Parsons' Master Principle of "Pressure Compounding."— The application of steam turbines on these lines could never, owing to the limitations described above, become general, and the master invention which has opened up an unlimited field of utility was supplied in 1884 by Sir Charles A. Parsons.
The high efficiency already attained by water turbines (q.v.) operating under very moderate "heads" of water, made it seem to him possible that if a number of "simple" one wheel steam tur bines were placed in series on the same shaft, one turbine exhaust ing its steam into another, and so on, it would be practicable to divide up the energy of expansion of the steam over a number of such "elementary" turbines so that the effective "head" of steam (in other words, the amount of expansion) in each would be small.
Thus, since the velocity of efflux of the steam jets in each "simple" turbine of the series would be correspondingly reduced, the latter would operate under conditions more analogous to the water turbine, that is with moderate and practical blade speeds, in conjunction with high "velocity-ratio" and high efficiency.
This fundamental concept was subjected to practical test by Parsons in a turbine of his own design at Gateshead-on-Tyne, England, in 1884, and has since proved to be the only solution.
a later date) to the simple turbine of de Laval. Figure 5 gives a diagram of a modern turbine of this kind, associated chiefly with the name of Rateau, who developed it from about the year 1896. The turbine consists of a series of de Laval wheels or steam "windmills" all mounted on the same shaft, each wheel being placed in a compartment. High pressure steam is admitted to the first wheel of the series through suitable nozzles. (Figure 6.) In the partition dividing the first wheel from the second wheel there is a second set of nozzles so proportioned as to maintain a suitable back pressure in No. i compartment and not allow the steam to escape from it too freely. By thus limiting the drop of pressure from boiler pressure to that in the first compartment, the steam velocity generated in the first set of nozzles can be adjusted to any desired value and consequently also the resulting jet velocity.
The passages which form the exhaust or exit ports from the first wheel are actually nozzles delivering the steam in the form of jets on the second wheel of the series. In the succeeding wheels
the same procedure is followed over and over again, the initial steam pressure being thus reduced by appropriate steps until finally the exhaust at exit pres sure is reached in the last wheel. From a correct knowledge of the properties of steam and of the losses which occur due to friction and eddies, it is possible to choose the proportions of the nozzles so that every wheel of the series operates at a proper velocity-ratio.
Difficulties Attending Pres sure Compounding.—There are certain unavoidable losses en countered in the practical utilization of the Parsons principle of pressure compounding.
It will be seen from fig. 5 that there are places in a "corn pound" turbine where serious leakage can take place, viz. round the periphery of the shaft where it passes through the "dia phragms" separating one wheel from another, and also at each end of the shaft where it passes into the outer atmosphere.
The necessary surface speeds in successful steam turbines are so high that generally speaking leakage cannot be entirely stanched by any form of packing which involves actual rubbing contact, on account of the great frictional heat that would be generated. It follows therefore, that such leakage, if it cannot be altogether suppressed, must be reduced to a minimum by the use of fine working clearances. Thus all compound turbines, of whatever design, have fine clearances in certain parts, such fine clearances being essential to steam economy.
The early pioneer work of Sir Charles Parsons was carried out against considerable obstacles. For 13 years after 1884, no notice whatever of his efforts was taken on the continent of Europe.
In dealing with the problem of the reduction of steam leakage, Parsons originated two principles, namely, that where the relative motion between two surfaces is very great and it is desired to limit the fluid or gaseous leakage between them as between the rotating shaft and the fixed casing, ) One of the surfaces should be provided with thin edges or so-called "contacts"; (2) Both surfaces must be serrated, i.e., given an interrupted contour in the direction of the pressure gradient.