Skimming 101

[Tiny Giants]

Yes this thread was very instrumental in my beginning to understand skimmers. I used the formulas from the Escobal book as the basis for my spreadsheet. I added a few logic routines of my own (inspired by crazzyreefer) for calculating bubble dwell time and air volume needed to maintain air saturation.

 

[ldrhawke]

Interesting thread.

One of the problems when running numbers is that frequently we make the assumption that the feed flow rate into the skimmer is the same as the rated flow of the feed pump. I often read that I am feeding xxx gph into my skimmer, which is based soley on the fact that I purchased a xxxgph feed pump.

The problem is the centrifugal pump laws are disregarded when running calculations and flow through the skimmer. Little aquarium pumps are rated at zero head or no back pressure. The moment they are piped into a system they see back pressure from static and friction loss head, they pump only a fraction of the zero head flow rate you are assuming. Odds are your little 300 or 500 gph pump may only nbe pumping 50 to 100 gph in your system if you could measure actual output.

This makes a significant difference in all of your calculations for skimmer sizing.

The same holds true for skimmers using recirculating pumps. Their true recirculation flow rates are not only reduced by the same static and friction loss head they see, but are even more dramatically reduced from the air entrained in the water and the pin wheel impellor. The little Sedra 9000 series pump with a conventional impellor is roughly rated at 900 gph at zero feet of head. Add the entrained air and the pinwheel impellor and I would guess the same 900 gph rating drops to less than 100 to 200 gph while recirculating.

 

[mojoreef]

Another huge one Idrhawlke is how is the water entering the skimmer?? If the water enters the top of the skimmer and flows out the bottom and the air comes in from the bottom and out the top we are talking about true dweel time. If the water is injected into the skimmer at the same place as the air then you cant use that as true dweel? as the water mole and the air mole are more likly to stay together, so then you loose the bombardment rates? dwell air and dwell water. Lots of things to take into account.


Tiny give me a day or tw to check things out, looks good though



MIke

 

[meitzler]

Has anyone ever done a controlled test of various skimmer types objectively. That is not a vendor who has a conflict of interest. If someone has the time, such a test would be very interesting and more meaningful. Comparing performance of various skimmers operating in different conditions simply has too many pitfalls.

 

[Scooterman]

meitzler
http://www.reeffrontiers.com/forums/showthread.php?t=5624

http://www.reeffrontiers.com/forums/forumdisplay.php?f=88

 

[mojoreef]

meitzler I think that would impossible. To try and recreate the exact tank conditions would be to hard to do. That was really the idea behind this thread. The basic laws that govern skimming can be used to figure out exactly how effective a skimmer can be.



Mike

 

[Tiny Giants]

My skimmer is under construction. I should finish up this week. My water flow meter for the feed line came in today. I still need to find one for the recirc line.

My air bubbles seem a bit large. I mistakenly ordered the medium pore airstones. However the difference according to aquatic eco is 2mm vs 3mm bubbles. Due to the larger size of my bubbles, I am getting a faster rise rate. I will have to do some expermintation to measure it. I will post my results.

I also have a air flow meter, so I can quantify how much air I inject.

Dale

 

[meitzler]

Think of a consumer report equivalent for skimmers. One could have a large tank feed "x" number of skimmers in small separate sumps, thus they would all have the same water source. This would be a good approximation of feeding all the skimmers with the same water and then determine which one extracts the most bad stuff. One would have to define what bad stuff is, but for a start one could use volume. The aquarium industry could benefit from true scientific tests of the products in the industry. Cool thing to do for someone who has money and time on their hands. Individual opinions will shed some light, but will always be flawed with all the variable among the different systems where they reside.

 

[Scooterman]

meitzler this reminds me of the great salt wars back in the day (yesterday or day before j/k) it will continually get mixed opinions even with all the studies in the world. I do think that here & other sites have recently taken a through look at skimmers & improved efficiency grandly, this will eventually lead to the market producing even higher efficiency skimmers ever made commercially at which time hopefully we all will have skimmers doing better than ever at a driven down cost to the average consumer. Yea this won't happen overnight but in time I'd beg to say IMO only it will eventually happen. Recently it seems the air stones used was getting mixed up at the supplier end, this opened their eyes to realizing what we need in making a custom skimmer, how long do you think people like Euro Reef will take before they start designing their own version, from there the fine tuning will just get better with competition, then eventually lower prices hopefully. Unfortunately now custom is the only way to get better performance than off the shelf but in the same breath, some if not most reefs still do just fine with a nice off the shelf skimmer, which is already been through the process & fine tuned & cheaper.


