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WINDMILL
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WINDMILL , a See also:term used, in the widest sense, for a See also:machine by which the See also:energy of the See also:wind is applied to useful purposes. Windmills were certainly used as See also:early as the 12th See also:century and are still largely employed in See also: The sails were rectangular, 5 to 6 ft. wide, and occupying five-sixths of the length of the whip. A triangular leading sail was sometimes added. Sometimes the sails consisted of a sail-See also:cloth spread on a framework; at other times narrow boards were used. The See also:oldest mill was no doubt the See also:post mill, the whole structure being carried on a post; to bring the sails to See also:face the wind, the structure was turned See also:round by a long See also:lever. The post mill was succeeded by the See also:tower, smock or See also:frock mill, in which the mill itself consisted of a stationary tower, and the wind shaft and sails were carried in a revolving cap rotating on the See also:top of the tower. See also:Andrew Meikle introduced in 1750 an See also:auxiliary rotating See also:fan at right angles to the See also:principal sails, which came into action whenever the wind was oblique to the See also:axis of the sails, automatically veering the sails or placing them normal to the wind. For safety, the sails must be reefed in high winds. In 1807, See also:Sir W. See also:Cubitt introduced automatic reefing arrangements. The sails were made of thin boards held up to the wind by weights. If the force of the wind exceeded a certain value the boards were pressed back and exposed little See also:surface. See also:American Windmills.—These generally have the sails, 18 or more in number, arranged in an annulus or disk. The sails consist of narrow boards or slats arranged radially, each See also:board having a See also:constant or variable inclination to the wind's direction. An American mill presents a larger surface for a given length of sail than the older type, and consequently the construction is lighter. To turn the mill face to the wind a See also:rudder is sometimes used projecting backward in a See also:plane at right angles to the plane of rotation of the sails. Various arrangements are adopted for reefing the sails automatically. (a) In some an action See also:equivalent to reefing is obtained by turning the sail disk oblique to the wind. The pressure on a See also:side See also:vane in the plane of rotation, controlled by a See also:weight, turns the sail disk edgeways to the wind if the pressure exceeds a safe amount. (b) In centrifugal See also:governor See also:mills the slats forming the sails are connected in sets of six or eight, each set being fixed to a See also:bar at the See also:middle of its length. By rotating this bar the slats are brought end on to the wind, the action being analogous to shutting an See also:umbrella. The slats are held up to the wind by a weight. A centrifugal governor lifts the weight if the speed becomes excessive and the sails are partially or completely furled. Many of the veering and reefing arrangements are very ingenious and too complicated to be described without detailed drawings. A description of some of these arrangements will be found in a See also:paper by J. A. Griffiths (Prot. Inst. Civ. Eng., 119, p. 321) and in a " See also:Report on Trials of Wind Pumping Engines at See also:Park Royal in 1903 " (Journ- See also:Roy. Agric., See also:Soc., 64, p. 174). See also:Warner's See also:Annular Sail Windmill.—Messrs Warner of See also:London make a windmill somewhat similar to American mills. The shutters or vanes consist of a See also:frame covered with See also:canvas, and these are pivoted between two See also:angle-See also:iron rings so as to See also:form an annular sail. The vanes are connected with See also:spiral springs, which keep them up to the best angle of See also:weather for See also:light winds. If the strength of the wind increases, the vanes give to the wind, forcing back the springs, and thus the See also: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 See also:storm is expected. The See also:wheel is kept face to the wind by a rudder in small mills; in large mills a subsidiary fan and See also:gear are used. Fig. 2 shows a large mill of this See also: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 a wind-mill should See also:control itself so that it See also:works efficiently in moderately strong winds and at the same See also: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 See also:miles per See also:hour for an inland station, See also:Kew, and a very ex-posed station, Scilly, for each See also:month during the See also:period 189o-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 See also:foot and v the velocity of the wind in miles per hour. It varies a little with the form and See also:size of the surface, but for the See also:present purpose this variation may be disregarded. (See experiments by Dr See also:Stanton at the See also:National See also:Physical Laboratory, Proc. Inst. Civ. Eng. 156, p. 78.) For velocities of 5, 10 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 crdinary 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.
See also:Jan. Feb. See also: See also:June. Kew . 8•o 8.5 8.5 7.5 7.5 7.0 Scilly . 2o•6 19.5 18.4 16'1 14.1 12.9 July. Aug. See also:Sept. Oct. Nov. Dec. Kew . 7.0 7.0 6•o 6.5 7.0 8•o Scilly . 12.4 13'9 14.6 17.2 19.3 22.0 Pressure on Surfaces oblique to the Wind.—Let fig. 3 represent a plane at See also: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 See also:rule 2 See also:cos 0 n =p1 +cos See also:alb per sq. ft. where p is the pressure in pounds per square foot on a plane struck normally by the same wind. mills of the European type, 0=12° to 18°, and the speed of the tips of the sails is 22 to 3 times the velocity of the wind. In mills of the American type, 0=28° to 4o°, and the speed of the tips of the vanes is to i 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 t=n sing=p2sinocosO I +cos' 0 ' 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 Oct 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,=v, is the velocity and direction of the wind relatively to the vane. v,= (v2+u2) =v sec 4,, tan 4,=u/v, and the angle between the relative direction of wind and normal to the vane is 0+0. It is clear that 0+0 cannot be greater than 90°, or the vane would See also: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 in = •003 v2 sec2 0 cos(o+0) +cos (o-}-~) The component of this pressure in the direction of See also:motion of the vane is t = •003 v'- sec202 sin1(++4))coscos(o+¢)(11+0) and the work done in driving the vane is to = tv tan 4) =•003 v3 sec2 4, tan , 2 See also:sin (0+0)cos (o+¢) + cos' (o+0) 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 v3 ft. lb per! O-sec. But parts of the vane or sail nearer the axis of rotation are less effective, and there are See also:mechanical See also:friction and other causes of inefficiency. An old rule based on experi- ments by See also:Coulomb on mills of the European type gave for the average effective work in foot lb per sec. per sq. ft. of sail W =o•oo11 v3. Sail Windmill. 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 See also:outer end to a larger angle near y O' b Y. the axis of rotation. In x--"'--"
B;
