With the peripheral speed of the buckets set at 1350 feet a second, the diameter of the turbine wheel will determine the number of revolutions a minute which in small machines is as high as 30,000, and in larger machines 10,000 to 20,000 revolutions. To reduce this high speed of rotation within practical limits a pair of helical spur gears are used, giving a.reduction of 10 to 1. These form by far the biggest part of the entire outfit. The necessity of using gears to reduce the speed from 10,000 to 30,000 down to 1000 to 3000 revolutions limits the size of the DeLaval turbine to 500 horsepower.
To avoid the use of gears Dr. Rieler and Prof. Stumpf have designed a wheel which is in successful operation in Germany that is essentially the same as the DeLaval except that the diameter of the turbine wheel is made ten times as large, thereby reducing the number of revolutions to one-tenth while keeping the peripheral velocity the same.
With this slower speed is also avoided the necessity for the flexible shaft. These machines have been built up to 2000 horsepower with a wheel 10 feet in diameter and running at 3000 revolutions a minute, and it is proposed to build a 5000 horsepower turbine with a wheel 16 1/2 feet in diameter and running at 1600 revolutions a minute.
In the impact type turbine when using saturated or wet steam, there is considerable wear on the blades owing to the tremendous velocity of the steam, but in the Parsons type turbine this wear is almost entirely avoided on account of the low velocity of the steam which seldom exceeds 400 feet a second as compared with 1350 feet a second in the impact type. Thi-low velocity of the steam is obtained by use of the large number of rows of vanes through which the expansion takes place.
In the 300 horsepower turbine there are 70 rows of vanes so that, in expanding steam from 150 pounds pressure to 26 in. vacuum, the drop in pressure between any two adjacent rows is only 2 1/4 pounds, which will produce a velocity of about 380 feet in a second, for the steam or proper peripheral velocity of the bucket of 180 feet a second. With this low peripheral velocity there is no trouble in getting the speed of rotation down to 3000 revolutions a minute, or less if desired.
In the Curtis turbine steam is allowed to expand in a nozzle similar to DeLaval's until it has a velocity of 2000 feet per second and this velocity is then reduced by compounding through six rows of moving vanes additional expansion taking place while passing through these rows of vanes as in the Parsons. By this means, the peripheral velocity, which amounts to 1320 feet a second in the DeLaval and 200 feet a second in the Parsons, is brought down to 400 feet a second. The diameter of the turbine wheel is then made four times as great as the Parsons, following the plan of Reidel and Stumpf, which gives a velocity of rotation only one-half that of the Persons.
In spite of the fact that the steam turbine elimin-inates the two largest sources of loss in the reciprocating engine - initial condensation and friction, - its economy has not come up to the high standard expected on account of leakage past the vanes due to the clearance that is necessary to prevent the revolving vanes striking the stationary ones with difference in expansion of the two when heated, and the unknown losses due to the friction of the steam at high velocity in the expansion nozzle or in passing through the thousands of small passages. The actual steam economy which the builders will guarantee is just about the same as that guaranteed by the makers of the best types of compound engines.
Cost of attendance in favor of the turbine, as one man can care for more of the larger units than can two men where the reciprocating engine is used. Cost of maintenance is bound to be less with the steam turbine on account of its greater simplicity and freedom from a multitude of moving parts. The item of lubricating oil is almost entirely eliminated in the turbine and the only parts subject to wear and breakage are the vanes which are easily replaced.
If the question of space is vital, the steam turbine requires only three-fourths as much floor space as the vertical engine and only two-fifths as much as the horizontal engine. As you can install 4000 horsepower of steam turbines in the same building that would be required for 1000 horsepower of reciprocating engines, and this with only one-half the head room, you have one of the most important considerations in the designing and building of a power station in our large cities, or increasing the output from one already built. With the steam turbine are avoided all vibration and shock due to the inertia of the reciprocating parts. The weight of the reciprocating parts of a 500-horse-power engine is about 30,000 pounds, with the engine running at 100 revolutions a minute and having a 5-foot stroke, this immense weight must be brought from rest to a speed of 1600 feet a minute and then slowed down again to rest and stopped 200 times every minute; it is evident, therefore, what an advantage the turbine has in the matter of foundation necessary, to hold it. As to relative first cost, the question is hard to determine, because it is quite as difficult to compare the costs of the turbine and piston engines as it is to compare the costs of different engines. At the present time, the cost of a turbine and generator installation is about the same as for a piston engine and generator installation of the same capacity.
Fig. 2. First Form of Parson's Turbine.
Running of direct connected, alternating-current generators in parallel has come to be a frequent requirement, but frequent as it is, its accomplishment has been anything but an exact science. But no difficulty exists with the steam turbine, as there is no fluctuation of angular velocity, but one direction of motion and no element to detract from an even turning moment. Due to its speed, more flywheel effect is stored up than is present in the piston engine, so that steam turbines easily run together in parallel as hydraulic engines always have done.
It is now quite generally recognized that superheated steam has an advantage, although there is still much to be learned about it. Future investigations, however, in which the turbine will take an important part, will reveal more precisely its economic status, and it may be hoped that before long the net advantages to be derived from different high steam temperatures will be known. The turbine may be used unreservedly with superheat of any feasible temperature. It has no internal rubbing surfaces and there are no glands or packings to become injured; as no cylinder oil is required, there is no opportunity for lubricating troubles; furthermore, there seems to be rather more proportionate benefit from superheat with the turbine than with the piston engine because of diminished skin friction.
in every essential aspect of its commercial utility the steam turbine appears to stand on solid ground. It has its field chiefly in electric lighting and power sta -tions, although in small sizes it has been extensively used for driving blowers, pumps and other devices; its speed, of course, prohibits belt or rope driving, but the direct connected electrical generating unit has been the aim of modern power development.