(These are just my opinions which is worth very little because of the lack of skimmer technology & experiences, so I may be just babbling & the names are changed to protect the innocent)

 

[ldrhawke]

Think of a consumer report equivalent for skimmers. One could have a large tank feed "x" number of skimmers in small separate sumps, thus they would all have the same water source. ................................

That is the only way you have any chance of making a comparison, but even then it is all too easy to skew the results.

The water in the tank should not be hooked up to a reef or fish tank. Too many potential variables in skimable waste load from test to test. I would suggest adding an exact amount of easily skimmed product to the tank water for each test and then compare skimmer performance.

We all know how easy it is to reduce or increase the skimmer performance. Stick your hand in the water and it stops skimming. Add a product like SeaChem's Reef Plus and the skimmer over flows with skimmate. Also, for the test to be of any value all variables need to be measured and equalized as much as possible.

The actual water throughput would need to be the same. Skimmer volumes would need to equalized by some formula. We all know a skimmer with a larger volume will pull out more skimmate, all things being equal. To compare a 25 gallon Bubble King to a 10 gallon ER is a waste of time other wise. I wouldn't be surprised if we found some unusual results, like....two small ER skimmer running in parallel or series, out performed a single larger skimmer of equal volume.

As an example: If we are striving for low nutrient conditions in a tank. You would want a skimmer to give the highest capture ratio of waste throughput. Simply skimming at 50% efficiency, and capturing the largest particles, and returning a large portion of the waste back into the system and could make it more difficult to capture on the next pass.

In a WWTP -waste water treatment plant, a frequent problem is capture of settled sludge. A lot of waste concentrating equipment,ie; centrifuges and belt presses, only scalp and get the largest particles capturing 80%; allowing the fines to be recycled back into the WWTP. This can quickly over load a plant production and shut a facility down. It is imperative to select and operate equipment so it capture 95% plus.

My point is that if you are striving for low nutrients a skimmer with a higher capture ratio on the first pass may have some strong pluses. This thinking leans toward a lot of air and low agitation, as frequently mentioned.

It isn't easy to make true comparisons with out having a good test proceedure.

All the recommended skimmer performance and design ratios should only be a starting point. I am sure new skimmer concepts will change our thinking, and what has been used to derive them for a tall air skimmer will not always apply.

Adding a measured volume of Reef Plus to the tank during the test and then measuring skimmate volume and concentration for each skimmer would need to be done. In this manner you could set up perfomance and test parameters for any skimmer and they would not need to be run side by side. As I stated before you also need the actual measured flow rates of feed and recirculation, and not simply what the pump rating is at zero head.

I get a kick out of Bubble King's claims of very low power consumption for their pumps. What they don't tell you is that they are adding such large volumes of air into the pump that is it is only pumping foam. The lower power consumption has little to do with the pump's efficiency. The same reduction in power consumption happens with any pump when you make it into more of an air fan than a liquid pump, by putting an efficient air venturi on the inlet and adding a large quantity of air it becomes part fan and part pump. That is not to say they don't make a good product.....but it is a little high in price / performance ratio for me. Sorry for rambling.....

Majoreef are you still using the airstone? I see where you gave another reefer a compressor and air stones. Didn't you state somewhere that you wanted to add a recirculation pump to your system? As I comment to you earlier I'm interested in your long term findings on going back to the original air skimmer concept.

 

[Forestal]

Hope y'all don't mind if i try to resurect this thread.... whew a lot of information...i think i am finally starting to understand some of this stuff..

i have 120 gallon tank and wanted a good skimmer, got a used MRC-1 skimmer...one problem was hooking up the tubing...so thought i needed a lot of flow for this big skimmer,
i am using a powerhead 901= ~900gph ...and with a skimmer volume of about 3.5-4 gallons (rough guess) my dwell time stinks....
now i get maybe 1/2 gallon of concentrated thick scum/month, i do feed heavy, and i have a lot of flow in the tank so my nutrients should be getting to the skimmer

but if i am understanding correctly, there isnt enough dwell time for the proteins to stick, so i have high turnover, but not effecient... i need a weaker pump... rather than getting a different skimmer

sound right? (btw i have 15-20%air in the skimmer)

 

[Anthony Calfo]

cheers, Dan

Your question reminds me of a common problem we run into as reefers using skimmers and/or giving advice about skimmers.

