DESIGN OF OPEN-HEARTH FURNACES 295
themselves. The difference in density created by temperature
changes creates local recirculating loops which eliminate stream-
A good idea of these confused currents may be gained by
observing any current of hot air rising alongside of a hot furnace.
These currents may be rendered visible by using a bright light
which will be partially polarized by the eddies created, causing
them to cast a shadow. A light which contains an appreciable
proportion of the blue end of the spectrum will render visible the
heat waves beyond the visible red of the spectrum.
In a gas-to-fluid heat transfer through a metal partition,
there are two very important factors, either one of which will
limit the value of the test. The heat transmission of the metal
will be limited by the manner in which the fluid circulates past
the wet surface, to a much greater degree than it will be limited
by the manner in which the hot gases circulate past the dry sur-
face. Practically all the recent experiments regarding gas-to-
fluid heat transfer through metal have entirely neglected the
part played by the fluid in carrying off the heat. Until the fact
is recognized that the gases cannot transfer heat to the metal any
faster than the water, in turning to steam, is able to carry it away
from the metal, very little progress will be made in boiler design.
In metallurgical furnace work it has long been recognized that
there are limitations upon the rate of temperature drop. In this
work there is frequently only a small temperature differential
between the gas giving up heat and the material to which heat is
imparted. In the steam boiler the temperature differential
between the hot gases and the water turning into steam is very
large, and the main obstacle to a high rate of heat transfer is the
poor arrangement of the water circulation.
Table 20 gives the quantity of heat available for a waste-heat
boiler at various initial and final temperatures, together with the
drop in temperature, initial and final gas volumes and the change
in the gas volume due to the drop in temperature. These values
clearly illustrate the large amount of heat lost by the drop in
temperature between the regenerator and the boiler and carried
away from the boiler by high waste-gas temperatures. With
leaky gas flues there is not only the drop in gas temperature, but
the added volume of air, which may increase the volumes to be
dealt with by 30 to 40 per cent, or more.