Latest Thoughts on Skimmer Designby Chris Paris for Reef Aquarium Information Depot
Updated November 29, 1998
The water on natural coral reefs is extremely low in dissolved nutrients, and relatively rich in particulate food sources. I will not try to prove it here, but I believe that even great reef aquariums have the ratio between dissolved nutrients and particulate food sources backwards. We have water that is dirtier than natural reefs, with far less particulate food available.
People look at Sanjay's tank (at right) and others like it and say, "Hey, we don't need cleaner water." In fact, one of the current fads in the hobby is to run skimmerless tanks, because "Skimmers starve the reef inhabitants." People who remove their skimmers are noticing a new abundance of certain life forms, especially filter feeding types such as feather dusters. I would expect that change if you shut off your skimmer without changing your feeding rate; skimmers certainly remove particles that are potential food, so removing skimming without changing the particulate input will increase the particle count in the water.
If more particulate food for the filter feeders is what you want--and that is my goal too--then why don't you just feed more particulate food? You don't do that because, traditionally, feeding more reduces water quality (increases dissolved nutrient levels). I think that there is a natural chemical or biological force that drives aquariums (and natural waters) from states of high particulates to states of lower particulates and higher dissolved nutrients. Without a matching opposite force, our aquariums will tend towards having eutrophic water without much particulate food in it. We need a force to stand opposite to the natural trend toward eutrophication. That force is a strong skimmer.
Ron Shimek has presented information from a paper in the scientific literature that showed that a particular natural reef receives about 10 ounces of wet food per 100 gallons per day. The reef aquarists who feed the most of all reef aquarists feed about 1/10 of that amount.
I am curious about how our tanks would do with cleaner water than they have now, but my primary motivation is to be able to feed a lot more while maintaining the same water quality that we have today in our best captive reefs. I want to use a strong skimmer as a driving force that enables me to maintain the dissolved nutrient level no higher than it is in our best captive reefs today, while maintaining a more natural particulate food level.
What animals are unable to live in our tanks because of the factor-of-ten food shortage that Ron Shimek suggests that we have? As lush as Sanjay's reef is, would it be even lusher if he could feed ten times as much food while maintaining the same level of dissolved nutrients? Would we be able to keep any interesting carnivorous fish that are currently considered not reef safe because their supply of tasty food is so small that they are forced to pick at the relatively unpalatable (to certain fish) corals? I don't have answers for any of these questions, but these are some of the questions that motivate me to explore more powerful skimmers.
However, at about the time that the ETS skimmer came out and became an instant hit, a few clever hobbyists started questioning the common belief. Anthony Tse, an advanced reefkeeper, explained an alternate position in a USENET article back in 1995. The argument for the position is fairly intuitive, and I held the position myself for a while. The argument goes as follows. Each skimmer has associated with it a certain nutrient level at which it effectively shuts down, which is to say that it no longer can remove "gunk" from the water. Any skimmer will be able to remove "gunk" at the rate at which you add it to the aquarium (in the form of food), but a more powerful skimmer will maintain a lower "gunk" level than the weaker skimmer, for the same food input. This particular statement is true. The final point of the alternate position is that all this implies that you should choose a skimmer size independent of your tank size (really, nutrient input rate), and based only on how low a "gunk" level you want to maintain. This last point does not follow from the previous points, and it is false.
