Page:Encyclopædia Britannica, Ninth Edition, v. 12.djvu/550

534 534 HYDKO MECHANICS [HYDRAULICS. pump with barrels 18 inches diameter, at speeds under 60 feet per minute, gave the following results : Lift in feet 14 34 47 Efficiency 46 66 70 The very large steam-pumps employed for waterworks, with 150 feet or more of lift, appear to reach an efficiency of 90 per cent., not including the friction of the discharge pipes. The Centrifugal Pump. 187. The efficiency of reciprocating pumps diminishes with the lift. When large quantities of water are to be raised on a low lift, no pump is so suitable as a centrifugal pump. The first pump of this kind which attracted notice was one exhibited by Mr Appold in 1851, and the special features of his pump have been retained in the best pumps since constructed. Mr Appold s pump raised continuously a volume of water equal to 1400 times its own capacity per minute. It had no valves, and it permitted the passage of solid bodies, such as walnuts and oranges, without obstruc tion to its working. Its efficiency was also found to be good. Fig. 203 shows a centrifugal pump differing from ordinary centrifugal pumps in one feature only. The water rises through a suction pipe S, which divides so as to enter the pump wheel at the centre on each side. The pump disk or wheel is very similar to a turbine wheel. It is keyed on a shaft driven by a belt on a fast and loose pulley arrangement at P. The water rotating in the pump disk presses outwards, and if the speed is sufficient a continuous flow is maintained through the pump and into the discharge pipe D. The special feature in this pump is that the water, discharged by the pump disk with a whirling velocity of not inconsiderable magnitude, is allowed to continue rotation in a chamber somewhat larger than the pump. The use of this whirlpool chamber was first suggested by &quot;Professor James Thomson. It utilizes the energy due to the whirling velocity of the water which in most pumps is wasted in eddies in the discharge pipe. In the pump shown guide-blades are also added which have the direction of the stream lines in a free vortex. They do not therefore inter fere with the action of the water when pumping the normal quantity, but only prevent irregular motion. At A is a plug by which the pump case is fillei before starting. If the pump is above the water to be pumped, a foot valve is required to permit the pump to be filled. Sometimes instead of the foot valve a delivery valve is used, an air-pump or steam jet pump being employed to exhaust the air from the pump case. ! 188. Design and Proportions of a Centrifugal Pump. The design of the pump disk is very simple. Let n, r Q be the radii of the inlet and outlet surfaces of the pump disk, di, d the clear axial width at those radii. The velocity of flow through the pump may be taken the same as for a turbine. If Q is the quantity pumped, and H the lift, ,-=0-25V27H (1). Also in practice Hence, &quot;- 257 Wvif (2). 203. Usually and d = according as the disk is parallel-sided or coned, the wheel radially with the velocity ,, and The water enters (3). Fig. 204 shows the notation adopted for the velocities. Suppose the water enters the wheel with the velocity v,, while the velocity of the wheel is V,-. Completing the parallelogram, v r i is the relative velo- city of the water *o and wheel, and is the pro per direction of the wheel vanes. Also, by resolving, iii and Wi are the component velo cities of flow and velocities of whir of the velocity r,- of the water. At the outlet surface, v n is Fig- 204. the final velocity of discharge, and the rest of the notation is similar to that for the inlet surface. Usually the water flows equally in all directions in the eye of the wheel, in that case r,- is radial. Then, in normal conditions of work ing, at the inlet surface,