Efficient Modern Adaptations of the Archaic Undershot and Overshot Water Wheels
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 to-day, 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.
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.
As the demand for farm water-powers in our hill sections becomes more general, the tangential type of water wheel will come into common use for small plants. At present it is most familiar in the great commercial installations of the Far West, working under enormous heads. These wheels are to be had in the market ranging in size from six inches to six feet and over. Wheels ranging in size from six inches to twenty-four inches are called water motors, and are to be had in the market, new, for $30 for the smallest size, and $275 for the largest. Above three feet in diameter, the list prices will run from $200 for a 3-foot wheel to $800 for a 6-foot wheel. Where one has a surplus of water, it is possible to install a multiple nozzle wheel, under heads of from 10 to 100 feet, the cost for 18-inch wheels of this pattern running from $150 to $180 list, and for 24-inch wheels from $200 to $250. A 24-inch wheel, with a 10-foot head would give 1.19 horsepower, enough for lighting the home, and using an electric iron. Under a 100-foot head this same wheel would provide 25.9 horsepower, to meet the requirements of a bigger-than-average farm plant.
The Pipe Line
The principal items of cost in installing an impulse wheel are in connection with the pipe line, and the governor. In small heads, that is, under 100 feet, the expense of pipe line is low. Frequently, however, the governor will cost more than the water motor itself, although cheaper, yet efficient, makes are now being put on the market to meet this objection. In a later chapter, we will take up in detail the question of governing the water wheel, and voltage regulation, and will attempt to show how this expense may be practically eliminated by the farmer.
To secure large heads, it is usually necessary to run a pipe line many hundreds (and in many cases, many thousands) of feet from the flume to the water wheel. Water flowing through pipes is subject to loss of head, by friction, and for this reason the larger the pipe the less the friction loss. Under no circumstances is it recommended to use a pipe of less than two inches in diameter, even for the smallest water motors; and with a two-inch pipe, the run should not exceed 200 feet. Where heavy-pressure mains, such as those of municipal or commercial water systems, are available, the problem of both water supply and head becomes very simple. Merely ascertain the pressure of the water in the mains when flowing, determine the amount of power required (as illustrated in a succeeding chapter of this book), and install the proper water motor with a suitably sized pipe.