The steam entering the annular space between the shaft and the case passes firstly through a ring of guide blades attached to the case, and is given a rotational direction of flow; it then passes to the succeeding ring of blades attached to the shaft, by which its direction of rotation is reversed, thereby impressing the difference of its rotational momentum in torque to the shaft. The steam then passes to the second ring of guide blades, and the process is repeated, and so on, gradually expanding by small increments at each ring of blades; the succeeding rings of blades get longer and wider, and at intervals the diameter of the turbine drums, cylinders, and rings are also increased. In condensing turbine engines of the larger size an expansion ratio in the turbines of one hundredfold and upward is attained before the steam passes to the exhaust pipe and condenser.
The loss of power present in engines of the piston class, due to cylinder condensation arising from the variation of steam pressure in the cylinder, is not present in the steam turbine, as the steam pressure remains constant at each turbine ring and each part of the cylinder and barrel, and the numerous tests of steam consumption that have been made have shown that compound steam turbine engines of moderate sizes when working with a condenser are comparable in steam consumption per effective horse power with the best compound or triple condensing steam engines of the piston type. They have been constructed in sizes up to about one thousand horse power for driving alternators and dynamos, and several sets of about two thousand horse power are nearing completion.
The application of the compound steam turbine to the propulsion of vessels is a subject of considerable general interest, in view of the possible and probable general adoption of this class of engine in fast vessels.
In the turbine is found an engine of extremely light weight, with a perfectly uniform turning moment, and very economical in steam in proportion to the power developed, and, further, it can be perfectly balanced so that no perceptible vibration is imparted to the ship. The problem of proportioning the engine to the screw propellers and to the ship to be driven has been the subject of costly experiments extending over several years, with the result that a satisfactory solution has been found, giving very economical results in regard to pounds of steam consumed in the engines per effective horse power developed in propelling the vessel, results which are equal or superior to those so far obtained with triple-expansion engines of ordinary type in torpedo boats or torpedo-boat destroyers. The arrangement adopted may be best described by saying that instead of placing, as usual, one engine to drive one screw shaft, the turbine engine is divided into two, three, or sometimes more separate turbines, each driving a separate screw shaft, the steam passing successively through these turbines; thus when there are three turbines driving three shafts, the steam from the boiler passes through the high-pressure turbine, thence through the intermediate, and lastly through the low, and thence to the condenser.
Fig. 3.—A Seventy-five Kilowatt Turbine Engine directly connected to a Dynamo. The turbine engine is on the right.
As to the propellers, these approach closely to the usual form. It has, however, been found best to place two propellers of approximately the same pitch on each shaft at some considerable distance apart, so that the after one shall not be seriously affected by the wash of the one in front. The advantage of this arrangement is that a sufficient blade area is obtained to carry the thrust necessary to drive the vessel with a lesser diameter of propeller, and so permitting of a higher speed of revolution of the engines.
The problem was complicated by the question of cavitation, which, though previously anticipated, was first practically found to exist by Mr. Thornycroft and Mr. Barnaby in 1894, and by them it was experimentally determined that cavitation, or the hollowing out of the water into vacuous spaces and vortices by the blades of the propeller, commences to take place when the mean thrust pressure on the projected area of the blades exceeds eleven pounds and a quarter per square inch. This limit has since been corroborated during the trials of the Turbinia.
This phenomenon has also been further investigated in the case of model propellers working in an oval tank of water, and to permit of cavitation at more moderate speeds than would otherwise have been necessary, the following arrangement was adopted: The tank was closed, plate-glass windows being provided on each side, through which the propeller could be observed, and the atmospheric pressure was removed from the surface of the water by an air pump; under this condition the only forces tending to prevent cavitation were the small head of water above the propeller, and capillary attraction.
In the case of a propeller of two inches in diameter, cavitation commenced at about twelve hundred revolutions, and became very pronounced at fifteen hundred. Had the atmospheric pressure not been removed, speeds of twelve thousand and fifteen thousand respectively would have been necessary.