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UMPS, Compressed-Air. During recent years much progress has been made in the art of pumping water by compressed air. At first all methods of lifting water or other liquids by means of compressed air were deemed extravagant, but the development of various systems has been such that water may be pumped with marked economy and rapidity by compressed air. The direct-acting piston or plunger pump, of simple or duplex type, is operated with little economy with steam as a motive power. One hundred and fifty pounds of steam per horse power per hour is a fair figure to take for this machine. Experiments have shown that under average conditions hardly 50 per cent of the indicated horse power of the driving cylinder is utilized in the pump cylinder, the rest being absorbed partly by the engine as machine friction and particularly by the friction of the water in passage through valves and chambers.

Compressed air is utilized in pumping in two distinct ways, (1) as the motive power in power driven pumps; and (2) directly upon the liquid in displacement pumps.

Power Driven Pumps.- The advantage of the power driven pump over the displacement pump lies in the fact that it permits the expansive use of the compressed air, resulting in a considerable saving and economy. Power pumps consist essentially of expansive air engines, which may be either directly connected, belted or geared to some form of duplex or triplex piston or plunger pump, or to the simple, compound or turbine types of centrifugal pump in its various forms. Compressed air is supplied to the air engine from some more or less distant air compressor and this air engine, when specially designed for the use of air involving considerations of air valves and low clearance, operates with efficiency and economy. Where the magnitude of the operation justifies it, the use of the "return pipe" system with expansive engines will, in connection with reheaters, secure the very highest efficiency.

This type of pump consists essentially_of two parts, an air end and a water end. The compressed air operates a piston which transmits its energy through the piston rod which, in turn, causes the pump piston or plunger to reciprocate and thereby pump water. In the "return pipe" or "closed circuit" system, the same air is used over and over again, the exhaust from the pumps being piped back to the compressor under a limited back pressure. In this case the compressed air is a transmitter of

power just as a belt or transmission rope, but the air never wears out, may be carried to any distance and at any angle, has little inherent friction and possesses the highest efficiency in transmission. The system eliminates all trouble from freezing since the air is used repeatedly and moisture once removed cannot be returned. While it requires two pipe lines instead of one, the pipe cost is frequently less because of the smaller pipe size permitted. The pump cylinders may be smaller because of the higher effective pressure per square inch and the losses in clearance, often enormous, are entirely eliminated. The great economy in this system is secured in the higher compression, since the scheme is based upon the well-known fact that the greatest losses in compression are thermodynamic, which losses are suffered chiefly at the lower pressures. For example 13.42 horse power will compress 100 cubic feet of free air per second to 60 pounds pressure, starting at atmospheric pressure. This same energy will compress 100 cubic feet of air to 350 pounds, when beginning compression at 60 pounds, giving on the return pipe system 290 pounds available pressure with 60 pounds back pressure. A reheater on the pump will secure additional economy. Losses through leakage and transmission are supplied by a small booster" compressor. Where the pressure is above 60 pounds two-stage pumps are employed; where above 300 pounds, three-stage pumps.

Another method of securing high economy is to employ compound or triple expansion pumps, reheating the air after each expansion. This may reduce the air consumption to 1⁄2 or 3 its volume in the simple pump of the same capacity. In some cases this same result may be secured by the use of three pumps in series, reheaters being used as before after each expansion and the exhaust from one pump being supplied to the next, with a larger air cylinder.

Displacement or Pneumatic Pumps.-The displacement pump is almost the essence of pumping simplicity and, if its first promises had been borne out, it would be a most powerful factor in pumping problems. As it is, within its recognized field, it has shown a fitness which qualifies it especially for the work. In brief, it consists of two chambers or cylinders which are filled and discharged alternately the liquid in each chamber being directly displaced by the admission of the required volume of compressed air through a valve operating automatically.

The fundamental requirement is a complete

submergence of three to six feet, or the setting of the chambers in a dry pit at a level so much lower than the water to be pumped that the chambers may fill rapidly by gravity. The filling is done at no expense of power and the discharge with a minimum of friction losses. Dirt, sand or grit will not interfere with its operation. Such a pump starts and stops automatically and uses air exactly in proportion to the amount of water discharged. It will run for weeks without attention and requires practically no repairs. Standard sizes have capacities up to 1,500 gallons per minute.

