tem uses compressed air expansively, and as a result there is considerable energy lost through the escape of the compressed air into the atmosphere-as with the high-pressure steam engine.

The "return-air» system overcomes the difficulty of not employing air expansively and in it no floats and no air valves, outside the engine room, are employed and the same air is used over and over again, thus eliminating the effect of clearance. This system employs two tanks and its operation consists in brief of pumping air out of one tank and forcing it into the other, and in so doing will draw water into the former and force it out of the latter. The content or volume of air in the system is so adjusted that when one tank is empty the other is full, and at that moment the switch will be automatically thrown, reversing the pipe action and thereby reversing the action of the tanks.

Figure 1 presents a conventional diagram of the displacement type of compressed-air pumps, drawn in this instance to show the action of the Harris or "return-air pump. The air is being pumped out of chamber A and into chamber B, the water from which is passing up the discharge-pipe D, under the pressure exerted by the air on the surface of the water in B. When chamber A is full the valves on the airpump reverse automatically and the air is pumped into A and out of B, the valves in the inlet box below the chambers reverse their positions and the water is forced out of A into the discharge-pipe, and a new charge of water

Using the air expansively, all the inherent energy of compressed air is used, and no mechanism being submerged, there is no chance for the system to become inoperative due to breakage or a demand for repairs. This system may be employed for pumping mixtures of sand and water, and operates with a marked degree of economy and reliability.

Air-Lift. Opinions differ as to the theory of the air-lift. A common air-lift is one where we have a driven well in which the water is comparatively near the surface. We place in this well a pipe for the discharge of the water." This is known as the "eduction pipe." This pipe does not touch the bottom of the well, but is lifted above it so as to admit freely the water through its lower end. Alongside this pipe, either on the outside or within, is a second but smaller pipe, properly proportioned; as soon as this pressure overcomes that of the water, the conditions are reversed and another "chunk of compressed air is discharged into the pipe, shutting off the water for an instant. This process is continuous and as regular as the movement of a pendulum.

As these "chunks" of air approach the surfact they are gradually enlarged, because of the reduced load upon them, and it is likely that before they reach the surface there is a general breaking up of the piston-like layer condition.

Figure 2 illustrates the action of the deepwell Pohlé air-lift, where the pressure of the air has to be relatively high. The pipe F leads the air from the compressor to the foot of the eduction pipe E. Where the diameter of the eduction pipe is properly proportioned to the weight of the column of water to be lifted, the compressed air pushes out of the air-pipe in little pledgets, one after another, holding their piston-like shape across the diameter of the

eduction pipe, and each lifting its little load of water toward the free air at normal atmospheric pressure above. Where the lift is not so high and the weight of the water column not very great, the air is released at the bottom of the eduction pipe in bubbles, perhaps from several orifices at once, as in the Frizell system, and thus the water is aerated, rendering it so light that it flows freely upward with the air hastening to escape into the lower pressure above.

The differential air-lift operates with a comparatively low air pressure, and by using the air expansively achieves a considerable lift. The device consists of two (or more) air chambers, with the general arrangement_shown in the explanatory diagram, Figure 3. It is to be supposed that the compressed air is being pumped into chamber A, exerting a pressure which forces the contained water upward through pipe G into chamber B. When the level of the water in A has sunk below the open end of pipe D the supply of compressed air is automatically shut off, and the compressed air then in A expands through pipe D into chamber B, there exerting a pressure which opens the valve at the foot of pipe F, up which the water contained in B is then forced. It is readily seen that the air pressure in B (supposing the chambers to be of the same dimensions) will be half that in A. If there is in the series a third chamber (C) of the same size and arrangement of pipes and valves, the air pressure in C will be one-third of the original pressure pumped into A. The possible lift from B to C will, of course, be just half that from A to B. Upon the final escape of the air at the top of the lift, chamber A fills with water by gravity and the cycle is renewed. A certain substantial amount of water pressure is necessary to smooth working of the air-lift. This is commonly secured by deep immersion of the eduction pipe. Where this cannot be secured an auxiliary air chamber is installed from which the air is pumped, the water flowing in to fill the vacuum as in the ordinary suction pump. On reversing the air, pressure is exerted downward on the water in the auxiliary chamber, thus forming an artificial "head" sufficient to lift the air-shaft.