One fault in this argument is that skimmers are not binary devices that go from a state of operating well to the state of being shut down. The rate of "gunk" removal of a skimmer varies linearly with the "gunk" level; skimmers gradually slow down as the water gets cleaner. In the equilibrium state, a skimmer removes nutrients from the water at exactly the rate at which they enter the water. When a weak skimmer is in its equilibrium state, a powerful skimmer is still removing "gunk" faster than the aquarist is adding it, so the water is getting cleaner. When a powerful skimmer is in its equilibrium state, a weaker skimmer cannot keep up with the nutrient input rate, so the water gets dirtier, until it is dirty enough that the weaker skimmer exactly keeps up with the input rate. Chen's paper contains an equation that captures the relationship that I have tried to explain in this paragraph. Unless I've made a mistake, you can read these sentences right out of the equation
When we buy or design a skimmer, we make a choice (hopefully) about what levels of efficiency we want. A skimmer's rate of removal per unit of time is limited by the amount of energy we throw at it. A well-designed 50 horsepower industrial skimmer is going to remove stuff faster than the best skimmer for a home aquarium. That's fine; we don't believe that the skimming problem is worth the cost of 50 horsepower, so we accept lower efficiency in this sense of the term. In contrast to that sense of efficiency, we generally want our skimmer to make the best possible use of the energy that we give it, whereas the big industrial skimmer user may not care about the electrical efficiency of a measly 50-horsepower device. The main limit to the efficiency in terms of energy use is how much money and time we are willing to invest in the design and manufacture of the skimmer. We should use our available time and money to design and build the skimmer that performs as best as possible for for the amount of energy we are willing to put into it. Now let us get to making concrete choices about the skimmer that we want.
I have used an industrial, motor-driven diaphragm air pump. It is similar to the pump that Cole-Parmer sells as the "Air Cadet," but I have replaced the original 1.1 amp motor with a 1.3 amp motor that turns faster than the original. The Little Giant 4-MDQX-SC water pump also uses a 1.3 amp motor. People have had good success with using that Little Giant pump in moderate size downdraft skimmers. I do not have the equipment necessary to measure the actual electrical current or power draw from my air pump motor. Its air flow, with the input and output unrestricted, is only 70 SCFH (cubic feet per hour, at standard temperature and pressure). My Quiet One water pump generates a flow of up to 100 SCFH through my small downdraft skimmer, which I modified with a Beckett 1408 foam head. The Quiet One has a 0.74 amp motor rating--little more than one half of the air pump's--yet it draws more air than the air pump. It would draw even more air if I had a taller injection column on the skimmer (I discuss this claim later).
This air measurement suggests that air pumps are not efficient after all, but large air pumps exist that are more efficient than my air pump. Aquatic Ecosystems sells a diaphragm air pump that pumps 258 SCFH at 2 psi while using 124 watts of electricity.
Despite the electrical inefficiency of my "large" air pump, I think that it is still useful for exploring larger airstone skimmers. Most hobbyists who compare downdraft skimmers to airstone skimmers base their opinions on comparisons between a downdraft skimmer driven by a 200 watt water pump and an airstone skimmer driven by a 7 watt air pump, such as the Tetra Luft pump. We should compare airstone and downdraft skimmers that share similar air flows, or similar operating costs. The Tetra Luft pump, turned all the way up and not pumping against any pressure, has too little flow to lift the float in my Dwyer flow meter. That means that it moves less than 40 SCFH. I have a smaller flow meter, which has a maximum reading of 15 liters per minute (31 SCFH). The Tetra Luft pushes the float of the smaller meter to the top. I might conclude that the air flow is between 31 and 40 SCFH, but I do not know whether these flow meters are accurate at the extreme limits of their measuring range; there may be considerable error in my measurement of the Tetra Luft pump.
When I tried to make a skimmer for my 70 SCFH air pump, I wanted to ensure that I had enough airstone area. I used an $18 fine-pore ceramic airstone from Aquatic Ecosystems, Inc. This airstone is 12" long and 1.5" on a side. Its air comes in through a 1/2" CPVC fitting. I experienced large bubbles from this airstone and pump combination, and the consequence of the large bubbles was poor skimming. I noticed that if I reduced the air flow, but kept everything else the same, I got smaller bubbles and better foam. It seems from my limited experience here that making an efficient airstone skimmer that can compete with the downdraft and HSA skimmers would require having a lot of airstones, and at $18 each, I don't want to do the experiment. Don't forget that airstones have to be replaced occasionally.
