FIG. 30.—UNDERSHOT, OVERSHOT, AND BREAST WHEELS
INVENTION OF THE PELTON WHEEL
FIG. 31.—SECTIONAL VIEW OF A PAIR OF PELTON WHEEL BUCKETS SHOWING HOW THE WATER JET IS DIVIDED AND FOLLOWS THE CONTOUR OF THE BUCKETS
In California, a number of years ago, they made use of what was known as the hurdy-gurdy wheel. This consisted of an ordinary wheel with bucket-shaped paddles against which was directed a stream of water at high velocity through a nozzle. There was a carpenter, named L. A. Pelton, who used to make a business of building and repairing such wheels and the flumes that carried the water to them. Although uneducated, he was possessed of considerable native ingenuity and was a very observant man. One day, when he was called in to repair a wheel, he noticed that one of the buckets which had been misplaced received the water from the nozzle without any splashing. The water struck the edge of the bucket with practically no shock, whereas the other buckets produced a great deal of splashing. Pelton had enough knowledge of the principles of mechanics to realize that a splash means a waste of energy, and that here was a bucket which, although out of plumb and apparently defective, was really more efficient than any of the others in the wheel. It occurred to him then that instead of having the jet of water strike the middle of the buckets it ought to strike the edge, so that all its power would be absorbed without any wasteful splashing. He might have displaced the jet laterally so as to accomplish this result, but he realized that that would have produced a considerable side thrust on the wheel, which, of course, would have been objectionable, and so he hit upon the plan of using double buckets and letting the stream of water strike the pair of buckets along their dividing line. (See Figure 31.) This would split the stream in two and let each half strike the slanting face of the bucket, and follow the surface around in the same way that it did on the single misplaced bucket, but the reaction or side thrust on one bucket would be counteracted by that on the other. This idea proved successful and out of it has grown the Pelton wheel which is now universally used in all power plants employing high heads of water.
A 4,000-FOOT HEAD OF WATER
A notable illustration of such a plant is the great installation at Big Creek, Cal. Big Creek, despite its name, used to be a small stream flowing down the mountains into a canyon. One would hardly suppose that it was capable of yielding much power, but it had its source high up in the Sierras and was fed mainly by melting snows. In the springtime, it swelled to a good-sized torrent. By building three dams near the top of the mountain, a lake was formed in which the water of the melting snow was impounded, so that a steady stream of water could be supplied the year round for power purposes. But even so, the stream hardly amounts to very much if we consider only the quantity of water that passes through it. The particular advantage of this installation is the fact that in a distance of six miles from the dam the creek falls 4,000 feet.
An inhabitant of the Eastern States who is unused to mountain heights may gain some conception of the meaning of this elevation by gazing up to the pinnacle of the Woolworth Tower, which rises 795 feet above street level, then mentally multiplying its altitude by five. Evidently even a small stream of water dropping from such an elevation would develop an enormous amount of power. In fact, it was considered inexpedient to use the entire fall at a single drop and so it was divided into two stages. The water is carried through a tunnel three-quarters of a mile long and then through a flow pipe along the face of the mountain to a point where it may drop 2,000 feet to the first power plant. After passing through this plant the water is discharged into the creek and is then diverted into a second tunnel four miles long and a series of steel conduits to a point from which it may drop 2,000 feet more to the second power plant. In each power house there are two electric generators, each fitted with a pair of Pelton wheels. These wheels are a little less than eight feet in diameter and each one develops 23,000 horsepower.