Turbine Steam

Passing over the blade-surfaces, the steam jets are deflected out of their paths OA, and shoot off the tails of the blades with velocity CD relative to the blades, which, if frictional losses are neglected, is equal to the relative velocity of entry OB. But as the blades are themselves moving with velocity "U" or ED, the actual path taken by the steam jets at exit, relative to the fixed turbine casing, will be CE.

It was mentioned previously that the best velocity-ratio for this type of blading is about 0.45; in other words the steam-jet velocity OA must be rather more than .wice the mean blade velocity BA. When this is the case, as will be seen from the diagram the steam finally leaves the wheel with axial velocity CE, which is evidently the minimum, for if it left in the direction C'E' (fig. 14b), it would evidently be greater. When CE is a minimum it is evident that the maximum energy has been extracted from the steam jets, and this occurs when is slightly CI less than one-half.

In Parsons' reaction blading, on the other hand, the best velocity ratio is about unity, that is to say, the mean blade-speed has to be nearly equal to the velocity of the steam jets. Thus OA (fig. 5a) represents the steam velocity. Circumferentially, this is equal to BA, the velocity with which the blades are running away from the steam-jets. To an observer at B therefore (mov ing round with the blades), the steam jets would appear to approach along OB, which is parallel to the axis of the turbine. Now this apparent or relative velocity of ap proach OB of the steam jets is quite small and further, being axial in direction, exerts no driving force on the moving blades. Hence the statement made that the func tion of the fixed nozzles is to bring the steam into the moving nozzles smoothly and without shock. Figure 1513 shows the diagram when the velocity-ratio is too low.

With either type of blading ("impulse" or "reaction") the ideal condition for theo retical maximum efficiency is reached when the steam leaves the moving blades with a relative circumferential velocity ED equal to the mean blade-speed BA, so that in relation to the turbine casing itself the actual velocity is a minimum, and the maximum energy has been extracted from the steam jets.

Combined Types of Turbines.

The velocity-compounded wheel of Curtis is frequently used for the first pressure stage of a turbine of which the succeeding stages are either of the com pound impulse or of the compound reaction type. A two-row Curtis wheel displaces three or four de Laval wheels in series, or alternatively seven or eight pairs of nozzle rings of the Parsons reaction type. Al though it has an inherently lower efficiency than single-row de Laval wheels or than reaction blading, its use has been justified by some designers on the grounds that at the high-pressure end of a turbine the efficiency of the latter is considerably reduced by steam leakages or windage losses, or both so that the net efficiency of the two-row Curtis is not much inferior.

Recently, however, in land turbines there has been a return to the use of the Parsons principle alone, the whole turbine, that is to say, being built up of simple pressure stages only (either of the de Laval or of the Parsons type), without any velocity compounding.

The large number of such stages required for high efficiency led Parsons as long ago as 1899 to divide up the casing into two sep arate "cylinders" or separate turbines, coupled together in line and driving the same dynamo. Parsons used three separate cylin ders in series on the steam-flow for the triple-shaft steam yacht "Turbinia" in 1897 (to be seen in the Science Museum, South Kensington, London).

In

modern land practice for the generation of electricity on a large scale in central power-stations, the two-, or even three cylinder turbine is widely used and in some continental examples even four separate cylinders have recently been adopted as a means of securing the highest efficiency, having regard to the ever increasing initial steam pressures and temperatures that are now being favoured.

Present Position of the Steam Turbine.

Essentially a high speed prime-mover, the steam turbine in the early days of its de velopment was unsuited to drive machinery which must run at a moderate or low speed of revolution. Such machinery might be grouped under the following headings :— (i). Direct current dynamos.