Small Recirc NW Skimmer Design and pump selection

[rishma]

I am going to build a gravity fed recirc skimmer for small tank I just set up. I have 4.5" acrylic tube left over from a previous project. It will have a 2" riser, 28" total height. I am considering a diffuser plate or an internal cylinder like the small H&S.

What pump should I use?

The aquabee is a little out of my price range right now. That leaves one of the small gen-x, ocean runner 1700 or 2700, or Sedra 3500.

The OR 2700 is pretty large, but only uses 38 watts (more efficient than sedra 3500). I have not found anyone who has used the OR 1700 and I am concerned it might be under powered for this height. I don't know anything about the smaller Gen-x.

Experiences and anecdotes appreciated! Data on air intake would be even better.

 

[RichConley]

In these cases, a sedra 3500 isnt going to draw much air. Thats too much head. The OR1700 is probably a good choice, although the 2700 could work great too, its just going to be tough to deal with turbulence.

 

[rishma]

Thanks for the reply Rich.

With 28 inches total height, I figure the reaction chamber will be ~22 inches. Is this really too much head for the Sedra 3500?

I too was worried about the 2700's turbulence. I don't think I can count on a diffuser to fix that problem in a 4" diamter chamber.

Has anyone used an OR 1700 for a recirc? I have not seen one posted.

 

[RandyStacyE]

Being such a small diameter skimmer and being so tall, couldn't you just mount the recirc pump a bit higher on the reaction chamber? Perhaps you can keep your height 28". You could diflect you pumps output down in an appropriate angle if you want.

If feesable, this would solve quite a few problems like 'lower head pressure' and the Sedra could be used. The Gen X 2400 could be used in this case as well, but from all I've read, I hear the OR is excellent.

I'm picturing a skimmer like shown below, but the pump mounted a tad higher.

 

[RichConley]

Randy, the problem is, that when you do that, most of the time you're just wasting reation chamber.

 

[quangtam7]

Recirculating skimmer works like (actually is) closed loop , water in skimmer is fed by overflow drain pipe or small pump and controlled by a valve to a certain level, all Nw pump has to do is recirculate water. There is no head or back pressure, this is why it works better than standard nw skimmer with same pump.

 

[RandyStacyE]

"Recirculating skimmer works like (actually is) closed loop , water in skimmer is fed by overflow drain pipe or small pump and controlled by a valve to a certain level, all Nw pump has to do is recirculate water. There is no head or back pressure, this is why they it works better than standard nw skimmer with same pump.Duhh why didn't I remember that?"

Duhh why didn't I remember that?

 

[David Grigor]

I retrofitted an old Top Fathom 200a ( if your at all familiar with them ) to be a recirculating. It's about 6"wide and no more than about 18-20" reaction chamber. Anyway, the OR2700 just couldn't produce enough bubbles ( I was reusing the venturi portion of the skimmer for the OR air intake. Not 100% sure how much that had to do with it. ) It does good on a Turboflotor but remember it's a pretty small skimmer footprint. I switched it out for a Sedra5000 and works tons better ( and it comes with the venturi air intake the OR2700 doesn't ). I have a feeling you would be in a similiar situation. I think because of the height a Sedra3500 may be too small and have a hard time pullin in air at that depth. If you have a turboflotor or know someone whoe does that you could borrow the pump, I would defintately do that before going out and buying one. I have my serious doubts.

I have a Sedra9000 on a ASM G4 recirc. and on a watt meter it is just under 50w. So I assume the 5000 would be something a little less ( although I never hooked it to the watt meter to see ). Wattage wise, difference from OR2700 and Sedra5000 is probably negligable and wouldn't add up to more than a few cents difference per month if that.

 

[RichConley]

David, I've seen nothing but good from the OR2700 I have. It was in fact, way too large of a pump for the TurboFlotor1000 I had. I have a feeling that the Venturi on your top fathom just didnt match up well with the pump. I've got a Sedra5000 sitting around, and IMO, the OR2700 is the better pump.

"Recirculating skimmer works like (actually is) closed loop , water in skimmer is fed by overflow drain pipe or small pump and controlled by a valve to a certain level, all Nw pump has to do is recirculate water. There is no head or back pressure, this is why it works better than standard nw skimmer with same pump."

The reason recirculating skimmers work better than non recircs has nothing to do with air flow. The above is NOT completely true. You're not dealing with a closed loop. The venturi ruins this. Because the pump has to pull air down (the standing level of water fills the venturi tube), you have to deal with suction head. Thats why pinwheel driven skimmers are generally short and wide as opposed to skinny and tall, even when recirculating.

 

[David Grigor]

The water pressure in the tubing definately has an inpact on the pumps ability to draw in air. That is what I was referring too. Since the OR doesn't come with a venturi fitting that's what I used. For my situation I wasn't impressed and think the Sedra5000 is more of a non-brainer to implement rather than "Matching up" a venturi for the OR2700 retrofit. That's just my experience and passing it along.

 

[rishma]

excellent discussion! Thank you.

