LANCASHIRE BOILER, contrived to remedy this inconvenience and also to
attain a more perfect combustion, the arrangement of the furnaces of
which is shown in fig. 19 and fig. 20.
It will be observed that there are two internal furnaces instead
of one, as in the Cornish type. These furnaces are sometimes each
continued as a separate flue to the other end of the boiler as shown
in the cuts; but as a rule they emerge into one internal flue. They
are supposed to be fired alternately, and the smoke and unburned gases
issuing from the fresh fuel are ignited in the flue by the hot air
proceeding from the other furnace, the fuel in which is in a state of
incandescence. Thus all violent changes in the temperature are avoided,
and the waste of fuel due to unburned gases is avoided, if the firing
is properly conducted.
[Illustration: LANCASHIRE BOILER—Fig. 18.]
The disadvantage of the Lancashire boiler is the difficulty of finding
adequate room for the two furnaces without unduly increasing the
diameter of the shell. Low furnaces are extremely unfavorable to
complete combustion, the comparatively cold crown plates, when they are
in contact with the water of the boiler, extinguishing the flames from
the fuel, when they are just formed, while the narrow space between
the fuel and the crown does not admit the proper quantity of air being
supplied above the fuel to complete the combustion of the gases, as
they arise.
On the other hand, though this boiler favors the formation of the
smoke, it supplies the means of completing the combustion afterwards,
as before explained, by means of the hot air from the second furnace.
[Illustration: Fig. 18 (_a_)]
Another disadvantage is the danger of collapsing the internal flue
already spoken of; this is remedied by the introduction of what are
called the galloway tubes, illustrated in the cut shown on this page,
which is a cross section of the water tubes shown in Figs. 18 and 20.
These tubes not only contribute to strengthen the flues but they add to
the heating surface and greatly promote the circulation so important in
the water space.