Bible Encyclopedias
Windmill

Warner's Annular Sail Windmill

Messrs Warner of London make a windmill somewhat similar to American mills. The shutters or vanes consist of a frame covered with canvas, and these are pivoted between two angle-iron rings so as to form an annular sail. The vanes are connected with spiral springs, which keep them up to the best angle of weather for light winds. If the strength of the wind increases, the vanes give to the wind, forcing back the springs, and thus the area on which the wind acts diminishes. In addition, there are a striking lever and tackle for setting the vanes edgeways to the wind when the mill is stopped or a storm is expected. The FIG. 1 - Windmill near Delft.

wheel is kept face to the wind by a rudder in small mills; in large mills a subsidiary fan and gear are used. Fig. 2 shows a large mill of this kind, erected in a similar manner to a tower mill. The tower is a framework of iron, and carries a revolving cap, on which the wind shaft is fixed. Behind is the subsidiary fan with its gearing acting on a toothed wheel fixed to the cap.

It is important that wind-mill should control itself so that it works efficiently in moderately strong winds and at the same time runs in very light winds, which are much more prevalent. It should also, by reefing or otherwise, secure safety in storms.

Table I. gives the mean velocity of the wind in miles per hour for an inland station, Kew, and a very exposed station, Scilly, for each month during the period 1890-1899.

The pressure of the wind on a plane normal to its direction, composed partly of an excess front pressure and negative back pressure, is given by the relation p=0.003 v2, where p is in pounds per square foot and v the velocity of the wind in miles per hour.

It varies a little with the form and size of the surface, but for the present purpose this variation may be disregarded. (See experiments by Dr Stanton at the National Physical Laboratory, Proc. Inst. Civ. Eng. 156, p. 78.) For velocities of 5, to and 20 m. per hour the pressures on a plane normal to the wind would be about 0.075, 0.3 and 1.2 lb per sq. ft. respectively, and these may be taken to be ordinary working velocities for windmills. In storms the pressures are much greater, and must be reckoned with in considering the stability of the mill. A favourable wind velocity for windmills is 15 m. per hour.

Table Pressure on Surfaces oblique to the Wind.-Let fig. 3 represent a plane at rest on which a wind current impinges in the direction YY, making an angle 0 with the normal Oa to the plane. Then the pressure n normal to the plane is given very approximately by Duchemin's rule n=p o cos + o lb per sq. ft.

where p is the pressure in pounds per square foot on a plane struck normally by the same wind.

In fig. 3 let AB be part of a windmill sail or vane at rest, XX being the plane of rotation and YY the direction of the wind. The angle 0 is termed the weather of the sail. This is generally a constant angle for the sail, but in some cases varies from a small angle at the outer end to a larger angle near y the axis of rotation. In.' mills of the European type, 0 = 12° to 18°, and the speed of the tips of the sails is 21 to 3 times the velocity of the wind. In mills of the American type, 0=28° to 40°, and the speed of the tips of the vanes is 4 to 1 time that of the wind. Then if Oa =n be the normal pressure on the sail or vane per square foot, ba=t is the effective component of pressure in the direction of rotation and 2 sin o cos t= n sin o- '1' -+-cos t B' When the sail is rotating in a plane at right angles to the wind direction the conditions are more complicated. In fig. 4 let XX be the plane of rotation of the vane and YY the direction of the wind. Let Oa be the normal to the vane, 0 being the weather of the vane. Let Ov =v be the velocity of the wind, Ou =u the velocity of the vane. Completing the parallelogram, Ov r =vr is the velocity and direction of the wind relatively to the vane.

y r = -4 (v 2 + u 2) = v sec 4), tan 4)=u/v, and the angle between the relative direction of wind and normal to the vane is 0+4). It is clear that 0+4 cannot be greater than 90°, or the vane would press on the wind instead of the wind on the vane. Substituting these values in the equations already given, the normal pressure on the oblique moving vane is 'n= 003 v2 sec2 cos(o-}-4) I +cos (o+4)) The component of this pressure in the direction of motion of the vane is sin (0+0) cos (0+0) t = 003 v 2 sec2¢ +cos t (o+0) and the work done in driving the vane is to =iv tan = 003 v 3 sec' 4) tan 45 sin (0+4)cos (0+0) I + cos t (0+4) foot lb per sq. ft. of vane per sec., where v is taken in miles per hour. For such angles and velocities as `are usual in windmills this would give for a square foot of vane, near the tip about 0.003 v 3 ft. lb perY sec. But parts of the vane or sail nearer the axis of rotation are less effective, and there are mechanical fric tion and other causes of ineffici ency. An old rule FIG. 4. based on experi ments by Coulomb on mills of the European type gave for the average effective work in foot lb per sec. per sq. ft. of sail W =oooI I v3.

Table -In 150 Working Hours. I. Goold Shapley and Muir, Ontario; wheel 16 ft. diameter, 18 vanes, 131 sq. ft. area (first prize). II. Thomas && Son (second prize). III. W. Titt. IV. R. Warner. V. W. Titt. VI. H. Sykes.

X; Some data given by Wolff on mills of the American type gave for the same quantity W = o 00045v3.

From some of the data of experiments by Griffiths on mills of the American type used in pumping, the effective work in pumping when the mill was working in the best conditions amounted to from 00005v 3 to 00003v 3 ft. lb per sec. per sq. ft.

In 1903 trials of wind-pumping engines were carried out at Park Royal by the Royal Agricultural Society (Journ. Roy. Agric. Soc. lxiv. 174). The mills were run for two months altogether, pumping against a head of 200 ft. The final results on six of the best mills are given in Table II.

A valuable paper by J. A. Griffiths (Proc. Inst. Civ. Eng. cxix. 321) contains details of a number of windmills of American type used for pumping and the results of a series of trials. Table III. contains an abstract of the results of his observations on six types of windmills used for pumping: eastern doorway of the Erechtheum, which formed part of the original building of 430 B.C., have lately been found; they were rectangular windows with moulded and enriched architrave, resting on a sill and crowned with the cymatium moulding. Of later date, at Ephesus, remains of similar windows have been discovered. Of Roman windows many examples have been found, those of the Tabularium being the oldest known. A coin of Tiberius representing the temple of Concord shows features in the side wings which might be windows, but as statues are shown in them they are possibly only niches. Over the door of the Pantheon is an open bronze grating, which is thought to be the prototype of the windows which lighted the large halls of the Thermae, as it was absolutely necessary that these should be closed so as to retain the heat, the openings in the gratings being filled with glass. In some cases window openings were closed with thin slabs of marble, of which there are examples still existing in the churches of S. Martino and the Quattro Santi Incoronati at Rome. Similar slabs exist in the upper storey of the amphitheatre at Pola; it still remains, however, an open question TABLE III.

I. Toowoomba; conical sail wheel with reefing vane. II. Stover; solid sai wheel with rudder; hand control.

III. Perkins; solid wheel, automatic rudder. IV. and V. Althouse; folding sail wheel, rudderless. VI. Carlyle; special type, automatic rudder.

Table IV. gives the horse-power which may be expected, according to Wolff, for an average of 8 hours per day for wheels of the American type.

Further information will be found in Rankine, The Steam Engine and other Prime Movers; Weisbach, The Mechanics of Engineering; and Wolff, The Windmill as a Prime Mover. (W. C. U.)