The old windmill had four sails against which the wind blew, driving the whole four round as everyone knows. The new turbine has a great many sails, only we now call them blades, and the steam blows them round. The old windmill had to have another smaller set of sails at the back for the purpose of keeping the main sails always in that position in which they would catch the full force of the breeze. In the turbine we need not do that, for we shut the windmill up in a kind of tunnel and cause the steam to blow in at one end and out at the other.
The difference between the various kinds of turbine lies simply in the manner in which the steam is guided in its passage through the machine.
After that general description we can take a more detailed view of the Parsons turbine. The casing or fixed part is a huge iron box suitably shaped for
standing firmly and rigidly upon the floor of the engine-room. It is made in two halves, the upper of which can be easily lifted off when necessary. Often, indeed, this upper half is hinged to the lower, so that it can be opened like the lid of a box.
Inside, the casing is cylindrical, comparatively small at one end but increasing by steps till it is very much larger at the other end. At each end is a bearing or support in which the rotor or moving part is held and in which it can turn freely.
The rotor or part which rotates is a strong steel forging shaped somewhat to follow the lines of the inside of the casing. It does not entirely fill the casing but leaves a space all round and all the way along, which space is intended to accommodate the blades. The ends of the rotor are smaller than the body since they are intended to fit into the bearings, and one of the ends is prolonged so as to be available for coupling to the propeller-shaft of the ship.
At one end of the casing, the smaller one, is the steam inlet and the steam after emerging from it passes along till it finds its way out at a very large outlet formed at the bigger end. On its way it has to pass thousands of small blades so that the progress of each individual particle of steam is not a straight line but a continual zigzag. There are rings of blades round the rotor, tightly fixed to its surface. There are likewise rings of blades affixed to the inner surface of the casing, the rings upon the casing coming in the spaces between the rings on the rotor.
Let us imagine that we can see through the casing of a turbine at work and that looking down upon it
from above we can trace the progress of a particle of steam. It rushes in from the inlet and at once makes straight for the outlet at the further end. Suddenly, however, it encounters one of the guide blades (those on the case) and by it is deflected to one side, we will suppose the left. That causes it to rush straight at one of the blades upon the rotor against which it strikes violently, giving that blade a distinct and definite push to the left. Rebounding, it then comes back towards the right but quickly is caught by another guide blade and by it hurled back upon a second rotor blade, giving it a leftward push just as it did to the first. Thus it goes zigzagging from one set of blades to the other until, tired out, so to speak, it finally flows away forceless and feeble through the outlet, having given up all its energy to the blades of the rotor against which it has struck in its course.
That, then, is the journey of one single particle. Multiply that by an unknown number of millions and you have a description of what takes place in the interior of a steam turbine. The blades are so proportioned, so arranged and so placed that it is very difficult indeed for a particle of steam to creep past without doing its share of work. Practically every one is made use of and while, of course, the action of a single particle of steam would have but a negligible effect, the vast number engaged cause the rotor to be powerfully blown round.