Breakthrough Discovery of the Affect of Bubble Size on the Efficiency of Airlift Pumps
High Efficiency Diffuser Airlift Pump
By Alan Burris, Ph.D. and Matt Prinsen
Most airlift pump technical literature describes pumping air through a hose to an injection point in rigid pipes immersed in water. Alab, LLC conducted experiments to determine if aeration and pumping efficiency could be improved by use of Alab’s fine bubble diffuser to introduce the air. We also evaluated the use of a lower cost flexible pipe. Such improvements could be especially important for aquaculture.
Our concept was that fine bubbles will improve oxygen transfer to water being pumped, compared to coarse bubbles from a hose because of the large increase in the bubble surface area.
Airlift pumping depends on the air in the pump tube reducing the density of the tube contents relative to the surrounding water. Slower moving fine bubbles will reduce the density of pump tube contents compared to fast rising coarse bubbles. In addition, fine bubbles will produce greater lifting friction with the water. Additionally we were able to confine the flow in a draft tube of inexpensive flexible thin tubing.
First, we conducted experiments using a 3 inch square (providing less than 9 square inches of diffusing surface) Alab fine bubble diffuser which produces bubbles only approximately 1/3 mm in diameter. This means that up to 10 times as much oxygen is dissolved compared with competitive diffusers, which produce bubbles ten times larger. Alab diffusers produce these tiny bubbles with only a 2-PSI pressure drop saving pumping energy compared to conventional diffusers with a 30-40 PSI pressure drop. Also, less expensive Alab fine bubble diffusers are clog resistant.
This 3” square diffuser was suspended 2 ¾” below the 4.5-inch diameter 6-mil polyethylene tube so as to not impede the flow of water into the pump tube (see photo 1). Water depth was 63 inches. 5 lpm airflow produced a maximum water lift of 2.75 inches. Using a 1-inch lift the water flow was 33.1 lpm. When the diffuser was removed, the water flow dropped to 6.2 lpm with the same 5 lpm airflow. So the diffuser increased the water flow in this test by 534%. Photos of this setup are shown below in photos 1 & 2.
Next, we tested the airlift pump with higher airflows using a 12”X12” Alab fine bubble diffuser. A 17” diameter plastic funnel was used to direct the liquid entrained with bubbles into the 4.5” tube as shown in the photos below: Photo 4 shows the entire assembly while photo 3 is a close-up of the top and photo 5 is a close-up of the bottom and the diffuser.
The plot below (figure 1) compares the airlift pump results with no diffuser, a 3X3” diffuser and a 12X12” diffuser with the same 5 LPM airflow used in the first tests: The results indicate better performance with smaller bubbles with the same airflow.
Then we tested the airlift pump with a 12X12” Alab fine bubble diffuser at higher airflow rates and compared it with a coarse bubble diffuser and an air hose with no diffuser at the same airflows. The test data is shown below (Test Data 1) with a plot (Figure 2) following the data:
The increase in water flow produced by the fine bubble diffuser should indicate a substantial reduction in pumping energy required for a given water flow. In order to evaluate the different methods of introducing air into the water stream on an equal basis all pressures where calculated at the diffuser or in the case of no diffuser at the outlet. Using the resulting pressures from the different depths and the constant airflow the mechanical power in was calculated to watts. The calculations do not take into account friction losses that may exist in piping to the airlift pump or electrical to mechanical losses to pump the air. To further evaluate and compare the different methods of introducing air into the liquid the power calculations obtained were then compared. The power in was compared to the power out for each air outlet configuration by the lift shown as “Head” (Column C of Table 1). The results are expressed as Mechanical Efficiency Percent. The energy reduction calculations are shown below and the results follow in Table 1.
On the face of the results shown in the test data (Test Data 1) and the plot in Figure 2 it would appear that the fine bubble diffuser is significantly better than no diffuser in energy efficiency. What we realize when the data is analyzed is that the diffuser greatly improves available performance and at the higher performance levels is much more efficient energy wise. For example the Alab Fine Bubble Diffuser has a mechanical efficiency of 18.07% at 4” compared to 7.11% with no diffuser. In table 2 we show the potential energy savings exhibited by an Alab Fine Bubble Diffuser in place of an open port with out a diffuser.
In summary, our tests show that the use of fine bubbles at a given airflow provides greater performance in terms of both available lift and water flow than course bubbles and when performance counts significant energy savings will be realized. Fine bubbles will also provide for better aeration of the pumped liquid. For more information on Alab Fine Bubble Diffusers see www.alabdiffusers.com.