With running or falling streams a large surface is required, and the wheels turn slowly. Two well-recognized forms of wheels have been employed, one called the undershot, or breast wheel, shown in [Fig. 1], and the other the overshot, illustrated in [Fig. 2].
In both types it is difficult to so arrange them as to shut off the power or water pressure when required, or to regulate the speed.
The Turbine.—Wheels which depend on the controllable pressure of the water are of the turbine type. The word is derived from the Latin word turbo, meaning to whirl, like a top. This is a type of wheel mounted on the lower end of a vertical or horizontal shaft, within, or at the bottom, of a penstock. The perimeter of the wheel has blades, and the whole is enclosed within a drum, so that water from the penstock will rush through the tangentially-formed conduit into the drum, and strike the blades of the wheel.
A column of water one inch square and twenty-eight inches high weighs one pound,—or, to express it in another way, the pressure at the bottom of such a column is one pound, and it is a pound for each additional 28 inches.
If there should be a head or height of water column of seven feet, the pressure on each square inch of water at the bottom of the penstock would be three pounds to the square inch. Assuming the opening or duct leading to the wheel blades should be 12 × 12 inches, and also the blades be 12 × 12 inches, the area would be equal to 144 square inches, and this multiplied by three pounds would equal 432 pounds pressure against the blades.
Calculating Power of a Turbine Wheel.—The power of such a wheel depends principally on two things. First, the arrangement of the blades with reference to the inflowing water; and, second, the discharge port, or ability of the water to free itself from the wheel casing.