Skimmer Geometry Forumulas?

[Brian Prestwood]

I'm working on the design for a skimmer my local reef club will produce in the next round of skimmer DIYs.

I've found the forumula (Hahnmeister, etc) that correlates neck diameter to air flow.

I can't find a formula for the main chamber size, e.g. something that correlates main chamber volume to air flow. Does anyone know where I might find this forumula?

I'm trying to come up with a skimmer design that supports either one ~1200 LPH (i.e. ~300 US GPH or ~40 SCFH) pump with a 4.5" dia neck or two ~1200 LPH pumps (~2400 LPH) with an 8.0" dia neck.

The body would be tapped for two pumps. The collection cup and "cone" would connect to the body with a flange.

When one 1200 LPH pump is installed I'd use the 4.5" dia neck. When two pumps are installed I'd use the 8" dia neck.

Assuming I have good control on the water level, how does a having an 8" main chamber and an 8" neck (i.e. no neck) affect performance?

 

[hahnmeister]

50-70lph per in2 is the sweet spot for necks. A maximum of about 35 lph per in2 for the body, but most are more like 25-30 lph per in2. You can go as low as 35 with the neck, but you start having a rather high waterline in the neck that can make height adjustments that much easier to result in an overflow. Likewise, when you are over 100, like 120 for instance, you are also likely to suffer from 'foam cannon' overflows and find it very hard to produce dry skimmate. As for the body; really depends on the combined air and water flow of the mixing pump. If you are able to find the combined air and water flow of your pump, then you are talking about 85-90 lph per in2 as the maximum flow per body diameter.

If you have a pump with enough turbulence diffusion on the output of the pump, and a pump with a good enough air to water ratio, yes, you can hit that spot where you can overlap the neck and body diameters (about 35-40lph per in2 of air)... one size for neck and body, no transition.

 

[Brian Prestwood]

Hahnmeister

Thanks, thats what I was looking for. Off to the drawing board.

Good luck with your pump

 

[Brian Prestwood]

Hahnmeister

I built a spreadsheet to calculate neck diameter based on air flow and body diameter based on total flow. For 2X 800 lph air pumps (1600 lph total) with a 3:2 water:air ratio is comes out approx 8" dia body and 6" dia neck as you recommended in another thread.

Is the total flow based body diameter recommendation for recirculating or single pass (in sump) skimmers?

The diameter of the body is fixed therefore the volume is determined by the height. What dictates the height (volume) of the body?

The Eheim pumps Deltec mods inject 800 LPH of air each. Deltec recommends pumping in water at a 3:2 ratio to air. What ratio do you recommend?

 

[hahnmeister]

The 'ideal' would be 1:1, but 1:2 air to water is the best we tend to see, maybe 1:1.5.

The flow provided is for either. If you have a recirc with significant counter current flow, just add that rate to the mixing pump rate.

In the past, volumes were based on a flow rate of 1.5-2 the tank volume per hour through the skimmer, and the skimmer should try to keep the water inside for up to 2 minutes according to ideals suggested before that.

More recently, this 'ideal' has been found to result in very large skimmers which can be outperformed by smaller ones. For example, a 200g tank would circulate about 300-400gph through a recirc skimmer, and that skimmer would need to hold about 10 to 14 gallons to keep the water in the skimmer for up to 2 minutes. You can go by this, but I prefer airflow calculations these days... I like to say 4lph per gallon of air for low-medium stocking, and up to 8lph per gallon for heavy stocking.

You COULD combine all of those and arrive at a skimmer for heavy stocked 200g that does 1600lph of air. Thats about 8" diameter... so height wise, that would make for a skimmer that is about 4' tall. IMO, this is overkill, and makes using a pump with a good air-water ratio just about impossible. If you use a bubble plate, you can cut that height in half, or at least to 30", and still have a very effective skimmer. Granted, this assumes you have a high enough load to need 1600lph. Lets say your 200g was only medium stocked, so say 4lph per gallon... thats 800lph, so a 6" body, and then you are talking 8' tall to keep the water in there for 2 minutes... things get a little absurd, and the recent research suggestst that this 2 minute ideal is not needed.

IMO, by making the foam head thick enough, and making sure the turbulence on the way to it is minimal, you get that 2 minute 'dwell' not in the time that a bubble rises freely through the water, but as it moves its way up through the body into the neck, stacked into a tall, slowly moving column of bubbles.

 

[FMarini]

out of curiousity is your formula more generic or specificlly for an air stone driven skimmer?
I ask because you hit on a major point in your last post, some smaller skimmers outperforming larger ones. For example; air induction via a beckett head, essentially require a high pressure/performance pump which liquifies the air in extreme concentrations and then allows for outgasing and microbubble formation. I was wondering if you had any comment on how these very small skimmers can have such performance increases w/out aligning w/ your formula for ideal sizing
thanks

 

[Brian Prestwood]

Tangential Injection and a Bubble Diffuser Plate

Tangential Injection and a Bubble Diffuser Plate

Hahnmeister

I'm still digesting your comments about body height (volume).

All other things being equal, are skimmers with bubble diffuser plates more effective than skimmers with tangential injection?

I'm considering a 2' tall 8" dia body with a bubble diffuser plate in the middle.

Tangential injectors just below the plate would maximize the contact time by creating a vortex. The plate would inhibit the angular velocity (break up the vortex) and all the bubbles to rise the last foot with minimal turbulance.

Do bubble diffuser plates serve any other purpose than eliminate turbulence?

FYI - Here's a link to the spreadsheet I created to calculate neck and body dia...

Skimmer Geometry Spreadsheet Download Page