The present invention relates to air lift pumps, particularly air lift pumps for transporting wet particulate matter from a lower level to an upper level.
U.S. Pat. Nos. 5,173,194 and 4,126,546 disclose deep bed filter units having a central air lift pump for transporting dirty filter media from a bottom region of the filter to a top region of the filter. Both patents disclose a coaxial pipe arrangement wherein air is forced down a first pipe ('194 down the center pipe, '546 down the outer pipe) and is discharged into the second pipe. The rising air provides the motive force to lift dirty filter medium up the second pipe ('194 up the outer pipe, '546 up the inner pipe). These designs therefore utilize a single media transport pipe and a single point of air injection.
As the size of such units is increased, the diameter of the airlift media transport pipe must be increased. Large diameter media transport pipes require an excessive amount of air to provide the proper media flow. Additionally, such large pipes may be more expensive and more difficult to install than smaller diameter pipes.
A single point of air injection may limit the capacity of such filters. The single point must be located sufficiently close to the transport tube intake to draw the dirty media into the tube. As the flow of water, air, and dirty media travels up the transport tube, friction and turbulence reduce the rate of flow. This flow reduction effectively limits the length of the transport tube, thereby limiting the size of the unit.
Such air lift systems spray dirty media and water upwards out of the upper end of the transport tube in an uncontrolled manner. The falling media and water causes splashing when it impacts the top surface of the water contained in the wash chamber. Airborne dirty media and water particles containing impurities may be free to leave the wash chamber contaminating the filtrate. Additionally, the water transported upwards with the dirty media contributes to the volume of dirty water which must be rejected as waste water. Therefore, a significant portion of the influent water (infilt) must be disposed of as waste.
U.S. Pat. No. 5,277,829, which issued on Jan. 11, 1994 to the assignee of the present invention, discloses a deep bed filter unit in which the infilt flows upwardly through a sand bed such that the filtrate accumulates above the sand bed while the dirty sand is continuously regenerated. Dirty filtration media is collected in the lower region of the filter vessel. An air lift pump system is utilized to transport the dirty media from the lower region to wash means located in the upper region of the filter vessel where impurities are washed out of the filter media. The filter utilizes a single air lift pump having two separate air injection points to provide the motive force necessary to transport the dirty filter media. The first air injection point is located in a horizontal section of the transport pipe and establishes an initial flow of the dirty media. The second air injection point is located in a vertical section of the transport pipe and provides scouring of the dirty media and the necessary motive force to transport the dirty media to the wash means.
The use of separate air pumps in a single air pump system requires additional piping, fittings, and diffusers. The additional piping may be comprised of multiple pipe segments or intricately bent pipe to avoid interference with other filter components.
The above filters employ a single air lift pump having a single suction point for the dirty media. Such single suction air lift pumps generally do not uniformly remove dirty media from the filter lower region. It is common for dirty media to be more efficiently removed from some areas and less efficiently removed from other areas. In addition, much of the energy imparted by the second air injection point is required for lifting the dirty media to the wash means thereby limiting the amount of media scouring.