Renewable Energy Solutions

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, such as would be
suitable for many power plants can be had for $75. A twelve-inch
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 horsepower. 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.

Reaction turbines may be either horizontal or vertical. If they are
vertical, 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 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 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.
This subject will be discussed more fully later.