Another problem with airstone skimmers is that large air pumps are extremely expensive. The 258 SCFH air pump that I mentioned previously costs $538 at Aquatic Ecosystems. I have reason to hope that I will be able to get that much air flow from my GRI 520 water pump and Beckett injectors, although in my current (highly unoptimized) configuration, I get only 150 SCFH from the GRI 520 and a single Beckett. You can also pump air with a regenerative blower. Those blowers move a lot more air than diaphragm pumps move, at low pressures, and they may be suitable for a skimmer. They, too, are expensive ($353 for a 1/8 horsepower (162 watt) blower from Aquatic Ecosystems).
Because I want to design and build my own skimmer, I have to consider whether the HSA really is the best possible design. The HSA may be the most efficient commercial hobbyist skimmer, but I think it's just a downdraft with a better injector (it uses the Beckett 1408) and injector mount. That is, the HSA's improvement over the ETS is not because of the fancy construction, the placement of the injection column in the middle of the foam column, nor the "bubble-free" water return. Although I like the Beckett injector, I have no confidence that the Beckett injector is the best that we can do. The Beckett foam head was designed to make a pretty fountain, not to create a lot of fine bubbles (although it may be that these two behaviors call for the same design, in which case we win). One problem is that the Beckett is small. I would like to have a bigger one. Large fountains use larger versions of the same design, but I don't know where to get one made from plastic. Tell me if you know of a source.
The parameters that produce the maximum air-water surface area will not necessarily produce the maximum air flow possible. For example, people are claiming that restricting the air input of their Beckett-equipped skimmers produces smaller bubbles and better foam. I wonder if it's possible to produce small bubbles with an unrestricted air input by tuning the other parameters (water inlet pressure and Beckett output pressure).
Consider the water pump to be a constant (we will choose a pump and then design a skimmer around it). Then if the skimmer has multiple Beckett injectors, you may achieve the most total air-water interface area by running each Beckett with parameters that would not be ideal if each Beckett were being considered alone. It may be that each Beckett's individual best performance requires a level of power that the pump cannot deliver to multiple Becketts simultaneously, and yet you get better performance by running all Becketts suboptimally than you get from using a single Beckett in the skimmer and operating it at its optimal parameters.
Second, choose a pump. The pump that I'm using to design my new skimmer is a GRI 520, which pumps 1500 gallons per hour at 5 feet of head pressure. This GRI is a commonly-used pump for the medium to large downdraft skimmers. I might find it sufficient, or I might want to try a larger pump in the future, but only if I convince myself that the GRI is limiting me, instead of my skimmer design limiting me. I think that one large pump is better than several smaller pumps, because of the principle that larger machines are generally more efficient than smaller ones.
Third, figure out how to maximize air-water surface area. Don't be too concerned yet with producing good foam or collecting the foam. We can design the air injection system separately from the foam concentration and collection system. We can measure air flow easily, but it's harder to measure air-water surface area. The best technique I can think of is to notice how easily the bubbles form collectible foam. So we do have to think about producing good foam, but don't make the common mistake of getting excited about seeing a small rate of production of beautiful-looking foam. In a later section of this document, I describe my attempts to maximize air flow.
Finally, after you think that you know how to maximize the air-water surface area, figure out how to make good foam and how to collect it fast. I don't believe that building up several feet of dry foam is a good thing to strive for, unless doing that is the quickest way to get the foam out of the skimmer, and I don't think that it is. I have noticed that, when I allow the foam to build up over a wide area, for example by lowering the water level in my skimmer to several inches below the top of the skimmer box, the wet foam becomes dry foam without rising very much. Within the space of only a few vertical inches, the foam transforms from runny water to foam that is still white, but has a fine-pored spongy look that tells me that it would be reasonable to collect that foam. This observation leads me to an idea that I hope will allow a skimmer to export dry foam with less than a foot of height between the wet water and the top of the foam riser tube. I can do this for any air throughput within certain limits, but wide limits. My new battle cry is "get it dry and get it out!"
