This is one of the oldest devices used to pump water for irrigating lands. It comprises a hose pipe that spirals along the length of a bigger cylindrical pipe which is kept inclined to the ground. The bottom end of the hose pipe is dipped in a water container and the cylinder is gently rotated in a clockwise direction. Water is steadily transported up through the hose pipe and comes out of the top end.
This mechanism finds use, among others, in transporting material such as grains and in generating power on a small scale.
Pumps are the devices used to lift liquids from a lower level to a higher level. Inlet, outlet and valves are the characteristic parts of the pumps.
However, Archimedes’ screw, which also does the pumping action doesn’t have any valve.
It is just a cylinder with a pipe or a strip wound around it in a helical manner. This cylinder has to be kept inclined to the ground such that the lower end is immersed inside the liquid completely.
As the cylinder is rotated, the liquid enters the lower end of the pipe and is forced to move up slowly as the rotation continues. With continued rotation, the liquid continues to enter the pipe, moves up and finally comes out at the top end. Then onwards for every rotation of the cylinder, some amount of liquid drops out of the top end of the pipe.
The mechanism being so simple, there are certain aspects that need to be paid attention. They are inclination, pitch and period.
Inclination is the angle between the axis of the cylinder and the ground. Pitch is the angle at which the strip or the pipe is wound around the cylinder. And the period is ratio of the number of turns of the strip or the pipe to the length of the cylinder.
One cannot choose any inclination for some value of pitch. There will be a range of inclination suitable for a particular pitch out the efficiency would be maximum for a particular angle of inclination.
Another important aspect is that the direction of rotation should be opposite to the direction in which the strip or the pipe is wound around the cylinder. There is no pumping action if both the directions are same.
Apart from lifting water from low lying water bodies to irrigation lands, the device finds application in many other fields.
They are used in sewage treatment plants as they cope well with the suspended solids and varying rates of flow. In fish hatcheries, where the physical handling of the fish has to be minimized, there screws are used in the percolators to lift fish safely.
They are also used in grain elevators and combine harvesters. Archimedes’ screws of decreasing pitch are used for the compaction of waste material.
Instead of lifting the water using the screw, if the water is poured into the top of the screw, then it will force the screw to rotate. As a result, the device can be used even to generate electricity.