FIG. 300.—THE SELF-INTENSIFYING PRINCIPLE OF PRODUCING COLD, USED TO LIQUEFY AIR.
This principle of self-intensification was first announced by Prof. C. W. Siemens in the provisional specification of his British patent No. 2,064, of 1857, but it does not seem at that time to have been carried out with any practical result. The first embodiment of the principle in a refrigerating apparatus is by Windhausen—United States patent No. 101,198, March 22, 1870. Solvay, in British patent No. 13,466, of 1885, gave further development to the idea, and following him came the operations of Prof. Tripler, who was the first to liquefy large quantities of air and to introduce it to the American people. Lindé, Hampson and Ostergren and Berger are more recent operators in this field of self-intensification, and Lindé’s British patent, No. 12,528, of 1895, may be regarded as a representative exposition of the principle. A simplified form of the Lindé apparatus is seen in [Fig. 300]. C is an air compressing pump, whose plunger descending compresses the air and forces it out through valve I, pipe 2, and coil 3. The coil 3 is immersed in a flowing body of water in the condenser W, the water entering at Y and passing out at Z. The cold compressed air then passes through pipes 4 and 5, pipe 5 being arranged concentrically within a larger coil 7. The cold air flowing down pipe 5 escapes through a valve adjusted by handle 6, into the subjacent chamber L, and expanding to a larger volume, produces a great degree of cold; this cold expanded air then passing up the larger and outer pipe 7 flows back over the incoming stream of air in pipe 5, chilling it still lower than the condenser W did, and this cold return flow then passing from the top of coil 7 descends through pipe 8 to the compressing pump C, and as its piston rises, it enters the pump through the inwardly opening valve 9, and here it undergoes another compression and circuit through the pipes 2, 3, 4, 5, but it is compressed on its second round of travel at a lower temperature than it had initially, and so this circulation of air going to the chamber L, expanding, and returning over the inlet flow pipe 5, successively cooling the latter and also successively entering the compressor at a continually lower temperature at each cycle of circulation, finally issues through the valve at the lower end of pipe 5, and expands to such a low temperature that it condenses in chamber L in liquid form. Fresh accessions of air are furnished to the apparatus through valve 10 as fast as the air is liquefied. The inlet flow to the liquefying chamber is shown by the full line arrows, and the return flow to the compressor by the dotted arrows, and the explanation of the term self-intensification is to be found in the cooling of the incoming air in pipe 5 by the outflowing air in the surrounding pipe 7, and the repeated reductions of temperature at which the air is returned to the compressor.
FIG. 301.—COMMERCIAL PRODUCTION OF LIQUID AIR.
FIG. 302.—VESSEL FOR TRANSPORTING LIQUID AIR.
In [Fig. 301] is shown the liquefier of a modern liquid air plant, in which liquid air is being drawn into a pail from the liquefier. Liquid air evaporates very rapidly, and produces the intense cold of 312° below zero. There is no known way to preserve it beyond a limited time, for, if put in strong, tightly closed vessels, it would soon absorb enough heat to vaporize, and in time would acquire a tension of 12,000 pounds per square inch, and would burst the vessel with a disastrous explosion. If left exposed to the air, which is the only safe way to transport it, it is quickly dissipated. A shipment of eight gallons from New York to Washington for lecture purposes shrunk to three gallons in two days’ time. It may usually be kept longer than this, however, as the jarring of a railway train promotes its evaporation and loss. A small quantity, such as a half pint, will boil away in twenty-five to thirty minutes. The only way to preserve it for any length of time is to surround it with a heat-excluding jacket. The simplest and most effective means for doing this in the laboratory is to surround it with a vacuum. [Fig. 302] shows a specially devised vessel for the commercial transportation of liquid air. A double walled globular vessel has between its walls air spaces and non-conducting packing. The liquid air in the interior chamber vaporizes gradually, and escaping through the outwardly opening valve at the top, expands around the air space surrounding the inner vessel. From this space it reaches the outer air by a valve at the bottom of the outer vessel. The liquid air in evaporating is thus carried around the body of liquid air in the center, and surrounding it with an intensely cold envelope, prevents the transmission of heat to the inner vessel. To withdraw the liquid air, a pipette or so-called siphon tube, shown in detached view, is substituted for the valve at the top.