[Illustration: FIG. 301.]
_The word duplex_ means two fold, or double, and has a wide
application, as the duplex lathe, the duplex watch, etc., hence, the
well-known duplex-pump is one in which _two direct acting pumps are
placed side by side_ and so connected that the steam piston of one
operates the valve of the other. See Fig. 300.
Fig. 301 shows one of the smallest manufactured patterns of this type
of pumps. Its dimensions are as follows: 2-inch diam. steam cylinder;
1-1/8-inch water cylinder; 2-2/3-inch stroke. Its capacity is .044
gallons per revolution; rev. per minute, 80; gallons per minute, 3.5.
Steam pipe, 3/8-inch; exhaust pipe, 1/2-inch; suction pipe, 1-inch;
discharge, 3/4-inch. Floor space occupied, 1´ 9″ × 7″ wide; requires
1/2 H.P.
The valve motion (see Figs. 302, 303) of one cylinder is communicated
or produced by the piston of the other through the medium of rocker
arms and links. By means of the small lost motion of the levers the
pistons have a slight pause at the end of each stroke, which allows the
water valves to seat quietly, thus preventing any slam or jar.
With this arrangement, as one of the steam valves must always be open,
there can be no dead point, thus removing the liability of the pump to
stick. The simplicity of the duplex movement is at once evident, each
valve is dependent upon its counter part, and both directly control
the action of the steam, which is supplied through one simple throttle
valve.
NOTE.—Of the effect produced by the steam-moved direct-acting pump of
much greater capacity it may be said there are now in use pumps of
this class, exerting over 250 horse-power, delivering five million
gallons of water in twenty-four hours through main pipes, say thirty
inches diameter and fourteen miles long, without the use of an air
chamber, and which do their work so quietly, steadily, and gently,
that a nickel coin set on edge on the extreme end of the pump would
not be overthrown by any jar or motion of the pump while it was doing
this work.
[Illustration: FIG. 302.]
[Illustration: FIG. 303.]
_The adoption of the Worthington design of duplex pumps_, has been
well nigh universal, especially so since the expiration of the earlier
patents. Nearly all leading manufacturers now make “duplex pumps.”
Single, compound and triple expansion.
Compounding consists of adding a second steam cylinder on the end of
the high pressure in use, both using the same piston rod, the steam
from the boiler being first used in the smaller cylinder, and at the
end of the stroke of the piston being exhausted behind the piston of
the larger cylinder on its return stroke. In this way the measure of
the expansion of the steam used, was the relation of one cylinder to
the other.
By this arrangement a much smaller steam cylinder, for using the high
pressure steam, could be adapted to do the same work, for in addition
to the pressure of steam working full stroke in the small cylinder,
was to be added the pressure of the steam being expanded in the large
cylinder.
In addition to this, for large compound and triple expansion engines
was added the further economy realized by attaching a condenser to form
a vacuum in the large steam cylinders.
Adapting these newer improvements in the marine engine coupled with
what had previously been accomplished in the direct-acting steam pump,
they were at once brought up in size, capacity, and economy alongside
the previously constructed rotative pumping engines.
The history of the invention of this pump is given on pages 69 and 70;
to these pages a careful attention is advised, as they briefly describe
also the fundamental principles of its operation. This form of steam
pump has been so long and generally in use that the valve mechanism is
already familiar to most engineers.
The simplicity of both its theory and its practical application
obviates the necessity of devoting very much space to its
consideration. The final improvement made by Worthington in connection
with the steam duplex pump was in the adaptation of triple expansion in
its steam ends.