what are the basic components for a large bubble mixing system?

A compressed air (or inert gas) source that includes compressors, storage vessels and air treatment components (dryers, filters, etc.) needed to deliver the quantity and quality required. In general, the air (or inert gas) needs to be clean (minimal particles & oil), dry (minimal condensate) and delivered at 40 to 90 PSIG to the pulsing valves (VertiMix mixing valve enclosures).
Mixing valve enclosures (MVEs) that contains pulsing valves, controllers and electronics to create short pulses of compressed air (or inert gas). The pulses are sent to vessels containing receiving air elements—bubble forming plates (BFPs) that form large bubbles/bubble masses.
The air elements are referred to as bubble forming plates (BFPs) and are located near or at the bottom of the vessel that is to be mixed. Each BFP creates a zone of mixing influence around it. The zones of influence have been determined via modeling and experience. The BFPs are specifically placed within the vessel to ensure that the total volume/area contained is mixed.
Leak-free piping delivers compressed gas to the MVEs from the gas source and delivers gas pulses from the MVEs to the BFP. The type of piping needs to be made of materials suitable for the pressures utilized and the liquids in which it is immersed. Pipe connections need to be mechanical. Solvent welded joints and PVC piping should be avoided.

what Size compressor does Vertimix require?

Since tanks for which a VertiMix mixer are used tend to be small (less than 10’ diameter), the compressor size can be smaller. But that depends upon how the VertiMix is operated. There are 3 operational parameters that the user can set to determine how much air is needed. First, the number of times per minute that a pulse is made can be selected (1 to 6) by the operator. Second, the length of time the pulsing valve stays open when a pulse occurs can be selected (0.2 to 0.8 seconds) by the operator. Third, the air pressure that is used can be changed (40 to 90 PSIG). All of these parameters determine how much air passes through the valve each time it pulses. At 50 PSIG and a pulse duration of 0.5 seconds, approximately 3 cubic feet of compressed air will pass through the valve. Those two settings are where the mixer should be set upon start-up and are effective for 90% of the applications. Adjusting the pulse rate up or down is normally the first parameter changed and results in the most air usage variance.

Most small pump station wet wells that have been cleaned before the mixing system is installed can be maintained in a clean state by pulsing only 2 or 3 times per minute per valve. So, a one valve VertiMix system using the factory settings (50 PSIG and 0.5 seconds), will use 6 cu ft of air when pulsing 2 times per min and 9 cu ft when pulsing 3 times per minute. That means a 3 to 4 HP compressor could be used (each HP produces about 4 cu ft) and still have a little capacity to spare.

Even a small “air tool” type of compressor could handle that small of a load if the mixing cycle was intermittent and did not last very long. A “air tool” compressor should not be used if continuous mixing is planned. Floating grease can be broken up in most small wet wells by a large bubble mixer in a 15 to 20-minute period which would mean an “air tool” compressor could possibly work pulsing 2 or 3 times per minute. The VertiMix can be programmed to stop in such instances after a max of 30 minutes to protect the small compressor from being overloaded.

For larger wet wells, the normal recommendation is for a one-valve system to use a 5 HP compressor and a two-valve system to use a 7.5 HP compressor.

Should any question of sizing remain, contact Bell Mixing Systems for assistance.

TECH BULLETIN

FAQ : Why do you use fewer Bubble Forming Plates than other bubble mixing manufacturers?

RESPONSE: THE QUESTION SHOULD BE - “WHY DO THESE OTHER MANUFACTURERS USE SO MANY PLATES, NOZZLES, OR AIR ELEMENTS?”

THE REASON WHY - Other manufacturers don’t provide TRUE large bubble mixing systems. What they sell are modified course air mixing systems. The bubbles their systems create are not large enough to provide thorough mixing.

The illustrations below give a layman's explanation for the physics of large bubble mixing. On the next page, Dr. William Arnold of the University of Akron provides a scientist's and engineer's explanation.

