Fig. 1783. T is produced by the arrange-, admitted at the axis of a hollow wheel traversed by ment shown in Fig. 1783, vanes, and made to revolve rapidly, is expelled at its which is a very old form of circumference. The pipe by which the water reaches pump. On referring to the the axis of the wheel, or the reservoir in which the force-pump, Fig. 1771, it will wheel is immersed, becomes under these circumstances be seen, that in the upward a suction-pipe; and if the reservoir into which the motion of the piston water water is received from the periphery of the wheel be is drawn up, and in its down- closed, and a pipe be carried from it upwards, such ward motion it is forced up; pipe becomes a force-pipe. Although, at the time of but as the piston contains no the Great Exhibition, rotatory pumps were regarded, valve, it was thought that in England at least, as novelties, they had long been its upper surface might be known in France and America, and a very slight made to raise or propel water research into books on hydraulic machinery was as well as the lower: so that sufficient to make known many varieties of this while the lower surface was form of pump, all agreeing, more or less, with the engaged in drawing water, general description just given. For example, Fig. the upper surface might be 1785 is a vertical section of the Massachusetts forcing it, and vice versa. These effects are produced pump in the plane of motion of the elevating blades, by the arrangement shown in Fig. 1783. When the and Fig. 1786 is a vertical transverse section of the piston ascends, as shown in the figure, the valves vl v2 are opened, and water is drawn up the tube T by the lower surface of the piston, and forced up T by its upper surface. When, on the contrary, the piston descends, water is drawn by the upper surface of the piston through the valve 3, and forced by the lower surface of the piston through 4. These double-action pumps are seldom used on account of the number of valves required, and their liability to get out of order; so that when a constant stream is required, 2 pumps are used, so arranged with reference to the moving power, that while the piston of one is ascending the piston of the other is descending. Four pumps are sometimes made to work together, in which case, while one piston is at the top of its stroke, the fourth is at the bottom, the second at a quarter, and the third at a half or three-quarter stroke. In one description of the Fire Engine [see FIRES], a force-pump without a piston is described. A still simpler form is shown in Fig. 1784, in which is a lever moving round c as a centre; the end cb works in a box b immersed in water, of which w is the level. When I c is pulled to the left, the end cb forces the water from the box up the pipe p, and through the valve v, where it is kept by the descent of the vaive. The water enters the box by the aperture a. Fig. 1784. A continuous stream is also produced by what is called the rotatory or centrifugal pump. The history of this form of pump has been traced back for upwards of a century.' In its most general form, water, (1) In the Practical Mechanic's Journal for September, 1851, is "A Historical Review of the Centrifugal Pump." Although the list of pumps here given is by no means complete, it is of great value. The following are the dates of the inventions, and the names of the inventors. 1732, Le Demour. Date unknownInverted Barker's Mill. 1816, Jorge-West. 1818, Massachusetts Pump. 1830, the same improved. 1831, Blake. 1839, an upright discharging passage F. The apparatus is | up into the pipe p, and is prevented from returning sunk below the level of the water which is to be lifted, by the valve », the rod and the vanes being made to revolve by means of the external bevel wheels, the water is sucked in at the central aperture c, and being impelled forwards by the revolving blades, is finally discharged by the centrifugal force through the passage F. It appears, from Fig. 1786, that the vanes are tapered towards their outer extremities. rising between the guide-rollers gg as 8 advances to r, for the purpose of preventing the water from getting through at r. The next wing, s, produces a similar effect, and carries the water above its natural level w w to the pipe p. In Fig. 1789 there are 2 revolving wheels, w w', working into each other, and fitting close to the elliptical cistern Fig. 1788. cc'. The water which rises through the pipe p into c is forced by these wheels round the outer teeth, and so up the pipe p'. A pump of this kind, with only one wheel, is described in Nicholson's Journal, vol. viii. In Fig. 1790 the same effect is produced by a wheel w, furnished with a number of vanes vv, which fall down on the circumference of the wheel at the side, and resume their other position by the action of a spring s attached to each of them: hence they force the water up from p to p'. Fig. 1789. Two centrifugal pumps in the Great Exhibition, one by Mr. Appold and the other by