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**Concept #1 - Flow of water through a pipe**

If you know anything about electricity you know that flow (amps) equals the pressure (volts) divided by the resistance (ohms). Water in a pipe acts the same way. If you want more water you either increase the pressure or decrease the resistance.

The flow of water through a pipe is described by this equation. Don't freak out!!! The math is actually pretty simple.

*Q*= (pD

*Pr*4)/(8h

*L*)

Q is the flow (pick your favorite unit - liters per minute, gallons per hour, etc.) Flow is what we pay for, and what we hope our pumps deliver.

p = 3.14159.......

P = Pressure (psi, meters of H2O, mm of mercury, or whatever)

r = radius of the pipe

h = the viscosity of the fluid - in our case water

L = the length of the pipe

We can't do anything about the viscosity of water or pi, so ignore those. From the equation you can see that flow is proportional to length of the pipe. A 100' garden hose will put out half as much water as a 50' garden hose. Check it out sometime. Fifteen extra feet of tubing will decrease your flow by quite a bit.

The pressure, for our purposes, is produced by the pump of interest. More on this later. Suffice it to say that if you double the pressure, you double the flow. Remember that canister filters are really nothing more than a pump attached to a media container.

Now, the grand-daddy of them all. The one that really matters. Flow is proportional to the radius of the pipe, raised to the fourth power. In english, this means that if you double the radius of your pipe, then you'll increase the flow rate by 16 times!!. Even small differences in diameter will make a large difference in flow rates. You'll get 2.1 times more flow from 3/4" tubing than you will from 5/8" tubing.

The reason that the radius is so important is that it affects the velocity of fluid in the pipe. Areas of high velocity are turbulent, which means the water molecules are crashing around randomly, wasting energy instead of flowing nicely along toward the destination like we want them to. Think of turbulent flow like friction. Even the very large turbines in the power plant at the bottom of Hoover dam are more efficient when velocities are low. This is why the intake and outlet pipes are so big.

Try to wrap your head around this idea too: The flow rate is the same everywhere in the system. If you use 1" intake tubing and 1/2" return tubing, the water simply moves 4 times faster (and has more turbulence) in the 1/2" return line.

Every fitting you put in the line also acts to add resistance. An single elbow will produce as much resistance as several inches of straight pipe. A partially closed ball-valve will act like many feet of extra pipe - dropping flow considerably (duh, that's what a valve is for). Even a completely open valve will introduce a small amount of turbulence in the line, robbing you of energy, and decreasing flow. Add 20 feet of extra tubing, an in-line heater, a UV sterilizer, and a CO2 reactor to a little Eheim 2213 and you'll see that flow rates go to almost nothing.

Why don't the filter manufacturers use huge tubing for everything? Economics. Big tubing is expensive, and isn't easy to work with. They compromise, and we're stuck with whatever they decide to produce. For the most part, the compromise is reasonable.

**Concept number 2 - how a pump works:**

The flow going into a pump is exactly equal to the flow coming out of a pump (conservation of mass). So what, exactly, does the pump do? It raises the pressure of water in the output line. The pumps that we use are really horrible at developing suction on the intake line. They require a constant supply of fluid, and if it comes in already under a bit of pressure, so much the better. This is why Eheim recommends that their filters be located below the tank. The water column in the intake line acts to "preload" the pump. If you place the filter above or level with the tank, the pressure inside the canister is negative. This will lead to impeller cavitation (noisy and shortens impeller life), and gasses will come out of solution (we'd like the O2 and CO2 to remain dissolved).

Every pump has a pump curve associated with it. The shape of the particular curve is dictated by the design of the pump (rpm, impeller shape, intake & outlet geometry, etc). Eheim is kind enough to publish theirs. For an example, I'll reference the 1260 hobby pump (orange line), which I'm using in my new setup.

You can see from the graph that the two variables are H and Q. We already know that Q is flow. H stands for head. Head is basically a measure of resistance but it's a little hard to explain. Let me try.

Eheim will tell you that the rating for the 1260 pump is 40 liters per minute, or 635 gallons per hour. This is true for a zero-head condition. Zero-head means no resistance - no attached tubing, no valves, no strainers, etc. Nothing. Squirting out into thin air. By the way, the flow ratings for canister filters are given for zero-head conditions too - no media, no tubing, no bends, and no strainers. Actual observed flow rates are much lower due to the resistance of all the stuff we hook up to the pump.

We whine and complain that our 1260 pump only produces 20 liters per minute of flow. From the graph we can see that we have about 2.0 meters of head. This is how much flow you'd see if the pump were delivering water to a location 2 meters higher than itself (like pumping up out of a sump). If you try to use this pump to deliver water up three flights of stairs - forget it. Once you get up to 4 meters, the flow rate goes to zero. We call this the shutoff head. This maximum pressure that this pump can deliver is equal to 4 meters of water or about 5.7 psi.

Here is the cool part. The entire resistance of your system (tubing, filter media, elbows, strainers, spraybars, gunk in the lines, etc.) can be represented by a single value for H. If your 1260 pump is only putting out 20 liters per minute, the resistance (friction) in your system is equivalent to pumping water straight up a 2 meter pipe. You can decrease H (and therefore increase Q) by using fewer fittings, shorter sections of pipe, cleaning the filter media, or most importantly, by using larger tubing.

One last thing to remember. Don't forget that for a canister filter, or an in-line pump, the water in the intake tube will help to preload the pump. Think of this example. If the level of the water in your tank is 2 meters above the pump, the pressure on the intake side is already equal to 2 meters of H2O (about 2.8 psi). The pump will add up to another 4 meters of H2O (5.7 psi), for a total of 6 meters (8.5 psi). No matter where it is located (within reason), this particular pump will be able lift water up to 4 meters above the level of your tank.