Substituting values, then, we have:
A head of 27 feet would give this stream a velocity of 41.7 feet a second, and, from formula (B) we find that the capacity of the wheel should be 30 square inches.
It is well to remember that the square inches of wheel capacity does not refer to the size of pipe conveying water from the head to the wheel, but merely to the actual nozzle capacity provided by the wheel itself. In small installations of low head, such as above a penstock at least six times the nozzle capacity should be used, to avoid losing effective head from friction. Thus, with a nozzle of 30 square inches, the penstock or pipe should be 180 square inches, or nearly 14 inches square inside measurement. A larger penstock would be still better.
CHAPTER IV
THE WATER WHEEL AND HOW TO INSTALL IT
Different types of water wheels—The impulse and reaction wheels—The impulse wheel adapted to high heads and small amount of water—Pipe lines—Table of resistance in pipes—Advantages and disadvantages of the impulse wheel—Other forms of impulse wheels—The reaction turbine, suited to low heads and large quantity of water—Its advantages and limitations—Developing a water-power project: the dam; the race; the flume; the penstock; and the tailrace—Water rights for the farmer.
In general, there are two types of water wheels, the impulse wheel and the reaction wheel. Both are called turbines, although the name belongs, more properly, to the reaction wheel alone.