idris said:
Does anyone know how much pipe diameter and length effect flow rate?
I know it's not a simple relationship, and I don't want to get into complicated maths, but it would bs useful to know roughly.
One factor remains constant; more hose length = less flow, as the pump has to work harder to shift a greater qty. of water.
Where it gets a little more complicated is hose diameter.
Ideally the hose diameter needs to appropriately match the filter pump rating.
Let's look at two scenarios.
1) You have a very powerful filter i.e. Fluval FX5, but with 12mm hose. This will restrict flow considerably. The ideal hose size for this filter is 1" (25mm).
2) You have a small filter i.e. Eden 501 nano external, but with a 25mm hose. This will restrict flow because the mass of water in the hose is too large for the pump to shift effectively.
So from these two examples you can see that there is no simple formula.
As a very rough rule of thumb I would say this -
Filters rated <600lph - 12/16mm hose
Filters rated 600>2000lph - 16/22mm hose
Filters rated >2000lph - 22mm+ hose
There are some tiny externals that come supplied with 9/12mm hose.
Most commonly available external canister filters on the market come supplied with 16/22mm hose (internal/external diameter).
If you use a 12mm hose on these you can expect the flow to drop by up to 50% or more. I have noticed this by using 12mm fitting on my Fluval G6, as it has an accurate built-in flow rate meter.
Another consideration is inline equipment i.e. external heaters and inline diffusers. Even if they are supplied with the same size fitting as the hose, they will reduce flow because the diameter of the fittings are smaller to allow fitment.
Finally another consideration is how powerful the pump motors are, in terms of actual ability to pump under load, not just flow rate. For instance a Koralia-type powerhead can shift several thousand litres per hour, but it's not designed to have any real load. Higher quality pump motors should be able to pump water at the claimed rates, despite some clogging.
In summary, bigger filters are generally better, but by using smaller and longer hoses, you are losing a lot of flow. I'm afraid there's no formula to determine actual qtys. as far as I know. There maybe a hydrodynamics expert among us that may be able to help further...