The Pohlé system of air-lift is probably the best known. Sand, grit and small stones are no obstacles in the satisfactory operation of this system. As a matter of fact, in many instances the capacities of the wells have been increased by opening up the well more thoroughly through this removal of the sand. Water pumped by this system is purified to some extent more so in the Frizell system by the aeration. The system is not limited as to the quantity of water that can be handled. This will depend upon the capacity of the wells to furnish the water. The height of the lift is limited only by the degree of pressure imparted to the air. Consult Greene, A. M., Jr., 'Pumping Machinery) (New York 1911); Ivens, E. M., Pumping by Compressed Air (New York 1914).

PUMPS AND PUMPING MACHINERY. In the modern acceptation of the term, a pump is a machine for exerting mechanical action upon fluids. It is commonly employed for raising liquids to a higher level; for propelling

them through pipes and orifices under (hydrau lic) pressure; and for the compressing and rarefying of gascous substances. The pumps used for compressing gases are discussed under the title AIR COMPRESSORS; those for rarefication under AIR-PUMP and VACUUM-PUMP.

Pumps operate by two quite distinct methods, and are consequently classified as (1) suction or "bucket" pumps and (2) force pumps. The suction pump depends for its operation upon the constant pressure of the atmosphere- 14.7 pounds per square inch. The action of the pump is to release from this pressure that area under the influence of the pump's piston: the

FIG. 1.-Section View of Modern Suction Pump. air pressure outside of that area causes fluids affected by it to flow into the space in which the pressure has been diminished. In the force pump the force is applied directly by the pump mechanism to the fluid to be moved, regardless of atmospheric pressure. In its simplest form, the modern atmospheric or suction pump consists of a cylinder (c) connected at the bottom with a pipe (p), the lower end of which is immersed below the surface of the water. In the cylinder are placed two valves (uv and lv), the lower stationary and the upper attached to a piston (f) at the end of a rod (r), which moves the piston up and down under the motion of a handle (h). A pipe or spout (s), attached to the cylinder near its upper end, receives and discharges the water raised by the working of

the piston. Both the valves open upward, and the action of the entire arrangement, based upon the physical fact that two bodies cannot Occupy the same space at the same time, is as follows: When the downward stroke of the handle moves the piston upward, the air in the space A is rarefied, having to occupy a greater space, and the partial vacuum thus formed relieves the pressure of the atmosphere from the lower valve, which being opened upward by the pressure of the air in the space (B) of the pipe, allows a portion of it to pass into the space (4). When the piston descends under the upward stroke of the handle, the air in the space (A) is compressed, the lower valve is closed, and when the density of the compressed air becomes greater than that of the atmosphere, the upper valve is forced open and the air passes into and out of the space (D). Thus, by the continued up and down movement of the piston, all the air in the space (B) is completely exhausted, the water rising in the suction pipe under the pressure of the atmosphere upon its surface in the well, until it fills the space (B) up to the lower valve. The next upward movement of the piston empties the air in the space (A), which is immediately filled with water by the opening of the lower valve. The downward motion of the piston relieves the pressure and allows the lower valve to fall into its seat. The water then in the lower part of the pump flows through the valve in the piston into the space (D), from which it is discharged through the spout by the next upward movement of the piston which at the same time refills the space (A) by suction. Under the laws of fluid pressure discovered by the experiments of Torricelli and others, the height to which a column of water will rise depends upon the atmospheric pressure at any point on the earth's surface, and varies with the altitude of that point. At the level of the sea, the atmosphere exerts a pressure of 14.7 pounds to the square inch, and will support a column of water in a closed tube from which the air has been exhausted, between 33 and 34 feet in height, while upon the top of Mont Blanc, or Pike's Peak, 15,000 or 16,000 feet above the sea, the atmospheric pressure will support a similar column of water only about 16 or 17 feet in height. A knowledge of this fact is important not only in determining the maximum distance at which the lower valve of a pump may be placed from the surface of water to be pumped, but also in every branch of hydraulic engineering in which atmospheric pressure is utilized.

By far the greater amount of pumping done in the economic world is accomplished by force pumps. It is, however, generally the case that the principle of the suction pump is used in combination to a greater or less degree, atmospheric pressure being relied on for preliminary lifts not to exceed 16 or 18 feet above the basic water level.