Agitation Potential of Large Bubbles for Mixing Applications

Energy is a key metric to evaluate the capability of bubbles to provide agitation in wastewater basins (essentially mixing). Assuming constant pressure at a depth below the surface of the water, the energy to create a bubble is:

Assuming the bubble (a) gently breaks the water surface when it rises to the surface, (b) has a negligible increase in volume during the rise, i.e., a few feet below the surface, and (c) has lost a negligible amount in viscous friction, then the energy used in creating that bubble has been transferred to kinetic energy during the rise. In large bubbles, most of this energy will be in the form of moving the water and contents rather than viscous heating. Note that the energy available to a specific bubble is proportional to cube of the bubble radius.

If smaller bubbles are used, where F is the radius ratio (Rlarge /Rsmall), requires that N=F3 bubbles are used to achieve the same gas volume. The surface area ratio (SAR) of smaller bubbles of equal total gas volume is

Hence, as the small bubble’s radius decreases, the surface area and the viscous friction energy dissipation increase proportional to the radius ratio. The viscous energy loss manifests itself in local heating of the water and does little for agitation and mixing. As a result, smaller bubbles have a smaller rise velocity.

The net result is that there is a clear advantage of using large bubbles for large scale agitation and mixing in wastewater applications.

Dr. William A. Arnold
Adjunct Professor
Department of Civil Engineering The University of Akron

BUBBLE FACTS:

Large diameter gas bubbles rise faster in liquids than do smaller diameter bubbles.
Large diameter bubbles will maintain their shape and cohesivity longer than
smaller diameter bubbles. This means that larger diameter bubbles will travel
further upward in liquid than will smaller diameter bubbles before breaking apart.
Larger diameter bubbles have more buoyancy than do smaller diameter bubbles.This can be translated as “lifting capacity.” See the chart below to see the magnitude of lifting energy difference between small and large diameter bubbles.
As gas bubbles travel upward in liquids, the natural forces of viscous friction act on the bubble to tear it apart into smaller bubbles and, ultimately, to dissolve it altogether. Once dissolved, it is absorbed into the liquid itself transferring the gas molecules to the liquid.
When dissolution occurs, the upward movement of the gas bubble stops and the mass of liquid and solids in the liquid stop moving upward as well. Thus, mixing of the total body of liquid stops. The solids are not evenly dispersed within the tank and often form a sludge blanket at the level of dissolution. Any remaining air begins to aerate the wastewater. That is not good for anoxic process tanks.
The use of large bubbles (> 24-inches in diameter) creates strong convection cells. Vertical velocities in the path of the bubble rise reach as much as 4 fps. The result is that the tank volume is thoroughly mixed, solids are not allowed to accumulate on the tank bottom so that the entire tank volume is available for BNR chemistry to occur.
All of the above facts are the result of the use of large diameter bubbles that are relatively compact when they were formed. The bubbles have been described as “beach ball shaped.” Fast acting pulsing valves that have very low maintenance are essential for this result to occur. Systems that develop elongated-shaped bubbles (hot dog shaped) are not effective in lifting water and solids. Continuous streams of air blown into water are equally ineffective.
Large bubble vertical mixing is a mixing technology that takes advantage of the natural forces of buoyancy and gravity. It has existed right before mankind’s eyes for eons. Vertical lifting of water and solids through large bubble mixing is the technology that will bridge existing water treatment technologies to new technologies that are being developed right now. No matter what the treatment process that is to be used, efficient mixing will always be required.
Large bubble vertical mixing costs less to buy, less to energize and less to maintain that anything else available in the market place. At the same time, it enables the use of more of the volume of process tanks than any other mixing technology. Systems that move material horizontally use fossil fuel energy to create the motive force and fights gravity in the process. Vertical mixing uses naturally occurring forces (buoyancy and gravity) to move material naturally using less fossil fuel. Natural application of science will always win such battles.
Those who have been schooled by aeration companies to design those systems try to equate the physics of aeration to that of mixing. Aeration and mixing are two completely different phenomena. It is a mistake to try use air dispersion aeration designs to achieve complete mixing as the late Dr. William Arnold expertly explained earlier in this bulletin.