In one design, the riser is in two sections, as is conventional. Only the lower section is tapered. Because I do not know where to buy tapered round acrylic tubing, I would make the tapered riser section from flat acrylic sheet. The riser would be a four-sided pyramid. The base of the riser would be 12x12". The top of the tapered section of the riser would be 3x3". The height of the tapered section would be only 3". On top of the tapered section, I would attach an untapered round acrylic tube of 2.75" inside diameter. The round tube should not be long, although I do not yet know what length it should be. I would begin with it long, and cut it down after I see how much height is required to ensure that the foam coming out the top of it is reasonably dry. I'm hoping that I can have it no taller than 6".
In the second design, the riser is a single part: an inverted large funnel. Suppiers such as US Plastic Corp. and Cole-Parmer list funnels with a maximum diameter as large as 21", as well as funnels with diameters in the 6" and 12" range. The spout of a funnel is very narrow compared to the foam risers that we are used to seeing on large aquarium skimmers. I believe that the narrow top will help eject dry foam. As the diameter of the funnel decreases, the air velocity increases. At the spout of the funnel, the air forms a veritable jet of air. As foam enters the high velocity region, the foam is ejected forcefully, as if by a canon. I have not yet tried this funnel idea on my experimental skimmer, but I am using a smaller funnel as the second stage foam riser on my small downdraft skimmer. That skimmer is now performing better than it ever has before.
The user can adjust a gate valve on the output of the skimmer in order to change the level of water in the skimmer. Because of the tapered riser, changing the water level in the skimmer will change the surface area over which the air-water mixture will initially begin separating into water and dry foam. The area that is best will be different for every pump and airflow combination. The taper allows the user to achieve the ideal area for a wide combination of pumps and air flows. The shallow shape of the tapered riser allows users of smaller pumps and air flows to achieve the ideal area without raising the water level in the skimmer excessively. Raising the water level puts backpressure on the air flow and reduces it. Raising the water level only a few inches will probably not decrease the air flow a lot. Compare this to the practice of raising the water level in conventional downdraft (or HSA) skimmers by many inches, in order to use the skimmers with undersized pumps. I have to do this when my Quiet One operates my downdraft skimmer. My air flow meter shows a dramatic decrease in air flow as I increase the water level in the skimmer.
In the following text, I will describe some of my observations that led me to the short and tapered riser idea. I have been watching bubbles turn into foam for a while now, and I have noticed something interesting. For a given air flow and skimmer, the behavior of the bubbles as they enter the foam riser tube falls into one of three categories:
I experience this difficulty in developing dry foam when I use my GRI 520 pump on my small downdraft skimmer, although with a lower riser section of 6" inside diameter, the skimmer is not especially small, at least in the dimension that we are considering now.
I have not been satisfied with my small downdraft skimmer, which I have attempted in vain to operate with a Quiet One pump since April 1997. The skimmer will produce beautiful dry foam, and push it all the way to the top of the foam riser column, but the skimmer collects little material. The waste gradually coats the inside of the foam riser tube and builds up a layer of baby shit, but it seems that the waste doesn't collect fast enough. My water quality stinks, so it's obvious to me that I need to do better.
The problem is that my collection cup is too small relative to my foam riser tube. The outside diameter of the foam riser tube is 4.5". The inside diameter of the collection cup is 6". That means that there is only 0.75" clearance between the two tubes. The lid of the collection cup is only about 1" above the top of the foam riser tube. The result is that, for all but wet foam, the foam simply gets blocked by the lid and the walls of the collection cup.
Why did I build such a small collection cup? Because I was cheap! Large diameter acrylic is very expensive, and I found a good deal on 6" inside diameter acrylic. I suspect that the skimmer manufacturers are being cheap with the collection cup, too, although not as cheap as I was.
