While, however, this enormous
advantage is gained, there is also the drawback that the apparatus is
not fitted for use with crude oils of heavy specific gravity, such as
can be dealt with in the big external superheaters of the Lowe class
of water gas plant, but the lighter grades of oil must be used in it
for carbureting purposes.
I am not sure in my own mind that this, which appears at first a
disadvantage, is altogether one, as, in the first place, the lighter
grades of oil, if judged by the amount of carbureting power which they
have, are cheaper per candle power, added to the gas, than the crude
oils, while their use entirely does away with the formation of pitch
and carbon in the pipes and purifying apparatus--a factor of the
greatest importance to the gas manufacturer.
The fact that light oils give a higher carburation per gallon than
heavy crude oil is due to the fact that the latter have to be heated
to a higher temperature to convert them into permanent gas, and this
causes an over-cracking of the most valuable illuminating
constituents; and this trouble cannot be avoided, as, if a lower
temperature is employed, easily condensible vapors are the result,
which, by their condensation in the pipes, give rise to much trouble.
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