Page:The New International Encyclopædia 1st ed. v. 16.djvu/620

* PUMPS AND PUMPING. 542 PUMPS AND PUMPING. plex, are the rule, except for small sizes and spe- cial purposes. Rotary pumps have revolving in- stead of reeiprocatiug pistons. Two pistons re- volve on parallel axes. Their longitudinal surfaces are formed into a series of curves, so they mesh closely on the inside as they revolve, while their outer parts fit tightly against a curved pump chamber. Screw pumps displace water when re- volved in a chamber ith proper inlets and out- lets. Chain piston pumps are a simple type, con- sisting of a series of pistons mounteil on an end- less chain. Both pistons and chain pass upward through a vertical cylinder, forcing water ahead of them, then descending again to repeat the pro- cess. The Pulsomctcr is a more elaborate appara- tus, in which the jmmp and motive power are combined. The displacement is eiTected by means of steam, acting in two connecting chambei-s. On admitting steam into one of these the water is forced out gradually, but with the increasing wall area to which the steam is exposed with the expulsion of the water the steam is condensed, a vacuum is created, more water rushes in, and the steam is automatically turned into the other chamber, whereupon the process is repeated. Such pumps are very largely used by contractors and others for pumping dirty water, or even water containing mud and small stones, on account of their com])aratively simple construction and great durability. The work done by a pump, when the pump is placed above the level of the water to be raised, and at the same time below its final elevation, is frequently classified as suction-lift for the first part, and force-lift for raising the water above the pump level. Pumps may perform either suction or force lift alone, or the two combined, according to their location. The accompanying diagram represents the ordi- nary suction pump. A is a cylinder, which is called the barrel ; with it is connected at the bottom a pipe, B, which communicates with the water to be ra'ised; and at its top is another pipe, C, which receives the water raised. In the barrel are placed two valves, D and E. D opens upward and is fixed in position at the bottom of the barrel; E also opens upward and is attached to and forms part of the piston F, which moves up and down the barrel when motive jiower is applied to the rod G. 'hen the valve E is opened, water or air can pass through it to the upper side of the piston; but when shut, none can pass from one side of the piston to the other. The other valve, B, is similar to it in all respects. On moving the piston up the barrel, the valve E closes, owing to the atmosphere pressure above it; no air can pass from above it into the part of the barrel from which it is moving. The air contained in the lower part of the barrel becomes rarefied, by hav- ing to occupy a greater space, and exerts less pressure on the valve D at the bottom of the barrel than the air in suction pipe B below it. This valve is thus opened, and the air from the suction pipe enters the barrel ; so that ^^■hen the piston has arrived at the top, a volume of air equal to the contents of the barrel has passed from the suction ])i]je into the barrel. When the piston descends, it compresses the air in the l>ar- rel. which shuts the valve D; and when the densi- ty of the compressed air becomes greater than that of the atmosphere, the valve E in the piston is forced open, and the air in the barrel passes to the upper side of the piston. The next upward stroke of the piston again draws a like quantity of air from the suction pipe into the liarrel; and, as none of this air again enters the pipe, but is passed to the upper side of the piston by its downward stroke, the suction jiipe is liy degrees emptied of the air it contained. During this pro- cess, however, motion has taken place in the water at the foot of the suction pipe. The surface of tlic water at H is pressed upon by the weight of the atmosphere with a jiressure of abo'ut 15 poiuids on every square inch; and, by the laws of lluid pressure, if an equal pressure is not exerted on the surface of the water in the suction pipe, the water will rise in it, until the pres- sure on the sur- face, plus the weight of its fluid column, bal- ances the pres- sure of the at- mosphere on the surface H out- side ; so that, as the air in the suction pipe is rarefied, the water rises in it, until, when all the air is ex- tracted from it, the water stands at the level of the valve D. By the next upward stroke of the pis- ton, the barrel being emptied of air, the water follows the pis- ton, and fills the barrel as it filled the suction pipe. The pressure produced by the downward stroke shuts the valve D, and forces the water in the barrel through the valve E. The succeeding upward stroke carries this water into the pipe above, and again fills the barrel from the suction pipe. In like manner, every successive upward stroke discharges a body of water equal to the content of the barrel into the pipe above it, and the pump will draw water as long as the action of the piston is continued. The ordinary forms of lift and force pumps are very similar to the suction-lift pump before de- scribed, with this exception, that the valve repre- sented by E, instead of being fixed on the pis- ton, is placed in the discharge pipe, the pis- ton itself being solid. The water is drawn up into the barrel by suction in the manner just de- scribed in the suction pipe, and then the pressure of the piston in its downward stroke forces it through the outlet valve to any height that may be required. In these pumps it will be observed that the water is forced into the ascending pipe or column only on the downward stroke; it will thus be dis- charged in a series of ru.shes or jerks. As it is a great object to procure a continuous discharge, both for its convenience, and for the saving of SINGLE SUCTION PUMP.