The collection cup should be of sufficient size so that dry foam can spill over the top of the foam riser tube without touching either the sides or the top of the collection cup. Dry foam holds its shape well, rather like sausage. Therefore, I will suggest the rule of thumb (rule of scum?) that the clearance between the collection cup and the foam riser should be no less than the inside diameter of the foam riser tube. This rule implies that we should be using huge collection cups, compared to what most of us use now. Large diameter acrylic tubing is expensive, but remember that you can make a square collection cup from flat acrylic sheets.
My idea to use an inverted funnel as a foam riser came after I formulated my ideas about collection cup design, and after the initial writeup of this section. Ejecting the foam out of a funnel spout may allow you to use a smaller collection cup, for two reasons. First, a narrow funnel spout allows you to meet my rule of thumb about collection cup size with a much smaller collection cup. Second, the forceful ejection of foam from the funnel spout should push any obstructing foam out of the way, so it seems less important to have a clear path for the foam.
The pump was a Quiet One. I performed the tests on my experimental skimmer (not my small downdraft skimmer about which I have a web page). The test skimmer is in some ways a cross between an ETS and a HSA. My skimmer looks like an ETS except for the following: there are no bioballs in my skimmer; the injector tube of my skimmer does not extend lower than the top of the box at the base of the skimmer, whereas in the ETS, the injector tube extends to the bottom of the box, and there are holes in the ETS's injector tube, within the box, to allow the water to escape into the box; I use a Beckett foam head to introduce the air, as on the HSA skimmer; there is no baffle in my skimmer, because I have found that a baffle is probably not necessary, and a tiny bit of bubble loss out to the aquarium sump does not bother me, because this is an experimental skimmer, rather than something that I plan to use for the long term.
My Beckett injector is in a sealed chamber. This is important. When the Beckett is sealed up, rather than simply hung from the top of an ETS-style injector tube, all air that enters the Beckett's air input holes must travel through the entire skimmer. If the Beckett is hanging in open air, it is possible for air that the Beckett sucks through to separate from the water flow and travel up the injection tube, and go through the intake holes in the Beckett, without passing through the the skimmer, and thus without contributing to the foaming process. In the worse case, which does not occur in practice, the Beckett would be sucking air at a high rate, but no air would actually pass through the skimmer. Sealing the Beckett makes a measurable difference in practice. In my Beckett-converted downdraft skimmer, I measured a 50 percent increase in air flow when I added a seal between the bottom of the Beckett and the wall of the injection tube.
One of the design parameters of this type of skimmer is the length of the injection tube. You can mount the Beckett injector directly on top of the skimmer box, which amounts to having a zero-length injection tube, or you can have a length of rigid PVC or acrylic tubing between the Beckett and the skimmer box. I found that the length of the injector tube significantly affects the amount of air that the skimmer draws. To my surprise, increasing the injector tube length increased the air flow.
Before doing the experiment, I expected that increasing the length of the injector tube would decrease the air flow. When the injector tube is longer, the pump has to generate more pressure to maintain the same flow. Centrifugal pumps don't work that way. The Quiet One in particular is known for having poor performance at high head pressures.
I witnessed increased air flow with injector column height increases only when the injector column was filled to the top with water. If you use a large diameter injector column, then the column will be filled mostly with air, and the output of the Beckett injector will simply fall through the air as a waterfall. If you use a narrow injector column, such as the 1" diameter pipe that the HSA uses, then the column will always be filled with water (actually with the uniform air-water mixture that the Beckett produces). The diameters of tubing used for ETS-style injection tubes are in a borderline range, where by slightly varying certain conditions, the injection column may be either filled with air and a waterfall, or filled with water. In my experiment, I found that I could change the injector column between filled with air with a waterfall and filled with a uniform air-water mixture. I accomplished this transition by very slightly changing the water level in the skimmer, using the gate valve on the skimmer's output. Some people describe the filled condition as "backed up." Some authors have written that ETS skimmers perform better when the injector tubes are not backed up. This claim seems suspicious to me now, because my results showed an increase in air flow flow when I transitioned the injection tube from unfilled to filled. The increase was more dramatic with the longer injection columns. Despite the increased in air flow after transitioning to the filled state, the entrance of the air-water mixture into the skimmer box looked more gentle and laminar than when the injector column had a waterfall in it. Judging visually, I guessed that there was less air flow with the injection column filled. Perhaps this illusion confused those authors who reported "better performance" when the ETS injectors are not backed up.
