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Wow. All of the posting lately about return pumps and flow got me thinking (sometimes a good thing, but usually not 
)
As usual in this hobby there is room for lots of variability, but in general I have come up with the following information.
<b>Sump only (no refuge) with display draining to sump:</b>
Calfo, somewhere burried in <a href='http://www.reefcentral.com/forums/showthread.php?s=&threadid=554786' target=_blank>this thread</a> recommends low flow from the display overflow in order to not mix up the surface gunk. Apparently the surface gunk, when remixed doesn't get skimmed efficiently. This, along with other posts on wetwebmedia (and perhaps other places) seem to form the basis for recommending a low flow return pump. Flow through the sump doesn't matter too much if it isn't being used as a fuge.
<b>Sump with tank draining directly to skimmer:</b>
It would seem that mixing of DOCs wouldn't matter if you directly feed a skimmer from the drain. In this case, a low flow return pump wouldn't really matter from a skimming efficiency standpoint. I suppose the flow from the return pump at whatever head would need to be somewhat matched to the skimmer input flow.
<b>Sump that has a fuge section:</b>
In this case, the recommendation is to have a minimum of 10x and ideally more flow in the fuge. I think this is best documented <a href='http://reefcentral.com/forums/showthread.php?s=&threadid=462212' target=_blank>here</a>. The 10x - 20x (or more) flow in this case refers to <em>fuge</em> turnover and doesn't include the display volume. Based on my cursory review (this could be wrong) people like Anthony Calfo recommend skimming prior to running display water to a fuge section -- to allow the skimmer to get as much of the nasties as possible. I'm not entirely sure about that, though. I've seen "turf scrubbers" that process just the raw water. The reason for the large turnover in the fuge is to maintain a healthy growth of macroalgae.
In this case it would seem the above return pump considerations would still apply -- i.e. If you are directly feeding a skimmer, then you can have more return pump, and if you aren't, lower flow from the display is recommended.
In all of the above, I believe the intent is to optimize the export of nutrients. You can, of course, export nutrients with high flow from the display to the fuge -- but that may not be an optimal setup. Optimum is not necessarily the goal, as a healthy tank is mainly the goal -- so lots of variation should still work fine.
Return pumps have to spend energy overcoming gravity, and so are less efficient at creating flow than they would be at 0' head (or close to 0') as would be the case for a closed loop or power heads. As I see it, the other possible benefits of lower flow return pumps are:
1) less energy useage (lower $ per month).
2) less heat introduced to the tank.
3) less issue with microbubbles going back to the display from skimmer or drain.
4) less noise.
In the case of a low flow return pump, display flow could be achieved by a closed loop or powerheads. In the sump, if sump only you may not need any additional flow. If a fuge is present, you could achieve that by power heads, a closed loop, or branching off some of the return flow (and using a larger return pump) so that the flow from return to display is still low.
All in all, it seems that in an optimized system the following factors are part of the equation:
1) the method of skimming
2) the capcity and size of the skimmer
3) the size of the sump
4) the presence or absence of a fuge
5) desire to reduce heat in the system
6) noise
This hobby is complex and challenging (part of why I like it). I thought I would throw this out there to hopefully get a discussion started and to get some input from some of you more seasoned reefers. Things are rarely black and white, and this is a case in point.
If you've read this far, thanks for sticking with me
Jack
)As usual in this hobby there is room for lots of variability, but in general I have come up with the following information.
<b>Sump only (no refuge) with display draining to sump:</b>
Calfo, somewhere burried in <a href='http://www.reefcentral.com/forums/showthread.php?s=&threadid=554786' target=_blank>this thread</a> recommends low flow from the display overflow in order to not mix up the surface gunk. Apparently the surface gunk, when remixed doesn't get skimmed efficiently. This, along with other posts on wetwebmedia (and perhaps other places) seem to form the basis for recommending a low flow return pump. Flow through the sump doesn't matter too much if it isn't being used as a fuge.
<b>Sump with tank draining directly to skimmer:</b>
It would seem that mixing of DOCs wouldn't matter if you directly feed a skimmer from the drain. In this case, a low flow return pump wouldn't really matter from a skimming efficiency standpoint. I suppose the flow from the return pump at whatever head would need to be somewhat matched to the skimmer input flow.
<b>Sump that has a fuge section:</b>
In this case, the recommendation is to have a minimum of 10x and ideally more flow in the fuge. I think this is best documented <a href='http://reefcentral.com/forums/showthread.php?s=&threadid=462212' target=_blank>here</a>. The 10x - 20x (or more) flow in this case refers to <em>fuge</em> turnover and doesn't include the display volume. Based on my cursory review (this could be wrong) people like Anthony Calfo recommend skimming prior to running display water to a fuge section -- to allow the skimmer to get as much of the nasties as possible. I'm not entirely sure about that, though. I've seen "turf scrubbers" that process just the raw water. The reason for the large turnover in the fuge is to maintain a healthy growth of macroalgae.
In this case it would seem the above return pump considerations would still apply -- i.e. If you are directly feeding a skimmer, then you can have more return pump, and if you aren't, lower flow from the display is recommended.
In all of the above, I believe the intent is to optimize the export of nutrients. You can, of course, export nutrients with high flow from the display to the fuge -- but that may not be an optimal setup. Optimum is not necessarily the goal, as a healthy tank is mainly the goal -- so lots of variation should still work fine.
Return pumps have to spend energy overcoming gravity, and so are less efficient at creating flow than they would be at 0' head (or close to 0') as would be the case for a closed loop or power heads. As I see it, the other possible benefits of lower flow return pumps are:
1) less energy useage (lower $ per month).
2) less heat introduced to the tank.
3) less issue with microbubbles going back to the display from skimmer or drain.
4) less noise.
In the case of a low flow return pump, display flow could be achieved by a closed loop or powerheads. In the sump, if sump only you may not need any additional flow. If a fuge is present, you could achieve that by power heads, a closed loop, or branching off some of the return flow (and using a larger return pump) so that the flow from return to display is still low.
All in all, it seems that in an optimized system the following factors are part of the equation:
1) the method of skimming
2) the capcity and size of the skimmer
3) the size of the sump
4) the presence or absence of a fuge
5) desire to reduce heat in the system
6) noise
This hobby is complex and challenging (part of why I like it). I thought I would throw this out there to hopefully get a discussion started and to get some input from some of you more seasoned reefers. Things are rarely black and white, and this is a case in point.
If you've read this far, thanks for sticking with me
Jack
