WIND AND WINDMILLS.
In small sewerage schemes where pumping is necessary the amount
expended in the wages of an attendant who must give his whole
attention to the pumping station is so much in excess of the
cost of power and the sum required for the repayment of the
loan for the plant and buildings that it is desirable for the
economical working of the scheme to curtail the wages bill as
far as possible. If oil or gas engines are employed the man
cannot be absent for many minutes together while the machinery
is running, and when it is not running, as for instance during
the night, he must be prepared to start the pumps at very short
notice, should a heavy rain storm increase the flow in the
sewers to such an extent that the pump well or storage tank
becomes filled up. It is a simple matter to arrange floats
whereby the pump may be connected to or disconnected from a
running engine by means of a friction clutch, so that when the
level of the sewage in the pump well reaches the highest point
desired the pump may be started, and when it is lowered to a
predetermined low water level the pump will stop; but it is
impracticable to control the engine in the same way, so that
although the floats are a useful accessory to the plant during
the temporary absence of the man in charge they will not
obviate his more or less constant attendance. An electric motor
may be controlled by a float, but in many cases trouble is
experienced with the switch gear, probably caused by its
exposure to the damp air. In all cases an alarm float should be
fixed, which would rise as the depth of the sewage in the pump
well increased, until the top water level was reached, when the
float would make an electrical contact and start a continuous
ringing warning bell, which could be placed either at the pumping
station or at the man's residence. On hearing the bell the man would
know the pump well was full, and that he must immediately repair to
the pumping-station and start the pumps, otherwise the building
would be flooded. If compressed air is available a hooter could be
fixed, which would be heard for a considerable distance from the station.
[Illustration: PLATE IV.
"DIVERTING PLATE" OVERFLOW.
To face page 66.]
It is apparent, therefore, that a pumping machine is wanted
which will work continuously without attention, and will not
waste money when there is nothing to pump. There are two
sources of power in nature which might be harnessed to give
this result--water and wind. The use of water on such a small
scale is rarely economically practicable, as even if the water
is available in the vicinity of the pumping-station,
considerable work has generally to be executed at the point of
supply, not only to store the water in sufficient bulk at such
a level that it can be usefully employed, but also to lead it
to the power-house, and then to provide for its escape after it
has done its work. The power-house, with its turbines and other
machinery, involves a comparatively large outlay, but if the
pump can be directly driven from the turbines, so that the cost
of attendance is reduced to a minimum, the system should
certainly receive consideration.
Although the wind is always available in every district, it is
more frequent and powerful on the coast than inland. The
velocity of the wind is ever varying within wide limits, and
although the records usually give the average hourly velocity,
it is not constant even for one minute. Windmills of the modern
type, consisting of a wheel composed of a number of short sails
fixed to a steel framework upon a braced steel tower, have been
used for many years for driving machinery on farms, and less
frequently for pumping water for domestic use. In a very few
cases it has been utilised for pumping sewage, but there is no
reason why, under proper conditions, it should not be employed
to a greater extent. The reliability of the wind for pumping
purposes may be gauged from the figures in the following table,
No. 11, which were observed in Birmingham, and comprise a
period of ten years; they are arranged in order corresponding
with the magnitude of the annual rainfall:--
TABLE No. 11.
MEAN HOURLY VELOCITY OF WIND
Reference | Rainfall |Number of days in year during which the mean |
Number | for |hourly velocity of the wind was below |
| year | 6 m.p.h. | 10 m.p.h. | 15 m.p.h. | 20 m.p.h. |
----------+----------+----------+-----------+-----------+-----------+
1... 33·86 16 88 220 314
2... 29·12 15 120 260 334
3... 28·86 39 133 263 336
4... 26·56 36 126 247 323
5... 26·51 34 149 258 330
6... 26·02 34 132 262 333
7... 25·16 33 151 276 332
8... 22·67 46 155 272 329
9... 22·30 26 130 253 337
10... 21·94 37 133 276 330
----------+----------+----------+-----------+-----------+-----------+
Average 31·4 131·7 250·7 330·8
It may be of interest to examine the monthly figures for the
two years included in the foregoing table, which had the least
and the most wind respectively, such figures being set out in
the following table:
TABLE No. 12
MONTHLY ANALYSIS OF WIND
Number of days in each month during which the mean velocity of
the wind was respectively below the value mentioned hereunder.
