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Full text of "The Flow Of Gases In Furnaces"

218

APPENDIX VII

P \

the Edgar Thomson Works (American Iron & Steel Institute meet-
ing, May 28, 1915), which showed that the checker openings
close to the shell took more gas when heating and less air on
blast.
In his work, Fours d Flamme, Professor Groume-Grjimailo
gives a mathematical demonstration of the reason why the portion
of the checkerwork close to the wall, losing a considerable amount
of heat by radiation, exerts a greater cooling effect on the hot
gases of combustion flowing downward and tends to concentrate
their flow through this portion of the checker, while the air, which
is being heated, tends to seek the central and for that reason more
slowly cooled portion of the checkerwork. It is interesting to
note that Professor Groume-Grjimailo cites the Cowper stove, as
follows: " For example, the checker openings of the Cowper
hot-blast stove located nearest to the shell lose a great deal of
heat by radiation; they therefore exercise a stronger cooling
effect upon the current of gas flowing through them and by reason
of this the velocity of the descending current of gas is increased,
since if fe<^i then V2>vi."
Convection currents in gases and air, due to small temperature
differences, are by no means inconsiderable. A temperature dif-
ference of 1 C. is sufficient to impress a vertical velocity of
0.268 m (0.88 ft) per second on a gas. This velocity will be
increased directly as the square root of the difference in tempera-
ture. Convection currents will act to carry the cooled gas or the
heated gas or air away from the heating or cooling surface. These
currents will exist in any chamber, regardless of whether a current
of gas is circulating through the chamber or not. When a hot gas
is giving up its heat to a surface the convection currents will be
downward, while when a cool gas is absorbing heat from a surface
the convection currents will be upward. It naturally follows that
the working current or circulation of the air or gases should be in
the same direction as the convection currents, as in this case they
wi]l tend to reduce the friction loss. When the convection cur-
rents are in the opposite direction to the working current they form
recirculating loops which entail a direct loss of heat capacity in the
checkerwork.