Duncan Griffiths
When I first started to write this piece I was not far into it when I realised, as yet there is no perfect solution to removing particles suspended in pond water going to a filter or we woulkd never have to clean our biological stages. There are varying reasons for this which I will try and explore. It also struck me that, limited as some systems may be the best life has to offer4 so far are the ones that involve a little bit of ingenuity, such as the vortex, sieve, eazy drum filter. Because although these systems have varying limitations they take up much less of a foot print in what has to be described as the limited space garden as opposed to the large ineffective inertial damped system because they simple cannot be made big enough to become effective so fail miserable, but more on that later
I have studied these things for some time now and reached an understanding about them based on sound engineering processes. I don’t begin to understand the maths involved in a vortex to which there are many and Isaac Newton I ain’t, but like it or not these are the forces at work inside a vortex, they are very, very complex but they are easy enough to discover for yourself as you will see and if you place an eazy in one or brushes or anything like that they cease to beehive in a vortex like fashion.
The heart of a koi pond is the filtration system; this system consists of two parts. First is the settlement section where suspended organic debris/particles are filtered out (making them exempt from the bio stage). These are known as POC’s (particulate organic carbons). This is closely flowed by the bio conversion section where water is altered at a molecular level taking out toxins making the water safe again for koi. You can’t really have one part without the other, if you allow the bio converter to become blocked with fish crud and other organic matter this section will cease to function and in some extreme cases can start partially reversing the nitrogen cycle. Therefore it could be argued the most important part and heart of the filter is the settlement where particles are filtered out leaving the bio section free to operate at peak efficiency.
There are basically several common ways to filter out these particles.
1. The mechanical system where particles are strained out (brushes, sieves, drum filters eazy, etc).
2. Inertia damped settlement, where fast moving water suddenly meets/hits a huge expanse of very much slower moving water. (Huge settlements chambers)
3. laminar flow settlement which employs a static boundary layer caused by friction (vortex)
4. Of course you can skip this stage and utilise none of the above as in nothing ( trickle towers, Bakki etc)
No matter which system you employ they all share a couple of things in common
1. They ALL need cleaning some easier than others
2. They all take up space some more than others
Which ever system you favour the one that has stood the test of time most hobbyists from beginner to high end user is the common vortex. So what is a vortex?
The vortex (plural of vortex =“vortices”)
A vortex is a spinning, often turbulent, flow of fluid. The motion of the fluid swirling rapidly around a centre ( its own axis )is called a vortex. The speed and rate of rotation of the fluid in a free (irrotational) vortex are greatest at the centre, and decrease progressively with distance from the centre, whereas the speed of a forced (rotational) vortex is zero at the centre and increases proportional to the distance from the centre. Both types of vortices exhibit a pressure minimum at the centre, though the pressure minimum in a free vortex is much lower.
What the basic difference between the two is, in none technical terms one spins faster the closer you get to the centre of axis and never slows down, (e.g sink plug hole vortex) the other spins fast to the centre, but unlike the plug hole because its going nowhere its starts to rotate on its own axis at which point there is no forward speed or movement, its stationary/static ( koi filter vortex)
What do common everyday vortices look like?
Its easy to see how we can put these natural occurring dynamics to good use in a koi vortex ,
so what does a common koi vortex look like?
Ok now we can get down to how they work.
Basically ignoring dimensions there are basically two types of vortices used in koi filtration a side exit and centre exit for water into the bio chambers.
They both usually employ around a 2” bottom drain. What make a vortex chamber different from any normal mundane round settlement chamber is, the vortex must have a tangential means of inputting the water to set up a rotation within the chamber. Without this type of input the vortex action cannot be established. It would then become just a normal updraft system and not a vortex. see fig
Water entering at a tangent to the side wall starts to turn/rotate the main body of fluid contained in the chamber creating a spin on the volume of water contained. Water enters at the bottom of the cylinder and exits at the top.
Most people don’t know what makes these pieces of kit work and probably don’t care, but when asked most peoples general opinion of how this action helps settlement is this:
The rotation once established, throws objects/particles contained within towards the outside, where a stationary boundary/ laminar layer exist. This layer is created by the friction of the water travelling against the wall of the chamber resulting in drag to the extent that the nearest few millimetres of water right up against the wall are almost motionless once particles enter this they fall out down the wall and collect the sump. This action is called centrifugal force,
Centrifugal force (from Latin centrum "center" and fugere "to flee") represents the effects of inertia that arise in connection with rotation and which are experienced as an outward force away from the centre of rotation. In Newtonian mechanics, the term centrifugal force is used to refer to one of two distinct concepts: an inertial force (also called a "fictitious" force) observed in a non-inertial reference frame, and a reaction force corresponding to a centripetal force. The term is also sometimes used in Lagrangian mechanics to describe certain terms in the generalized force that depend on the choice of generalized coordinates
[ExtracT] from Wikipedia
This boundary layer phenomena can bee seen in any river where the centre of the river moves through much faster than the first few feet from the bank you see this boundary layer a few feet out as a series of eddy’s where debris settles out. Eddies are good for settleemt because they are usually static.
