A selection of articles

Airlift Pumps 1

Airlift Pump

Part 1

      This article discusses the practical use of airlifts in aquaculture and garden ponds. A little beter understanding of how it works will make it easier to apply more efficiently. First we look at the history and theoretical framework and then the practical application.

   In 1786 Bavarian Carl Emanuel Löscher reported his observation that mine water rose with injected air and followed with further research into the phenomenon. He produced an article “Aerostatic Kunstgezeug" (1787) the following year describing a practical airlift pump for mining purposes.

Carl Löscher (1750-1813) experimented by blowing air from a 6mm tube into a submerged pipe 283mm long and a diameter of 120mm. He studied the effect of rising water in the pipe and developed the airlift pump for pumping water from wells.

Löscher was a multi-facetted man. After his studies at the Mining Academy in Freiburg, he worked as assistant to the geologist Abraham Werner relieving the famous doctor of his tedious administrative duties. Later he qualified as a model maker designing bridges and devices for coal mining. He took on running the pharmacy, “Zum Schwarzen Elephants" (The Black Elephant) of his late father-in-law. Of his inventions was a fire truck with quick discharge, a sponge-machine for pumping water from a mine, innovative bridge designs, the airlift pump and the “mammoth pump” widely used in oil wells to the present day.

He was a prolific writer and covered a wide range of topics from engineering and mining matters; crystallization transitions in fossils, the structure of crystals and their formation, and improvements to the cyanidation process for gold and silver extraction from ore. He wrote on “mathematic for the countryman” and even ventured into political issues. A characteristic of his technical descriptions was his use of seemingly endless, difficult to understand, complex sentences.[1],[2]

Today the pumping effect of airlifts is widely used in aquaculture to pump, circulate and aerate water in closed, recirculation systems and ponds. It finds many applications in areas as diverse as industrial waste water treatment, mining, dredging, underwater archaeology, salvage operations and collection of scientific specimens. A more exotic application may be perhaps the bringing of diamond-bearing gravels to the surface along the west coast of Africa as Figure 1 illustrates.

Closer to home we can seen how the water level seems to rise above an air stone[3]or aerated dome in a pond. That is actually an airlift without a riser pipe in operation.[4]Airlifts can be very efficient movers of water and depending on the plumbing, features added and design employed in the pond, less than 4W energy per 1 m3 pond water may be needed to circulate, aerate and filter a pond[5].

 figure1 mining 330

Figure 1. Mining with airlift

Why do airlifts work?

Various explanations are given for the effect like “the air bubbles act as pneumatic pistons, pushing or drawing (the) water” or the “viscous drag” of the bubbles as they rise and expand up a submerged tube. These explanations are redundant and from a simple conceptual viewpoint because they do not offer much theoretical basis. However, from the field of fluid statics it is easy to derive the basic formulation describing the phenomenon. (later we’ll get back to the ‘pneumatic pistons’ analogy.)

 figure2 800

 Figure 2. Demonstrating the principle of airlift.

 Figure 2 illustrates what happen when air is introduced at the bottom of a submerged tube. The water in the tube will rise by a “lift” or “head” that is the function of submerged depth, the diameter of the tube and the amount of air being introduced.

From fluid statics we can write:      hmgm = hsgl                          (1.1)

Where            hm = height of the liquid–air mixture in the tube

                      gm = specific gravity of the liquid–air mixture in the tube[6]

                        hs = height of submerged tube from where air is introduced

                    gl = specific gravity of the liquid outside the tube

(It helps a lot that specific gravity is a dimensionless quantity.)

Because the specific gravity of the water-air mixture is less than the specific gravity of the water, the water will rise in the tube until the Equation 1.1 is satisfied. That demonstrates the principle. It is only the difference in density that causes the lift just as oil will float on water.

In this mathematical statement 1.1 is also ‘hidden’ fact that the only energy you need to create a difference in height, however small, is to overcome the pressure to form those bubbles at the water pressure of the chosen depth. Once you have a difference in height, you have potential energy and gravity can be employed to do work.

 figure3 800

Figure 3 – Principles of an airlift

 Therefore, if we allow the water to “overflow” of “flow out” of the tube as is demonstrated in Figure 3, an upward current is created. Once you have flow you have momentum and however small, a little added momentum over time can soon bring a large body of water in motion. That is why it works and why we have a pump action.