We cannot just say that slowing water flow through the skimmer is often good or better. While true stand alone as a statement, we must consider that some designs (many popular ones) harness the action of the feed pump to also create the bubbles/foam! On such models, reducing the water flow may reduce the bubbles for an overall net loss in performance. Or, increasing the water flow may help by increasing the amount of bubbles produced. Yet ultimately, we must examine if some such designed are merely flawed from the start... meaning: increasing water flow to the amount that produces a satisfactory or optimal amount and size of airbubbles may also mean the flow is to high for optimal dwell time as well! Indeed... there are more than a few skimmers that fit this bill. Hence the old recommendation you hear some of make when advising on how to diagnose a skimmer to simply add a fine pore glass or wooden airstone to see if skimmate production increases. on the aspirating/venturi/injection style models... the addition of an airstone allows you to increase air without increasing water flow (and hence reducing dwell time).

 

[Forestal]

thanks Anthony...

im in the process of hooking up a smaller pump as an experiment...i know the nutrients are in the tank, and after 8 hours on regular, i get tiny bit of clear foam making it up the neck.
the air flow comes from the neck (? venturi) and not dependent on the pump for flow...i plugged numbers into the formula given by MIke above and got a 6 minute dwelling time for the skimmer, so i am seeing what i can do to tweak this.
also i am still trying to get my head around how they work...one of those things i know is basically simple so once the light clicks on i will make one to try out...have your design from your book, but i want to know the how's and why's before making it
will let you know how the smaller pump works...

 

[Forestal]

experiment failed ...ph of 200gph not enough to generate bubbles...since i now realize the vacuum force sucking in the air on top of the skimmer is from stregnth of pump...this skimmer wii be tough but not impossible to tweak with an airstone...may try that...
have a gate valve on the pump to the skimmer, but didnt want to close it off too much for fear of burning out the pump... here's a pic...again excuse my ignorance:
how far up the 2nd riser should the water level be...i have had typically where you see it...much higher and i get tons of skimmate, but is dilute, my understanding was to get the thick dry stuff.

 

[mungus]

Dan

I have the exact same skimmer as you. (MRC-1) One thing that I have noticed is if you use a pressure rated pump you will get much better production with a beckett. I have a mag 12 running on mine and foam production is good. When I had my noisy gen x pcx-40 on it I got a much better water to air ratio and also a lot smaller bubbles. Production was a lot better than with the mag 12.

I am thinking of building a skimmer in the next month or so and I would like to stick with the beckett since I can use parts from my current skimmer. Since the flow rate is so high through the skimmer I am thinking if I made it a recirculating skimmer I would get better results. I am also leaning toward feeding the skimmer with the overflow so it would it would receive more raw water and a slower flow rate. Is this a good idea/bad idea?

TIA
Brian

 

[Forestal]

Hi Brian...i do have a decent pump, tried it witha small powerhead but didnt work well, less bubbles...

can anyone tell me how far up the 2nd riser the water level should be... i have a cup of fine foam from 24 hours, nothing liquid in the cup after 24 hours...

any specific recommendations on how to tweak this particular skimmer to optimum...my bubble dwell time is so small

 

[Forestal]

in case anyone with same skimmer is reading, the makers suggest the water level be kept 1-2" above the black box, which seems low, but will make for some pretty concentrated stuff by the time it gets up top.

 

[ToddLaCo]

I am not sure if anyone is still monitoring this thread, but if so I have a question.

First off I want to say this is by far hands down the most informative thread I have ever seen concerning skimmers.

I am planning to build a counter current skimmer for a system that holds approximately 400 gallons after subtracting sand, rock, etc..

First off I noticed that "Tiny Giant" asked in an earlier post if 10 was an optimum bombardment rate, and there was no answer. Is there a value given for the optimum bombardment rate?

I came up with the following skimmer for 400 gallons using Tiny Giants spreadsheet considering the value of 10 was correct for bombardment.

Skimmer Height - 72 inches
Skimmer Radius - 9 inches
Skimmer Feed Rate - 6 GPM (360 GPH) after head loss

This sounds large, but very doable. Please let me know if this looks correct.

One thing I didn't see mentioned in this thread, how do you determine the length and diameter of the riser between the main body and the collection cup? I wouldn't think this could be just any random size. Are there certain dimensions for the collection cup also?

 

[LakeEd]

ToddLaCo,

1st... WELCOME TO REEF FRONTIERS!!!

Unfortunately, I don't have any answers for your questions listed above. However I'm responding to you so this post moves off the bottom of the "last posts" to the top (giving ya a "bump")... so those that do have the answers are more likely to notice!

Keep in touch with every one here, I know we will all be interested in seeing your progress on your new skimmer!

 

[LakeEd]

Okay... I'll give this one more bump here this evening... hoping it will catch the right people's attention.