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294                                 APPENDIX VII
the refractory is subject to erosion or chemical action, as both of
these forms of attack are more rapid at high temperatures. Reduc-
ing their temperature increases their resistance to such attack.
Another function of water cooling is in reinforcing the rigidity of
the binding of the furnace. Metal work exposed to heat will
warp; cooling the metal prevents warping and holds the furnace
to line. It is possible, however, to overdo the cooling; it, there-
fore, requires careful consideration, and the apparatus must be
designed in such a manner that all steam and air pockets are
avoided. With an open-hearth furnace, as with a blast furnace,
cooling should only be used to hold vital points which cannot
be held in any other way.
Thermal insulation for the conservation of heat will only be
successful where the maximum temperature to which the refrac-
tories are exposed is less than their yielding temperature and
where a sufficient thickness of the refractory is interposed to
protect the insulation from temperatures above its yield point.
In some cases the insulation will replace a certain amount of
refractory brick, which serves to reduce the cost of the insulation;
in other cases the insulation will be an addition to the cost of the
refractories. The whole question must be settled on the basis of
"Will it pay?" The balance between the cost of the thermal
insulation and the value of the heat which it makes available for
other purposes must not only pay the interest upon the investment,
and supply funds for maintenance, etc., but must replace the
capital invested. It is needless to say that the output of the
waste-heat boiler will be increased by the delivery of high-tem-
perature gases.
Waste-heat boilers are an indirect means of heat recovery.
The main purpose of the furnace is the production of steel, and
auxiliary apparatus cannot be permitted to interfere with this.
Considerable diversity of opinion exists in regard to waste-heat
boiler design and installation, the type of boiler and the method
of baffling.
High gas velocities have been considered necessary for rapid
heat transfer from gas to water. Such velocities mean a con-
siderable draft differential through the boiler and are based upon
the idea that, at high velocities, stream-line flow of the gases is
replaced by confused eddies. Stream-line flow cannot occur with
gases which are in contact with surfaces hotter or cooler than