The turbine wheel may revolve at a speed of 30,000 revolutions per minute. Such a tremendous speed has its disadvantages. If a wheel is to run smoothly it must revolve on its center of gravity. A lopsided wheel, or one that is mounted a little off center, produces a pounding action which imposes a serious strain upon the bearings and the revolving parts. The wheel tries to turn on its own center of gravity and will do so if permitted to. It is impossible to balance a wheel so perfectly that the axis it turns on passes exactly through its center of gravity. At ordinary speeds this slight eccentricity is so slight that it is practically negligible, but when we have to deal with 30,000 revolutions per minute the least divergence between the center of rotation and the center of gravity will produce dangerous strains. For this reason the wheel of the De Laval turbine is mounted on a flexible shaft and on floating bearings, so that it will automatically find and turn on its own center of gravity. In order to utilize the power developed in the wheel, gearing must be used to step down the speed.
PARSONS EXPANSION TURBINE
The Parsons steam turbine resembles a water turbine in its action. Instead of having a few nozzles directing steam against the buckets there is a nozzle for each bucket, and instead of a single wheel there is a series of them through which the steam passes, successively passing through a set of nozzles between each wheel. The nozzles are formed of blades on the periphery of stationary wheels. These blades are curved in the opposite direction to the blades on the revolving wheels, as shown in Figure 49. Steam in passing through the ring of curved stationary blades is divided up into a series of jets which strike the curved blades of the first wheel. In passing through this wheel the direction of the stream is reversed, and it enters between the second set of stationary blades, which turn it back again and direct it against the next wheel. Thus the steam pursues a sinuous course through the series of wheels. To allow for the expansion of the steam the blades are made progressively longer and the wheels of progressively larger diameter from the inlet to the exhaust end of the engine.
FIG. 49.—PERIPHERAL VIEW OF THE BLADES OF A PARSONS TURBINE
The Curtis turbine combines the De Laval and the Parsons principles. The steam enters through a series of nozzles which are of the expanding type, then it goes through a series of moving and stationary blades, as in the Parsons turbine, from which it enters another set of expanding nozzles and gains velocity and momentum before passing through the second series or stage of moving and fixed blades. (See Figure 50.)
Steam turbines are particularly adapted for use in electric power plants. The speed of rotation of the Parsons and Curtis types is much lower than that of the De Laval and hence the electric generators may be directly driven by them without the interposition of any gearing. They can be built of larger power than the reciprocating engines because they are so economical of space.
FIG. 50.—PERIPHERAL VIEW OF THE BLADES AND NOZZLES OF A CURTIS TURBINE