Let us try to make sense of my test results. I said that the air flow increased with increasing injector column heights. I did not measure the water flow. The pump pushes the water up to the top of the injection column, but then the injection column brings the water back down to the base of the skimmer. Gravity creates a suction in the injection tube to pull the water down. For each increase in injection column height, the pump does more work to lift the water to the top of the injection column, but gravity does work in return by pulling the water down the injection column. (I know that this explanation is abusing the proper terms of physics. Someone help me here.) As the injector column increases in height, the only extra effort that the pump has to do is to overcome a little bit more pipe friction. That requires a minor increase in effort. If this explanation is correct, then the water flow rate should not change significantly with increasing injection column heights. But this explanation is an oversimplification. The fluid moving down the injection tube is an air-water mixture. Its density is less than that of the water that the pump is lifting. Therefore I would expect that the suction on the downside does not fully compensate for the height on the upside. But the velocity of the water on the downside is higher than the velocity on the upside. Does that make up for the density difference? Now look at it in terms of mass flow. The mass flow on the downside must be higher than that on the upside. On the upside, we move a certain mass of water per unit time. On the downside, we move exactly that much water per unit time, but we also move the air that the Beckett sucked in.
It's 1:00 am and I'm into the physics way over my head. I need to stop and get some Professional Help. I now understand my results less well than I thought I did when I started writing this section. I'll tell you where I was going with it anyway.
The pressure at the bottom of the injection tube should be just atmospheric pressure, because the injection tube empties into the top of the skimmer, which is where the surface of the water is, and the skimmer is open to the atmosphere through the foam riser tube. The pressure difference between the top and the bottom of a vertical column of fluid increases as the height of the column increases. Because the bottom of the injector tube is at a constant pressure, no matter how high the injector tube is, the pressure at the top of the injection tube (at the output of the Beckett) must be lower than atmospheric pressure. The pressure gets lower as the column height increases. This means that the pressure drop between the Beckett's air inputs and the Beckett's output gets larger with taller injection columns. The air flow rate through the Beckett depends on that pressure difference, so air flow must increase with increasing column height.
Common sense tells me that, for a given pump, there is some height above which increasing the injection column will begin to decrease the air flow rate, because of the decreasing water flow with increasing height. That critical height may be higher than your ceiling. I will perform more testing soon. In the meantime, my gut feeling is that you should maximize your skimmer's performance potential by using an injection tube, or injection tubes, that are as high as your ceiling and your wife will allow.
I would like to make several other observations that follow from the above.
Right now, I have absolutely no information nor guesses about whether there is an advantage to using multiple Beckett injectors rather than just one. If you use multiple injectors, you have to decide whether to give each injector its own injector column. In any case, you have to decide on the widths of the injector columns. Testing on all these details should happen sometime this century.
Shulin Chen, Michael B. Timmons, James J. Bisogni Jr, Danel J. Aneshansley. Modeling Surfactant Removal in Foam Fractionation: II -- Experimental Investigations. Aquacultural Engineering, 13 (1994) pp. 183-200.
Uraizee, F., and Narsimhan, G. A Model for Continuous Foam Concentration of Proteins: Effects of Kinetics of Adsorption of Proteins and Coalescence of Foam. Separation Science and Technology. Vol. 30, No. 6, pp. 847-881, 1995.
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