TURBINE: STEAM. A turbine is a rotary motor in which the shaft is rotated steadily in its bearings, not by means of cranks as for example in a reciprocating engine (q.v.), but by a current of water, air, steam or any other fluid. Thus a steam turbine is a "prime mover" which generates motive power in the same manner as the familiar country-side "wind mill," which is used for grinding corn or pumping water. Instead of a current of air being used to rotate the shaft by means of "sails" as in a "windmill," a current or blast of steam issuing from a number of fixed nozzles, is employed to rotate the shaft of a steam turbine by means of "vanes," "buckets" or "blades." The relatively small power obtainable from the wind with a reasonable sail area, and the discontinuity of the breeze, render the windmill of comparatively little economic importance, but with the steam turbine on the other hand, the matter is very different. Not only is there the convenience of uniform motion applied to a shaft direct, but also the powerful and un failing steam blast generated with coal or oil fired boilers, and the enormous power output obtainable from a turbine of very moderate dimensions.
The economic value of the steam turbine has, therefore, been the incentive to its development to the utmost, and its evolution has been carried out in the face of all obstacles,—a story worthy of an epic of mechanical engineering—for many years almost entirely owing to the efforts of the Hon. Sir Charles A. Parsons, 0.M., K.C.B., etc., and his associates and in more recent years by a host of other workers as well.
General Mechanical Features.—In the steam turbine, in which a pure twist or driving torque is the only force applied to the shaft, the entire mechanism may be described under the fol lowing six headings.
(I) . The system of "blading" by means of which the motive power is produced. The term "blading" (as distinct from "blades") is used to denote collectively the fixed nozzles and the rotating "vanes" or "blades." (2). The rotating shaft, or "rotor," which carries the rotating ele ments of the blading, and collects and transmits the driving torque.
(3). The casing, which carries the nozzles or fixed elements of the "blading," balances or provides the reaction to the driving torque (see torque tube in a motor car) , and embodies the supports or bear ings for the rotor.
(4). The speed regulating mechanism.
(5). The lubricating system for the bearings.
The blading may consist of a set of fixed nozzles (see fig. I) directing high-velocity steam jets on to "vanes" or "blades" mounted round the rim of the wheel. If there were no blades or other obstructions in front of the steam jets, the latter would travel forward in a straight line.
The curved surfaces of the blades, however, forcibly deflect the jets from the free path, the blades themselves receiving in conse quence an "impulse" in the direc tion opposite to the deviation which they produce. Hence the name "impulse turbine" applied to a turbine of this kind. The steam jets in passing through the blade passages and doing work on the wheel, will be continuously slowed down and will leave the moving blades with a velocity much less than the initial.
Special Difficulty of the Problem.—Such a very simple ar rangement (the counterpart of the well-known Pelton water wheel for utilizing waterfalls) although widely adopted for small powers, is not applicable to large powers.
The underlying reason for the unsuitability of the simple steam wheel as compared with the water wheel is to be found from a comparison of the physical properties of steam and water. Water is an in-elastic fluid ; that is, it has practically the same density (weight per cubic foot) at all pressures, and in a water turbine, the water jet velocity generated by a given fall in pressure is de termined from the static head corresponding to that pressure. If h is the height of a column of water, the difference of pressure between the top and the base of the column is that due to the weight of flip water in the column pressing on an area equal to the sectional area at the base. Denoting this sectional area by A then hA=the volume of the water, the weight*Of the column ph A, and p, the pressure per unit area at the base, =ph where p is the density or weight per unit volume.