Placement Of External Filters?

[fry_lover]

I read something today, that got me thinking, it's kind of obvious, but apparently if you place the filter directly underneath the tank this is the worst place for maximising the flow / power of the filter as it's fighting directly against gravity?

Is there any physicists amongst us apart from Dave?

Can someone shed some light on this?

 

[NoRp]

i would think that was right.

below the tank maybe the worst for flow but its also the most convenient as i dont know many tanks with hiding places above them ( or even to the side for that matter)

i try to place my filters as close to the top portion of my tank as posible

hope thta helps

 

[noop]

i'm not sure the pump has to work any harder depending on where the filter is positioned. remember that the water in the intake pipe is pushing down with about the same pressure as the water in the outlet pipe. all the pump has to do is transfer water from one to the other

but i'm no physicist

anyway, the filters are designed to be put there, and i would expect the stated through-flow to be available when the filter is positioned like that

 

[Lateral Line]

Noop is right, the inlet pipe is working as a siphon pulling water out of the tank and forcing it into the bottom of the canister. In most canisters, once you have that siphon running, the canister and outlet pipe will fill with water by themselves until the level in the return pipe reaches the same level as the water in the tank, all that without switching the pump on. The pump then has the job of getting the water to flow that last bit of height to get it back into the tank, and to apply some pressure/flow.

The thing that really can slow filters is if you have the hoses longer then they need to be. The water flowing through the pipes is subject to friction along the pipe walls which tries to slow the water down. Short pipes, less friction, better performance.

If you want a demo of this friction, blow through 1m of airline, then try blowing through a 100m roll!

 

[fry_lover]

thanks for feedback thus far,

A fish keeping buddy of mine thinks the lower the filter the more more head flow is required so you will lose some efficiency, and he always tries to place his externals level with the bottom of his tank or maybe slightly higher, but obviously below water surface level, what do people think about this view?

 

[rabbut]

Length of hoses as stated above is more important than head height. The longer the hoses, the more resisance you get and thus the less flow you get.
Gravity acctualy helps the filter return water to the tank. Due to gravity, water will try to find it's own level, i.e. of it can flow to a lower area then it's presant it will, so the water in the pipes without the pump running will fill the outlet pipe to the same level as the waterline in the tank All you motor does is push the water over the edge, battling against the weight only of the water above the level of the water in the tank. If you positioned the outlet say half a foot above the level of the water in the tank then mebe the flow would reduce noticably, otherwise it should stay the same.
Positioning the filter closer to the water line reduces the hose length, and this may explain why there is a increase in his filters flow rate. I honestly don't see it being due to head height

HTH
Rabbut

 

[nmonks]

All external filters will have a quoted "delivery head", which is the height to which the pump will push water upwards against gravity. For the Eheim 2217 for example, the delivery head is 2.3 metres. If you increase the distance between the waterline and the pump beyond that, the filter isn't strong enough to move the water and nothing will happen.

Since flow rate will be zero at 2.3 metres, it's obvious that you get the greatest flow rate when the distance between the pump and the waterline is as small as possible. If you make the distance zero, i.e., have the pump at the same level as the waterline, you'll get maximum turnover. That's the way filters are tested (as I understand it) simply because it's an easy benchmark to understand. So an Eheim 2217 with a turnover of 1000 litres per hour only performs at this level when the filter is placed next to, not below, the aquarium. (It's also a good marketing ploy, since you're offering a "best case scenario" rating.)

In the real world we usually place the filter in a cabinet under the aquarium. So the Eheim 2217 under my aquarium is positioned with the pump about 1 m below the waterline. Turnover is going to be well below the optimal 1000 litres per hour.

It's actually debatable whether this matters overly much. You lose a *lot* more turnover once you add media to the filter, particularly if that media is comparatively fine and gets clogged with silt. And yes, I believe it is true that manufacturers rate their filters *without* media installed, so again, the turnover quoted on the box is a bit hypothetical to say the least!

Cheers, Neale

 

[fry_lover]

Thanks Neale and other's, i am bit clearer on the whole issue now

 

[waterdrop]

Lateral Line and rabbut are right about water friction in overly long hoses being a bigger factor than people think.

Its not intuitive to us because we live our whole lives in air and incorrectly judge actions in other mediums by that.

A similar confusion happens when people think swimming is more about muscular arm pulling or leg kicking. Water resistance is a much larger force in swimming, so the skill of body positioning on the top of the water to minimize frontal resistance is actually a larger factor than strength, and the skill takes more years to develop than the muscles.

Thanks Neale for the interesting observations about how manufacturers rate their filters. Certainly seems the recommendations on TFF to choose a filter turnover rate a little higher than what the manufacturer says it is makes a lot of sense.

~~waterdrop~~

 

[fry_lover]

Well all this "i have this and that turnover in my tank", has always made me laugh anyway, i go by what i see and my water readings

 

[Lateral Line]

If you increase the distance between the waterline and the pump beyond that, the filter isn't strong enough to move the water and nothing will happen.

I'm not sure I'm reading that right, but if I am, then I disagree.

The delivery height is the height between the water level and the outlet, (spray bar or whatever). With the rider on resistance to flow by the walls of the pipes, which is considerable, but let's ignore it for now, a filter taking water from a tank and returning it to the same tank via a spray bar, say 20mm above the water level, is delivering at 20mm height, how far below the tank the pump is will not make any difference.

As I said above, if you start a siphon into the bottom of the canister, the water will flow, by gravity, until the water level in the return is equal to the water level - this without the pump doing anything, and regardless of whether the pump is 5mm, 5cm or 5m below the tank.

As the delivery height above the water level increases, the pump has more gravity to overcome and the flow rate reduces. At the "maximum delivery height", this flow reduces to zero. Try it with a filter. As with most things, I suspect the maximum delivery height quoted by manufacturers is a "best possible" figure and is unlikely to be acheived in any real setting.