The reaction turbine is best adapted to low heads, with a large supply of water. It is not advisable, under ordinary circumstances, to use it under heads exceeding 100 feet, as its speed is then excessive. It may be used under falls as low as two feet.
Five thousand cubic feet of water a minute would give approximately 14 actual horsepower under such a head. A sluggish creek that flows in large volume could thus be utilized for power with the reaction turbine, whereas it would be useless with an impulse wheel. Falls of from five to fifteen feet are to be found on thousands of farm streams, and the reaction turbine is admirably adapted to them.
Reaction turbines consist of an iron “runner” which is in effect a rotary fan, the pressure and momentum of the column of water pressing on the slanted blades giving it motion and power. These wheels are manufactured in a great variety of forms and sizes, and are to be purchased either as the runner (set in bearings) alone, or as a runner enclosed in an iron case.
In case the runner alone is purchased, the owner must enclose it, either with iron or wood. They vary in price according to size, and the means by which the flow of water is controlled.
A simple 12-inch reaction turbine wheel, using 18 or 20 square inches of water, would generate about 7 1/2 horsepower under a 20-foot head, with 268 cubic feet of water a minute. Under a 30-foot head, and with 330 cubic feet of water such a wheel will give 14 horsepower.
A 36-inch wheel, under a 5-foot head, would use 2,000 cubic feet of water and give 14 horsepower. Under a 30-foot head, this same wheel, using 4,900 cubic feet of water a minute, would develop over 200 horse-power.
If the farmer is confronted by the situation of a great deal of water and small head, a large wheel would be necessary. Thus he could secure 35 horsepower with only a 3 -foot head, providing his water supply is equal to the draft of 8,300 cubic feet a minute. From these sample figures, it will be seen that the reaction turbine will meet the requirements of widely varying conditions up to, say a head of 100 feet.
The farmer prospector should measure first the quantity of water to be depended on, and then the number of feet fall to be had. The higher the fall, with certain limits, the smaller the expense of installation, and the less water required.
When he has determined quantity and head, the catalogue of a reputable manufacturer will supply him with what information is necessary to decide on the style and size wheel he should install. In the older settled communities, especially in New England, a farmer should be able to pick up a second-hand turbine, at half the price asked for a new one; and since these wheels do not depreciate rapidly, it would serve his purpose as well, in most cases, as a new one.
Vertical and Horizontal Turbines
Reaction turbines may be either horizontal or vertical. If they are vertical (as shown in the illustration), it is necessary to connect them to the main shaft by means of a set of bevel gears. These gears should be substantially large, and if the teeth are of hard wood (set in such a manner that they can be replaced when worn) they will be found more satisfactory than if of cast or cut metal.
The horizontal turbine is keyed to its shaft, like the impulse wheel, so that the wheel shaft itself is used for driving, without gears or a quarter-turn belt. (The latter is to be avoided, wherever possible.) There are many forms of horizontal turbines; they are to be had of the duplex type, that is, two wheels on one shaft. These are arranged so that either wheel may be run separately, or both together, thus permitting one to take advantage of the seasonal fluctuation in the water supply.
A convenient form of these wheels includes draft tubes, by which the wheel may be set several feet above the tailrace, and the advantage of this additional fall still be preserved. In this case the draft tube must be airtight so as to form suction, when filled with escaping water, and should be proportioned to the size of the wheel.
Theoretically, these draft tubes might be 34 feet long, but in practice, it has been found that they should not exceed 10 or 12 feet under ordinary circumstances. They permit the wheel to be installed on the main floor of the power station, with the escape below, instead of being set just above the tailrace level itself, as is the case when draft tubes are not used.
Reaction turbines when working under a variable load require water governors (like impulse wheels) although where the supply of water is large, and the proportion of power between the water wheel and dynamo is liberal say two to one, or more this necessity is greatly reduced.
Reaction wheels as a rule govern themselves better than impulse wheels, due both to the fact that they use more water, and that they operate in a small airtight case. The centrifugal ball governor is the type usually used with reaction wheels as well as with impulse wheels.