71

Source: Schutte and Koerting.

Source: Schutte and Koerting.

Small units (less than 1 gpm, 0.23 m3/h) can be supplied for limited discharge pressures, as indicated by Figure 21. With air as the motive fluid, the suction liquid can be very close to its boiling point and only a very slight NPSH is required.

When air is used as a motive fluid, the smaller sizes operate more efficiently because the air is more intimately mixed with a suction fluid. In larger sizes, the tendency is for the fluid to be discharged in slugs because intimate mixing does not readily occur. This has a detrimental effect on the performance and especially on available discharge head.

Air-Lift Eductors Air-lift pumps are frequently used for difficult pumping operations. Compressed air is forced into the bottom of a pipe submerged in the liquid to be pumped. The expanding air, as it rises up the pipe, entrains the suction fluid.

If compressed air is not available, it is possible to lift water higher than 34 ft (10 m) with the use of an eductor-air-lift combination. Figure 22 illustrates the suction capacity

FIGURE 21 Air pumping liquid (Schutte and Koerting)
FIGURE 22 Suction capacity of eductor-air-lift combination (gpm X 0.227 = m3/h)

of a 2-in (51-mm) eductor drawing water from a 50-ft (15-m) depth and discharging it to the atmosphere. In operation, an air line from the atmosphere enters the suction pipe near the water level. As the eductor creates a vacuum in this line, atmospheric pressure forces air into the suction pipe. After it is in the line, the rising air carries the suction fluid to the surface and both fluids are discharged to the atmosphere through the eductor.

No sizing data are presented because this type of pump is best specified according to specific conditions.

Boiler Injectors The boiler injector is a jet pump utilizing steam as a motive fluid to entrain water, and it is used as a boiler feedwater heater and pump. It differs from a siphon in that the discharge pressure is higher than either motive or suction pressure. This is achieved by the double-tube design shown in Figure 23. In operation, the lower nozzle is activated by pulling the handle partway back. The lower jet creates a vacuum in the chamber, causing water to be induced into the unit. When water is spilling out the overflow, the handle is drawn back all the way. This closes the overflow and simultaneously admits motive steam to the upper jet. This second jet, which is of the straight or forcing type, picks up the discharge from the first jet and imparts a velocity to the water through the discharge tube. The energy contained is sufficient to open the check valve and discharge against the boiler pressure.

fEEO WATER

FIGURE 23 Boiler injector, starting position (Schutte and Koerting)

fEEO WATER

FIGURE 23 Boiler injector, starting position (Schutte and Koerting)

TABLE 12 Capacities of boiler injectors, gph"

Size no.

Size iron pipe conn., in (mm)

Size copper pipe OD, in (mm)

Size overflow (drip funnel)

pipe, in (mm)

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