Mr. Gwynne, excited much attention and interesting discussion. Mr. Appold's pump consists of a revolving fan F, Figs. 1791, 1792, 1 foot in diameter, and 3 inches wide, formed of 2 sheets of copper or sheetiron bevelled outwards towards the centre like shallow dishes. There is an opening one-half the total diameter in the centre of each side, for the admission of the water, and a central division-plate, extending to the circumference, to give a direction to the 2 streams of water, and for the convenience of fixing to the shaft s. Between the outer discs and the centre plate are 6 arms or blades passed through slots in the centre plate, and soldered to the inner surfaces of the outer disks. These arms are curved backwards, and terminate nearly in a tangent to the circumference. The Fig. 1790 revolving fan is fixed to the end of the driving shaft | s, which passes through a stuffing box in the side of sible loss of the vis viva of the water in its access to the wheel, and when there remains the least possible vis viva in it when it leaves it. For if there be any loss of the vis viva of the water in its ingress to the Fig. 1791. the casing, and the fan is made to work between 2 circular cheeks cc, as close to them as possible without actually touching, the object of these cheeks being to shield the outer revolving surfaces from the water, but at the same time to allow a free P pump which might have been avoided, it is evident producing that vis viva. And in like manner, if any that power must have been expended unnecessarily in vis viva remain unnecessarily in the water when it leaves the wheel, it is evident that the power by which that vis viva was created might have been saved. The expedients by which the water may be brought to the wheel with the least loss of vis viva are common to this and to other hydraulic machines; those by which it enters and is delivered from the wheel are peculiar to the centrifugal pump. If the vanes be straight (as at BC, Fig. 1797), it is evident that whatever may be the velocity of the water in the direction of a Fig. 1792. ingress for the water at the centre. To facilitate the escape of the discharged water, a large space ss is left round the circumference of the fan. The water to be raised is, as already stated, admitted through the central openings in the outer disks, and the fan being made to revolve with considerable velocity, the water is discharged by the centrifugal force through the openings in the circumference, and so up the force-pipe to the discharge opening. In Mr. Gwynne's pump, there is one straight radial arm, as shown in Figs. 1793, 1794, which represent a longitudinal and a transverse section. The arms are also straight in Mr. Bessemer's pump, shown in vertical section, Fig. 1795, and plan, Fig. 1796. A number of experiments were tried at the Great Exhibition on the working power of these pumps. The results are given in the Jury Report in a tabular form, together with the following remarks:-"The greatest economy of power, in such a pump, may be expected to be attained when there is the least pos Fig. 1795. Fig. 1796. radius, when it leaves the wheel, its velocity in the direction of a tangent will be that of the circumference of the wheel, so that the greater the velocity of the wheel the greater will be the amount of vis viva remaining in the water when discharged, and the greater the amount of power uselessly expended to create that vis viva. If, however, the vanes be curved backwards (as at A, Fig. 1797), as regards the motion of the wheel, so as to have nearly the direction of a tangent to the circumference of the wheel at the points where they intersect it, then the velocity due to the centrifugal force of the water carrying it over the surface of the vane in the opposite direction to that in which the wheel is moving, and nearly in the direction of a tangent to the circumference, will,-if this velocity of the water over the vane in the one direction be equal to that in which the vane is itself moving in the other-produce a state of absolute rest in the water, and entire exhaustion of vis viva. And in what- | raising water to different heights. The power em A B C Fig. 1797. Α' ever degree the equality of these two motions-of the water in one direction over the vane, and of the vane itself in the opposite direction-is attained, in that same degree will the water be delivered in a state approaching to one of rest. "With regard to the admission of water to the wheel, it is obvious that it should pass directly from the suction-pipe into the wheel without the intervention of any reservoir in which the vis viva of the influent stream,-communicated in the act of rising through the pipe - may expend itself, and that such space should be allowed at the centre as not to alter the dimensions of the influent stream. It would further seem expedient, by means of properly constructed channels, to divide the water into separate streams, and to give to these divergent streams such curvatures