So we initially have a body of water spinning that is throwing particles it contains outward towards the wall under centrifugal force. At this point, due to friction with the chamber wall a boundary laminar layer exists that’s not moving so once there the particles can drop out . see fig
If only it were that simple, so why is not that simple?
If centrifugal force is too believed then once the particle hits the static layer it should drop dead vertical down the wall into the sump, but some do not and escape into the bio chamber, why?
First , centrifugal force occurs very rarely in the natural world and this is manily because of people perception of it is not quite what the phenomena actually is
Most people’s perception of centrifugal force is an object being rotated around a fixed centre of axis. Like in the shot put in the Olympic Games. If this object is released; lets say at the 360 degree or the 12 o’clock point it will continue to travel on the 360 degree or 12 o’clock path. This is kinda only a small part of it, because involved in centrifugal force or always some kind of forward motion which has an impact on the direction of travel.
Imagine the cricket bowler bowling to the bats man. His arm rotates with the ball clenched in his hand his aim is the wickets straight ahead. If after gathering pace he let the ball go when his hand pointed at the wicket the ball would be propelled down at his own feet. If you think about it and anyone who has done any bowling will know you let the ball go at the very top of the arc of your arm ( the 12 o’clock position) but under centrifugal force laws if you did this the ball should head for the sky’s. If you had a sling shot rather than release the shot when its in line to target as you would when firing an arrow or a rifle you maybe let it go around 90 degrees prior to lining to the target. This action is called centripetal force.
This action/term is hard to explain, but basically centrifugal force is a term for describing an outward force. If you corner to fast in your car you feel the force throwing your body towards the side of the car on outside of the curve, but it gives you no indication of the direction of travel once released or the car leaves the road because forward velocity is involved. Now if we could factor in the forward velocity we can predict where the car will roll to. This is centripetal force
Picture courtesy of Wikipedia
The so what is! When the particles hit the wall and enter the layer, rather than drop vertically down the wall as you would expect they have inertia (forward energy) they will continue the forward momentum while also descending or some this will bounce them out of the laminar static layer because once here there is no outward pressure holding them there, for some other’s they will have to run the gantlet of passing right in front the water inlet again from the pond ( incidentally this is why the long empty settlement fails its because the particle has motion so does the settlement it not the length that counts its mass/volume verses flow ) but ill try get to that at the end.
The centripetal type vortex settlement only works for fines or fine particles, so what you may ask happens to the heavy particles, the big faeces ,this brings me to the “parabola effect ” and a little experiment you can do at home.
Open a tea bag if you don’t buy tea loose put say a third of a spoon in the in a cup of warm water and stir . You should see the big particles go straight to the centre and the fine bits of tea dust go to the outside. How does this happen?
Well far from what most people think the spin on a vortex gets faster the closer you get to the centre of its axis, its only when you get to the dead centre what we call the point of axis where there is no velocity and it simply spins on its own axis and is in effect stationary
Experiment for you again drop two pieces of K1 in your vortex both at the same time one 6 inches in from the edge and the other 6 inch from the centre of axis and watch which is going the fastest
This leads me to an example. a set of photos. The vat was set to spin from a hose pipe at the edge then at the edge a solution of neutral Acriflavine was added watch what happens look at the leading part of the dye
As the dye advances it gets faster and closer the middle this is what happens in your vortex this has an effect on the surface far from being dead flat it gets dished/saucer shaped depression at the surface this is called a Parabola. What this means is, the actual head pressure in the centre is much less than the edges this lower pressure pulls the bigger particles to the centre where the pressure is at its lowest. From this you can probably gauge that a centre extraction is probably not the best solution as faeces with neutral bouancy is going to be less likely to sink as quick.
Ok we come to flow rates for these devices, obviously for any given flow its safe to assume that bigger is better. So for any given size you want the flow to be as slow as is humanly possible. But obviously we have not got unlimited space or resource the rule of thumb and guide for a vortex is aim for 120 GPH flow for every square foot of surface area . (the depth is not as important)
Example: a 4 foot diameter vortex has 12.5 square foot of surface area 12.5 X 120 = 1500gph flow rate you can stretch and exceed this but obviously you will get more overspill of particles into your bio stage
Lets take the typical vortex that sits at the front of the majority of the every day filter systems 2.5 ft diameter
1.25 X 1.25 X 3.142 = 4.9 square feet = 589 GPH
As mentioned these figures are practical figures and are not ideal in a perfect world you would want to be pulling half maybe even a quarter of this for every square foot of surface area, to get a better process if you need it put another one4 along side the one you have but run it in parallel NOT series. This in effect will half your flow rate.
I’m sure many of you will be saying those figures cant be right? All I would say is don’t shoot the pianist. But ask yourself one question how often are you forced to clean out your bio stage ? If the answer is a lot or more than you would like, take an even lower flow rate as the ideal, or consider some other means of particulate removal. i recommended a drum filter you will probably get away with zero cleaning but dig deep my friend
I’m sure after reading all this some lab rat somewhere is going to crawl out of the wood work and say “well that point is not quite right” I say, this is an overview a very brief look into it the forces at work, in not writing a piece for a Nobel prize if it gives folks a greater understanding my work is done
thanks for all the effort you have put into this
