COMBUSTION AND BOILER SETTINGS 347
Bulletin No. 40, Fig. 3, page 15. This setting comes the nearest
to being in agreement with the flow conditions existing in a boiler
set as recommended by the builder. Mr. Bement, however, has
altogether too great an area in his first and last pass.
Fig. 177 shows the gas flow in a two pass horizontally baffled
boiler. This design of baffling is better than the single pass. The
gas velocities computed by the Bureau of Mines are much higher
(see curve, Fig. 178) than those of the single-pass boiler. In
reality the gain in this boiler is due to the longer path of the gases
in contact with the tubes, but it is extremely doubtful whether
the gas velocity is any higher than in the single-pass boiler.
Fig. 179 shows curves of temperatures made by locating the
thermo couples at different distances from the water legs. The
drops in curves D and C indicate the same pockets of cooler gas
which occurred in the single-pass setting. While the regularity
of the curve E would seem to show that this couple had not at
any time come in contact with the stream of hot gases.
The location of the last thermo couple in all of these tests
seems to the writer to cause some doubt as to whether it really
showed the temperature of the stream of hot gases leaving the
boiler. This stream of ascending gas would have a fairly high
velocity due entirely to its temperature and it is extremely unlikely
that it filled the full area of the gas uptake, being surrounded by a
layer of relatively cooler gases.