A./cans of Driving.—Pumps are driven—(i) by hand, catching direct hold of the piston-rod; (2) by hand, through a lever attached to this rod; or (3) by belts or gearing operated by hand, by foot-power, by the wind, or by animal power; they are also (4) attached directly to motors which drive them without the intervention of belts or gears; and (5) in some cases the motor is a part of the pump proper, its piston being a prolonga tion of the piston of the pump. This fifth class is so important that it will be given special consideration by itself after the others have been briefly reviewed. Examples of nearly every one of these methods are given in the illustrations, reference to which in detail would be unneces sary.
Role-finV.—The rope-pump is a very crude device, consisting merely of an endless rope hanging in the reservoir and running rapidly over a wheel at the top. The water which clings to the rope by friction and inertia is led off at the top by a suitable conduit.
Canc-pump.—In the cane-pump (pi. r r 3, fig. 7) there is a cylinder whose lower end dips below the surface of the reservoir and whose upper end is attached at the discharge point. A foot-valve or check-valve below the water-level admits the liquid when the cylinder is lowered, and prevents its exit when the barrel is raised.
the diaphragm-pump the raising of the water is caused by the motion of a flexible diaphragm attached to a rod, cord, or chain, so that the volume of the chamber, of which this diaphragm forms part of the wall, is caused to change.
Jet-pr110.—An example of the jet-pump is shown in the boiler-feeding injector (pl. 79, fig. i). In this the flow of the liquid is caused by the impact of another fluid, which may be a vapor, as in the case of the injector, or a permanent gas, as in that of the tromp, where an air-jet is emplo) ed, or a liquid itself; as where a jet of water at high pressure draughts and forces a current of other water.
Oscillating Pump.—The oscillating pump corresponds very closely to the motor in which a hinged piston works in a case or chamber which is a sector of a circle. (See p. 26o.) Double or Twin-cylinder Pumps.—The steadiness of discharge of a pumping-machine is greatly increased by having two cylinders with pis tons so arranged that one is forcing while the other is discharging. (The double or twin-cylinder pump must not be confused with the duplex pump.) Steadiness of action is still further increased by having three charging into a common delivery. In the latter case they arc generally driven from a crank-shaft having three cranks 12o° apart.
The Duplex Pump.—Tlic term " duplex " is applied to a direct-acting pumping-machine in. which there arc two pumping-cylinders and two
steam-cylinders, each steam-cylinder working the piston of one pump cylinder, and the valves of each steam-cylinder being driven by the action of the other side of the machine, whether by the piston-rod of the other half or by steam discharged at a certain point in the stroke of the other; the essential characteristic of the duplex pump being that each half or side of the steam end is driven by the other half or side of the machine. While two pump-cylinders :,rive steady action to a direct-acting pumping engine, the feature of making the valve-action of one side depend upon the working of the other causes trouble, iii that any slowing of one side retards the other, and this again slows the first side, and so on; so that a trifling difference in the tightness of packing or a little air-pocketing in one cylinder may cause each side to make less than the full stroke.
The Hall Duplex of which only one of the steam- and water-cylinders is shown in Figure 5 (A/. Ir3), has for a steam-valve (on each side) a plain flat slide, which stands at the left of the stroke, admitting steam to the right of the piston and moving it to the left. When the piston nearly reaches stroke-end and passes by port A (port 11 being closed), some of the steam which is moving the main piston passes through port A and follows a passage which leads to the steam-chest of the other steam-cylin der, whose slide-valve it shifts. The plunger is a long cylinder fitting air tight in a short cylinder, which is practically the face of a packing-ring or partition which divides the pump-casing in two crosswise. There are two sets of suction-valves (r) and two sets of discharge-valves (c), each set acting for one end only. The Figure shows one set of each of the valves open.
Compound Duplex 4 shows a duplex pump in which the steam-ends are " compounded ;" that is, the exhaust steam of the high-pressure cylinder A discharges into the steam-chest C, and is expanded in the low-pressure cylinder B. There is a short, solid water piston playing in a long cylinder, above which are all the water-valves E, there being two sets of suction-valves on the lower plate and two sets of discharge-valves on the upper plate. In the Figure the steam-valves arc in position steam in at the right-hand end of the high-pressure cyl inder and to dri,•e the piston to the left. The steam in the right-hand end of the high-pressure cylinder acts by expansion only, and the left-hand end exhausts into the steam-chest C of the large cylinder. The left-hand end of the large steam-cylinder exhausts into the air or into a condenser.