B. - High-duty Apparatus. -
3. Compressors for Pneumatic Telegraphs.-These machines are used in large cities for producing the pressure or vacuum for impelling packets through underground systems of pneumatic tubes (see Pneumatic Telegraphy, in TELEGRAPH).
Fig. 137 represents the machine used in the Post-Office, London,
England. The walking-beam is connected with two compressing cylinders,
also to steam-cylinders built on the compound system, and to the
fly-wheel. The absolute pressure of the compressed air is 1.7
atmosphere, and the corresponding volume of air furnished is 588
cubic feet when cylinders are single-acting; when double-acting,
twice this total. The dimensions are as follows: Motor: Diameter
of small piston, 16.7 inches; large piston, 24.9 inches; stroke
of small piston, 48.7 inches; of large piston, 65.1 inches. Cut-off
at 3/8 stroke; condensing; steam-pressure, 75 lbs. Air-cylinders:
Diameter of air-piston, 84.7 inches; stroke, 35.5 inches; useful
volume of cylinder, 20 cubic feet 22 cubic inches; revolutions
per minute, 25; volume generated per minute, one cylinder, 998
cubic feet; two cylinders, 1,996 cubic feet. The cylinders are
made as shown in Fig. 137, so as to be both exhausting and compressing,
or either exbausting or compressing. To this end the valves are
placed in chambers on each side of the cylinder, as shown. The
inlet-valves are on the left, the delivery-valves on the right.
The two upper ones communicate with the atmosphere; the lower
pair communicate, one with the receiver to be exhausted, the other
with the compressed-air reservoir. It will readily be seen how,
by suitable adjustment of these valves, the apparatus may be made
to act in the different ways above described. The total valve
openings aggregate air-area .0087 that of the piston-surface.