the keying
course. Such a driving in of this course should raise the arch as a
whole from the center. The center should be so constructed that it may
be dropped free of the arch when the key course is in place and removed
from the furnace without being burned out.

[Illustration: A Typical Steel Casing for a Babcock & Wilcox Boiler
Built by The Babcock & Wilcox Co.]

Care of Brickwork--Before a boiler is placed in service, it is essential
that the brickwork setting be thoroughly and properly dried, or
otherwise the setting will invariably crack. The best method of starting
such a process is to block open the boiler damper and the ashpit doors
as soon as the brickwork is completed and in this way maintain a free
circulation of air through the setting. If possible, such preliminary
drying should be continued for several days before any fire is placed in
the furnace. When ready for the drying out fire, wood should be used at
the start in a light fire which may be gradually built up as the walls
become warm. After the walls have become thoroughly heated, coal may be
fired and the boiler placed in service.

As already stated, the life of a boiler setting is dependent to a large
extent upon the material entering into its construction and the care
with which such material is laid. A third and equally important factor
in the determining of such life is the care given to the maintaining of
the setting in good condition after the boiler is placed in operation.
This feature is discussed more fully in the chapter dealing with general
boiler room management.

Steel Casings--In the chapter dealing with the losses operating against
high efficiencies as indicated by the heat balance, it has been shown
that a considerable portion of such losses is due to radiation and to
air infiltration into the boiler setting. These losses have been
variously estimated from 2 to 10 per cent, depending upon the condition
of the setting and the amount of radiation surface, the latter in turn
being dependent upon the size of the boiler used. In the modern efforts
after the highest obtainable plant efficiencies much has been done to
reduce such losses by the use of an insulated steel casing covering the
brickwork. In an average size boiler unit the use of such casing, when
properly installed, will reduce radiation losses from one to two per
cent., over what can be accomplished with the best brick setting without
such casing and, in addition, prevent t