OF THE CONSTRUCTION OF CONDENSING ENGINES.
385. The distinguishing feature of this class of engines is, that of condensing
the steam to the state of water. The moving force is nearly equivalent to the
force of the steam, as in the boiler, moving through the difference between the
space in the state of steam and that in the state of water. Different systems of
construction render the effective or useful power more or less, but I will endeavour
to give the general principles in a few words, and then proceed to more minute
detail.
386. The essential parts of a single condensing engine consist of a cylinder
having a passage to admit steam at the top, and one from the bottom to convey
the steam to another cylinder, called a condenser. The condensing cylinder has
a passage from the lower part of it to an air pump; and both the air pump and
the condenser are immersed in a cistern of cold water, a jet of which plays into
the condenser. The cylinder has an air-tight piston fixed to a rod, which moves
in an air-tight box in the top of the cylinder. Conceive there to be a valve
in the piston, which, whenever the piston arrives at the bottom, opens and allows
the steam to pass from the upper to the lower side of the piston. Then, let the
jet be stopped, and the cylinder and condenser be filled with steam from the
boiler, and the piston be raised to the top of the cylinder by a counter weight at
the other end of the beam, to which the piston rod is fixed. The cock being open
to admit the steam from the boiler to the cylinder, if the jet of cold water be
allowed to play into the condenser, nearly the whole of the steam in the condenser,
and in that part of the cylinder below the piston, will be reduced to water, and
the pressure on the top of the piston being equal to the elastic force of the steam
in the boiler, while the elastic force of the vapour remaining below it is very small,
the difference of the forces will press down the piston, and, consequently, raise
an equivalent weight at the other end of the beam.
