In the so-called vacuum ice-making machines water is used as a working fluid, and it is cooled to the freezing by its own evaporation. In these machines the water is injected into a refrigerating chamber in which a vacuum is maintained and a portion of the water is frozen by the cooling effect produced by the evaporation.. In some machines the vacuum is maintained entirely by mechanical means and in others by the combination of an air pump and a reservoir of strong sulphuric acid for absorbing the water vapor. Where acid is used ,it is heated after it has become weak through the absorption of water vapor and after concentration is returned to the ma chine and used over again as an absorbent. Where a vacuum is maintained entirely by me chanical means a very large cylinder displace ment is required in the air-pump. One machine constructed on this principle had a cylinder 20 feet in diameter and a stroke of 10 feet, and made 20 double strokes or revolutions per minute. The ice produced is white, opaque and hard. It is produced in large cakes and cut by a saw to the size required for commercial use.
Bibliography.— Anderson, J. W., 'Refrig eration) (New York 1908); Arrowood, M. W.,
Wright (New York 1904) ; Leask, A. Ritchie,
Machinery' (4th ed. London 1907); Ledoux, M.,