WATER PRESSUREMACHINES
WATER PRESSUREMACHINES
WATER WHEELS.
Hydraulic machinerymay be broadly divided into1. Motor machines, and,2. Pumps.
Water motorsmay be divided into1. Water wheels,2. Turbines, and,3. Water pressure engines.
In hydraulic motor machinesa quantity of water descending from a higher to a lower level, orfrom a higher to a lower pressure, drives a machine which receives energy from the water and applies it to overcoming the resistances of other machines doing work.
In the next general class, work done on the machine by a steam engine or other source of energy is employed in lifting waterfrom a lower to a higher level. A few machines such as the ram and jet pumpcombine the functions of both motors and pumps.
The subject of water wheels is but a continuation of much that has been illustrated and defined in the historical introductionto which is now added the following summary.
In every system of machineryderiving energy from a natural water-fall there exist the following parts:
(1)A supply channel, leading the water from the highest accessible level,to the site of the machine; this may be an open channel of earth, masonry, or wood, or it may be a closed cast or wrought-iron pipe; in some cases part of the head race is an open channel, part a closed pipe.
(2)Leading from the motorthere is a tail race, culvert, or discharge pipe delivering the water after it has done its work.
(3)A waste channelplaced on or at the origin of the head race by which surplus water, in floods, escapes.
(4)The motor itself, which either overcomes a useful resistance directly, as in the case of a ram acting on a lift orcrane chain, or indirectly by actuating transmissive machinery, as when a turbine drives the shafting, belting, and gearing of a mill. With the motor is usually combined regulating machinery for adjusting the power and speed, to the work done.
The great convenience and simplicity of water motors has led to their adoption in certain cases, where no natural source of water power is available. In these cases, an artificial source of water power is created by using a steam engine to pump water to a reservoir at a great elevation, or to pump water into a closed reservoir in which there is great pressure.
Water flowing from the reservoir through hydraulic engines gives back the energy expended, less so much as has been wasted in friction. Where a continuously acting steam engine stores up energy by pumping the water, while the work done by the hydraulic engines is done intermittently,—this arrangement is considered the most useful.
Note.—“Wherever a stream flows from a higher to a lower level it is possible to erect a water motor. The amount of power obtainable depends on the available head and the supply of water. In choosing a site the engineer will select a portion of the stream where there is an abrupt natural fall, or at least a considerable slope of the bed. He will have regard to the facility of constructing the channels which are to convey the water, and will take advantage of any bend in the river which enables him to shorten them. He will have accurate measurements made of the quantity of water flowing in the stream, and he will endeavor to ascertain the average quantity available throughout the year, the minimum quantity in dry seasons, and the maximum for which bye-wash channels must be provided. In many cases the natural fall can be increased by a dam or weir thrown across the stream. The engineer will also examine to what extent the head may vary in different seasons, and whether it is necessary to sacrifice part of the fall and give a steep slope to the tail race to prevent the motor being flooded by backwater in freshet time.In designing or selecting a water motor it is sufficient to consider only its efficiency in normal working conditions. It is generally quite as important to know how it will act with a scanty water supply or a diminished head. The greatest difference in water motors is in their adaptability to varying working conditions.”—Encyc. Brit.
Note.—“Wherever a stream flows from a higher to a lower level it is possible to erect a water motor. The amount of power obtainable depends on the available head and the supply of water. In choosing a site the engineer will select a portion of the stream where there is an abrupt natural fall, or at least a considerable slope of the bed. He will have regard to the facility of constructing the channels which are to convey the water, and will take advantage of any bend in the river which enables him to shorten them. He will have accurate measurements made of the quantity of water flowing in the stream, and he will endeavor to ascertain the average quantity available throughout the year, the minimum quantity in dry seasons, and the maximum for which bye-wash channels must be provided. In many cases the natural fall can be increased by a dam or weir thrown across the stream. The engineer will also examine to what extent the head may vary in different seasons, and whether it is necessary to sacrifice part of the fall and give a steep slope to the tail race to prevent the motor being flooded by backwater in freshet time.
In designing or selecting a water motor it is sufficient to consider only its efficiency in normal working conditions. It is generally quite as important to know how it will act with a scanty water supply or a diminished head. The greatest difference in water motors is in their adaptability to varying working conditions.”—Encyc. Brit.
Fig. 110.
Fig. 110.
Water wheelsare large vertical wheels driven by water falling from a higher to a lower level: they are motors on which the water acts, partly by weight, partly by impulse.Turbinesare wheels, generally of small size compared with water wheels, driven chiefly by the impulse of the water. Before entering the moving part of the turbine, the water is allowed to acquire a considerable velocity; during its action on the wheel this velocity is diminished, and the impulse due to the change of momentum drives the turbine. Roughly speaking, the fluid acts in a water-pressure engine directly by its pressure,in a water wheel chiefly by its weight causing a pressure.
A flutter-wheelis shown in Fig. 110. This is a water wheel of moderate diameter placed at the bottom of a chute so as to receive the impact of the head of water in the chute and penstock. Its name is derived from its rapid motion, the effect of which is to cause a commotion of the water like “the fluttering” of a fowl.
Impact Wheels.—The simplest and most imperfect of the horizontal wheels are the so-called impact wheels or impact turbines, such as shown in Fig. 111.
Fig. 111.
Fig. 111.
