An airlift pump is a pump that has low suction and moderate discharge of liquid and entrained solids. The pump injects compressed air at the bottom of the discharge pipe which is immersed in the liquid. The compressed air mixes with the liquid causing the air-water mixture to be less dense than the rest of the liquid around it and therefore is displaced upwards through the discharge pipe by the surrounding liquid of higher density. Solids may be entrained in the flow and if small enough to fit through the pipe, will be discharged with the rest of the flow at a shallower depth or above the surface. Airlift pumps are widely used in aquaculture to pump, circulate and aerate water in closed, recirculating systems and ponds. Other applications include dredging, underwater archaeology, salvage operations and collection of scientific specimens.
Principle
The only energy required is provided by compressed air. This air is usually compressed by a compressor or a blower. The air is injected in the lower part of a pipe that transports a liquid. By buoyancy the air, which has a lower density than the liquid, rises quickly. By fluid pressure, the liquid is taken in the ascendant air flow and moves in the same direction as the air. The calculation of the volume flow of the liquid is possible thanks to the physics of two-phase flow.
Utilization
Airlift pumps are often used in deep dirty wells where sand would quickly abrade mechanical parts.. However airlift wells must be much deeper than the water table to allow for submergence. Air is generally pumped at least as deep under the water as the water is to be lifted.. It is also sometimes used in part of the process on a wastewater treatment plant if a small head is required.
In an aquarium an airlift pump is sometimes used to pump water to a filter. Another aquarium use pulls water up to the surface of an aquarium from under a lifted mesh floor up a tube using an aeration stone fed air from a cheap vibrator pump; the water/bubble stream is confined by a cheap plastic tube. This method uses the sand and gravel on this 'false bottom' of the tank as a support medium and growth matrix for naturally occurring bacteria, which breaks down waste. Not only does this circulate water from bottom to top, and aerate lifted water, such a filter system is downright CHEAP to build.
In a coffee percolator the airlift principle is used to circulate the coffee.
Inventor
The first airlift pump is considered to be invented by the German engineer in 1797.
Advantages and disadvantages
Advantages
The pump is very reliable. The very simple principle is a clear advantage. Only air with a higher pressure than the liquid is required.
The liquid is not in contact with any mechanical elements. Therefore, neither the pump can be abraded, nor the contents in the pipe.
Act as a water aerator and can in some configurations lift stagnant bottom water to the surface.
Since there are no restrictive pump parts, solids up to 70% of the pipe diameter can be reliably pumped.
Disadvantages
Cost: while in some specific case the operational cost can be interesting, most of the time, the quantity of air to compress is high compared to the liquid flow required.
Conventional airlift pumps have a flow rate that is very limited. The pump is either on or off. It is very difficult to get a wide range of proportional flow control by varying the volume of compressed air. This is a dramatic disadvantage in some parts of a small wastewater treatment plant, such as the aerator.
the suction is limited.
this pumping system is suitable only if the head is relatively low. If one wants to obtain a high head, one has to choose a conventional pumping system.
because of the principle, air dissolves in the liquid. In certain cases, this can be problematic, as, for example, in a waste water treatment plant, before an anaerobic basin.
Design improvements
A recent variant called the "geyser pump" can pump with greater suction and less air. It also pumps proportionally to the air flow, permitting use in processes that require varying controlled flows. It arranges to store up the air, and release it in large bubbles that seal to the lift pipe, raising slugs of fluid.
