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Full text of "The Flow Of Gases In Furnaces"

OWING to the fact that water may be converted into vapor at
rather low temperatures, there is no particular difficulty in con-
structing a boiler that will produce steam when heat is applied
to it. The earliest boilers were modeled upon the familiar kitchen
kettle. To-day there are so many different designs of boilers
that it would be difficult to enumerate them. A few years ago
boilers were operated and rated at the nominal figure of 10 sq ft
of heating surface per boiler horsepower. To-day the nominal
rating remains the same, but in practice the boiler is operated at
from 150 to 300 per cent of rating. The water-tube boiler was
devised to increase boiler efficiency by securing improved water
circulation and breaking the circulating water up into multiple
streams surrounded by hot gases. In this it has met a certain
degree of success, as is proved by the modern method of boiler
The most widely used design of the water-tube boiler consists
of one or more steam drums and an assemblage of inclined tubes,
the pitch or slope varying from 8 per cent to 30 per cent. The
tubes are generally arranged in multiple, but there are designs
tha?t employ series operation of a number of multiple banks of
tubes. Circulation generally depends upon the thermal syphon
principle, that is, upon the hydrostatic head developed by the
difference in weight of two columns of water of slightly different
temperatures. Additional circulating velocity is supposed to be
due to the fact that one of these columns of water is solid and the
other partially displaced by steam bubbles.
The accompanying table shows the weights of water and
steam at different temperatures, by 20° C. increments, together
with the corresponding pressures. At the boiling temperature,
100° C., the ratio between the weights of equal volumes of steam
and water is about 1 to 1600, which is sufficient to supply an
•ascensional velocity to the steam of about 177 m (580 ft) per
second. At 200° C. the ratio drops to 1 to 110 and the ascensional
velocity to 46.5 m (153 ft) per second. The coefficient of friction
between the bubble of steam and the surrounding water is un-
known, but even if an extremely liberal allowance is made for this