m.
and 7 a.m.
The first cost of installing a small windmill is practically
the same as for an equivalent gas or oil engine plant, so that
the only advantage to be looked for will be in the maintenance,
which in the case of a windmill is a very small matter, and the
saving which may be obtained by the reduction of the amount of
attendance necessary. Generally speaking, a mill 20 ft in
diameter is the largest which should be used, as when this size
is exceeded it will be found that the capital cost involved is
incompatible with the value of the work done by the mill, as
compared with that done by a modern internal combustion engine.
Mills smaller than 8 ft in diameter are rarely employed, and
then only for small work, such as a 2 1/2 in pump and a 3-ft
lift The efficiency of a windmill, measured by the number of
square feet of annular sail area, decreases with the size of
the mill, the 8 ft, 10 ft, and l2 ft mills being the most
efficient sizes. When the diameter exceeds l2 ft, the
efficiency rapidly falls off, because the peripheral velocity
remains constant for any particular velocity or pressure of the
wind, and as every foot increase in the diameter of the wheel
makes an increase of over 3 ft in the length of the
circumference, the greater the diameter the less the number of
revolutions in any given time; and consequently the kinetic
flywheel action which is so valuable in the smaller sizes is to
a great extent lost in the larger mills.
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