Fitz Water Wheels

Engineering News

"Test on a Overshot Water Wheel"

January - June 1913


BY C. R. -WEIDNER

A series of tests on a 10-ft. steel overshot water wheel have recently been completed at the hydraulic laboratory of the University of Wisconsin, the results of which may be of interest to engineers who have occasion to develop small water powers.

A large number of American engineers consider the overshot water wheel as out of date relic of the past which has been replaced by the turbine, and for modern hydro-electric developments of large power there call he no question as to the superiority of the latter. The overshot wheel, however, is still capable of serving a distinct field, where the power to he developed is small and the speed of the machinery is slow and requires but little regulation. The fact that wheels of this type are still being installed in Germany and France, after careful engineering investigation, for small factories, pumping plants, and even for the development of electrical energy, may he taken as an indication of satisfactory and economical use of this type of motor.

Eng News Waterwheel Picture

The overshot wheel was formerly constructed of wood and a number of picturesque and interesting examples of this type of construction still remain in this country. In recent years, however, the wheel has been built entirely of steel and iron. Although the initial cost of a wooden wheel is less than one built of steel, the latter possesses certain advantages which makes it more desirable to install. Not-withstanding the increased cost. Unless a wooden wheel he kept continually in operation, the constant swelling and drying of the wood soon causes all parts to get loose; the buckets leak and a considerable percentage of the available energy is wasted. In a steel wheel the buckets can be shaped more readily to accord with the design; a larger percentage of the total head is made available on account of the smaller thickness of the metal; and the friction in the bearings is decreased because a steel wheel is lighter than a wooden wheel well soaked with water. For these reasons the steel wheel is more efficient. Moreover, it is more durable, the cost of maintenance is less, and the parts are more readily assembled.

Fig. 1, shows an example of a steel overshot wheel of modern design - the wheel tested at, the University Wisconsin. Wheels of this type have been installed in various parts of the United States and South America. The buckets are formed by curved vanes attached to the cylindrical surface or inner circumference of the wheel, called the soling, and to the segmental circular housings on the sides. The vane thus forms the front or outer part of the bucket, the soling the back or inner flanged steel riveted together. To the housings are bolted the radial arms, made of flat bar steel, which in turn are bolted to cast-iron center flanges keyed to the shaft. Power is usually taken from the wheel by means of gearing and is transmitted by shaft, belt, or rope drive. In some designs the segmental spur gearing is bolted to the arms of the wheel, as in Fig. 1. In others, a gear wheel is keyed directly to the shaft. The manufacturers furnish either ball or roller bearings as specified.

The advantages of the overshot wheel are;
(1) High efficiency.
(2) Adaptability to varying discharge,
(3) Simplicity in construction,
(4) Reliability,
(5) No interference of operation on account of clogging with debris or ice.

Of these advantages the first two are of the greater importance. The results of the experiments discussed below show high efficiencies under a wide range of operating conditions. Reliable tests of turbines have reported yielding as high as 89% efficiency, but it is rarely that this figure is attained in an actual installation. In the smaller plants especially, where an overshot wheel would be capable of competing with a turbine, it is doubtful whether the turbines operate with efficiency higher than 70%. It is, however, extremely difficult to make any comparisons as to the efficiency of two types of motors, unless both have been tested under exactly similar conditions of operation.

The installation of a turbine requires a technical analysis of the problem, and unless the turbine is set properly and selected for the particular conditions under which it is to operate, the efficiency will fall far below that of which it is capable when operating under the proper conditions. Very few of the small water powers developed receive the proper engineering supervision to promote a high degree of efficiency in their operation. The overshot wheel, on the other hand, suffers but little from a lack of proper design or selection, although to obtain the highest efficiencies of which the wheel is capable, it is not advisable to dispense with a technical analysis.

To engineers familiar with the variation in efficiency turbine at part gate, a glance at the curves obtained from the Wisconsin experiments ( Fig. 4) will be convincing as to the superiority of the overshot wheel, in respect to its adaptability to varying discharge. The experiments show that the range in the discharge may be as much as 400% with only a difference of 5% in the efficiency or the wheel. The other advantages cited are of minor importance, though in particular cases they may influence the choice of type motor to be selected. The limitations or disadvantages are;
(1) An economical limit in respect to the head and discharge to be developed,
(2) A limit in respect to the speed of the machinery to be operated,
(3) A limit in respect to the variation in the water levels in the head and tail races,
(4) Larger space requirements than a turbine.

