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Drilling an air bubbler manifold

Air agitation, while easy to implement, presents a few challenges to the tank designer. Foremost among these is providing the right amount of VERY clean, dry air to the sparger. Generally speaking, better agitation is achieved if the air injection results in a uniform "curtain" of large bubbles delivered at 2 CFM (cubic feet per minute) per square foot of solution surface area. Unfortunately, large bubble generation usually translates into a requirement for a high volume air source such as a regenerative blower or high capacity air compressor. In the case of a process tank for PCB prototyping, the surface area is usually less than 2 square feet, so a fairly small compressor can be employed if we use a simple trick to generate large bubbles while minimizing air consumption.

Start making the air sparger by drilling the two sections of PVC pipe as follows.
  1. Fixture one pipe length onto the table of a drill press with an accurate depth stop. You will need to be able to slide the pipe underneath the spindle so that a line of holes can be drilled through one side. It is very important that all of the holes fall along a line parallel to the central axis of the pipe section. This will insure that every hole is at the same height above the bottom of the tank when the sparger is installed, resulting in uniform bubble generation along the length of pipe.
  2. Set the depth stop on your drill press so that the tip of the 1/8" dia. carbide PCB drill bit penetrates half way through the side wall (hole A). This is referred to as drilling a "blind hole" into the pipe wall.
  3. Drill a line of blind holes on 1" centers along one side of each pipe section.
  4. Leave enough room so that an end cap can be mounted on one end and a 90o elbow on the other.
  5. Using the set of blind holes drilled above as pilots, drill holes through the wall of the pipe using a NEW #60 (0.040") PCB carbide drill bit (hole B). A new bit is required to make sure that no burrs form inside the pipe that could interfere with the air flow through the hole.
  6. Repeat for the second section of pipe.

When assembled into a sparger, the holes face inward towards the PCB and are rotated 45o below horizontal. Air flowing through the 0.040" dia. holes collects in the 1/8" dia. cavity until a 1/4" to 1/2" diameter bubble forms and breaks away. This process continues, forming a turbulent stream of relatively large and "active'" air bubbles. Without the downward facing cavity, the bubbles formed would be no more than 3/16" in diameter and unable to adequately disturb the process solution.

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