III. HIGH-PRESSURE COMPRESSORS.
B. High-duty Apparatus.-
4. Compressors refrigerated by Water introduced at the Periphery of the Compressing Piston.-In 1872 Messrs. Benjamin Roy & Co., who built the first air-compressing machines for the St. Gotbard Tunnel adopted a system of construction which involved a hollow compressing piston, receiving by its rod water under pressure which it distributed uniformly over the piston by means of a channel at the middle. This disposition is analogous to the water-layer on the piston in the preceding clam of machines, but it Is considered preferable because it is applicable to double-acting, to horizontal, and to fast-running compressors, since the piston is no longer formed of two independent portions, of which one (the liquid packing) may be disadvantageously affected by high velocities.
In Fig. 151 is given a section of the air-cylinder and pump of
a compressor of this type, made for mining purposes by the French
Compagnie de l'Horme. The piston is hollow, and has on its periphery
five channels, of which one communicates by apertures with its
interior, and serves as an escape for the water. The others receive
the metal packing-rings. In rear of the piston is a hollow rod
which is screwed in the piston-rod, and which, passing through
the rear cylinder-head, connects with a pump. The latter is composed
of two small barrels and a spring accumulator. The pistons consist
of two solid rods connected by a crosspiece and a sleeve to the
prolonged hollow piston-rod. These plungers pump the water from
a reservoir into the accumulator chamber, whence it is forced
through the piston-rod and out through the piston,as already described.
The water then escapes into a reservoir, in which is an automatic
valve which opens when a certain level is reached, and
allows the water to pass into the receptacle, from which it is
pumped. Dimensions and data: Absolute air-pressure, 6 atmospheres;
volume of air furnished at this pressure by two cylinders, 142
cubic feet. Motor, two direct-acting horizontal cylinders. Di.
ameter of pistons, 19.5 inches; stroke, 31.2 inches. Variable
cut-off. Condensing. Fly-wheels, 11 feet 5 inches in diameter.
Two double-acting air-compressing cylinders. Diameter of pistons,
15.6 inches; stroke, 31.2 inches; useful volume of cylinders,
3,547 cubic feet; revolutions per minute, 60. Volume delivered
at this velocity, one cylinder, 422 cubic feet; two cylinders,
844 cubic feet.