III. HIGH-PRESSURE COMPRESSORS.
B. High-duty Apparatus.-
6. Compressors refregerated by Injection of Water in Form of Spray into the Cylinder -
(Fig. 153) - Four sets of compressing cylinders, three in each
group, and all belonging to this type, are in use at Airolo, St.
Gothard Tunnel. To each group motive power is communicated from
gearing connected with the shaft of a turbine wheel. There is
a circulation of water in the head and around each cylinder, and
also in the piston and rod, besides an injection of spray at each
end. The circulation around the body and ends of the cylinders
is obtained from small pumps operated from the rod of the compressing
piston. This pump injects water into the hollows in the cylinder-ends
and into the annular space formed around the cylinder by a sheet-iron
envelope. The piston-rod, which is of steel, is bored through
to receive a copper tube of smaller diameter. This tube is nearly
as long as the rod, and is connected to it at the rear end by
a screw-threaded bronze plug. The mode of connection at the opposite
extremity is clearly shown in the drawing. The water injected
by the pump passes through this tube and returns by the annular
space between the tube and piston-rod, as far as a diaphragm which
is formed of a bronze ring fixed on the rod just inside the piston,
and which compels the water to penetrate the latter, cooling the
two faces as indicated by the arrows. This water then escapes
by the rubber tube attached to the rear end of the piston-rod.
The water which enters at each end of the cylinder does so through
spouts so constructed that two fine streams are emitted by each,
and the two jets on escaping are caused to meet at nearly a right
angle, so that the water becomes turned to spray by the impact.
The quantity of water introduced is regulated so as to maintain
the air completely saturated. Under these conditions even when
circulation inside the piston is not continuous, it has been found
possible to maintain the temperature of the entire apparatus at
about 86° Fahr. Three valves are placed at each end of the
cylinder, two inlet and one delivery. The total area of the inlet-valves
is .11, and of the delivery-valves .04, that of the piston-surface.
Dimensions and data: Absolute air-pressure, 6 atmospheres; volume
of air furnished at this pressure by the three compressors of
a group, 236.9 cubic feet. Motor for each group turbine of 46.8
inches diameter, under 528 feet head velocity, 390 turns per minute.
Transmission by gearing in ratio of 1 to 4.35. Triple compressors,
double-acting. Diameter of pistons, 17.9 inches; stroke, 17.5
inches; useful volume of cylinders, 26.4 cubic feet; revolutions
per minute, 90; theoretic volume delivered at this velocity for
each compressor, 475.2 cubic feet. Final temperature of air on
leaving cylinders, 104°. It will be seen that the four groups
of compressors employed produce at the above average velocity
of 90 revolutions 947,600 cubic feet of air at a pressure of 6
absolute atmospheres.