METHODS OF COMPUTING FOR FURNACES 73
determined from data including its production, that is, the number
of ingots to be heated per hour, the time required for their reheat-
ing and their arrangement upon the hearth of the furnace.
The distance from the hearth to the roof of the furnace will
be established by keeping the following fa'cts in mind: sufficient
space must be provided to obtain complete combustion of the hot
gases, and constant contact of the hot gases with walls or material
at a high temperature is an essential condition. A very serious
fault of many furnace designs is the premature cooling of the
flame or burning gases by directing them in such a manner that
they impinge upon cold ingots or other cold material. On the
other hand, it would serve no useful purpose to make the chamber
too large, but it is desirable to make the roof of sufficient height
to give the hot gases of the flame a sufficient time to complete their
reaction before they come in contact with bodies sufficiently cold
to impede or prevent the completion of the reaction of combustion.
When the lines and working method for the heating chamber
have been determined, its volume may be computed.
The same method of procedure is used for tempering furnaces,
annealing furnaces, iron-melting furnaces, puddling furnaces, etc. ;
the very first thing that is necessary is to determine the dimensions
of the heating chamber and its volume. When the foregoing have
been fixed, the composition of the furnace gases, or, more exactly,
the volume of air required with reference to that theoretically
required to burn the fuel, is assumed.
Calculations show that a good coal from central Russia,
burned with the theoretical volume of air required, gives a theoret-
ical calorific intensity of 2082°. With 70 per cent excess air
supply this coal will give a calorific intensity of 1400°; and with
double the theoretical air supply (100 per cent excess air) the
calorific intensity will be 1250°.
Now, it is evident that these temperatures can only be obtained
with instantaneous combustion in an athcrmal chamber, the total
amount of heat released being absorbed in raising the temperature
of the gases of combustion. In reality the best means of lowering
the temperature of the jet of burning gases is to operate the furnace
so that combustion takes place with only the theoretical air supply.
Such combustion requires a certain length of time; the combustion
of the last traces of combustible gas requires a considerable amount
of time in its combination with the small amount of oxygen