I currently own 3 euroreef sedra 7000 (a little wierd I know, long story) and a euroreef sedra 5000 w/ a CS8-3. Years ago I had a skimmer with 2 NW ehiem 1060's which I sold (and regret)

Overall, I am not totally impressed with the sedra's performace. The ehiems pulled a tremendous amount of air by comparison. The 5000 is a little underpowered for my CS8-3. The sedra 7000's good, but this seems like an enourmous pumps for the 4" skimmer I am considering, even with the height of 28". I am very concerned about having too much turbulence at the body. That is why I am considering selling the Sedra 7000's and trying a sedra 3500 or OR 1700/2700. Once I drill holes, I dont have much choice on pump type.

David Grigor - the OR2700 reviews have been very good. You are likely correct that the venturi was not appropriate forthe pump. THe one that comes with the sedra's leaves a lot of room for improvement. H20ENG has a good adjustable venturi that can be tuned for better performance than most stock venturi's.


RandyStacyE- the height does make a big difference on air flow. I think this is properly explained by Rich below. I can definitely tell you that less air is pulled on taller skimmers with the same pump.


So - back to my original question. It seems the consensous is: 3500 is too small for this height. I assume the same is true for the OR1700 because is has less power and a lower max head than the sedra 3500. I am not sure how the gen-x 2400 compares.

THe OR2700 seams a kind big for this diameter. I think my Sedra 7000 would be in the same boat.

In summary I still down know what to do, however and underpowered skimmer still skims, an overpowered turbulent NW skimmer does very little.

 

[quangtam7]

The reason recirculating skimmers work better than non recircs has nothing to do with air flow. [/B]

For same heigh skimmers, one main reason why recir skimmer is better than standard skimmer using same pump is no back pressure on pump, which means lot stronger water flow which also means stronger air flow, heigh of skimmer of course does impact air flow a bit but not as much as back pressure on pump. You can test it by using airflow meter.

 

[rishma]

quangtam7 - agreed!

 

[rishma]

Other options -

I could use a smaller pump and just supercharge (a la Spazz) it with a little air flow to compensate for the skimmer being 28 inches, or I could just cut the skimmer down to 22 inches like the RC80.

 

[quangtam7]

I would go with shorter chamber, more air does not mean skimmer will work better, it could be worse. If you don't want to cut it down, you can place the pump higher on skimmer body, this will eliminate lot of bubbles on output, best option imo.

 

[RichConley]

Cutting the skimmer down defeats the purpose. Increased height means increased contact time.

And
"For same heigh skimmers, one main reason why recir skimmer is better than standard skimmer using same pump is no back pressure on pump, which means lot stronger water flow which also means stronger air flow, "

Is completely incorrect. A recirc pump is NOT a closed loop. It DOES have to deal with head height in exactly the same way as a feed pump. The second you start opening that venturi up, its now trying to move water uphill.

The reason some recircs pull more air is because it doesnt matter how much water you move. The more water, the more air, so they can use less restrictive venturis.

 

[quangtam7]

<a href=showthread.php?s=&postid=7862743#post7862743 target=_blank>Originally posted</a> by RichConley
Cutting the skimmer down defeats the purpose. Increased height means increased contact time.

Height has to be balance with the pump, increase contact time does not always make the skimmer better. Got it?

Is completely incorrect. A recirc pump is NOT a closed loop. It DOES have to deal with head height in exactly the same way as a feed pump. The second you start opening that venturi up, its now trying to move water uphill. [/B]

Very bold statement, the second you start opening that venturi up, it sucks the air in, water level is controlled by feed pump and valve, recir pump DOES NOT move water uphill, if you fill up water to a certain level and close the valve then start the pump, would it empty the chamber by moving water uphill ? Here you really don't know what you are talking about.

The reason some recircs pull more air is because it doesnt matter how much water you move. The more water, the more air, so they can use less restrictive venturis. [/B]

Not some, all recirculating pull more air than comparable stardard skimmers.
Bottom line, recirculating skimmers are much more expensive than regular nw skimmers for a reason... no need to argue.

 

[Masterofnonsense]

Very bold statement, the second you start opening that venturi up, it sucks the air in, water level is controlled by feed pump and valve, recir pump DOES NOT move water uphill, if you fill up water to a certain level and close the valve then start the pump, would it empty the chamber by moving water uphill ? Here you really don't know what you are talking about.

I don't think you fully understand how a recirc works. If you turn off the recirculating pump the air intake line will fill up with water equal to the height of water in the skimmer. When you turn on the recirculating pump it has to generate enough suction in its venturi to pull all that water down out of the air intake tube. The taller the skimmer, the higher the level of water in the air intake tube, and the more suction you need to overcome that.

This is what Rich and the other are talking about when they speak of head pressure. Your pump has to be able to generate enough suction through its venturi to pull down all the water in its air intake line and start to pull in air. Some pumps just can't do this on tall skimmers.

 

[quangtam7]

I fully understand how it works, I am talking about how the back pressure on the pump affects the air volume.

 

[Masterofnonsense]

I fully understand how it works, I am talking about how the back pressure on the pump affects the air volume.

So then you understand that the taller a skimmer is, the lower the volume of air the pump can pull.