mojoreef said:
2 cfm is alot of air, I figure my skimmer is pushing 2.25 to 2.5 cfm just figured I would say that. I think the pump model are built with psi to cfm ratios built in. From the way it was put to me, psi has to do with the stone but mostly to do with how deep your going into water. So if only a few feet 1 to 5 psi linear type pump, 10 feet to twenty then you would go wit a compressor style pump, deeper even the a blower. So what I am saying is get an air pump that has the cfm you want and don't worry so much about the psi as the pump is matched proportionately?? that make sense?? lol
On the pump if you go with the whitewater line then that would be the LT19 which pushes 2.5cfm
mike
Mike,
It's a little more than simply selecting a compressor based on ACFM (actual cubic feet per minute) which is measured at compressor inlet. Depending upon the compressors ability to develop pressure this will vary greatly.
If you select simply based on CFM and use a fine bubble air stone, which normally requires higher pressure for a fixed air flow, you may not deliver anywhere near the air flow required. Example: Your LT19 is rated 2.5 cfm @ 0 psig. If the airstone has a 2 psig pressure drop to make fine bubbles, it will only deliver 1.5 cfm or a 3 psig pressure drop only 1 cfm. Which also shows the need for selecting the right size and number airstones also.
Also, the compressor needs to take into account the static liquid head from the skimmer. Example: a 4.6 foot high skimmer needs 2 psig pressure just to overcome the liquid pressure.(2.31 feet of water =1 psig) Then you add on the pressure drop of the airstone, assume 2 psig. If you go to the LT19 curve you will see it can't deliver more than 1 ACFM of air.
No matter what size compressor you select it will only pass a certain amount of a per air stone. Airstones are often rated in fractions of a CFM at a certain pressure, the pressure often being 5 or 10 psig. Which the small compressors used for aquariums often can't even develop.
I agree a compressor should be slightly over sized that is able to overcome increasing back pressure from the airstone blinding with age, but it is foolish to buy one too big and restrict the flow. Since an airstones can only pass a set amount of air, oversizing a compressor too much is a waste of energy and will make the compressor run too hot and reduce its life, and not give any better airflow to the skimmer.
Like a centrifugal pump a compressor will always remain some place on it perfomance curve. Too large a capacity and it will just force it up its perfomance curve and waste energy. If the compressor delivers too much air it is better to put a small valve on a T in the line and bleed off air, rather than valve it down and restrict the air flow; which will reduce the load on the compressor and motor.
