The Zoelly Steam Turbine.

Progress, Volume IV, Issue 8, 1 June 1909, Page 268

The Zoelly Steam Turbine.

The importance of this turbine, known to many engineers before the Franco-British exhibition, was revealed to the world in general at that international gathering. It was described at once by many authorities, amongst them by our contemporary Engineering , from the columns of which journal we make a few extracts. The Zoelly turbine is of the compound impulse type — that is, the steam is expanded successively through small ranges of pressure, the velocity acquired during each expansion being utilised by a separate single wheel. Each wheel runs in a compartment by itself, the number of wheels required being determined by the range of expansion through which the turbine has to work, the pressure in each compartment being never less than 58 per cent, of that in the previous one. This limitation enables the full expansion at every stage to be obtained with simple convergent nozzles, which are easier to design and more efficient than the flared nozzles, which become necessary with greater expansion ratios. The impulse type of pressure-compounded turbine seems to have originated with Moorhouse over thirty years ago. In view of its many merits, it is somewhat surprising that a number of British firms have not developed it on their own account, as it is a type particularly easy to design. It is free from the mechanical difficulties which have oeen so difficult to surmount in the case of the reaction type of turbine, and which have gone far to discount the higher efficiency theoretically possible with the latter. Blade-stripping has no doubt occurred with impulse turbines, but arose solely from the bad design of the discs, and all patterns now made are safe in this regard, even if using highly superheated steam. Steam enters the annular steam-chest on the extreme left of the casing and passes thence to the first wheel-chamber through nozzles cored in the circular casting shown. After doing its work on the wheel, the steam enters the next chamber through

similar nozzles formed in the diaphragm between the chambers, and gives up the velocity acquired in these nozzles to the second wheel. Thence alternately through nozzles and wheel-blades it passes away to the exhaust at the right-hand end of the casting. The nozzles are divided by curved iron plates about an eighth of an inch thick,

which form their sides, the top and bottom of the nozzle passages being the main easting itself, into which the division-plates are east. The latter are spaced at about 2in. pitch, and direct the steam at an angle of 20deg. to the plane of the wheel. The nozzles at the high-pressure end are about three-eights of an inch deep radially, and are disposed in two groups on opposite ends of a diameter. The depth is main-

tamed constant for as long as possible, the extra steam- way necessitated by the greater volume of the steam in successive compartments being obtained by adding extra nozzles to each group. Finally, when all the available circumference is filled by nozzles, the latter are increased in depth to accommodate the increasing volume of the steam. The wheels are also increased in

diameter in the low-pressure half of the turbine, so as to get the requisite steam-way without using unduly long blades. For overloads high-pressure steam" is admitted around the belt in the casing shown in the illustration, whence it passes into the turbine proper by means of ports in the flange of the fourth diaphragm. The effect is analogous to that of by-passing live steam to the intermediate or low-pressure cylinder

of a reciprocating engine; and though, of course, the steam is not used so efficiently, a large increase in the power may be obtained. The turbine casing is divided horizontally, so that all the wheels are exposed when the top half is raised. This will be well understood from the illustration, which shows the turbine open, with its rotor lifted. Eye-bolts are provided for lifting the cover, and with it come the upper halves of all the diaphragms, for these latter they are also split horizontally. The diaphragms are of cast iron, slightly dished, and with horizontal flanges along the joint, through which temporary bolts are put to hold the halves together while being machined. Nozzles are cast in as already mentioned, and the diaphragms are located longitudinally by circumferential flanges, which form spacing-pieces between them. Set-screws through the casing hold the half-diaphragms from moving when the rover of the turbine is lifted. The central hole in the diaphragms is bushed with bronze, the bore of the bushing being turned with numerous grooves to prevent leakage between compartments. „ The turbine- wheels are made from about 5 per cent, nickel steel, and are very highly finished all over, to minimise disc friction. This is an important consideration with large wheels running at a very high speed in the dense steam prevailing in the highpressure end of an impulse turbine. The Zoelly wheels vary from about 2 centimetres in thickness at the boss to about 1 centimetre under the rim. About half way between the boss and the rim the wheel is thickened up by a facing on each side, and through this thickened part half a dozen large holes are drilled. The function of these holes is to permit cf the instantaneous equalisation of the pressure on each side of a wheel when the load is suddenly thrown on or off. Normally, of course, there is almost perfect steam balance, as no expansion takes place in the moving blades. Any end thrust there may be is taken by a simple thrust block, the function of which, however, is more to locate the rotating parts endwise than to do anything else. . The blades are of nickel steel ground to a good finish, the leading edge ground very fine at the back, their thickness being about a millimetre and the distance pieces are of the same metal machined, all over, and polished on the upper face, which is in contact with the flow of steam. By their shape they hold the blades so that the exit angle of each bucket is much less than the entrance angle. The whole job is obviously of great mechanical strength, and the blade angles are of necessity accurate and fixed. As a testimony to the strength of the blading, we have been told of a ease in which a nut, carelessly left in the easing, wore a deep groove in the blading without stripping any blades or putting the turbine out of action, or even giving any evidence of its presence until the machine was opened; and we are informed that no single instance of the stripping of blades has been reported to the makers.

A correspondent states that a patent automatic stop, invented by Messis. jiimmott, Howarth, and J. W. Horsfall, of Wesham, near Preston, lias been expeiimentally tiied with success on the Lancashire and Yorkshire R'dlway ft can be so arranged that it will shut off steam and apply the vacuum brake without any assistance from either the driver or any one else.