If the steam ways, valve chest, and steam
pipe also are jacketed, an increase in efficiency will be gained, though
perhaps somewhat at the expense of appearance.
Boiler.--The boiler described on pp. 211-216, or a vertical multitubular
boiler with about 800 sq. inches of heating surface will drive this engine
satisfactorily.
XVI. MODEL STEAM TURBINES.
Steam turbines have come very much to the fore during recent years,
especially for marine propulsion. In principle they are far simpler than
cylinder engines, steam being merely directed at a suitable angle on to
specially shaped vanes attached to a revolving drum and shaft. In the
Parsons type of turbine the steam expands as it passes through successive
rings of blades, the diameter of which rings, as well as the length and
number of the blades, increases towards the exhaust end of the casing, so
that the increasing velocity of the expanding steam may be taken full
advantage of. The De Laval turbine includes but a single ring of vanes,
against which the steam issues through nozzles so shaped as to allow the
steam to expand somewhat and its molecules to be moving at enormous
velocity before reaching the vanes.
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