Water-Lifting Devices – Pumps - Practices of Irrigation and On-farm Water Management - page 459

and air is lighter than the water outside the pipe and it rises in the pipe (Fig. 12.5 ) .
The head which can be produced depends on the depth of submergence of the air
tube.

Discharge

Fig. 12.5 Schematic of air
lift pump

Pump

Water level

Air-lift pumps are relatively inefficient. Typical efficiencies range between 30
and 50%. Generally, air-lift pumping is most efficient when the static water level is
high, the casing diameter is relatively small, and the well depth is not excessive in
relation to the pressure capability of the compressor.
The volume of air needed to lift the water depends on the total pumping lift, the
submergence, the length of air line, and the casing length and diameter. A useful
rule of thumb for determining the proper compressor capacity for air-lift pumping
is to provide about 3/4 cfm (cubic feet per minute) of air for each 1 gpm of water at
the anticipated pumping rate.
Air-lift pumps have some advantages over the other pumps discussed above.
They do not have any moving parts, can be used in a corrosive environment, and
are easy to use in irregularly shaped wells where other deep-well pumps cannot fit.
Air-lift pumps are not available from suppliers, but they are very simple to build.
The main disadvantages of air-lift pumps are their low efficiencies and requirement.

12.6.2 Jet Pump

A jet pump is a combination of a volute centrifugal pump and a nozzle-venturi
arrangement. The driving force lifting the water in this type of pump is provided by
a high-pressure nozzle, which creates a low pressure region in a mixing chamber
(Fig. 12.6 ) . This low pressure causes water to flow into the pump. A diffuser fol-
lowing the mixing chamber slows down the water and converts velocity head into
pressure head. The jet nozzle is installed in the pipe conveying the water. For a shal-
low well the nozzle is frequently located outside the well next to the centrifugal
pump. However, for a deep well, the nozzle can be placed inside the well in the
intake line. This location increases the jet pump lift capability considerably beyond
that which is practical for the volute centrifugal pump. The role of the centrifugal