The compressed air is forced by the compressing cylinder A into the refrigerating cylinder BB, just at the moment when the position of the plunger, PP, is close upon the upper cone D. This air, which fills the space ar, between the plunger, PP, and the lower cone E, is of course heated by the compressicn; and in order to cool it again, cold water is made to circulate hi the cone E, by an entrance-pipe F, and an exit-pipe at G. The next movement of the machine draws the piston in the cylinder A to the opposite end, and consequently allows the compressed air to expand again ; but at the same moment the plunger, PP, descends close upon the cone E, thus allowing the space between the plunger and the upper cone D to he at its fullest when the expansion of the inclosed air is at its greatest.
By this arrangement, the patentee secures that the air while being compressed will always be at the lower, or what he calls the hot end, of the refrigerating cylinder B; and while being expanded, it will always be at the upper or cold end. There is a regenerator constructed as in the calorie engine (q.v:) of wire-gauze, placed in the middle of the plunger at C. This, while it allows the air to move freely through it, prevents the con veyance of heat or cold from one end of the cylinder to the other. The plunger, PP, is filled internally with sawdust or sonic non-conducting material, We may now explain that the low temperature of the air which surrounds the cone D during expansion is not produced by simple rarefaction. That alone would not pro duce cold. It is necessary as well to abstract heat from the air by giving it some work to do, and here it unavoidably does work, in assisting to force back the piston of the compressing cylinder, while in the act of expanding. The air loses an equivalent of heat exactly in proportion to the amount of force which it expends in moving, or in assisting to move, the piston. See FORCE.
Before air is admitted into the cylinders, it is necessary to dry it thoroughly, by first passing it through a box containing chloride of calcium, because, if any moisture were present, it would freeze in the regenerator, and stop the action of the machine. In the particular form of the apparatus shown in the figure, the substance to be cooled is placed inside the cone D, which is furnished with a lid L. Here not only water, but even mer cury, can be frozen with facility.
Most of the machines of this kind which have as yet been made were required for working on a more extensive scale than the one shown in fig. 2 could easily do. In the larger-sized machines, accordingly, instead of one hollow cone like D (fig. 2), a series of circular V-shaped corrugations are fixed to the top cover of the refrigerating cylinder. These form annular passages through which a continuous current of some fluid not easily frozen, such as brine, flows, This is of course cooled by the expanded air (in the manner already explained) at the cold end of the cylinder, and can be conveyed away in pipes to cool any substance at a greater or less distance from the machine.
After a trial of more than 10 years, the use of Mr. Kirk's machine has, in several large chemical works at least, been given up in favor of those machines which. like
Siebe's, evaporate ether or some other very volatile liquid by mechanical power. Here, as in most other applications of it, the use of compressed air has as yet proved too costly. In some recent trials, an ether-machine required only 5 cwt., while Kirk's consmned 20 cwt. of coal to make a ton of ice. However, notwithstanding the difficulties attending the economical employment of compressed air for this purpose, as shown by the imperfect success of both Kirk's and Windhausen's refrigerators, Messrs. Bell & Coleman of Glas gow have quite recently introduced another compressed-air machine,which in 1870 was on its trial on board one or two large steamers engaged in bringing fresh meat from America to England.
The ether-machine of Siddelev & Mackay of Liverpool, which is an improvement on Sick's, is now a good deal used 'for making ice, and for other purposes. In it a steam engine is employed to work two vacuum-pumps, and to supply motive-power to other parts of the apparatus. The refrigerating vessels contain thin pipes, through which brine or chloride of calcium flows. Ether surrounds these pipes; and under the reduced pressure produced by the vacuum-pumps, with which the refrigerators communicate, a pardon of the ether evaporates, producing cold in the act of doing so, as already The ether vapor is then condensed at a slight pressure, cooled by a stream of cold water, and returned to the refrigerator. Some paratfine oil makers who use this machine have found that a very light shale naphtha answers as well as ether for their purpose.
Having already described Carre's small portable ammonia-machine, we may add here that this kind of ice-machine, even in its larger and more complicated continuous form, since it works by heat and has few moving parts, is really the simplest of all. Reece's ammonia-machine is more recent than Carre's, and is worked with anhydrous ammonia, instead of an aqueous solution of it. In Reece's process less fuel is required to distill the ammonia, less water to condense it, and less power to work the moving, parts of the machine. Some imperfections which it nevertheless had appear to have been overcome, as it is now introduced into several of the Loudon breweries.
It is only within the last twenty years that much attention has been given to the con struction of machines for the production of artificial cold on the large scale; but they have already received several important applications. In this country, besides being available for the production of ice; the extraction of certain salts from mixed solutions, such as sulphate of soda from common salt—the former separating at a temperature above that which keeps the latter in solution; the separation of paraffine from mineral oils; and in other chemical operations, as well as for cooling worts in breweries and dis tilleries, they are now turned to good account in bringing fresh meat from distant coun tries to our shores. In warm countries, besides other uses, they could be applied to cool large hospitals and public buildings, by sending a current of some cold liquid through pipes, just as we in Great Britain heat buildings with pipes through which hot water flows.