CENTRIFUGAL PUMPS.
Many ingenious men have vainly attempted to apply what has been erroneously called "centrifugal force" as a motive power, conceiving that the effort made by bodies to fly off when whirled round in a circle was occasioned by a force generated by their rotation. The experiment of the "whirling table," which is commonly shown to illustrate centrifugal action, tends to confirm the notion that force is generated; for it is there seen that, when the velocity of rotation is doubled, the centrifugal force is quadrupled, and that it continues to increase in a geometrical ratio. It has, therefore, been conceived that a power might be generated of indefinite amount; for as a double velocity can be communicated by doubling the moving power, whilst the tendency to fly off at the circumference is quadrupled, there appeared to be a creation of power which, if properly applied, would realize perpetual motion.
A working engineer known to the author was so fully possessed with the notion that power might thus be created, and that its application would be of the utmost benefit, that he imagined he had been specially appointed to reveal the principle to man, as a boon of inestimable value to the manufacturing arts. The plan he adopted was to employ what he called a generating engine, consisting of a centrifugal pump; and the force with which the water was projected from the ends of two rotating horizontal arms was directed against pistons working in cylinders, as the force of steam is in a steam engine. Having once set this machine in action, he expected to be able, by means of the self-creating centrifugal force, to generate the power that worked the generating engine, and thus to have a reservoir of force of any magnitude constantly at command. So completely satisfied was he of the practicability of the plan, founded, as he supposed, upon one of Newton's laws of motion, and he felt so happy in the thought of being charged with an important mission for the benefit of mankind, that it was almost cruel to attempt to correct his notions of the power of centrifugal force. He spent all his money in endeavouring to realize this impossible project, and even its failure did not convince him of his error.
The simple kind of Centrifugal Pump applied in that chimerical scheme was known upwards of one hundred years ago. It consisted of a vertical hollow shaft, into which were inserted two horizontal arms. The shaft was supported on a pivot at the bottom, and was turned by a handle at the top, as represented in the accompanying drawing. The lower end of the vertical shaft was immersed in water, and when rotary motion was given to the machine, the centrifugal action propelled the water from the ends of the arms, and the water rose in the vertical shaft to supply its place.
The effect in a pump of this construction is due to the pressure of the atmosphere, for the outpouring of the water from the rotating arms tends to produce a vacuum in the shaft, in the same manner as the lifting of the plunger in a common pump. It is evident, therefore, that a Centrifugal Pump of that construction could not raise a column of water higher than the pressure of the atmosphere would force it up, which would be about thirty feet.
Mr. Appold's Centrifugal Pump, which constituted one of the most remarkable features of the Machinery Department of the Great Exhibition, is constructed on a different plan, though the principle is the same. The rotating arms are immersed in the water to be raised, and to diminish the resistance which would be produced by the rotation in water of two or more exposed arms, they are enclosed within discs of metal, about one foot in diameter, and three or four inches apart. The arms are formed by curved partitions between the discs, which radiate from the centre to the outer rim, towards which the space between the discs is contracted. This pump is fixed on an axis, to which rapid rotary motion can be given; and it is fitted into a case connected with the pipe that conveys the water to the discharging orifice. The water enters the rotating disc through a large aperture in the centre, and it is forced through the spaces formed by the radical arms with increasing velocity, until it escapes from the circumference. Sections of Mr. Appold's pump are shown in the accompanying diagrams, in which A is the central opening for the admission of water; C, C, C, the curved radical partitions which form the arms by which motion is communicated to the water, and through the ends of which it issues into the external case, connected with the lift-pipe, D.
In the Great Exhibition there were two other Centrifugal Pumps shown in action, one by Mr. Bessemer, and one by Mr. Gwynne, from America; but neither of them exhibited such striking effects as Mr. Appold's, which was so arranged as to throw out a continuous cascade of water from an aperture six feet wide, at a height of twenty-six feet. The Jury of Class V., who made numerous experiments to determine the practical efficiency of Centrifugal Pumps, and the relative merits of the three exhibited, reported very favourably of that of Mr. Appold, to whom a Council Medal was awarded. When rotating at the rate of 788 revolutions in a minute, and lifting the water 19·4 feet, the greatest practical effect, compared with the power employed, was attained. The discharge of water per minute at that height, with the pump rotating with a velocity of 788 revolutions, was 1,236 gallons; and with a lift of 8 feet, 2,100 gallons per minute were discharged, when the rotating velocity was 828 revolutions per minute. In Mr. Gwynne's and Mr. Bessemer's pumps, which had straight vanes, the ratio of power to the effect did not exceed 0·19. One of Mr. Appold's pumps, only one inch in diameter (the exact size of the small diagram), will discharge ten gallons per minute. The greatest height to which water has been raised by the pumps that are one foot in diameter is 67·7 feet, with a velocity of 4,153 feet per minute.
The velocity with which the pump should revolve depends upon the height to which the water is to be raised. Beyond a certain height, the required velocity is practically unattainable, but long before that limit is reached the waste of power becomes so great, that the pump is of no value, for the pressure on the circumference counteracts the force with which the water is expelled. It is, therefore, only at comparatively low levels that the Centrifugal Pump is a useful engine. The absence of all valves renders it very suitable for draining marshes, and for other similar purposes, as the muddy water and suspended matters will not obstruct its action.
In the Report of the Jury the influence of the curved shape of the radial arms is considered very important in producing the effects. "If the vanes be straight," the Report observes, "it is evident, that whatever may be the velocity of the water in the direction of a 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 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, the velocity due to the centrifugal force of the water carrying 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." It is an interesting fact in the history of the invention, that the curved form was formerly adopted in some of the American pumps, and afterwards abandoned.
There are competing claims to the invention of Centrifugal Pumps in the form now adopted. This kind of pump is stated to have been used in America in 1830. M. Charles Combe took out a patent in France for a similar pump in 1838; but though Mr. Appold was later in the field with his more perfect machine, he appears to have proceeded independently of previous inventors.