how to run an oversized filter in a small tank

[alweek]

Hi,

I did this simple bypass system for my 2 foot tank.

I managed to get an Eheim Pro 2228 from a friend and ran it on my 2 foot tank. Now, the 2228 is rated for a 600 litre tank... way over the 60-70 litres in my tank (!!!)

to keep my fishies from getting blown away, I made a simple bypass system from 2 t-joints and a gate valve. This allows me to divert a portion of the output back into the input. If i feel like it, i can also shut the valve and run the filter at full "blast".

Dunno if the gate valve is easy to obtain. I got mine from c328, seemed to be the last piece there. The uncle said it was from italy. Using ball valves (eheim type) will work but they require more tuning to get the flow right

[woodduck]

Hi

just wondering will the gate value will be better after the T-joint towards the tank direction.
you will have more control for the tank outflow.

BTW did your fishes manage to keep "still"?

[alweek]

that would work too. but, i wouldn't go for that since having the valve on the left of the outlet tee would mean that there would always be some flow recirculating in the filter. i prefer the option of varying from zero return to 70-80% return (estimated).

and yes, my fishies are keeping still now... sort of ehehehe

[vinz]

Eh... actually you can simply put a valve on the filter output (never the inlet) and partially close that to control the flow.

[hwchoy]

But the 2228 already comes with that valve control (always close using the output), but not sure if doing this long term is detrimental to the filter.

[bclee]

According the Eheim website, "The flow of any EHEIM External Filter may be reduced without negative effect on the appliance."

Check this.
BC

[hwchoy]

so there you have it! no need for the contraption

[juggler]

But looks good anyway. Maybe you can change it to a tap system such that you can draw water from there to water your plants. And can also use it to pump fresh water from a pail to your tank.

[robin]

Hi, I got the same problem, but what i did was way more primitive then you.

I just turn both the i/p lever and o/p level 1/2 open or 1/4 open depending on how strong i need the water to be.

Works fine for 1 year + already, no leaks all is well.

Robin

[vinz]

Robin,

I'm not exactly sure why, but the recommendation is not use the input valve to control the flow. Read the link provided by BCLee.

I think, the idea is to never prevent water from flowing into the filter, but its ok to control the flow of water out of the filter.

[robin]

o okay thanks for the warning.
Anyway, will control the o/p instead

Robin

[alweek]

choking back on the inlet is potentially bad because it can lead to a phenomenon called cavitation. when there is not enough input, the impeller causes a vacuum to form. because of this vacuum, tiny bubbles will appear in the water and these can wear out the impellor quite fast.

the reason why i went with this method is that i was anticipating that i had to cut back on the flow by a lot. i didn't want to stress the motor by adding this extra pressure drop so i came up with this bypass instead. at least i know that the motor is running at full efficiency this way

[BFG]

cavitation occur at high rpm. It usually happen to ships n submarine. It doesn't appear at the blade but at the tip of the propeller. I doubt cavitation happen in an external filter.

[vinz]

One other problem is this... before the filter, anything can come thru the inlet hose. If its too big to pass thru the partially closed valve, you end up with a blockage. Whereas at the output, the water would have been filtered and unlikely anything large enough to block the valve would come through.

Yes, there is a "guard" at the filter inlet, but there's always a chance something gets thru and get snagged at the inlet valve, whereas at the outlet valve, there is virtually 0 chance of that happening.

[alweek]

BFG, actually cavitation does not only occur under those conditions you stated.

cavitation can be caused on purpose in your external filter. all cavitation is is the formation of air bubbles due to a reduction in pressure.

try this out, especially after washing the filter or something. run it normally, then suddenly block off the inlet with your thumb. you should be able to hear some bubbles being swirled around by the impeller. at least it happened for me. this is cavitation.

[BFG]

er, that's not cavitation.

I will tell you to the best of my knowledge regarding cavitation.

Look at the blade of your fan. It is slanted rite. Now look at the centre part of the entire fan blade where the 3 blade joined together. The whole piece that u can take off to clean. It is flat. Now a ship propeller blade is nearly as similar to as the fan blade but the centre part of the blade is not flat as the blade on your fan. It is curved like the half of a ball. This is the point where cavitation occur at high rpm. The tip of this curve is where cavitation occur. It looks like a tornado but it's actually made of very fine bubbles that appears at high temperature. Don't ask me the physics of how the bubbles appear but from what I know, those bubbles are way hotter than boiling water. The length of the cavitation( the ones that looks like a tornado ) will grow longer the faster the propeller blade spin. Once cavitation occur, there will be noise but not the kind you'll hear in the filter.

Compare your filter blade to your fan blade. The filter blade is not slanted. Also the filter blade has a piece of rod sticking thru the entire blade n rest on a housing. After flipping thru my filter manual, I realised that the filter blade is sucking water from the filter out to the pipe to your tank. Cavitation occur when the propeller blade is pushing water, not sucking in the water. Again by looking at the manual, there is not enough space for the tornado effect( the tiny bubbles ) to occur in the filter compartment itself. So I would say that cavitation does not occur in the filter.

