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Full text of "The Flow Of Gases In Furnaces"

COMBUSTION AND BOILER SETTINGS                 329

which forces them to turn, horizontally, the gases lose their vertical

velocity.

Applying this formula to the case in hand, it is found that the
depth ht will be 1.27 m or 4.17 ft. The horizontal velocity of the
gases flowing out at the end of this arch, will be about 15.25 ft
per second and their mean velocity will be about 10.20 ft per
second. As soon as they pass beyond the end of the arch the hot
gases "will have an ascensional component, due to their density, of
zero at the end of the arch but increasing so that their rise "will be
on an arc of a parabola unless interfered with by other forces. In
the boiler setting these other forces are a current of colder gases
descending among the tubes and the tubes themselves, and while a
portion of the gases flowing out from under the arch will penetrate
the colder layer of gases and possibly reach the baffle behind the
tubes, this action will occur intermittently according as the current
of hot or cold gas is stronger in its surges. Naturally these sudden
changes in. the temperature of the gases at the baffle "will cause
cracks and ultimately failure of the baffle as shown in Mr. Smith's
article. When this occurs the hot gases will pass through the
break as indicated in. Fig. 169.
At 226 Ib pressure absolute, the steam temperature is 200 C.
The temperature of the outside of the iron tube should not exceed
220, allowing a liberal drop between the "wet surface and the
emulsion of steam and water, and assuming that the tube is clean,
and free from both scale and soot. The temperature in the firebox
is around 1400 C., and it has "been customary to assume that the
curve representing the drop in temperature from the fire to the
smoke-flue outlet is a regular logarithmic curve, this curve being
based upon assumptions which violate the simplest principles of
elementary physics. The difference of 1200 between the hot
gases and the water tube is nearly five times the difference in tem-
perature between that of the workroom and the temperature
of liquid hydrogen, 259. The refrigerating effect of the cold
tube on the hot gases is almost equivalent to that of dipping
a red-hot bar of steel into liquid hydrogen. The ignition tem-
perature of the gases varies between 350 and 800; any portion
of the unburned gases coming in close contact with the tubes will
be chilled belo^y the ignition point. In the case of CO this may