REFRIGERATION AND REFRIGER ATING MACHINERY. Refrigeration is the operation of cooling substances by artificial means. Natural ice forms the greatest source of refrigeration, but there are certain conditions where cold produced through mechanical means possesses advantages over natural ice, which have led to the development of a number of re frigerating machines. The advantages of me chanical refrigeration over that produced by natural ice are that the temperatures of the objects cooled can be more readily regulated, lower temperatures can be obtained and in many cases the desired cooling effect can be secured at a lower cost. The industrial im portance of mechanical refrigeration is of ever-increasing importance in modern life. One of its earliest and largest applications is to brewing. Moreover, it has revolutionized the industry which relates to the distribution and sale of ice. It has given us a product which is not only better than natural ice but can be sold at a price with which natural ice in many in stances cannot compete. It has further created an enormous new industry in the transport and storage of the perishable necessaries of life in a cold state. Broadly speaking, the methods that are employed for the production of cold may be classified under two heads, the second of which is by far the more important in practice. There is, first, the production of cold directly by the agency of heat, and, second, the production of cold by the expenditure of me chanical work. It is by the expenditure of mechanical work that nearly all modern refrig eration is carried out, namely, by compressing a gas or a vapor and afterward allowing it to expand under conditions which allow it to absorb heat from the thing that is to be cooled. To maintain the low temperature of a cold storage room or refrigerator space one must have a continual extraction of heat going on from a thing which is already colder than its surroundings. The same is true in the case of a brine tank for the production of ice. The brine has to be maintained at a temperature somewhat lower than that at which water freezes, considerably lower if the process of freezing is to go on at a reasonably fast rate. The heat which comes into the brine from the water to be frozen, the heat which the water gives out when it freezes and the beat which leaks in from all sources has to be continuously extracted from the brine at this comparatively low level of temperature. The heat so ex tracted is not destroyed but is raised to a higher,level of temperature and is discharged by being given up to some substance which acts as a receiver of heat. In all actual cases of re frigeration the substance which absorbs the rejected heat is circulating water, which be comes more or less warmed by the heat which it takes up. The refrigerating machine accord ingly works between two temperatures; the lower temperature is that at which heat is taken in from the body which is to be kept cold, and the higher temperature is that at which heat is rejected to the circulating water or other substance which absorbs it. In order
that this process may go on an expenditure, of mechanical power is necessary.
Classification of Refrigerating Machines. — Refrigerating machines are generally classi fied by reference to the particular working sub stance they employ. A broad distinction may be drawn between machines which use air as their working substance, and those which use a liquid which is alternately vaporized and liquified during the cycle of operations. Gen erally speaking, in this second class of machines the liquid winch is used is one whose vapor pressure is higher than the pressure of the atmosphere, under the actual conditions of temperature at which the machine works. If we take, for instance, ammonia or carbonic acid as the liquid which is alternately liquefied and vaporized, we find that under the conditions of temperature usually obtaining, the pressure of the vapor is higher than the atmospheric pres sure. It is, however, quite possible to use a liquid which can only be vaporized by sub mitting it to a lower pressure than that of the atmosphere. Water is an instance in point. When water is the working substance, the whole action is taking place in what is, rela tively speaking, a vacuum. It has to take place in chambers which are maintained at a pressure much below the pressure of the atmosphere, and consequently machines using water as their working substance are frequently spoken of, for this reason, as ((vacuum* machines. Fur ther, when a vaporized liquid is used for work ing substance one may either adopt simply mechanical compression as the means of restor ing the vapor to the liquid state, or one may adopt a chemical action between the vapor and some other substance which has an affinity for it. This alternative gives rise to the grouping of machines under the title of *compression* machines on the one hand or ((absorption* ma chines on the other. The latter are those in which a quasi-chemical action or species of solution goes on as a substitute for the me chanical compression used in other vapor ma chines. In absorption machines there is usually a direct application of heat instead of mechani cal power. Machines of the vapor-compression type are further classified according to the particular vapor which they employ, as am monia, carbonic acid, sulphurous acid or ether machines. Air machines are divided into two groups, in one of which the same volume of air is made to pass again and again through a cycle of operations without leaving the _hine. Such machines are called closed cycle machines to distinguish them from those air machines in which air is discharged from and taken back again in the cycle at atmospheric pressure.