Canisters and head height

[czcz]

Does head height matter for canister GPH? If it does, is there some standard height for measuring GPH, since its likely canister is below tank and quoting 0 head height is misleading? Thanks.

 

[Bearfan]

Yes it does matter, as the head heights increase, the flow and effectiveness decrease.

 

[czcz]

Are rated GPH at 0 head? Can someone test some canister filter at measured head height with no media? (Say, time how long it takes for output to fill 1g jug.) Thanks.

 

[TomK2]

I bet most cannister outputs are measured exactly that way, 0 head and empty baskets. The manufacturer stated GPH is therefore only the best you can get, not what you will get. I don't know this for certain, but If I were going to market a product I would use the data that gives the best result. Yes, you could run the output into a known volume, measure how long it takes to fill it, and then calculate your actual GPH for your set up. But since many of us are in the 10 X plus turnover range using the manufacturer ratings, it hardly seems worth it. Although it wouldn't be that hard to do.

 

[czcz]

I hear you Tom, but disappointing. If I look at some sub pump, site or box probably tells me GPH at given head heights (Mag, Rio, etc). Canisters are almost always under tank, and would expect companies to at least use realistic standard for these numbers (say 2ft even so outlet is really close to bottom of tank). Powerhead rated at 0 head, on the other hand, is reasonable standard.

I was considering buying Eheim or Fluval btw, but now am even more encouraged to just build some canister as at least the pump I buy will be closer to honest -- was thinking cost difference between MagDrive and those filters was worth it. I'm thinking of doing these tests myself to see if maybe a company with great rep like Eheim does it right, in which case I would buy thier product, and simply to satisfy my curiosity. Didn't mean to ask anything unreasonable, but thought others who had these products would be curious as well. Thanks again.

 

[Jchillin]

This subject has come up a few times czcz. As Tom has pointed out. They would love to give "better data" but they leave it up to us since we constantly alter what media we put into those beautiful little baskets.

When I first set up the fluval, the intake tube actually hung down almost to the bottom of the tank. I wound up having to cut the tube and do away with the suction cup at the bottom. Of course, now that I've altered the device...the gph will be totally different than the manufacturer's suggested gph.

O head...how misleading...LOL

 

[TomK2]

you know, you got my wheels turning. When I do water changes, I could leave the cans running and do a measurment. I have the fluvals on the floor, but with a little extra tubing to put them in the next room instead of under the tank. The head is the same as under the tank, but I bet there would be some dropoff from extra tubing too. I could time how long it takes to fill a known volume. I know they pump out quite a bit, since the outflow stream makes it about 18 inches from the nozzle when the water level drops.

It sure would be a great article for TFH, stated GPH versus real world GPH? I don't think your curiosity is unreasonable at all. Being cycnical, I made the assumption that the stated GPH would be higher than real world, and bought more filter with that in mind. And I was never motivated enough to verify my assumption, just happy that I was overfiltering.

If I can find a jug with volume markings on it, I'll give it a go.

 

[czcz]

You rule. If numbers are close to rated Fluval might have new customer If you do this and no one else does, I'll buy Eheim and post trade of info if you're interested (would keep it if tests good, would tell Petsmart what I'm doing - if they declined after my agreeing to pay restocking fee, would just order from BigAl's).

Would be interesting article... Bueller? Bueller?

 

[TomK2]

Ok, heres the experiment. Fluval 404, 1 basket of coarse ceramic prefilter, one basket of generic floss, one basket of fluval biomedia, one basket of seachem biomedia. All baskets are full to capacity. The media has been running undisturbed for about 6 weeks or more. Fluval is on floor in next room, which requires about an extra 3 or 4 feet of 5/8 tubing. The addition of the extra tubing really makes the comparison to manufacturers claims invalid, but what the heck. I put a 2 liter jug under the output for ten seconds, then measure the output volume for ten seconds in a 2 liter calibrated jug. 10 second results are multiplied by 360 for liters per hour.


Manufacturer claims :
Pump = 1300 liters per hour
filter flow = 850 liters per hour (225 gals)
notice how the pump output of 340 gals is what is often what the aquarist is told, but the box and manual state filter flow is 225 gals.

My set-up: @ 1600cc every ten seconds. My measurements are crude (wristwatch and trying to be consistent, gotta figure there is some error here)

= 576 liters per hour.

This is about 68% of the manufacturers advertisement. So, My set up, with extra tubing, lots of basket stuff, and a filter that hasn't been cleaned for a while is two thirds of the manufacturer claims for filter flow. I bet the extra tubing makes a big impact.

This means that I am 153 gals per hour per fluval, or 300 gals per hour for my 50 something gal tank, which is a measured, actual 6 times hourly turnover.

With my consumners cynicism, I had hoped for half the manufacturers stated GPH. I am at two thirds of this, but was slightly hoodwinked because I never noticed that the pump capacity and filter flow capacity were different (1300 L/Hr vs 850 L/Hr.). This is my fault for not being diligent.

Now, what does it all mean? not much, since the two fluvals are doing a fantastic job on my tank at a reasonable price, which is what I wanted.

 

[czcz]

Kudos! Guesstimate on head height? (3-4 ft stated was mostly horizontal, yes?) Will compare to Mag's stated and guesstimate ratio for my height maybe. Fluval's rep is certainly good enough for me, but tank environment/experiment makes GPH matter. Measured numbers look great to me with your details (only ~30% loss with all that? Wow), but what do I know. I'll probably test Eheim this weekend. Want data?

 

[TomK2]

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?

 

[TomK2]

Oh yeah, and definitely post your data too. I am curious.

 

[czcz]

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.

 

[TomK2]

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.

 

[Jchillin]

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.

 

[BASSNMAN]

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.

 

[jsoong]

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!>

 

[hashbaz]

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.

 

[mattrox]

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.)

 

[czcz]

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.