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Old 04-28-2005, 06:38 PM   #11
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The highest part of the filter tubing is 58 inches above the floor, the top of the canister is 16 inches above the floor.

You have to remember that I have added 3 or four feet of tubing, the tubing has never been cleaned in 6 months, and the filter hasn't been cleaned in nearly two months. The inside of the filter is never really clogged or dirty when I have cleaned it in 4 to 6 week intervals, so now I am stretching it out to 2 months or so. Just lazy, I guess. But if I ever clean the filter and the tubing, I will repeat the measurements to see how much plumbing grunge decreases flow.

I would love to see data on a fluval without any extra tubing added, just to see how much flow is lost to the extra tubing. Anyone?
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Old 04-28-2005, 06:43 PM   #12
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Oh yeah, and definitely post your data too. I am curious.
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Old 04-28-2005, 06:45 PM   #13
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Head height is measured from lowest part of filter tubing in tank/actual exit (I think). If too much trouble its cool, but set of data with that is great. Thanks. Will follow your model for Eheim test.
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Old 04-28-2005, 07:24 PM   #14
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Quote:
Head height is measured from lowest part of filter tubing in tank/actual exit (I think).
I thought it was the maximum height up it had to go up, but you are right. Makes sense, it only matters where the water comes out of? Like with a siphon, if the tubing goes up 6 feet and then down twelve, the head height is still minus 6. There is some initial work in priming the tubing to the maximun height, but once it is flowing it is only the differences in height of where the water goes in and where it comes out, plus the resistance of flow in the tubing. So, take about 4 inches off the 58 inches. The top of the outflow tube is about 4 inches below the tubing as it goes over the tank.
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Old 04-28-2005, 07:45 PM   #15
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I'm gonna jump in with this also. Since I have the traditional set up (canister in tub directly under tank...tubes go straight up from there). I am due for a PWC on Saturday. I will measure all components and fill a marked container (hope my stop watch is still working). I assume the flow rate should be set to max. I currently have:

Basket #1 - Ceramic rings
Basket #2 - Filter floss
Basket #3 - Bio-max

Great idea you two.
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Old 04-29-2005, 12:36 AM   #16
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Eheim litrature states they are rated with media but it does not mention head pressure. I think that is why their stated flows appear low. The two main factors in flow rate are head and friction loss, or the drag in the hose. A one inch square by 1 foot high column of water exerts about 1/2 pound pressure at the bottom so you need to overcome this weight. The friction is determined by the size of the hose plus bends. As the flow increases, the friction increased dramatically. I do know that if you double the pressure, you quadruple the flow I can't give you figures on the small amounts flowed by aquarium pumps.
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Old 04-29-2005, 02:33 AM   #17
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The usual setup of a cannister doesn't involve much of a head pressure .... The column of water in the intake pushes down & help the pump along, so to speak. The head pressure is therefore the height difference between the intake & output, regardless of where you put the canister. Since both are in the tank, the head pressure is at most a foot or so. I bet the less than rated flow is due to media & tubing resistence.

I'll give you a rough idea of my Magnum 350. My head height is much more than usual as I am using that as a return pump for my sump - about 2 1/2 feet. I usually gavel vac out 10 gal from the main tank, then fill from the sump, so the time it takes for the tank to return to full gives a pretty good indication of the flow.

Without any filter (empty basket), my return rate is around 200 gph, about 60% of rated flow. If I add the micron filter, flow rate is more like 150.

BTW, to get a rough estimate of flow & pressure, you can use the laminar flow equation:

Flow = [ (Pi x r^4) /( 8 x n x l) ]x Pressure

where r = radius of tubing
l = length of tubing
n = viscosity of water (constant)
Pressure = pressure of pump - head pressure

If you have a lot of bends, etc ... flow may become turbulant ... in which case you'd need to figure the reynold's number & use a whole bunch of other equations. < Too complicated dfor me!>
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Old 04-29-2005, 02:55 AM   #18
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I assume that canister filters are completely sealed. Based on this, there will be no head losses from elevation gain - and the only only head will be from friction in the tubes and friction of the filter media. The "elevation" head is measured from the surface of the water and not the elevation of the tube inlet or outlet.

In a sump, the pump has to fight the elevation gain because the filter's water surface is open to the air. But this is not the case with a canister filter where the pump receives help from siphoning action to exactly counter the effort needed to push the water up 3 feet.

I would venture a guess that ANY canister filter with clean tubes, and no filter media, would come very close to the specified flow rate. Friction from clean tubes should be very small compared to friction of the filter media.

I sincerely hope I don't sound like a know it all, but I recently had a class on fluid dynamics and am always interested in applying what I learn in school to something useful.

If we are trying to maximize the flow rates, a canister filter with a larger diameter will have less loss from friction and hence higher flow rates.

I have never owned a canister filter so I can only theorize. Am I way off base here?

Hope this helps.
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Old 04-29-2005, 04:54 AM   #19
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The "head height" is is the distance from the top of the water to the outlet. (*edit ok this doesn't sound how I originally intended, I didn't see Jsoong's post but he put it much more eloquently) Since my outlet is just at/under the surface the head height is about 12 inches at most. But I am not sure that this height needs the pump to work against.

Experiment..... get a syphon going with an ordinary hose.... keep the end in the tank at the level and raisethe outside end until the flow stops. This is the point where the pump has to work against gravity.

The height (or lack of) of your pump has nothing to do with it. It is simple physics. The tubes are rigid and the syphon and water pressure will counteract gravity up to the waterlevel of the tank (where your outflow tube is).

However, a full basket of media will affect flow rate. So will surface area of tubing... since the fluval tubing is not smooth it will have a greater impact on flowrate. These two will affect the filters performance much more than the height of the tank (stand... etc.).

However actual flow rate is not the critical acpect (it has been stated in another thread somewhere too), it is volume of water per unit surface are per second, for biological filtration that is utmost. A filtration rate if you will. You need to balance the flow rate with the appropriate amount of media for biological filtration, especially on highly stocked tanks.

Consider a filter with no extra surface area and a maximum flow rate. Now consider a filter so packed with material that the water does not flow. Ok, so in those models we have exteremes. Somewhere in between there is a maximum filtration rate. (If we did the intergral calculus we could find that point, but the equation would be to hard to derive. I'll leave that for the applied mathematicians.)
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Old 04-29-2005, 11:21 AM   #20
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Thanks for detailed explanation in both layman's terms and with forumla, guys. Thanks for the thought and time, Tom and Jchillin. Might just KIS,S and order the Fluval already.
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