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.
Impulse wheels derive their power from the momentum of falling water. Reaction wheels derive their power from the momentum and pressure of falling water. The old fashioned undershot, overshot, and breast wheels are familiar to all as examples of impulse wheels. Water wheels of this class revolve in the air, with the energy of the water exerted on one face of their buckets.
On the other hand, reaction wheels are enclosed in watertight cases, either of metal or of wood, and the buckets are entirely surrounded by water. The old-fashioned undershot, overshot, and breast wheels were not very efficient; they wasted about 75 per cent of the power applied to them. A modern impulse wheel, on the other hand, operates at an efficiency of 80 per cent and over. The loss is mainly through friction and leakage, and cannot be eliminated altogether. The modern reaction wheel, called the turbine, attains an equal efficiency. Individual conditions govern the type of wheel to be selected.
The Impulse, or Tangential Water Wheel
The modern impulse, or tangential wheel (so called because the driving stream of water strikes the wheel at a tangent) is best adapted to situations where the amount of water is limited, and the head is large. Thus, a mountain brook supplying only seven cubic feet of water a minute a stream less than two-and-a-half inches deep flowing over a weir with an opening three inches wide would develop two actual horsepower, under a head of 200 feet not an unusual head to be found in the hill country. Under a head of one thousand feet, a stream furnishing 352.6 cubic feet of water a minute would develop 534.01 horsepower at the nozzle.
Ordinarily these wheels are not used under heads of less than 20 feet. A wheel of this type, six feet in diameter, would develop six horsepower, with 188 cubic feet of water a minute and 20-foot head. The great majority of impulse wheels are used under heads of 100 feet and over. In this country the greatest head in use is slightly over 2,100 feet, although in Switzerland there is one plant utilizing a head of over 5,000 feet.
The old-fashioned impulse wheels were inefficient because of the fact that their buckets were not constructed scientifically, and much of the force of the water was lost at the moment of impact. The impulse wheel of today, however, has buckets which so completely absorb the momentum of water issuing from a nozzle, that the water falls into the tailrace with practically no velocity. When it is remembered that the nozzle pressure under a 2,250-foot head is nearly 1,000 pounds to the square inch, and that water issues from this nozzle with a velocity of 23,000 feet a minute, the scientific precision of this type of bucket can be appreciated.
A typical bucket for such a wheel is shaped like an open clam shell, the central line which cuts the stream of water into halves being ground to a sharp edge. The curves which absorb the momentum of the water are figured mathematically and in practice become polished like mirrors. So great is the eroding action of water, under great heads especially when it contains sand or silt that it is occasionally necessary to replace these buckets. For this reason the larger wheels consist merely of a spider of iron or steel, with each bucket bolted separately to its circumference, so that it can be removed and replaced easily.
Usually only one nozzle is provided; but in order to use this wheel under low heads down to 10 feet a number of nozzles are used, sometimes five, where the water supply is plentiful. The wheel is keyed to a horizontal shaft running in babbited bearings, and this same shaft is used for driving the generator, either by direct connection, or by means of pulleys and a belt. The wheel may be mounted on a home-made timber base, or on an iron frame. It takes up very little room, especially when it is so set that the nozzle can be mounted under the flooring. The wheel itself is enclosed, above the floor, in a wooden box, or a casing made of cast or sheet iron, which should be water-tight.
Gates
Since these wheels are usually operated under great heads, the problem of regulating their water supply requires special consideration. A gate is always provided at the upper, or intake end, where the water pipe leaves the flume. Since the pressure reaches 1,000 pounds the square inch and more, there would be danger of bursting the pipe if the water were suddenly shut off at the nozzle itself. For this reason it is necessary to use a needle valve, similar to that in an ordinary garden hose nozzle; and by such a valve the amount of water may be regulated to a nicety.
Where the head is so great that even such a valve could not be used safely, provision is made to deflect the nozzle. These wheels have a speed variation amounting to as much as 25 per cent from no-load to full load, in generating electricity, and since the speed of the prime mover the water wheel is reflected directly in the voltage or pressure of electricity delivered, the wheel must be provided with some form of automatic governor. This consists usually of two centrifugal balls, similar to those used in governing steam engines; these are connected by means of gears to the needle valve or the deflector.