Month | Year of least wind (No. 8) | Year of most wind (No. *8*) |
| 5 10 15 20 | 5 10 15 20 |
| m.p.h. m.p.h. m.p.h. m.p.h. | m.p.h. m.p.h. m.p.h. m.p.h. |
------+-------+-----+-------+-------+-------+------+------+-------+
Jan. 5 11 23 27 3 6 15 23
Feb. 5 19 23 28 0 2 8 16
Mar. 5 10 20 23 0 1 11 18
April 6 16 23 28 1 7 16 26
May 1 14 24 30 3 11 24 31
June 1 12 22 26 1 10 21 27
July 8 18 29 31 1 12 25 29
Aug. 2 9 23 30 1 9 18 30
Sept. 1 13 25 30 1 12 24 28
Oct. 5 17 21 26 0 4 16 29
Nov. 6 11 20 26 3 7 19 28
Dec. 1 5 19 24 2 7 23 29
------+-------+-----+-------+-------+-------+------+------+-------+
Total 46 155 272 329 16 88 220 314
During the year of least wind there were only eight separate
occasions upon which the average hourly velocity of the wind
was less than six miles per hour for two consecutive days, and
on two occasions only was it less than six miles per hour on
three consecutive days. It must be remembered, however, that
this does not by any means imply that during such days the wind
did not rise above six miles per hour, and the probability is
that a mill which could be actuated by a six-mile wind would
have been at work during part of the time. It will further be
observed that the greatest differences between these two years
occur in the figures relating to the light winds. The number of
days upon which the mean hourly velocity of the wind exceeds
twenty miles per hour remains fairly constant year after year.
As the greatest difficulty in connection with pumping sewage is
the influx of storm water in times of rain, it will be useful
to notice the rainfall at those times when the wind is at a
minimum. From the following figures (Table No. 13) it will be
seen that, generally speaking, when there is very little wind
there is very little rain Taking the ten years enumerated in
Table No. 11, we find that out of the 314 days on which the
wind averaged less than six miles per hour only forty-eight of
them were wet, and then the rainfall only averaged .l3 in on
those days.
TABLE No. 13.
WIND LESS THAN 6 M.P.H.
-----------+-------------+------------+--------+----------------------------------
Ref. No. | Total No. | Days on | | Rainfall on each
from Table | of days in | which no | Rainy | rainy day in
No. 11. | each year. | rain fell. | days. | inches.
-----------+-------------+------------+--------+----------------------------------
1 | 16 | 14 | 2 | .63 and .245
2 | 15 | 13 | 2 | .02 and .02
3 | 39 | 34 | 5 | .025, .01, .26, .02 and .03
4 | 36 | 29 | 7 | / .02, .08, .135, .10, .345, .18
| | | | \ and .02
5 | 34 | 28 | 6 | .10, .43, .01, .07, .175 and .07
6 | 32 | 27 | 5 | .10, .11, .085, .04 and .135
7 | 33 | 21 | 2 | .415 and .70
8 | 46 | 40 | 6 | .07, .035, .02, .06, .13 and .02
9 | 26 | 20 | 6 | .145, .20, .33, .125, .015 & .075
10 | 37 | 30 | 7 | / .03, .23, .165, .02, .095
| | | | \ .045 and .02
-----------+-------------+------------+--------+----------------------------------
Total | 314 | 266 | 48 | Average rainfall on each of
| | | | the 48 days = .13 in
The greater the height of the tower which carries the mill the
greater will be the amount of effective wind obtained to drive
the mill, but at the same time there are practical
considerations which limit the height. In America many towers
are as much as 100 ft high, but ordinary workmen do not
voluntarily climb to such a height, with the result that the
mill is not properly oiled. About 40 ft is the usual height in
this country, and 60 ft should be used as a maximum.
Mr. George Phelps, in a paper read by him in 1906 before the
Association of Water Engineers, stated that it was safe to
assume that on an average a fifteen miles per hour wind was
available for eight hours per day, and from this he gave the
following figures as representing the approximate average duty
with, a lift of l00 ft, including friction:--
TABLE NO. 14
DUTY OF WINTDMILU
Diameter of Wheel.
10
12
14
16
18
20
25
30
35
40
The following table gives the result of tests carried out by
the United States Department of Agriculture at Cheyenne, Wyo.,
with a l4 ft diameter windmill under differing wind
velocities:--
TABLE No. 15.