They consist of 16 or 20 rectangular blades fastened to the wheel at an inclination of 50° to 70° with the horizon. The water is brought on through a race of 40° to 20° inclination, so that it strikes at about right angles upon the blades.
These wheels are used in falls from 10 to 20 ft., where a large number of revolutions is necessary, as in grain mills, where the moving millstone is hung on the vertical shaft of the wheel, hence intermediate gearing is unnecessary. These crude machines are found in Southern Europe, North Africa, in the Alps, Pyrenees, and in Algiers. They are about 5 ft. in diameter, and the blades are 15 inches high and 8 to 10 inches long (measured radially).
Fig. 112.
Fig. 112.
Fig. 112 shows an “undershot water wheel.” In this style of wheel, the work is done by impact alone, as the running water acts only on a few immersed buckets on the under side of the wheel.
In thebreast wheel, Fig. 113, the water is admitted on a level or slightly above the center of the shaft, so that the water acts by impact and weight.
Fig. 113.
Fig. 113.
Note.—“A weiris a dam erected across a river to stop and raise the water, as for the purpose of taking fish, of conveying a stream to a mill, of maintaining the water at a level required for navigating it, or for the purposes of irrigation.”For facilitating the computation of the quantity of water flowing over weirs, Weir Tables, are used, based upon approved formulas, of which “Francis’ Formula” is perhaps the most reliable. These tables are applicable to the subject of water wheels but cannot be printed in this work.
Note.—“A weiris a dam erected across a river to stop and raise the water, as for the purpose of taking fish, of conveying a stream to a mill, of maintaining the water at a level required for navigating it, or for the purposes of irrigation.”
For facilitating the computation of the quantity of water flowing over weirs, Weir Tables, are used, based upon approved formulas, of which “Francis’ Formula” is perhaps the most reliable. These tables are applicable to the subject of water wheels but cannot be printed in this work.
Fig. 114.
Fig. 114.
Fig 114represents anover-shot water wheel(F G H L, with axis at O) in which the water flows upon the top of the wheel ath, in the same direction in which it revolves, therefore the impact of the water is utilized upon the upper buckets H, a, b, after which the weight of the water acts in the buckets c, d, e, F, e´, d´ and c´. At b´ the buckets begin to overflow and empty themselves as shown at a´. It will be seen that the water acts upon almost one-half the circumference of this wheel, thus realizing the greatest mechanical effect with the smallest quantity of water.
The current-wheelis perhaps the first application of the force of water in motion, to drive machinery. In the first century B. C., water-wheels for driving mills were used in Asia Minor and on the Tiber. In the former case we suppose, but in the latter case we know, that these were current-wheels.
Fig. 115.
Fig. 115.
The tide or current wheel, (Fig. 115) erected in the vicinity of the north end of London Bridge, and subsequently under itsnorthern arch, was erected by Peter Morice, a Dutchman, in 1582, and operated force-pumps which supplied a part of London with water. The stand-pipe from the pump was 120 feet high, and conducted the water to a cistern at that height. The amount raised was about 216 gallons per minute. The wheel worked sixteen pumps, each 7 inches in diameter, and having a uniform stroke of 30 inches.
During the seventeenth and eighteenth centuries the works were extended from time to time, and occupied one after another of the arches. In the first arch of the bridge was one wheel working sixteen force-pumps. In the third arch were three wheels, working fifty-two pumps. The united effect was 2,052 gallons per minute, raised 120 feet high.
In 1767 Smeaton added wheels in the fifth arch. Steam-engines were added about this time to assist at low water and at neap-tides. Thus the matter remained till 1821. Stow, the antiquarian and historian, describes the works in 1600; and Beighton in 1731 gives an account of them at that date.
The water-wheels at that time were placed under several of the arches. The axis of these wheels was 19 feet long 3 feet diameter. The radial arms supported the rings and twenty-six floats, 14 feet long by 18 inches wide. The axis turned on brass gudgeons supported by counterpoised levers, which permitted the vertical adjustment of the wheel as the tide rose and fell. On the axis of the wheel was a cog-wheel 8 feet in diameter and having forty-four cogs; meshing into a trundle-wheel 41⁄2feet in diameter and having 20 rounds, or pins and whose iron axle revolved in brasses.
The axis of the trundle was prolonged at each end, and had quadruple cranks which connected by rods to the ends of four walking beams 24 feet long, whose other ends worked the piston-rods of the pumps. The axis of oscillation of the lever supporting the wheel, and by which it was adjusted to the height of the tide, was coincident with the axis of the trundle, so that the latter engaged the 8-feet cog-wheel in all conditions of vertical adjustment. Cranks operated one end of the beams while pumps were attached to the other end.
Fig. 116.
Fig. 116.
Fig. 116 exhibits anovershot water wheelemployed at Laxey, Isle of Man, for driving the pumps which drain the mines at that village; these have an extreme depth of 1,380 feet. The wheel is 72 feet 6 inches in diameter, 6 feet in breadth, exerts a force of about 200 horse-power and is capable of pumping 250 gallons per min. from a depth of 1,200 feet. Its crank-stroke is 10 feet. The water for driving it is conducted by pipes from a reservoir on a neighboring hill, and ascends in the column of masonry shown to the left of the wheel. (Knight Vol. III.) An extra crank appears to be shown in the foreground of this reproduction of an old drawing.