as would facilitate their entrance upon the channels formed by the vanes; as in the turbine. It is obvious that the tendency of the centrifugal force continually to increase the velocity of the water over the vanes as it recedes from the centre, cannot take effect in respect to all the particles of water in the same section, unless the sections of the channels diminish. If they do not, some of the particles of water in each section must be continually retarded, and power be uselessly expended in producing this retardation; whilst the current cannot but suffer from it a disturbance destructive of its vis viva. This diminution of the sections of the channels might probably best be effected by giving to the sides of the wheel the forms of conical disks; an expedient which is adopted in Mr. Lloyd's blowing machines and in Mr. Bessemer's centrifugal pump. The communication of motion to the water of the reservoir in which the wheel revolves and into which the water is discharged, should by every practicable expedient be avoided; and for this object the water should be kept as much as possible from the sides of the wheel. This is effected in Mr. Appold's pump by fixing the wheel between two cheeks which project from opposite sides of the reservoir. The velocity with which the wheel must be driven depends upon the height to which the water is to be raised. Beyond a certain height this velocity is practically unattainable. But long before this limit is reached, it becomes inconsistent with an economical application of the power which drives the pump. It is probably therefore only in comparatively small lifts, where a large quantity of water is to be discharged, that the centrifugal pump will be found useful." The experiments tried with Appold's pump at the Great Exhibition were for the purpose of ascertaining the per-centage of useful effect yielded by it when ployed in each experiment was measured by means of Morin's dynamometer,' arranged as follows:-The driving strap from the steam-engine was passed over the first pulley of the dynamometer, and the pump was driven from the second pulley running loose on the same shaft and connected to the first by means of a spring, through which all the power was transmitted: the amount of the driving power was therefore indicated by the extent to which the spring was bent, and this was shown by a continuous pencil mark upon a paper cylinder connected to the instrument, and from which the actual tension of the driving strap at all periods of the experiment was accurately ascertained. By this means the following results were obtained :APPOLD'S CENTRIFUGAL PUMP, with curved arms. A, Fig. 1797. B Fig. 1798. D At a meeting of the Institution of Mechanical Engineers, held at Birmingham 28th June, 1852, Mr. Appold made some interesting statements respecting centrifugal pumps. He illustrated the superior action of oblique arms, B, Fig. 1797, to radial arms, C, Fig. 1797, by supposing a vertical arm A B, Fig. 1798, to move in a straight line to CD, instead of moving round in a circle in the pump, and the body A, representing a particle of water, would then be simply moved along to c with the arm, without having any tendency to be propelled outwards along the arm to B. But if an oblique arm AE is employed, moving in the same direction as before to the position CB, it propels the particle A outwards towards B, having an inclined-plane action to push the particles of water outwards from the centre towards the circumference. When this was applied to a circular motion, and the direction A C bent into a circle, the inclined arm A E became curved in a spiral direction like the arms in the pump. The comparative value of the different forms of arms proved by the experiments at the Great Exhibition, and as shown in the above table, give a duty of 68 per cent. for curved arms, 43 per cent. for inclined arms, and only 24 per cent. for radial arms. It is stated that the other centrifugal pumps in the Exhibition which had straight arms did not give a higher duty than 24 per cent. With respect to the velocity of (1) This instrument is described in our INTRODUCTORY ESSAY, page cliii. Figs. 1xxii-lxxiv. (2) Although Mr. Appold appears to have been the first practical centrifugal pump maker who fully appreciated the superior value of curved vanes over straight ones, it appears that the curved form was adopted in some pumps erected in the United States of America about 1839; but it is a remarkable fact that curved vanes the circumference of the wheel, this must be constant | for the circumference of the pump to be driven, to for all sizes of pump for the same height of lift: that raise the water to a certain height, is equal to is, a pump 1 inch diameter must make 12 times the number of revolutions per minute compared with one 12 inches in diameter, and both pumps will then raise the water to the same height, but the quantity of water delivered will be 144 times greater in the 12-inch pump, being in proportion to the