Pumps and flow rate

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Haters' gonna hate
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There's a lot of confusing information in here.

For starters, flow into the start of a pipe = flow out. Simple as that.

The pressure will be different through the pipe. Highest at the discharge of the pump, lowest/zero (assuming guage pressure, atmosphere is zero) where the water comes out.

The little green pumps are centrifugal pumps which means they have an impeller that spins to apply pressure to move the water. Picture this -
  • You have a hose that goes vertically. The pump will push water up the hose until it reaches it's max operating pressure at the discharge of the pump. The height of this column of water is referred to as maximum head / pressure (pressure = density x height x accel. due to gravity. Or simply, 10m = 100 kPa). For these pumps, max head = 3.4m or 34 kPa. Hence the water will be pumped up to a height of 3.4m REGARDLESS of the daimeter.
  • Imagine you trim the pipe off at 3.39m. A tiny, little bit of water will trickle out. Your pump is now pumping. Pressure will be 33.9 kPa at the pump discharge.
  • If you continue to trim the pipe off lower and lower, more flow will come out because there is less water in the pipe vertically applying pressure at the pump.
  • Continue this until the pipe is gone and the pump is spitting out water. This is the max flow it is able to acheive, at essentially zero pressure.
A pump manufacturer normally supplies a flow chart so you can determine flow at a given pressure as it is not linear. For a cheap $65 pump, we don't really care because we just want it to move fluid. So it will probably be specced for the highest numbers - max flow with no pressure, and max pressure with no flow. Bigger numbers sound impressive though right?

Hydraulic (gear, piston etc.) and peristaltic style pumps are positive displacement - that means they push a fixed volume of water, and the pressure is induced by the system. This is a completely different scenerio.

There is nothing wrong with your pump. The flow is what it is, and yes the pipe diameter will provide quite a bit of backpressure. Blow through it with your mouth and if you feel resistance, this resistance is much higher pushing liquid through it. I put in a HERM-IT coil and my flow basically halved.

If you want more flow - move the two tank HEIGHTS closer together. It's the difference in pressure that determines your static head, so lifting the bottom tank has the same effect as lowering the top tank. Moving the pump and and down between the two won't make a difference once it's primed for a home brew scenerio.


Supremely mediocre brewer
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shaunous said:
I studied to be a busted arse diesel fitter and I can tell you there is a law of pressure.

Pascal's law is a principle in fluid mechanic's that states that pressure exerted anywhere in a confined incompressible fluid is transmitted equally in all directions throughout the fluid such that the pressure variations (initial differences) remain the same.

I clearly made the 'law of flow' part up.

Your getting to technical for what is as you said, a 'Non-positive Displacement Pump', best example being the water pump on our cars. As long as he is within the spec's of mounting the pump, and his inlet and outlet hoses/fittings/valves are not a smaller i.d. than the inlet and outlet of his pump, then he should get within the spec of 19L/min.
Bernoulli's principle builds on Pascal's law and is the principle that is applicable to this situation. And yes, if everything is set up reasonably well the OP should get close to the spec of 19 L/min. We seem to agree on what the outcome for the OP should be. i.e. if he has quite low head pressure then he should get close to 19 L/min. The specs say "max head: 3.4 m". That means the pump will work with 3.4 m of head pressure but doesn't mean you will get the max flow of 19 L/min at all head pressures below 3.4 m. Clearly lower head pressure means you will get higher flow rate. We can agree on that, right?

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