POWER or l4-rx WINDMILL IN VARYING WINDS.
Velocity of Wind (miles per hour).
0--5 6-10 11-15 16-20 21-25 26-30 31-35
It will be apparent from the foregoing figures that practically
the whole of the pumping for a small sewerage works may be done
by means of a windmill, but it is undesirable to rely entirely
upon such a system, even if two mills are erected so that the
plant will be in duplicate, because there is always the
possibility, although it may be remote, of a lengthened period
of calm, when the sewage would accumulate; and, further, the
Local Government Board would not approve the scheme unless it
included an engine, driven by gas, oil, or other mechanical
power, for emergencies. In the case of water supply the
difficulty may be overcome by providing large storage capacity,
but this cannot be done for sewage without creating an
intolerable nuisance. In the latter case the storage should not
be less than twelve hours dry weather flow, nor more than
twenty-four. With a well-designed mill, as has already been
indicated, the wind will, for the greater part of the year, be
sufficient to lift the whole of the sewage and storm-water,
but, if it is allowed to do so, the standby engine will
deteriorate for want of use to such an extent that when
urgently needed it will not be effective. It is, therefore,
desirable that the attendant should run the engine at least
once in every three days to keep it in working order. If it can
be conveniently arranged, it is a good plan for the attendant
to run the engine for a few minutes to entirely empty the pump
well about six o'clock each evening. The bulk of the day's
sewage will then have been delivered, and can be disposed of
when it is fresh, while at the same time the whole storage
capacity is available for the night flow, and any rainfall
which may occur, thus reducing the chances of the man being
called up during the night. About 22 per cent, of the total
daily dry weather flow of sewage is delivered between 7 p.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.
Any type of pump can be used, but the greatest efficiency will
be obtained by adopting a single acting pump with a short
stroke, thus avoiding the liability, inherent in a long pump
rod, to buckle under compression, and obviating the use of a
large number of guides which absorb a large part of the power
given out by the mill. Although some of the older mills in this
country are of foreign origin, there are several British
manufacturers turning out well-designed and strongly-built
machines in large numbers. Fig. 19 represents the general
appearance and Fig. 20 the details of the type of mill made by
the well-known firm of Duke and Ockenden, of Ferry Wharf,
Littlehampton, Sussex. This firm has erected over 400
windmills, which, after the test of time, have proved
thoroughly efficient. From Fig. 20 it will be seen that the
power applied by the wheel is transmitted through spur and
pinion gearing of 2 1/2 ratio to a crank shaft, the gear wheel
having internal annular teeth of the involute type, giving a
greater number of teeth always in contact than is the case with
external gears. This minimises wear, which is an important matter,
as it is difficult to properly lubricate these appliances, and they
are exposed to and have to work in all sorts of weather.
[Illustration: Fig. l9.--General View of Modern Windmill.]
[Illustration: Fig. 20.--Details of Windmill Manufactured by Messrs. Duke and
Ockenden, Littlehampton.]
It will be seen that the strain on the crank shaft is taken by
a bent crank which disposes the load centrally on the casting,
and avoids an overhanging crank disc, which has been an
objectionable feature in some other types. The position of the
crank shaft relative to the rocker pin holes is studied to give
a slow upward motion to the rocker with a more rapid downward
stroke, the difference in speed being most marked in the
longest stroke, where it is most required.
In order to transmit the circular internal motion a vertical
connecting rod in compression is used, which permits of a
simple method of changing the length of stroke by merely
altering the pin in the rocking lever, the result being that
the pump rod travels in a vertical line.
The governing is entirely automatic. If the pressure on the
wind wheel, which it will be seen is set off the centre line of
the mill and tower, exceeds that found desirable--and this can
be regulated by means of a spring on the fantail--the windmill
automatically turns on the turn-table and presents an ellipse
to the wind instead of a circular face, thus decreasing the
area exposed to the wind gradually until the wheel reaches its
final position, or is hauled out of gear, when the edges only
are opposed to the full force of the wind. The whole weight of
the mill is taken upon a ball-bearing turn-table to facilitate
instant "hunting" of the mill to the wind to enable it to take
advantage of all changes of direction. The pump rod in the
windmill tower is provided with a swivel coupling, enabling the
mill head to turn completely round without altering the
position of the rod.