Bubble Flow Path
We have measured the pressure within the housing of the Bubble Generators . It is difficult to measure pressure in a cavity space that fills with bubbles because the bubbles will block the small diameter tubing that is inserted into the cavity to detect pressure. We never really solved this problem but assumed that the air within the bubbles is at the same pressure as existed within the housing at the time that it is formed. This is also supported by a visible (but not obvious) expansion of the volume of the bubbles after the bubble blowing ceases. The bubbles at rest will continue to "expand" to fill the cavity for a few moments as the mass of bubbles reach an equilibrium with still air. The air in front of the moving mass of bubbles (the "Bubble Flow") could also be measured but there is no back pressure and what would be detected is a dynamic pressure drop. Especially if two Bubble Generators are working in tandum. In that case the air is not only forced out of the cavity by incoming bubbles but the air is being sucked out of the cavity by the BG that is using this air to make bubbles.
Look at this YouTube Video
Larger roof systems (the largest built was 12,000 square feet) have roof modules where the cavity spaces are long duct-like channels (see pictures of these roof channels in this Album:
Bubble Flow is much like air flow in a duct. In a straight run there is almost no back pressure. So the pressure at the making of the first bubbles and the last bubbles that flow into the channel is almost the same. The longest flow path in the field was 150 feet and there was only about 0.5 inches water gauge pressure difference. It is the wet screen itself that gives all the backpressure. This will depend on the size of the screen and the type (how much liquid it will hold) and how porous is the "open cell" foam or the open space in a preforated sheet. The screen must be uniformly wet so that it has a uniform pressure and the air flows through and mixes evenly with the air over the whole surface area. Dry spots will let more air through and become drier and then there will be a mixture of bubbles and free air coming from the screen. It will look like the bubble process is failing. Sometimes it is difficult to see the free air and the bubbles come out slowly and then seem to stop. Also, if the srceen becomes too wet it can block the flow of the air because the air mover (blower or fan) cannot overcome the backpressure of the very wet screen. Since air flow may begin to decrease when the backpressure reaches the "stall pressure" of the fan the the screen becomes wetter and the final result is that no more bubbles are made even though the BG continues to operate. The Bubble Flow will stall and it looks like the fan has no power to make the bubbles. What often will solve this problem is to use a nozzle that will deliver less flow so that the screen is drier. The bubbles will look more dry and light and will flow quickly from the screen.
So with most systems it is possible to adjust the various ratios untill the Bubble Generators unit is making good bubbles. I like to use propeller type fans that have shaped blades that can deliver the airflow at 1 or 2 inches water guage pressure before stalling. Some ordinary exhaust fans will stall at a back pressure of 0.25 inches - and that is too low. If the fan is rated for 1000 cfm at 1 inch then it should produce about 1000 cubic feet bubbles per minute (cfbm). Filling a channel that has a cross section of say 25 ft sq would produce a foam flow of 40 feet per minute and a 120 foot long cavity space would take 3 minutes to fill to the far end.