A De Laval wheel revolves at terrific
speeds, the limit being tens of thousands of turns per minute for the
smallest engines. The greatest efficiency is obtained, theoretically, when
the vane velocity is half that of the steam, the latter, after passing
round the curved inside surfaces of the vanes, being robbed of all its
energy and speed. (For a fuller description of the steam turbine, see How
It Works, Chap. III., pp.74-86.)
The turbines to be described work on the De Laval principle, which has been
selected as the easier for the beginner to follow.
A Very Simple Turbine.
We will begin with a very simple contrivance, shown in Fig. 67. As a "power
plant" it is confessedly useless, but the making of it affords amusement
and instruction. For the boiler select a circular tin with a jointless
stamped lid, not less than 4 inches in diameter, so as to give plenty of
heating surface, and at least 2-1/2 inches deep, to ensure a good steam
space and moderately dry steam. A shallow boiler may "prime" badly, if
reasonably full, and fling out a lot of water with the steam.
Clean the metal round the joints, and punch a small hole in the lid, half
an inch from the edge, to give egress to the heated air during the
operation of soldering up the point or joints, which must be rendered
absolutely water-tight.
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