level. Notice that the foot piece is suspended within a discharge pipe which, in turn, is contained within the well casing. Notice that the discharge pipe is open at the bottom, directly beneath the foot piece. When compressed air is discharged through the foot piece, a column or mixture of air is formed above the foot piece in the discharge pipe. The solid column of water in the well casing, resting high above the foot piece and discharge pipe inlet, now has greater weight or static pressure. This effect forces the air-water mixture upward in the discharge pipe where it is vented to the atmosphere through an open discharge outlet. In effect, the flow of water has a U-shape down the well casing, around the foot piece, and up the discharge pipe. The air-water discharge then strikes a separator or deflector that relieves the water of air bubbles and entrained air vapor. The discharge then settles in a collector tank.

The air-lift pump can deliver considerable quantities of water in the manner just described. The discharge pressure at which it is delivered, however, is relatively low. For this reason air-lift pumps cannot be used to discharge directly into a water distribution system. They do not develop sufficient pressure to distribute water horizontally above the ground for any appreciable distance, and the discharge can only be collected at the well for ground storage.

The capacity of the air-lift pump depends largely on the percentage of submergence of the foot piece; that is, the greater the submergence of the foot piece below the water level in the discharge pipe, the greater the volume (column) of water the pump can deliver per unit of time. However, the deeper the foot piece is submerged, the greater the compressed air pressure must be to lift the column of water. In other words, a higher column of water (in the discharge pipe) above the foot piece exerts a greater weight or pressure at the foot piece. The greater the static water pressure at the foot piece, the greater the air pressure must be to infuse air with the water.

Starting air pressure is always greater than working air pressure. When the pump is started, the static (at rest) level of water is drawn down somewhat to a pumping or working level. In effect, the column of water above the foot piece is decreased or lowered, and this, in turn, decreases the air pressure required to infuse the water with air. In wells where the drawdown is rather large, the pump is sometimes equipped with an auxiliary air compressor, connected in series with the main compressor, for starting. Once the pump has been started and the pumping level reached, the auxiliary compressor is no longer required and is secured.

Air-lift pumps have a low discharge pressure and require more depth so the foot piece can submerge deep enough. Additionally, the entrained oxygen in air-lifted water tends to make it more corrosive. In spite of these drawbacks, air-lift pumps have several advantages especially their simplicity of construction and lack of maintenance problems. Particularly useful in emergencies for deep well pumping, air-lift pumps can be used to pump crooked wells and wells with sand and other impurities. They can also pump hot-water wells with ease.

In air-lift pump operation, compressed air has to be regulated correctly. The amount of compressed air should be the minimum needed to produce a continuous flow of water. Too little air results in water being discharged in spurts, or not at all. Too much air causes an increase in the volume of discharge but at lower discharge pressure. If air is increased still further, discharge volume begins to decrease.

The air-lift pump is so simple in design that nearly all operating and maintenance inspections and procedures relate to the air compressor, which is described later.

Jet Pumps

Pumps that use the rapid flow of a fluid to entrain another fluid and thereby move it from one place to another are jet pumps. A jet pump contains no moving parts.

Jet pumps are EJECTORS that use a jet of steam to entrain air, water, or other fluid, and EDUCTORS that use a flow of water to entrain and thereby pump water. The basic principles of operation of these two devices are identical. The basic principle of operation of a simple jet pump of the ejector type is shown in figure 6-27. Steam under pressure enters the chamber through pipe A, which is fitted with a nozzle, B. As the steam flows through the nozzle, the velocity of the steam is increased. The fluid in the chamber at point F, in front of the nozzle, is driven out of the pump through the discharge line by the force of the steam jet. The size of the discharge line increases gradually beyond the chamber to decrease the velocity of the discharge and thereby transform some of the velocity head into pressure head. As the steam jet forces some of the fluid from the chamber into the discharge line, pressure in the chamber is lowered and the pressure on the surface of the supply fluid forces fluid up through the inlet, D,