[Larger plate.]

[465]. Fret saw or jigger.

[466]. Three-cutter tube shears, with worm gear motion.

Section 25.—CONDENSING AND COOLING APPLIANCES.

Their uses generally are to condense steam, to cool heated gases, air, or articles of food requiring a low temperature; distilling, and other purposes. For cooling purposes, compressed air machines are in most demand. The air is compressed in a cylinder, then cooled to ordinary temperature again in a surface condenser, such as [No. 468], and then expanded into the cooling chamber, through a cylinder and piston, the expansion reducing its temperature usually to 10° or 20° below zero. Other cooling appliances are ammonia machines, fans, and blowers of all kinds, punkahs, or waving fans, freezing mixtures, &c.

[467]. Gravity condenser. The pipe should be 34 feet high or more, in which case no air pump is required, as the condensed steam and air are discharged below. In place of the pipe an air pump and foot valve are required, and are commonly used, as it is seldom convenient to have a vertical pipe 34 feet long with a water supply at the top.

[468]. Surface condenser, multitubular. The steam may be led into the tubes, and the water around them, or vice versâ.

[469]. Worm, or coil condenser, chiefly used for distilling.

[470]. Still condenser for essences, spirits, &c.

[471]. Condensing chambers for gases, &c. Horizontal or vertical.