Hydraulic Reference has already been made to the potential power for work by water stored at a height above the level where it is to be used. This power is exercised only in case the water is allowed to move. The power developed is that of the weight of the water used multiplied by the speed or velocity at which it moves; this product being termed its "momentum." From the previous discus sions it is seen that in malting use of moving water a much greater power can be secured by confining it in pipes than by conducting it in open channels. In the latter case, while the weight of the water used may be the same, the velocity factor in the momentum is quite small, whereas in a pipe it may be very large if the grade of the pipe is steep. So that a small quantity of water operating under a great head may be made to do much more work than a large quantity with practically no head at all, as in the case of an open channel. For this reason the open water water-wheels are much less efficient with the same volume of water than the enclosed Pelton and turbine wheels operated by pipe lines.
. The Turbine An overshot wheel is unscientific, because it carries a small part of the water beyond its lowest point, back on to the rising side of the wheel, where its weight is opposed to the forward movement; the undershot wheel gets the push of the water dur ing only a small part of its revolution, and also carries up waste water. The best construction aims to throw the water quick and hard into the wheel and get it out again without dragging on the wheel. The theoretically best construction has been found in the turbine wheel, which is fundamentally a wheel rotating on its• sides, on a perpendicular or upright axis, and taking in water at the sides, from gates in a stationary case, which admit the water at a desirable angle so that it strikes the curved blades of the runner or running wheel, gives them a quick thrust and falls through, delivering usually at the centre. It should be here noted that in English and Continental usage all water wheels are called turbines, while in American usage the word turbine is reserved for the style of water wheel just described, having spiraled flanges and runners. The turbine has been built in many forms, in the endeavor to secure the most power from the flowing water. The style which has become most common is the impulse or action turbine, having stationary gates with curved guides that direct the flow of water.
Immediately below is the runner, having flanges curved hut substantially at right angles to the fixed guides, so that the water delivers a push to the runner, and falls down, perhaps to a second, third or fourth runner. This developed type is now commonly called the Francis tur bine, after an engineer who developed it about 1850. The reaction turbine' has spiral-shaped flanges positioned to receive the water axially and deliver it tangentially.
The turbine is now almost universally adopted for all water powers of low head, that is under 300 feet head. It should be stood that ahead* signifies the height of the source of water above the point where it enters the turbine or •other water -wheel. This head gives a pressure the equivalent of the weight of a column of water of that height, or slightly less, owing to the friction of the tube or container.
Impulse or Pelton The impulse or wheel, after going through evolutionary development as overshot, under-, shot, breast wheel, etc., has settled into the form known as the Pelton wheel as the best type. It is in almost universal- use for high heads of 500 feet and over and in hydraulic street service. It is named after L. A. Pelton, because he did more than any other man to perfect the dividing wedge principle, which serves to direct the water to the best advantage, securing the highest efficiency deemed possible with a direct-impulse wheel. The Pelton is a wheel in its simplest form, usually rotating on a horizontal axis, having buckets bolted to its periphery, against which the impelling water is directed from one or more nozzles. The buckets are like twin hollowed hemispheres or half globes that are pushed together so that they divide by a straight line and sharp edge. The jet from the nozzle is aimed directly at this dividing ridge, which separates the water and gives it a direction that tends to throw it out of the hemispherical twin buckets as soon as it has done its work. The impulse wheel for utilizing high heads of water was first intro duced by the California miners of 1850, and gradually improved, Pelton doing his work about 1878 to 1880. In modern practice the construction has improved. See HYDRO-ELECTRIC