Force pumps are of two general types: (1) the piston pump, and (2) the plunger pump. In the former a well-fitted piston is driven to and fro in a smoothly-bored cylinder to which the water to be pumped is admitted. The pressure of the moving piston upon the water forces it out of the cylinder by any valve opening freely outward, and thus into pipes through which it may be raised to heights varying with the power exerted on the piston; or it may

5

transmit the pressure of the piston to all water in a long pipe line for power purposes. The plunger pump is similar in results, but develops its pressure by the forcible thrusting of the comparatively large mass of the plunger into a chamber already filled with water. The water being incompressible, a volume of it equal to the bulk of the entering plunger is driven out

Y

of the chamber into the transmission pipes. Both piston and plunger pumps are usually designed to act both on the outward and the return strokes the piston sucking in a supply of water behind it as it moves in either direction; and the plunger being double-ended, and working alternately into two chambers divided by a wall or a diaphragm carrying a packing ring or sleeve. These types are known as double-acting pumps. In the sectional diagram of a piston force-pump shown in Fig. 2, the piston P is represented as starting on its outward stroke. The water W before it is being forced upward through valve E into the discharge-pipe system D, while valves F and K are

valve F opens for the discharge of the water which has just come through H, and valve K opens to allow the supply to again fill the cylinder. The same description applies to the diagram of the plunger force-pump, Fig. 3; in which P represents the plunger which drives outward and back alternately into chambers W and Y, divided by the packing-ring R, R.

A variation of the plunger pump is the "differential" pump, in which the valves and double-acting plunger are so arranged that while suction of the supply water takes place only on the outward stroke, a discharge occurs upon the strokes in both directions. Fig. 4

presents a sectional diagram of the most common form of the differential pump, in which P represents the plunger, working in the sleeve R, R. On the inward stroke just beginning, the water in chamber W is forced out into the discharge system D, valve K opening to allow a supply to be drawn in (by suction) through pipe S into chamber Y: On the return stroke valve K is closed by the pressure, and valve E opens, and another discharge into D occurs.

While there are undoubtedly a larger number of pumps worked by hand than by any other power, such pumps are relatively small, and are not operated continuously. By far the greater part of the world's pumping, as to volume, is done with what are called power pumps, many of them being very large. The power generally employed is steam, although compressed air is coming more and more into use, being found more economical. (See PUMPS, COMPRESSED-AIR). The attachment of the power to the pump has generally taken the tandem form, with the power cylinder at one end and the pump cylinder at the other, the piston rod of the steam cylinder being extended to form the piston rod or plunger rod of the pump. This type is known as the direct-acting pump. In other arrangements the steam cylinder may act upon a walking-beam to which the pump rod is connected; or it may work upon a shaft on which is a fly-wheel. Frequently the work is divided, two or three cylinders being provided for both steam and pumps, working alternately or in succession, and thus discharging a more nearly continuous and uniform volume of water. All of these power pumps discharge the water against a

cushion of air, the elasticity of which relieves pump, engine and piping of shocks which otherwise would soon be disastrous to their mechanism. These air chambers are, therefore, to be considered an essential part of the pump. They are proportioned to the volume of water delivered per minute, and the pressure which is to be preserved in the pipe line into which the water is delivered, a certain specific volume of air being necessary to transmit the particular pressures of the individual installation. In some makes of pumps air chambers are placed also on the suction pipe, to take up and distribute (in point of time) the strains of operation, aiding materially in maintaining a uniform discharge.

Rotary pumps, formerly in limited use, have become very numerous, as the electric motor offers an ideal power for small installations. These pumps are of two general types, called conventionally "piston" and "plunger." In the piston rotary pump the piston-rod carries longitudinal wings or vanes which produce a "wiping" pressure on the water in the cylinder as the piston revolves on its axis. The more modern types are fitted with two "pistons" set parallel, the wings of which interlock to prevent a backward flow of the water. They are run usually at great speed by small dynamos, and do excellent service in raising comparatively small quantities of water to the service tanks on the roofs of buildings of moderate height. The plunger type of rotary pump has a cylindrical plunger revolving on a OUTLET