The height to which these pumps will lift water is limited only by the air pressure used, and by an arrangement of several pumps and reservoirs in series or steps almost any height may be attained with ordinary moderate air pressures. In mine work the displacement pump is especially useful in handling water accumulating in sumps, dips, entries, etc. It is also peculiarly effective in subways and tunnels, in automatically discharging the seepage or leakage water which accumulates. It may also serve in the basement of factories and warehouses which are subject to occasional inundations by floods or high water. Another very important economic use is the lifting of sewage from low-level catch-basins into the trunk sewer at a higher level. The same apparatus, but in modified form, is employed for elevating acids and heavy chemical solutions or for pumping marl, paints and other semi-fluids. In this case the air valve is so located that it is not affected by contact with the liquid or by corrosive fumes arising therefrom. The capacity of the displacement pump is determined by the size of the cylinders or chambers and, the volume of air available. Employing air at ordinary pressure from a common single stage air compressor with a lift not exceeding 250 feet, the efficiency of the displacement pump is higher than that of the usual reciprocating plunger pump under the same conditions of lift and pressure. There are three distinct types of these pumps known as the Latta-Martin, Halsey and Harris or "return-air."

The Latta-Martin system employs two tanks, side by side, with a valve arrangement to convey compressed air to a point near the bottom of the eduction pipe. The air pipe is connected with a compressed-air reservoir on the surface, which is in or near the engine room in which free air is compressed. Before turning on the air the conditions in the well show water at the same level on the outside and inside of the eduction pipe. At the first operation we must have sufficient air pressure to lift to the surface the column of water which stands in the eduction pipe. This goes out en masse, after which the pump assumes a normal condition, the working air pressure being then lowered to stand at a point corresponding with the normal conditions in the well. This is determined by the volume of water which a well will yield in a certain time and the elevation to which the water is discharged. It was first supposed that in all air-lift cases the water was discharged because of the aeration of the water in the eduction pipe due to the intimate commingling of air and water. Bubbles of air rising in a water column not only have a tendency to carry particles of water with the air, but the column is made lighter, and, with a

submergence or weight of water on the outside of the eduction pipe there would naturally be a constant discharge of air and water. This is known as the Frizell system and where the lifts are moderate, that is, where the water in the well reaches a point near the surface, it is very likely that the discharge could be effected by simple aeration.

Most air-lift propositions are deep-well cases, that is, the water is lifted a distance greater than 25 feet. Aeration will not suffice to expel such water, so the idea of piston-like layers of expanding air alternating with blocks of water is developed. The upward urge of the air to reach the lower degree of pressure in the atmosphere and thus restore its equilibrium as free air carries or impels the blocks of water ahead of it to the discharge gate where separation takes place, the air escaping into the atmosphere and the water flowing away in the conduit provided. The economy of the air-lift system is in direct proportion to the capacity of the well to form these piston-like layers and the reason why they are formed is after the first discharge there is kept up a constant struggle between the air under pressure and the head of water on the outside of the pipe, each one seeking to enter the lower end of the eduction pipe.

When the air pressure is greater than that due to the head of water, a certain volume of compressed air is admitted into the eduction pipe. If a sufficient quantity of air has been admitted in proportion to the diameter of pipe, and if there is a sufficient pressure in this pipe to prevent the free discharge of the air, it is readily seen how this bubble of air spreads itself across the diameter of the pipe in a pistonlike condition. The reason why this piston is not elongated and continuous is that the free discharge of the air, aided by the velocity with which everything in the eduction pipe is moving, causes a fall in the air pressure just sufficient to allow the water head to press the water into the open end of the eduction pipe. In other words, as the air pressure is for the moment slightly lower, the water pressure which was nearly equal to the air pressure, becomes a little greater and the piston-like layer of water enters the pipe, shutting off the air. This. "chunk" of water rises in the eduction pipe with a velocity equal to that of the rising bubble of air and as it has plugged off the air nozzle, there is a momentary rest, during which the air has a chance to accumulate greater pressure, and just controlling the admission and discharge of water and air. In operation the pump is completely submerged and one cylinder fills by gravity while air is forcing out the water from the other. When this cylinder is empty, the air is automatically released, escaping into the atmosphere, and the water enters, the compressed air meanwhile being applied to empty the other cylinder. The discharge water flows continuously.

The Halsey system employs either a single tank or double tanks, as desired, and must be submerged to insure good operation. The tank fills by gravity, and as the water enters it causes a float to rise, which, when near the top of the tank, drops a valve and permits the influx of compressed air to discharge the water. The flow of water is intermittent with the single tank, but continuous with the double tank. Neither the Latta-Martin nor the Halsey sys