My knowledge on cavitation is based on my experience as a captain on board a US Los Angeles class submarine. Sad to say, I never sank any Russian submarine as their Akula captain are very protective of their missile launching capable submarine that they are escorting. [] [] []

Hope this enlighten all!

[BFG]

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On 8/21/2003 12:00:36 AM
try this out, especially after washing the filter or something. run it normally, then suddenly block off the inlet with your thumb. you should be able to hear some bubbles being swirled around by the impeller. at least it happened for me. this is cavitation.
----------------

That occur because the blade is spinning but the water in the filter compartment is not moving out. You can get the same effect by using a spoon n a glass of water.

[alweek]

BFG, still have to disagree with you there. what you described IS cavitation but is not the only type of cavitation there is.

cav.i.ta.tion kav' i ta' shun n [1. the rapid formation and collapse of vapor pockets in a flowing liquid in regions of very low pressure. 2. such a pocket formed.] (Webster)

when the inlet to the filter is blocked, no flow occurs. the region near the centre of the filter impellor (the hub) experiences a sudden drop in pressure and this causes dissolved gases to form. this is ALSO cavitation. it doesn't have to be the fancy type that occurs on a multi-million dollar submarine. it can also happen on your humble aquarium filter as well.

http://cavity.ce.utexas.edu/kinnas/cavphotos.html

[BFG]

Alweek, thanks for showing me the site. Now back to the topic of cavitation.

Look carefully at the pics from the site that u have provided. It shows the use of PROPELLER n the creation of cavitation by the degree of the blade. The hub vortex is formed as a result. Look at the pics titled 'HUB VORTEX'. The edge that I have previously mentioned, is completely free n not blocked by anything. That is where the hub vortex appear as the blade spin.

Back to the external filter.

The external filter uses an IMPELLER, not a PROPELLER. Again the blade of the impeller is not slanted but straight. Also, the impeller has a rod sticking thru the whole piece of impeller n rest on a rubber part, which rest on a cap or sieve to prevent anything large from entering or reaching the impeller n damaging it. This cap or sieve is why I say cavitation won't occur.

1) If u look again at the pic titled 'HUB VORTEX', there is nothing blocking the area behind the propeller. As the propeller spin faster, the hub vortex appear. This is essential, in my opinion to the formation of cavitation. Look at the length of the hub vortex. It need a large space behind the propeller to form. If u block this area, there will be either turbulence or drag or maybe both. So no cavitation will appear if this area is blocked.

2) The space from the top of the basket tray to the cap/sieve is a FEW centimetre ONLY. There's not enough depth for the hub vortex to appear. The cap/sieve is also blocking the essential area where the hub vortex should appear. If you don't believe me, open up your external filter and find a way to measure from the top tray to this cap/sieve.

Function.
A propeller is used to push water whereby the impeller is sucking in water. If the impeller cause the same basic action as the propeller, why not call it propeller instead of impeller? This is where the difference in function started. The site that you have provided has clearly show how a propeller looks n what it does. The author also shows how each blade function when the angle of the blade is increased or decreased. Look at the picture and study the shape of the propeller.

[BFG]

Next, take out the impeller from the external filter and compare it to the propeller. Look carefully at the design of the blade of the impeller. The impeller blade will NOT look the same as the propeller blade. Why is this so? The impeller is sucking water from the compartment tray out to the tank. Remember the function of the propeller? It is use to PUSH water instead of drawing. The faster the blade spin, the faster the ship travel, the longer the hub vortex appear and the higher the noise level appear in the water. It works great in one direction but if u reverse the direction of the propeller, it won't work as well as the other direction. that's why in my humble opinion, ships travel faster in forward than in reverse because of the way the angled blade cut through n push the water.

If u don't believe me, please try the above mentioned example. Measuring the space between the top tray to the cap/sieve that holds the impeller. Look at the cap/sieve n tell me whether a hub vortex is able to fit in any of the hole. Chances are the small space itself will prevent the hub vortex from appearing. One more example that you can try out is by using the fan( if u have one). Switch it on and you will enjoy the breeze. Now try to spin the blade of the fan in the opposite direction that it normally spin. Chances are there will be very little wind created. This will show that the slanted blade on the fan will collect n push the air better in one direction than the other. This will show you that a propeller is used to push, rather than pull while the impeller is better in pulling/drawing. Most pump will have the same design or near to similar design to the impeller for them to work.

Hope this short explanation might clear your doubt. If you are still doubtful, just post again and I will try my best to explain to the fullest of my knowledge.

1st Rear Admiral BFG.
P.S- I'm not in a submarine right now. I have my own ship in space!