We all have heard of air-lift pumps. These pumps are usually found near agricultural lands and irrigational fields where the availability of electricity is rare and intermittent. But have you ever wondered how air can lift water and pump it from deep wells? How intermittent availability of electricity does not interfere with the operation of these air-lift pumps? Let us discuss the operating principle, advantages and disadvantages of air-lift pumps.
Working Principle of Air-lift Pumps
An air-lift pump is a device which is used to lift water from a well or a sump with the use of compressed air. The compressed air is made to mix with the water. It is well known that the density of water is more than the density of air. So it is obvious and evident that air floats higher than water or to understand better, water has more weight than air. So the main principle used in air-lift pumps is the density difference between water and air. Air is made to mix with the water and thus allowed to form froth. Froth here consists of mixture of water and air. So the density of this mixture is less than that of the water. It is the mixture of air which makes the density less than water. Thus a very small column of pure water can balance a very long column of air-water mixture. This is the working principle of air-lift pumps.
The system has a compressed air pipe with a nozzle introduced into the sump or the well. The compressed air is introduced into one or more nozzles at the foot of the delivery pipe, which is fixed in the well from which water is to be lifted. In the delivery pipe, which is partly open to the well or sump which contains water, a mixture of air and water is formed. As already discussed, this density of air and water becomes less than the density of pure water. Hence a small column of pure water is sufficient to balance a very long column of air-water mixture. This air-water mixture is discharged through the delivery pipe. The flow will continue as long as the compressed air supply is maintained.
Let us assume
h = height of static water level above the tip of the nozzle,
H = height to which water is lifted above the lip of the nozzle.
Thus (H-h) is known as the useful lift. The results are optimum if the useful lift (H-h) is less than the height of static water (h) above the tip of the nozzle. Hence for best results,
(H-h) < h.
The ratio (h/H-h) generally varies from 4 to 1.
Also when h = 30m, then the ratio (h/H-h) is about 4.
When h = 90m, then the ratio (h/H-h) is about 1.
For h = 30m,
so, 30/H-30 = 4
30 = 4H – 120
4H = 150
H = 37.5 m.
For h = 90, h/H-h = 1.
90/H-90 = 1.
90 = H – 90.
H = 180m.
So it is evident that when h increases H also increases.
Advantages and Disadvantages
These pumps do not require any electrical power from the power mains. Wondering how? There are systems installed in villages to irrigate agricultural fields. The system has a windmill which drives an air compressor. The compressed air from the air compressor is led into the pipe which has a nozzle in the deep wells. This system can also be modified by windmill driving a generator and power from this generator is used for compressing air and then used in these pumps.
- No moving parts,
- Less maintenance,
- Simple & reliable,
- No mechanical parts below ground level,
- It can handle mud, sand, and gritty water too.
- This pump can raise more water through a bore hole of given diameter than any other pump types.
- Worst efficiency, 20 to 30% operating efficiency, when compared to expenditure of energy in compressing air.
- Running cost of an air-lift pump is high in terms of energy expenditure terms.
- Bore holes have to be drilled very deep in order to get enough static head. If not, the discharge will be less and probably no discharge. Boring is considerably a costly operation.
Applications: These pumps, in spite of their poor efficiency, are commonly used in many areas where conventional pumps usage is difficult. These pumps are used in
- Petroleum fields,
- Handling some hazardous liquids,
- Sewage plants,
- Deep sea mining,
- Recovery of archeological artifact and many more.