Like most pumps, a centrifugal pump converts mechanical energy from a motor to energy of a moving fluid; some of the energy goes into kinetic energy of fluid motion, and some into potential energy, represented by a fluid pressure or by lifting the fluid against gravity to a higher level. For more details on this topic, see Centrifugal compressor. The transfer of energy from the mechanical rotation of the impeller to the motion and pressure of the fluid is usually described in terms of centrifugal force, especially in older sources written before the modern concept of centrifugal force as a fictitious force in a rotating reference frame was well articulated. The concept of centrifugal force is not actually required to describe the action of the centrifugal pump. In the modern centrifugal pump, most of the energy conversion is due to the outward force that curved impeller blades impart on the fluid. Invariably, some of the energy also pushes the fluid into a circular motion, and this circular motion can also convey some energy and increase the pressure at the outlet. The relationship between these mechanisms was described, with the typical mixed conception of centrifugal force as known as that time, in an 1859 article on centrifugal pumps, thus: To arrive by a simpler method than that just given at a general idea of the mode of action of the exterior whirlpool in improving the efficiency of the centrifugal pump, it is only necessary to consider that the mass of water revolving in the whirlpool chamber, round the circumference of the wheel, must necessarily exert a centrifugal force, and that this centrifugal force may readily be supposed to add itself to the outward force generated within the wheel; or, in other words, to go to increase the pumping power of the wheel. The outward force generated within the wheel is to be understood as being produced entirely by the medium of centrifugal force if the vanes of the wheel be straight and radial; but if they be curved, as is more commonly the case, the outward force is partly produced through the medium of centrifugal force, and partly applied by the vanes to the water as a radial component of the oblique pressure, which, in consequence of their obliquity to the radius, they apply to the water as it moves outwards along them. On this subject it is well to observe that while the quantity of water made to pass through a given pump with curved vanes is perfectly variable at pleasure, the smaller the quantity becomes the more nearly will the force generated within the wheel for impelling the water outwards become purely centrifugal force, and the more nearly will the pump become what the name ordinarily given to it would seem to indicate_a purely centrifugal pump. When, however, a centrifugal pump with vanes curved backwards in such forms as are ordinarily used in well-constructed examples of the machine, is driven at a speed considerably above that requisite merely to overcome the pressure of the water, and cause lifting or propulsion to commence, the radial component of the force applied to the water by the vanes will become considerable, and the water leaving the circumference of the wheel will have a velocity less than that of the circumference of the wheel in a degree having some real importance in practice. The statement "the mass of water ... must necessarily exert a centrifugal force" is interpretable in terms of the reactive centrifugal force_the force is not an outward force on the water, but rather an outward force exerting by the water, on the pump housing (the volute) and on the water in the outlet pipe. The outlet pressure is a reflection of the pressure that applies the centripetal force that curves the path of the water to move circularly inside the pump (in the space just outside the impeller, the exterior whirlpool as this author calls it). On the other hand, the statement that the "outward force generated within the wheel is to be understood as being produced entirely by the medium of centrifugal force" is best understood in terms of centrifugal force as a fictional force in the frame of reference of the rotating impeller; the actual forces on the water are inward, or centripetal, since that's the direction of force need to make the water move in circles. This force is supplied by a pressure gradient that is set up by the rotation, where the pressure at the outside, at the wall of the volute, can be taken as a reactive centrifugal force. This is typical of 19th and early 20th century writing, to mix these conceptions of centrifugal force in informal descriptions of effects such as that in the centrifugal pump. Differing conceptions and explanations of how a centrifugal pump works have long engendered controversy and animadversion. For example, the American Expert Commission sent to the Vienna Exposition in 1873 issued a report that included observations that "they are misnamed centrifugal, because they do not operate by centrifugal force at all; they operate by pressure the same as a turbine water wheel; when people understand their method of operating we may expect much improvement." John Richards, editor of the San Francisco-based journal Industry, in his in-depth essay on centrifugal pumps, which also downplayed the signficance of centrifugal force in the working of the pump, remarked: This extraordinary report stands printed in a Government publication, signed by men who were, or are, eminent in mechanics, and we can only deplore the stupidity, as well as presumption of the commission who thus disposed of a subject that had twenty years before been carefully investigated by such men as Sir John Rennie, Professor Cowper, Mr. Whitelaw, Dr. James Black, Professor Rankine, and many others. The most astonishing part is, however, that this report was passed and signed by men who we can hardly suppose would fail to perceive its absurdity. Modern sources say things like that the fluid "flows radially under centrifugal force", or "centrifugal force flings the liquid outward". Others counter that "there is no force at all, and a great deal of confused thinking." Some are more careful, attributing the outward force to the impeller, not to centrifugal force: "the impellers throw the water to the outside of the impeller case. This centrifugal action is what creates the pressure..." Even serious texts that explain the working of the pump without mention of centrifugal force introduce the pump as one in which "the mechanical energy is converted, into pressure energy by means of centrifugal force acting on the fluid." Source: wikipedia