FIG. 5.-Sectional Diagram of Piston Rotary Pump. P, P, pistons revolving in opposite directions. longitudinal axis placed eccentrically as to the plunger, but in the centre of a larger cylinder in which the plunger revolves. A longitudinal backstop, actuated by springs, or by its own weight, prevents the water crowded out of onehalf of the cylinder from flowing back into the half whence it was forced by the plunger. Another form of rotary pump is the "screw> pump, a cylinder having a longitudinal centre shaft set with a series of blades like those of a screw propeller. This is sometimes called an "impeller" pump, and is of great utility in pumping sewage and similar heavy and heterogeneous liquid mixtures which could not be

moved by the forms of pumps commonly in use. One of the last-named type was constructed at Milwaukee in 1889 to flush the Milwaukee River of its sewage-polluted waters. It has a screw wheel 13 feet in diameter, which, revolving at a rate of 60 revolutions per minute, deM

FIG. 6. -Sectional Diagram of Plunger Rotary Pump.

Lmlet; M, outlet; N, back stop; P, rotating plunger.

livers 550,000,000 gallons of water a day on a lift of four feet. This water is forced through a brick-lined tunnel 12 feet in diameter, and 2,500 feet long. A battery of 11 of these screw pumps are in use in New Orleans for discharging storm water from the city's drainage system. They are 12 feet in diameter and run at a speed of 75 revolutions per minute. The screw is placed at the summit of a flattopped siphon and is operated by an electric motor. The discharge is about 550 cubic feet per minute, the height raised varying from five to nine feet. The efficiency of these pumps as shown by actual tests is 80 per cent.

R

V

FIG. 7.-Sectional diagram of centrifugal pump. I, inlet; R, rotor; D, diffusing chamber; V, volute casing; O, outlet.

The "centrifugal pump" is the best known representative of the "impeller" type. It embodies the principle that a body revolving around a centre tends to move away from it with a force proportional to its velocity; thus the rim of a revolving wheel imparts a portion

7

of its velocity to any substance adhering to it, and throws it off when the force of the velocity thus imparted exceeds the force of adhesion, as the mud thrown off by the wheels of a carriage, or the water from a mop or towel, rapidly revolved by the hand. The first pump of this type was invented by M. Le Demour, who sent a description of it to the Academie Française in 1732. It consisted of a straight tube attached in an inclined position to a vertical axis, around which it was whirled by a crank-handle. Later forms were constructed by tubes joined in the form of a T, the vertical tube of which was placed in the water. The lower end perforated to admit the water, and fitted with a valve to retain the water in the vertical tube when the pump was not in operation, was placed under the surface of the water and supported upon a pivot. The ends of the horizontal discharge tube were bent down into a circular trough over which they were revolved. When the machine was revolved rapidly the centrifugal force which discharged the water in the horizontal tube was communicated to the water in the vertical, which was also drawn out, but in the meantime refilled by the atmospheric pressure on the reservoir. When it is required to lift large quantities of water to a low elevation, centrifugal pumps may be used with greater efficiency than reciprocating pumps, the efficiency of which diminishes with the lift. A pump of this kind, constructed in Massachusetts in 1818, was equipped with four blades set at right angles like those of a fan blower, and was used for many years. An improved form was exhibited by Appold in England, in 1851, which embodied all of the principal features employed in the best pumps constructed since that time. It is stated that Appold's pump raised continuously a quantity of water equal to 1,400 times its own capacity per minute. The "whirlpool chamber," designed to utilize the energy developed by the whirling water which in most pumps of this type is lost as eddies in the discharge pipe, was suggested by Thomson (Eng.), and consisted of a chamber somewhat larger than the pump, in which the water discharged by the pump disc, with considerable velocity, was allowed to rotate and impart its energy as an auxiliary aid to the driving power. A pump constructed by him for drainage purposes, in the Barbados, was equipped with a whirlpool chamber 32 feet in diameter, around a pump disc 16 feet in diameter. Others of large size were constructed by Easton and Anderson, for the North Sea Canal, in Holland, which were capable of delivering 670 tons of water per minute to a height of five feet; while the Gwynne pumps employed to drain the Ferrarese marshes in Italy, at the junction of the Po di Volano and the minor rivers of lower Lombardy, are capable of a combined delivery of 2,000 tons of water per minute.

A large class of pumps, involving the principle of centrifugal action in some degree are the turbine pumps.

The turbine pump is practically a reversal of the turbine water-wheel (q.v.), being a turbine run by power, generally steam, to produce motion or pressure in a body of water or other liquid. It differs from the centrifugal pump in adding the power advantage of the screw to that of centrifugal force. A common application of this form of pump where the lift is