DESIGN OF OPEN-HEARTH FURNACES
total cost will be about the same for both methods of installa-
Another factor that has
to be considered, is the area
of the gas passage through
the boiler must be sufficient
to permit the maximum vol-
ume of the products of com-
bustion to pass without adding
unduly to the draft resistance
through the boiler.
In the foregoing computa-
tion the temperature of the
boiler feed water was taken
as zero C. In practice this
will not be the case, if the ^ „ „„ ^ , « .
. , , . ,, FIG. Io7.—Proposed Setting of Marine
average temperature of the Type of Waste Heat Boiler,
water is considered.
THE MATTER OF THE DRAFT
Stationary or land practice has persistently used brick boiler
settings in spite of their many disadvantages. Brick settings are
undoubtedly less costly than air-tight steel-sheathed boiler settings,
and the fact that the weight of the boiler setting on land is unim-
portant has led to the almost exclusive use of the porous brick
setting, owing to its lower cost. There are a number of places
where the extremely low-draft pressure makes the brick setting
comparatively unobjectionable, but in open-hearth and other
waste-heat installations where a large draft differential is required
the porosity of the brick setting is objectionable. In addition
the brick setting will be badly damaged by any explosions which
may occur; the brickwork is readily cracked and these cracks
admit air which will produce explosions at each reversal.
Practically all waste-heat boilers installed upon open-hearth
furnaces have necessitated the installation of an induced draft fan.
With the commercial types of boiler set and baffled to the makers'
drawings, these fans will be necessary, or else an unduly high
chimney. The pressure in the heating chamber of the furnace
is equal to the atmospheric pressure and it is necessary to supply