# Air flow in relation to water depth!

I am building a home aquaculture system using air lift pumps. I have formulas for calculating water flow and for metering my cfm with orfices. My question is how does water depth effect air flow. I know that 1 psi = 27.68 inches of water. So my question is do I need to increase my psi proportional to inches of water, or will my flow be unaffected as long as my psi is greater than the submergence. Example 1/4" orfice @ 5 psi = 4.43 cfm 5 psi = 138.4" wc. If I submerge the air line 27.68" would I simply increase to 6psi to get 4.43 cfm? Or would my flow be unaffected as long as it was submerged less than 138.4"? Or do I need another equation? Any help would be greatly appreciated.

Thanks

Dave

## Answers and Replies

The short answer: YES you need to increase air pressure as you increase depth. If you have 10psi @ the surface, and your air line is 277 inches below the surface, no air will flow.

Now, the question about flow rate at a particular depth is a bit more complicated, especially as the depth increases. As pressure increases, the air volume decreases, but the mass of air per unit volume increases...

PV = nRT

for your purposes simply assume V is proportional to 1/P. This implies that if the pressure is 10 times higher than atmospheric pressure, a 1mm^3 air bubble coming out of your hose will be ~10mm^3 at the surface (assuming the water does not absorb any of the air).

To further complicate the mathematics of your aquaculture system, gas absorption is also a function of pressure, temperature, surface area of the aeration gas and a function of the water's existing gas content. The equations required to resolve all of these conditions simultaneously even on a first order basis (that is, ignoring fish, plants etc) are a bit daunting.

Real world aqua-farms typically simply monitor gas levels (primarily O2 & CO2) and adjust aeration accordingly. Since water absorbs CO2 much more readily than oxygen, many commercial interests use pure O2 for aeration, and will employ separate CO2 removal techniques. In the case of a simple aquarium, most hobbyist simply add an aerator and some plants and hope for the best.

If your aquarium is saltwater, you might consider a foam-fractionator or "Protein Skimmer". For freshwater, if you want to increase your aeration efficiency w/o creating a "bubble hazard" in your tank, you might consider adding a vertical PVC column floor to ceiling near your tank. The idea is to flow water and air into the bottom of the column, and let the water gravity feed out of the top of the column back into your tank. This gives the air considerably more contact time with your water and returns the water to your tank "highly gassed" but w/o bubbles. A slighly more complex, but considerably more efficient system employs two columns joined at the bottom. The water enters the first column at the top, and the air is introduced in the first column at the bottom. The water has to flow against the air flow until it reaches the bottom of the first column, then the water has time to allow small bubbles, unabsorbed bubbles to travel all the way up the second column prior to gravity feeding back into the tank. This is called a "counter-flow" system.

There is a wealth of information about these and other areation methods on aquarium based forums.

Fish

The short answer: YES you need to increase air pressure as you increase depth. If you have 10psi @ the surface, and your air line is 277 inches below the surface, no air will flow.

Now, the question about flow rate at a particular depth is a bit more complicated, especially as the depth increases. As pressure increases, the air volume decreases, but the mass of air per unit volume increases...

PV = nRT

for your purposes simply assume V is proportional to 1/P. This implies that if the pressure is 10 times higher than atmospheric pressure, a 1mm^3 air bubble coming out of your hose will be ~10mm^3 at the surface (assuming the water does not absorb any of the air).

To further complicate the mathematics of your aquaculture system, gas absorption is also a function of pressure, temperature, surface area of the aeration gas and a function of the water's existing gas content. The equations required to resolve all of these conditions simultaneously even on a first order basis (that is, ignoring fish, plants etc) are a bit daunting.

Real world aqua-farms typically simply monitor gas levels (primarily O2 & CO2) and adjust aeration accordingly. Since water absorbs CO2 much more readily than oxygen, many commercial interests use pure O2 for aeration, and will employ separate CO2 removal techniques. In the case of a simple aquarium, most hobbyist simply add an aerator and some plants and hope for the best.

If your aquarium is saltwater, you might consider a foam-fractionator or "Protein Skimmer". For freshwater, if you want to increase your aeration efficiency w/o creating a "bubble hazard" in your tank, you might consider adding a vertical PVC column floor to ceiling near your tank. The idea is to flow water and air into the bottom of the column, and let the water gravity feed out of the top of the column back into your tank. This gives the air considerably more contact time with your water and returns the water to your tank "highly gassed" but w/o bubbles. A slighly more complex, but considerably more efficient system employs two columns joined at the bottom. The water enters the first column at the top, and the air is introduced in the first column at the bottom. The water has to flow against the air flow until it reaches the bottom of the first column, then the water has time to allow small bubbles, unabsorbed bubbles to travel all the way up the second column prior to gravity feeding back into the tank. This is called a "counter-flow" system.

There is a wealth of information about these and other areation methods on aquarium based forums.

Fish