area of the discharging orifices at the circumference, or the square of the diameter, when the proportion of breadth was kept the same, namely 4th of the diameter in each case. A small pump 1 inch in diameter gave proportionate results with a pump 12 inches in diameter, the former discharged 10 gallons of water per minute and the latter 1,440, and consequently it is assumed that a wheel 10 feet in diameter would discharge 144,000 gallons per minute. A velocity of the circumference of 500 feet per minute raised the water 1 foot high, and maintained it at that level without discharging any; and a double velocity raised the water to 4 times the height as the centrifugal force was proportionate to the square of the velocity; consequently Feet per min. ft. 500 raised the water 1 without discharge. 1,000 2,000 4 16 64 The greatest height to which the water had been raised without discharge, in the experiments with the 1-foot pump was 67.7 feet with a velocity of 4,153 feet per minute, being rather less than the calculated height. A velocity of 1,128 feet per minute raised the water 5 feet without any discharge, and the maximum effect from the power employed in raising to the same height 5 feet, was obtained at the velocity of 1,678 feet per minute, giving a discharge of 1,400 gallons per minute from the 1-foot wheel. The additional velocity required to effect the discharge is 550 feet per minute; or the velocity required to effect a discharge of 1,400 gallons per minute, through a 1-foot pump, working at a dead level without any height of lift; so that adding this number in each case to the velocity given above at which no discharge takes place, the following velocities are obtained for the maximum effect to be produced in each case: Feet per min. ft. 1,050 velocity for 1 height of lift. 4 " 2,550 550+(500/height of lift in feet.) Mr. Appold considers that when his pump works at the most effective velocity it yields a duty of 70 per cent. One of the great advantages of a pump of this kind is the ease and celerity with which it can be erected; and situations occur where it is highly important to be able to discharge a very large quantity of water in a short time. For example, in laying the foundations of the harbour works at Dover, a large quantity of water-2,000 to 3,000 gallons per minute -was pumped out by one of these pumps, an effect which could not have been produced in the time by any other means, in consequence of the difficulty and delay of fixing ordinary pumps of great capacity. The centrifugal pump had another important advantage for such applications from having no valves, which enabled it to pass large stones, and almost anything that was not too large to enter between the arms. The largest pump constructed on the present plan was erected at Whittlesea Mere for the purpose of draining. The wheel is 4 feet in diameter and its average velocity is 90 revolutions or 1,250 feet per minute; it is driven by a double cylinder steam-engine, with steam 40 lb. per inch, and vacuum 13 lb. per inch; it raises about 15,000 gallons of water per minute an average height of 4 or 5 feet. The cost of the engine and pump was about 1,600%. The centrifugal pump is more advantageous for low lifts (below 20 feet) than for high lifts. Its most advantageous application is as a tidal pump, where the height of lift is continually varying, because it discharges more water the lower the lift, the pump still going at the same speed. Valve pumps generally discharge their cubic contents only, however low the lift. The centrifugal pump is also a useful adjunct to a water-wheel, to assist in keeping it at work by returning a portion of the water when the supply is short. PUNCH-PUNCHING MACHINE. In the ordinary acceptation of the word, a punch is a circular or a four-sided or other form of chisel, for making a hole in any thin substance, or for cutting out blanks, as for buttons, steel pens, and numerous other objects. Punches be arranged into two classes, viz. duplex may and single. The former are used in pairs, and partake of the nature of shears. Single punches have usually acute edges with one perpendicular side but sometimes the edges are rectan Or in general terms, the velocity in feet per minute gular. The punch is driven through the were in most if not all cases rejected for straight ones. In a memoir by M. Ch. Combes, "Sur les Roues de Réaction," read before the Academy of Sciences at Paris on the 23d July, 1838, "the theory of the centrifugal ventilator is discussed, and its obvious relations to the theory of the centrifugal pump are pointed out. The curved form proper to the vanes is insisted on in this paper, and its theory investigated." In 1838-9, M. Combes appears to have taken out a Brevet d'invention for a centrifugal pump, a model of which still exists in the collection of the "Ecole Nationale des Mines," at Paris. In 1843 he published a work, "Sur les Roues à réaction ou à tuyaux." material to be cut by the blow of a hammer, Fig. 1799. |