On account of the increase in the weight of the wheel, as the head or discharge is increased beyond a certain limit, the cost of the overshot wheel becomes so great that it can then no longer compete advantageously with the turbine. The economical -field of the wheel, therefore, lies in developments which range approximately between 2 and 30 cu/ft. per sec. discharge, with heads varying from 10 to 40 ft., corresponding to a maximum development of about 75 hp. Within this field, the question whether to install a water wheel or turbine must be decided on the basis of the particular conditions of the power to be developed, the class of machinery to be operated, and the cost of installation. Only a rough approximation can be made as to the cost of the two types of motors, from the figures furnished by the manufacturer

In general it may be said that the initial cost of an overshot wheel up to a diameter of 16ft, will be about twice that of a turbine of equal horsepower, and above that diameter a little more than twice the amount.

Eng News Test Results

The peripheral velocity of the overshot wheel, when operating efficiently, varies approximately between 3 and 7ft. per sec. depending on the diameter, discharge, and velocity of the entering water. Hence, there, would be a practical limit to the speed of the machinery to he served beyond which the loss of power through the necessary gearing would probably offset any gain In efficiency. It is therefore particularly adapted for the operation of slow-speed machinery, and should find a field of usefulness for the operation of small factories, the machinery on farms or country homes, and especially for pumping plaints where the pumps may be connected directly to the wheel shaft. For the operation of high speed machinery, a loss from 3 to 10% may be estimated to occur through the necessary gearing or belting.

The principal application or use of -water power at present is the development of electrical energy. In general the opinion is held that only turbines on account of their high speed are applicable for this purpose. However, a number of electrical plants, operated with water wheels, have been built and are giving satisfactory service. The inertia of the heavy wheel and gearing provides a very uniform motion, and high efficiency at part load is a very desirable feature in electrical plants. It is a matter for the designing engineer to decide, which type of motor will give the highest total efficiency, for a plant of this kind.

The efficiency of the overshot wheel drops off quite rapidly if the bottom of the wheel is submerged, even though this is but a few inches; therefore, the wheel is poorly adapted to conditions where there is a considerable variation in the tail-water level. The turbine will operate almost as efficiently when submerged as when not submerged, and by use of the draft tube there is but little occasion to have it submerged. In this connection it should he noted, that in periods of flood, when dm water wheel is likely to he submerged, there is an excess of water and as the wheel is particularly adapted for over load capacity, the extra power developed would probably make up for any decrease in efficiency.

Within these limitations the overshot wheel is capable serving a field in water-power development, and of competing successfully with the turbine efficiency and cost of operation. The decision as to water wheel or turbine must lie made after careful investigation the particular conditions to be met with; each type or motor should receive equal consideration. The considerations which influence this choice are:
(1) Head and discharge available and probable variations in these,
(2) Cost of the motor including setting and foundation.
(3) Relation or motor cost to available capital for plant construction.
(4) Class or machinery to lie served, and whether continuous or intermittent power supply is required.

Laboratory tests of a machine, when properly interpreted, undoubtedly have a great value, but it must be borne in mind, that any test so made represents results under the exact conditions of the test. The conditions under which the Wisconsin experiments were performed approached practical conditions very closely. The wheel lusted was of a standard pattern taken from the stock of the manufacturers. The structural features are simple, and none of these features, of the wheel itself, were changed during- tile tests. Apart from the structural features of the wheel, the chief factors which influence the efficiency are the velocity of the entering water, the relation between this velocity and the peripheral velocity or the wheel, and the discharge. The results should, therefore, be readily duplicated in actual service, if the wheel is set properly, and they may he taken as a guide in the design and installation of this type of water wheel.

A summary of the conclusions reached in the experiments, the details of which will be published in a University bulletin, is given below.

Summary of Conclusions from Experiments

(1) The maximum efficiency obtained was 89% of the theoretical input delivered, at the wheel shaft or 86% at the jack shaft.
(2) The transmission loss, through the spur gear and pinion, including the friction in the bearings, reduced the efficiency of the wheel 3% at maximum efficiency of operation. Within the limited range of the experiments this loss increased with the speed and decreased with the horsepower transmitted.
(3) The efficiency of the wheel, other conditions remaining constant, increased with the decrease in the entrance velocity of the water. This statement is limited, however, by the fact that for maximum efficiency a definite relation exists between the peripheral velocity and the entrance velocity, so that a limit is reached when the entrance velocity equals or slightly exceeds the peripheral velocity.
(4) The variation ill the discharge within reasonable limits. so that the coefficient of does not exceed approximately 0.50, has little if any effect on the efficiency of the wheel.
(5) For maximum efficiency or operation, the ratio of tile peripheral velocity of the wheel to the velocity a the entering water is a constant, whose value is approximately
(6) Submergence the wheel in the tail water causes a serious decrease in the efficiency. Submergence of 3in. caused a loss of 6% at maximum efficiency of operation, which loss increased rapidly as the submergence was increased.
(7) For maximum efficiency, the point of impact of the water should ire as high in the wheel as possible. The point is regulated by the distance of the entrance spout or orifice from the wheel and must be adjusted for each installation; otherwise a serious loss of energy occurs at entrance.


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