 From standard treatment of Equation 1.1 the minimum air flow needed for pumping to begin was deduced[7]

 

formula1 2

From this we can deduce the minimum air pump requirements to start start pumping at a specific head in terms of flow and pressure. We can therefore establish the principle factors involved to get the water flowing.

 We can also see that if hS, depth of submergence, is equal to the maximum height of the mixture hm , a special condition is when MS is equal to 1 and there is no flow Qam possible. Precisely what we intuitively expect. Also that is the weight difference of the two fluids driving the process.

 Now that the pump is pumping the previous static arrangement is replaced by a dynamic model and all sorts of additional factors come to play on the fluid in motion. The predictability ceases. Flow rate affects the friction in the pipe which is affected by the diameter of the pipe, the material it is made of, the finish of the inside surface (work surfaces) of the pump, viscosity of the fluid, temperature and other physical properties of the fluid.

Finally, importantly, the bubble size, bubble number, rate of bubble formation, rate of bubble formation relative to flow, bubble dynamics and flow dynamics are all factors changing a near linear relationship between pump speed and air flow into a polynomial curve.

 rise velocity

Figure 4 – Rise velocity as a function of bubble size of riser.

It is easy to make an airlift to work, it is difficult to predict the optimum conditions across the different designs we devise. To attain the most energy efficient pump with the highest lift and largest pump flow is not simple. These factors work against each other and we have to define our needs and design accordingly.

To be continued.

Servaas de Kock

(For those who want to experiment, choose an air pump operating on the most efficient and safe part of the work curve to overcome the water pressure you are predicting. Ask if you need advice.

Additional information


[1]              Fischer, Walter, "Löscher, Carl Imanuel" in: New German Biography (1987) 15, S. 64 f [online version]; Retrieved 15/8/2015. URL: http://www.deutsche-biographie.de/ppn117156299.html

[2]              Wikipedia. Carl Emanuel Löscher. [online version]; Retrieved 15/8/2015. URL https://de.wikipedia.org/wiki/Carl_Emanuel_Löscher

[3]              Although in common use, the word “air diffuser” is preferred to “air stone”. It sounds technically more correct.

[4]               We use an air diffuser to inject air for aerobic activity into a pond, but we erroneously think that is all aeration is the result of the air bubble’s contact with the water. Depending on the design of the pond, placement of the diffuser and amount of air injected, a large portion of the aeration is derived from the current created by the airlift effect replacing the oxygen-rich surface water with the oxygen-poor water below.

[5]               Author’s experience. One must of cause, design with sufficient oxygen in mind for the aerobic processes demanded by the fish load, feeding regime and biological filters.

[6]               Specific gravity: the ratio of the density of a substance to the density (mass of the same unit volume) of a reference substance. The reference substance is nearly always water at 4°C (it’s densest) and one atmosphere pressure. The specific gravity of water is 1.00 at that state.

[7]               Todoroki, I., Y. Sato, and T. Honda, 1973, Performance of Air-lift Pumps, Bulletin of JSME, Vol. 16, pp. 733-740.

also

Castro, W.E., Zielinski, P.B., Sandifer, P.B., (1975), Performance characteristics of Airlift Pumps, World Mariculture Society Meeting, 6: 451-460

Nicklin, D.J., (1963), The airlift pump: theory and optimization, Transactions of the Institution of Chemical Engineers, 41: 29-39.

Whaton, F.W. (1993) Aquacultural Engineering. Krieger Pub Co. Florida. USA. Pp728.

Wurts W.A., McNeill S.G., and Overhults D.G., (1994), Performance and design characteristics of airlift pumps for field applications. World Aquaculture, 25(4): 51-54.

Johnson, B. S. 2008. Airlift assisted waste water treatment. M.Sc. Thesis.Dept of Civil and Environmental Engineering. Louisiana State Univ.