View Full Version : Skimming Basics article

01/27/2004, 10:28 PM
Frank- I re-read your excellent skimmer basics article again, and have a couple of questions.

Water swirling patterns will also effectively increase reaction times.

Could you explain the physics behind this comment? Here is my thinking. Picture a tube with an inlet at the top, and an exit at the bottom. Water flows though the tube at the rate of 10 gallons per minute. Capacity of the tube is 1 gallon.

If the water flows straight though (laminar) then each molecule will take 6 seconds to exit the tube (1g / 10 gpm) = 1/10 minute or 6s.

Picture the same setup, but with the water swirling down though the tube.

If the water swirls, it must move faster to make way for the water behind it, as we are fixed at a rate of 10gpm. If any portion of the water takes "longer" to exit the tube, then a corresponding portion must exit quicker, to compensate. Each molecule will still take 6 seconds to exit the tube (1g / 10 gpm) = 1/10 minute or 6s. The only difference is that the spinning water has a higher velocity in relation to the tube wall.

Other issues with swirling:
- If the water spins around in a skimmer, then either the bubbles on top of the water spin too, or they are stationary in respect to the tube. If they spin, the shear between the bubbles and the tube wall will tend to pop bubbles. If they don't spin (in relation to the tube) then the shear between the spinning water and the static bubbles will tend to pop bubbles.
- The method of creating the spin is often to use a pair of nozzles, creating more back-pressure on the pump.

I see no increase in benefit here, only drawbacks.

Do you have other information that would support your claim?