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.  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.

[Illustration:  Fig. 67.—­Simple steam turbine.]

For the turbine wheel take a piece of thin sheet iron or brass; flatten it out, and make a slight dent in it an inch from the two nearest edges.  With this dent as centre are scribed two circles, of 3/4 and 1/2 inch radius respectively.  Then scratch a series of radial marks between the circles, a fifth of an inch apart.  Cut out along the outer circle, and with your shears follow the radial lines to the inner circle.  The edge is thus separated into vanes (Fig. 68), the ends of which must then be twisted round through half a right angle, with the aid of a pair of narrow-nosed pliers, care being taken to turn them all in the same direction.