanship are most absolute essentials.
Paper lately read before the Civil and Mechanical Engineers' Society.