a
I. II. III. IV. V. VI.
Revolutions of-wheel . 208,000 308,000 264,000 322,000 222,000 202,000
See also:Double strokes of See also:pump 40,000 122,000 264,000 160,000 78,000 202,000
Gallons lifted 78,000 40,000 46,000 40,000 36,000 48,000
Average effective See also:horse-power 0.53 0.27 0.31 0.27 0.24 0'32
I. Goold Shapley and See also:Muir, See also:Ontario; wheel i6 ft. diameter, 18 vanes, 131 sq. ft. area (first See also:prize). II. See also: J. W. Titt. IV. R. Warner. V. J. W. Titt. VI. H. Sykes. Some data given by See also:Wolff on mills of the American type gave for the same quantity W =0.000450. 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 0.0005v3 to 0.00030 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 (See also:loam. Roy. Agric. Soc. lxiv. 174). The mills were run for two months altogether, pumping against a See also: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 (Prot. Inst. Civ. Eng. cxix. 321) contains details of a number of windmills of American type used for pumping and the results of a See also:series of trials. Table III. contains an abstract of the results of his observations on six types of windmills used for pumping:-eastern See also:doorway of the See also:Erechtheum, which formed part of the See also:original See also:building of 430 B.C., have lately been found; they were rectangular windows with moulded and enriched See also:architrave, resting on a See also:sill and crowned with the cymatium moulding. Of later date, at See also:Ephesus, remains of similar windows have been discovered. Of See also:Roman windows many examples have been found, those of the See also:Tabularium being the oldest known. A See also:coin of Tiberius representing the See also:temple of See also: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 See also:door of the See also:Pantheon is an open See also: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 See also:heat, the openings in the gratings being filled with See also:glass. In some cases window openings were closed with thin slabs of See also:marble, of which there are examples still existing in the churches of S. Martino and the Quattro Santi Incoronati at See also:Rome. Similar slabs exist in the upper See also:storey of the See also:amphitheatre at See also:Pola; it still remains, however, an open question L II. III. IV. V. VI. Diameter of wheel, feet 22.3 11.5 16.o 14.2 10.2 9.8 Sail area, square feet 392 104 201 157 81 8o Weather angle, outer ends . 18° 47' 43° 36° 30° 28° 5o° inner ends 38° 2o' 43° 36° 30° 28° 14° See also:Pitch of vanes, outer ends, feet . 23.8 33'7 36.5 25.7 17.0 22.4 inner ends, feet . ! 20.6 I 13'1 1 13'7 8.2 6.4 7.2 Height of lift, feet . . . 25 10o 29.2 61.2 I 39.0 66.3 38.7 30.7 Velocity of wind at maximum efficiency, miles 4.3 7.0 ~ 5.8 6.5 I 6•o 7.o 8.5 6•o per hour Ratio of velocity of tips of vanes to velocity of wind •93 '77 •92 •82 .65 '91 .87 '73 Revolutions of mill, per See also:minute . 5.o 6.8 13.0 13'3 7'5 12.6 20.5 12.5 Actual horse-power 0.018 0.098 0.011 0.025 0.024 0.065 0.028 0'012 In 10o average hours in a See also:calm locality- 495 306 153 135 259 267 115 145 Quantity of water lifted, gallons per hour . In too average hours in a windy locality- 816 629 287 271 525 540 237 270 Quantity of water lifted, gallons per hour . as _ I. See also:Toowoomba; conical sail wheel with reefing vane. II. Stover; solid sail wheel with rudder; See also:hand control. automatic rudder. Table IV. gives the horse-power which may be expected, according to Wolff, for an average of 8 hours per See also:day for wheels of the American type. Diaeter of Velocity of Horse-power of Revolutions of ~V'ind in ~'liles Wheelmin Feet.. Mill. Wheel per Minute. per Hour. 81 t6 0.04 70-75 to 16 0.12 6o-65 12 16 0.21 55-6o 14 16 o-28 50-55 16 16 0.41 45-50 t8 16 o•61 40-45 20 t6 0.78 35-40 25 16 1'34 30-35 Further See also:information will be found in See also:Rankine, The See also:Steam See also:Engine and other See also:Prime See also:Movers; V6'eisbach, The See also:Mechanics of Engineering; and Wolff, The Windmill as a Prime Mover. (W. C. Additional information and CommentsThere are no comments yet for this article.
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