VACUUM PROCESS. The cooling of liquids on this principle depends upon the conversion of the sensible heat into latent heat (see HEAT) during evaporation, and has been practiced in all ages. A primitive example of this process in its crudest form is the practice in India and other warm countries of placing earthen vessels of water in a natural or artificial draught so that the liquid may be cooled by surface evaporation. The first machine for the production of ice by the vacuum process appears to have been invented in 1755 by Dr. Cullen, who in that year made the dis covery that the evaporation of water could be facilitated by the removal of the atmospheric pressure by means of an air pump, to such a degree as to enable him to freeze water even in summer. This apparatus was the parent of all those subsequently designed, but seems not to have been a commercial success. In 1777 .Nairne found that by the introduction of sulphuric acid into the receiver for the exhaust the aqueous vapor could be absorbed from the rarefied air and the latter dried, thus preventing the forma tion of a permanent atmosphere over the water and hindering the continuity of the evaporation. Nairne was followed by other inventors, but it was not until the second quarter of the nine teenth century that Edouard Carre invented a commercially successful machine adapted to pro duce the carafes frappes used in Paris ian and restaurants. This machine con sisted of a cylindrical vessel intended to contain the charge of concentrated sulphuric acid, of an air pump so arranged that it could be connected to the mouth of the carafe, and of an agitator cempled to the air-pump lever for the purpose of keeping the sulphuric acid in motion. The
Carre machine proved most successful for its purpose, and improved forms of the device are still manufactured, the largest of which are capable of producing SO pounds of ice per day. In IS7S Franz Windhausen patented a vacuum machine, an improved form of which was in stalled in 1SS1 at the Aylesbury Dairy, London, England. This machine was nominally capable of producing from 12 to 15 tons of ice per 24 hours. The ice-forming vessels in this machine were six in number, circular in transverse 'sec tion and slightly tapered. The mouths of these vessels were connected with the sulphuric-acid chamber and the vacuum pump, and water was admitted to them in fine streams, which offered extended surfaces for evaporation and almost instantly congealed into ice globules which fell to the bottoms of the molds and there froze together. To facilitate the release of the ice from the molds, they were surrounded with hollow jackets into which steam could be forced until the ice was melted loose. Various other forms of vacuum machines have been devised, but the two described are typical examples and explain the process sufficiently. Like the liquefaction process, the vacuum process cannot compete in economy with the more strictly mechanical processes, and vacuum machines are now used only for domestic ice-making and similar small installations.