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FLOW  THROUGH  ORIFICE                            31

the waste gases are taken off by a port through the roof, the hot
gases cannot distribute themselves uniformly; the hottest currents
tend to rise and pass out of the waste gas port in the roof, and
there will be currents of cold and therefore heavy gases in the neigh-
borhood of the ingots on the hearth, which accordingly will not
be well heated.

Therefore the waste gases should always be carried away from
the heating chamber by a port at the level of the hearth of the fur-
nace; the colder currents of gases will then pass out of the heat-
ing chamber and their place will be taken by currents of hot gases;
under these conditions the ingots will be well heated. These
remarks apply to furnaces of all kinds, and are a general rule for
the construction of all furnaces of rational design. It has been
well established that the port for the waste gases must be located
at or below the level of the hearth of the furnace.

Returning to the computations, the equation (B) may be
considerably simplified, if it is assumed that the weight of a cubic
meter of waste gases is the same as the specific weight of air
(1 kg 29). This is very nearly true. Therefore,


Aair — Agas                   1 +orf         .



Q = K-I *2co '\/2gIIott.......((7)

The expression Hat is that of the velocity head ft, because

Therefore                 h = Hat}
h_   t
and                           #~273'
Therefore the ratio of the head required to generate the velocity
to the height H is equal to the ratio of the temperature t to 273.
This expression will be found used later in connection with
jets of gas, page 65.