140 STEAM BOILERS, ENGINES, AND TURBINES
by the motive column furnished by the difference between
the weight of the hot gases in the chimney and the equivalent column outside, and in those cases the fan has really
only to provide the pressure that is not furnished by the
chimney.
On the other hand, with a fan forcing air into the
furnace, it is not necessary to provide that the gases ascending the
chimney shall have any appreciable temperature at all if the heat
which they carry can be economically absorbed before they reach the
chimney, and this is one of the advantages of forced draught even
where a chimney is already in existence. It will be remembered
that it is the common practice to deliver the hot gases to the chimney
at about 600° F., because this is about the temperature up to which
an increased pressure is obtained, and that with a furnace temperature
of 2400° F., this means that 25 per cent, of the heat units are lost.
When forced draught is applied, the temperature of the hot gases
may be reduced to any figure the engineer pleases, providing that he
height,
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40.—Diagram showing the course of the Air and Hot Gases, with Forced Draught and an Eoonomiser.
can get them away comfortably to the outer atmosphere. It is quite common now to take out 300° of temperature from the hot gases by means of economisers, and to deliver them to the chimney at 300° only, this meaning that a further Vl per cent, of the heat units delivered to the hot gases in the furnace are usefully employed. It appears to the author, subject to practical considerations, that where forced draught is employed this might be carried very much further. Fig. 40 shows diagrammatically the course of the air and hot gases, in a Lancashire or Cornish boiler, with forced draught, where an economizer is used. In addition to enabling a larger percentage of the heat delivered to the hot gases to be usefully employed, forced draught enables a higher pressure to be maintained in the boiler furnace, and hence enables a thicker fire to be employed, the fuel lying on the grate-bars in larger quantities, and therefore a larger combustion to be obtained from a given grate area ; and further, the quantity of air supplied to
