These immense pressures necessitate extreme care in the construction of the apparatus, thereby enhancing the cost; and the difficulty of keeping the joints tight often occasions loss of material and reduced production. (3) Equally necessary to be taken into consideration, is the condensation of the vaporised body, in order that it may be used over again. This condensation is effected by means of a supply of cold water. In some industries, and in certain localities, the scale of consumption of water for this purpose is such as to altogether preclude the use of certain machines. (4) The chemical properties of the substances employed must be studied in relation to their action upon the metal or other material with which they will come into contact. Having said so much concerning the general principles and conditions involved in the artificial production of a low temperature, or ice itself, some space may now be devoted to a description of the principal machines devised with this object.
Carry's intermittent portable apparatus, in which ammonia is employed, is shown in Fig. 19. A boiler k containing the ammonia is connected by the pipe r with the refrigerator t, into the well of which are pat vessels tilled with water to be frozen. The boiler k is placed over a portable furnace, and the apparatus is purged of air, which is driven by the evolved gas out at the stop-cock m. This being closed, and the refrigerator immersed in a lank of cool water, the temperature of the liquid ammonia is raised to 230°-240° F. (110°- 115° C), at which heat the ammonia is expelled, and condensed in a liquid form in the refrigerator t. The boiler being now removed from the furnace, and placed in the water-bath, the temperature of the water in it will fait, and the power of the water to dissolve ammonia will be restored. The gas will be rapidly re-dissolved, reducing the pressure, as the liquid ammonia, will evaporate with corresponding rapidity, drawing for its latent heat upon the sensible heat of the water to be frozen. The result will be the complete evaporation of the liquefied ammonia, ami the restoration of an aqueous solution in the boiler, of the original strength.
In the large refrigerator, an ammo-niacal Solution is placed in a boiler and heated in the ordinary way by a fire underneath. The ammonia is given off rapidly as a gas, and is collected at pressure in a coil of pipes placed in a tank, through which a constant stream of cold water runs. The ammonia is here liquefied, both by its own pressure and by the extraction of all heat above that of ordinary cold water. From this liquefied condition, the ammonia will, on removal of the pressure, fly at once into gas. The liquefied gas is then need in a species of water engine or meter, which serves to pump back the reunited ammoniacal solution into the boiler again. The liquefied gas, after having here done its work, immediately on release flies into gas; and this re-evaporated gas is conducted in circuitous tubes through the freezing tanks or chamber. By reason of this sudden re-evaporation of the ammonia, upon release from high pressure, a large quantity of heat is taken up and rendered latent, and this is of course abstracted from surrounding objects, or from the liquid to be frozen. After having served its purpose, the ammonia Is led into a chamber, meeting and mixing with the water from the boiler, out of which the ammonia has been evaporated.
It is thus re-absorbed, and then pumped, by the water engine before refefred to, back again into the boiler. The ammonia thus is continually circulating round; first evaporated by heat, giving the motive power to the arvirtue of its own pressure of 8 to 10 atmospheres, and being cooled by a stream of running water, it then re-evaporates in doing work, thereby causing a large absorption of heat, and effecting the freezing operation. It is lastly remixed with the unaerated water from the boiler, and is pumped back, as a solution, once again into the boiler.
The ether refrigerator consists essentially of an engine to give the motive power to the various operations. To this engine is attached, probably on the same piston-rod, a suction pipe on the one side attached to the refrigerating vessel, which is partially filled with ether. By reason of the reduction of pressure in this vessel produced by the pump, a portion of the ether evaporates, being an exceedingly volatile liquid. In evaporating, the ether renders latent a large quantity of heat, thus extracting it from the remainder of the ether, producing a very low temperature. This reduction of temperature is made use of by circulating through the ether in thin pipes, a fluid such as brine, or calcium chloride, which will not freeze at 32° F. (0° C). This circulating medium is then made use of to freeze water in blocks for commercial purposes. The circulation is effected by means of a suitable pump. On the other side of the main vacuum pump, the volatilised ether is delivered at slight pressure into a pipe, circulating through a large tank through which a constant stream of cold water is flowing. This causes the recondensation of the ether into a liquid, which then falls by gravitation back again into the main refrigerating vessel.
Thus a constant circulation, without loss of the ether, is kept up; the heat abstracted in the refrigerator by evaporation on the suction side being carried off by the constant stream of cold water on the delivery side. This is the most usual form, perhaps, of refrigerating machine, and may be represented by that made by Siddeley and Mackay, of Liverpool.