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306                                 APPENDIX VIII
ture differential for the blast in the checker is about 580 and of
the gases about 940. Through the checkerwork the convection
currents act in the same direction as the flow of the blast and the
gases. Therefore the convection circulation tends to make every
particle of the gases brush every unit of surface. This fact was
demonstrated many years ago by Peclet at the Institut des Arts et
Metiers, at Paris, but is still ignored by many.
In the combustion chamber the convection currents will be in
the reverse direction to the flow of the blast and the gases. For
that reason, it is probable that temperatures taken slightly below
the top of the dome would show a higher blast temperature than
would be found at the hot-blast valve, lower down.
The study of Table No. 4 reveals several interesting facts in
regard to the two-pass stove which was tested. Hot-blast stove
temperatures are comparatively low, both as regards the tempera-
tures realized from the burning of the gas and the hot-blast tem-
perature. In this stove the gas-to-gas differential was about 600,
which is nearly double that for other types of regenerators. The
rate of heating and cooling, that is, the average temperature
change per second in the blast heating and the gases cooling is low.
The checker contains 28 m2 32 of heating surface per cubic meter
of free air per second, or, if the average volume of the blast in the
checker is considered, this ratio becomes 24 m2 35. The rate of
heat absorption by the blast averages 0.659 calories per square
meter of heating surface per second. The fact that the isotherms
at the upper end of the checker are much closer together than those
lower down may be interpreted as indicating that this end of the
checker worked too cold, due, possibly, to the greater heat loss
through the walls and shell of the stove.
The usual thickness of the outer walls of stoves is 18 inches of
brick, 450 mm, with an air space of from 1 to 2 inches. A cyclic
change in temperature occurs in the inner portion of this wall
while the outer portion will give a temperature gradient toward
the air space. Due to the fact that the heating and cooling periods
are not equal and there is a constant heat or temperature loss
through the wall, the analysis of this cycle is not simple. The
central portion of the wall has a small temperature change. There
is a constant flow of heat to the outer surface and a periodic flow
from and to the inner wall surface, according to whether the stove
is at the beginning or end of the heating or cooling period. The