A. For each 100 degrees of superheat added to saturated steam, at
temperatures ordinarily used in locomotive practice, the volume of a
given weight is increased roughly from sixteen to seventeen per cent.
88. Q. Why is the superheated steam so much more economical on coal and
water than the saturated steam?
A. Because for a given amount of water evaporated you can increase the
volume of steam 33 per cent. by superheating. It is readily seen that
the coal does not have to be burned if the steam used has 33 per cent.
more volume for filling space, or in other words, only so much steam can
be admitted to the cylinders for every movement of the valve, and what
can not be used must remain in the boiler, so if the engine can not use
all of the steam that the boiler is capable of generating, the saving
must show in coal and water. If you can not use all of the steam you do
not have to burn coal to make it.
89. Q. Which is the better practice, to close the feed valves or water
valve while waiting on sidings, etc.?
A. Close the feed valves; the water valve may leak.