Mechanics: The Science of Machinery

by A. Russell Bond

The water is directed into the buckets of the Pelton wheel in a stream six inches in diameter, and it issues from the nozzle with a velocity of 300 feet per second or about 210 miles per hour. A jet of water is almost like a solid bar of wood. In fact, it is impossible to chop through it with an ax. The water would swing the ax out of one’s hand before it got part way through the jet. Traveling at such a high speed the friction is so great that it would tear the skin off one’s hands, if it did not actually tear the hand off the arm, and yet it strikes the buckets of the wheels with no shock at all, for the first part of the bucket it touches is nearly parallel to the jet, and as the water sweeps around the curved face of the bucket it loses practically all of its pressure and velocity and falls into the tail race. The electric power generated by the two plants is stepped up to 150,000 volts and sent out over transmission lines to points of service. The street cars of Los Angeles are connected by a 240-mile electric harness to the hydraulic horses of Big Creek.

Powerful as this stream is, a still higher head is used in Switzerland, at Lake Fully, where there is a drop of over a mile in a distance of 2.8 miles. The water is carried by a short tunnel through the mountain, and then makes a drop of over 5,000 feet to the power plant, where it strikes the Pelton wheels at a velocity of 400 miles per hour, or about seven times the speed of a fast express train.

HARNESSING THE MISSISSIPPI

In contrast to such high heads, we have the low-head power plants which are employed where a large volume of water is available. The most notable installation of this type, and the largest in the world, is that at Keokuk, Iowa, where a dam has been thrown across the Mississippi River. For many years it was thought impossible to make any use of the vast volume of water that flows through this great river. But above Keokuk there used to be a rapid extending back about twelve miles. By building a dam across the river just below the rapid it was possible to obtain a working head of about thirty-two feet, and with the enormous volume of water available this provided sufficient energy to make the development worth while. In marked contrast to the installation at Big Creek, it is volume rather than velocity that is employed, and hence turbines rather than Pelton wheels are used. More water goes through a single turbine than is used in the whole of the city of New York with all its elaborate aqueduct system. Enormous turbines are used, fifteen feet in diameter, and when the installation is complete there will be thirty units, each yielding 10,000 horsepower, or a total of 300,000 horsepower. A turbine, it may be explained, differs from the ordinary water wheel in the fact that the water runs through the wheel instead of around it (Figure 32). The water may enter at the center and then flow out at the periphery, or it may enter at the periphery and then be discharged from the center of the wheel, or it may run axially through the wheel. In a Pelton wheel there is a single jet which strikes but one pair of buckets at a time, but in a turbine there are many jets distributed all around the circumference of the wheel. The water is divided into a series of jets by being forced through a stationary set of curved vanes. The blades of the rotor or revolving part of the turbine are oppositely curved. If the rotor were immovable the jets would have to change their direction in passing through the rotor, but as the rotor is free to turn, the jets react against these blades and set the wheel to revolving. The turbine may be designed to run either on a horizontal axis or on a vertical one.

FIG. 32.—TURBINE WHEELS; INFLOW TYPE SHOWN ON THE LEFT AND OUTFLOW TYPE ON THE RIGHT

The turbines used at the Keokuk plant are of the inflow type. The rotor is mounted on a vertical shaft in a scroll-shaped concrete chamber, something like a snail shell. Water pouring into this chamber is thus given a swirling motion in the direction of rotation of the wheel. As it flows into the wheel it passes first through a ring of fixed vanes, which divide it into the jets.

The highest velocity of a wheel is naturally at the periphery and the advantage of an inflowing turbine such as this is that the water is traveling at its highest velocity when it strikes the periphery of the rotor. As it loses its velocity it flows in toward the slower-moving portions of the rotor. Finally it reaches the center, after giving up practically all its energy, and falls into the tail pool through a draft tube at the center of the rotor.

The scroll chambers at Keokuk are thirty-nine feet in diameter and the draft tubes are eighteen feet in diameter. Water enters the scroll chambers with a velocity of fourteen feet per second and comes out of the draft tubes into the tail pool with its velocity cut down to but four feet per second. Compare this with the velocity of the water jets at Big Creek!

The current generated at Keokuk goes to St. Louis and surrounding towns and serves a population of 1,120,000.