Its subjective at best, I've used venturi and needle wheel designs myself, have never used downdraft. I like the needlewheel designs myself they dont require a pressure pump like downdrafts and venturi kinda seemed ok. The needle wheel type just seemed to be a little easier to dial in for me... But in the end they all make bubbles:rollface:
As with most responses, the answer will inevitably me "mine". Whichever skimmer most people have will be what they suggest.
As far as designing one, you will want to go with one that can move the most amount of water with the highest dwell time. These are opposing factors which usually means that when you want increase the size of the tank you will need to increase the flow and while keeping the dwell time. All things being equal dwell time or the amount of time a particular 'piece' of water is in contact with the bubble column will be the deciding factor in skimmer performance.
For instance, if you have a 100 gallon tank and a skimmer with a 1 minute dwell time (almost unheard of in hobby skimmers), but only moves 5 gallons a hour through it, it won't be of much use.
Conversely, the opposite is also true on the same size tank, moving 250 gph through the skimmer with a 10 second dwell time will be completely useless except for possibly oxygenating the water.
Most people have no idea what the dwell time or flow amount is for their skimmer, they are usually going by the coolest looking or pier review which very subjective. You will want to look up designs for industrial skimmers and scale down to the size tank you want to put the skimmer on. The industrial skimmers will show the sizing of every part of the skimmer and give you the flow and dwell times. If you can get a hobby skimmer close to a minute dwell time, then you will be doing very well.
I have saved these basic rules and found them helpful over the years--
First rule: Flow rate
Flow rate through the skimmer. The formula for figureing it out is Hours=9.2 purity coefficient (which means that 99.9% of the water has been treated (gallons/gph) so this formula gives you an idea on how to see how many times a day the tank water runs through the skimmer in order to treat all the tank water completely. So an example of say my skimmer would be:I have a pump pushing 600 gph and my tank has about 1000 gallons of water and I want to have a treatment of 9.2 coefficient (which means 99.99% of the water is treated in the tank). Or (1000/600)9.2=15.3 So all my water will hae seen the skimmer every 15.3 hours. This formula works for all equipment like skimmers, uv, ozone and so on.
2nd Rule: Bombardment rate
Which is the number of times an assending air bubble hits a descending water drop with in the skimmer. This concept deals with the time the tank water spends in the skimmer and the diameter of the skimmer. Ok the formula for figuring this out is Bombardment rate (R) = the dwell time of the water/the dwell time of the air. So an example: my skimmer holds 30 gallons of water and is feed by that 600 gph so that means that the water completely passes through the skimmer every 180 seconds So my dwell time for the water is 180sec. The dwell time for air is a long formula (like about a page of math) and I go cross eyeed just looking at it so here is my simpler method. I turned the air pump super low so thier was only a few bubbles in the skimmer. With the bubbles coming in and hitting the water flow in the skimmer I timed it out at a very approximate but it came out to 12 seconds, so my bombardment rate is 180/12= 15 bombardment rate. 10 is concidered to be perfect.
Rule 3: Volume of air
This is the amount of air as compared to the amount of water in the skimmer. The forumla is that the air volume should be 20% of the total water volume. So if you have a skimmer that holds 5 gallons of water, you should only have 1 gallon of air in thier. If you have more air then this figure in your skimmer the bubbles will join togehter and be to large and thus will not hold as much of the protiens/organics on their surface. Also it will reduce your bombardment rate as thier wont be as many bubbles hitting the water moles. An easy way to figure out what your percentage is, is to run the skimmer normal with air being injected (or however you get air into the skimmer) and make a mark on the skimmer body with some tape. Then kill the air and make another mark, the water level should go down the cylinder by 20%
Rule 4: Configuration.
The injection points for both water and air are also critical for the skimmer to work efficiently. Example of this would be, if the water enters the top of the skimmer and exits the bottom and that your air enters the bottom and leave via the top, it would mean that all water must pass through the air bubbles in order to leave the skimmer. This method is the best. If the water and the air are injected at the same point, then your bombardment rate is reduced as the air bubble will travel with the same water mole all the way to its exit, thus it only hits a few water moles instead of many. Now I know some are going to get their undies in a not on this one (Skimmy I see that strained look ) So I will say that the water and air can be injected through the same point, but I have yet to seen the math to prove its not less effective or as good.
Rule 5: Contact time.
Since the function of a protien skimmer is to first mechanically strip the protien/organics from the water mole and then to chemically attract the protien/organic to the surface of the air bubble, it is important for the organic/protien to spend as much time in contact with the air bubbles. Now with some protiens it take a long time for them to actually bond to the surface of the bubbles, so the more the better. We have to remember that some protiens dont want to let go of their bond and thus some can take minutes to be mechanically seperated.
Ok those are the basic laws of skimming, but their are some other thoughts that should be concidered. A big one is the amount of violence with in the skimmer, if the water and air mixture is so violent, then the air bubbles will break and will thus release their chemical bond on the protien mole and put you back to square one. Or they will join together and form a bigger bubble which will then reduce the surface area available for protiens to attach.
Another is the what I call the freight train effect. This is where the [ump is so big and moves water through the skimmer so fast that it basically acts as a frieght train and takes much more out of your water then just the protiens we are looking to remove.
Good question... Everyone thinks they have the best skimmer (it's normal). Me personally, I'd never go based on a brand name because then your paying for the sticker on the skimmer.. The main thing is having the best skimmer pump.
What I look for in a skimmer:
1. Size in regards to price (meaning I am not spending $800 on a tiny bubble king when I could buy a reef octopus diablo or Skimz - twice the size at half the price).
2. Would never go for a skimmer that's like 2 vertical PVC pipes (body and collection cup). Does not have to be completely cone shaped but I think this is the best design thus far. Best part is you no longer have to spend thousands owning a cone w/ a bubble plate.
3. I feel the collection cup tube in a cone shape is great. Turboflotor's collection cup is way to small but that collection cup sends bubbles to the collection cup like no other skimmer i have used. Great skimmer for tanks with space constraints that need versatile skimmer (hang on sump, hang on tank, free standing)
4. Something that does not have to be adjusted every time I do something to the tank (like water changes).
5. Look for something that is versatile and easy to maintain. example, something I can upgrade to a larger system, easy to clean air tube, easy to clean collection cup,
6. I don't go by skimmate..Everyone loves posted pictures on what there skimmer pulled out. It's a skimmer it's suppose to skim green smell stuff.. I don't care if you put a air stone in a tube you will eventually get skim, might take longer but you'll see it eventually.. lol.