not exactly.
i just want to point out that pressure at pump increases when the water column height increases.
so when the pump is running, it is pushing against pressure from ''complete" water column (pump outlet to discharge point). the inlet pressure does not negate the pressure experience by the outlet. under operating condition the inlet pressure will be lower than outlet due to suction action from the eye of the rotating impeller.
the eheim operation manual for 2422/2424 defines "a maximum of 180cm between the surface of the water and bottom of the filter". the specified pumping head are 130cm and 150cm respectively while the height of the canisters are 29cm and 34cm respectively.
thomas liew
I tent to agree with tawauboy. In my experience, filter flowrate will considerably reduce if you add additional item like chiller or CO2 regulator on the outlet. If the equilibrium theory discussed here true then there should not be any flowrate change since inlet and outlet at the same height. In other word the additional water inside chiller or regulator must have effecting the flow. I rule out friction because chiller is basically empty chamber.
tawauboy: You are right to say that the pressure at the pump increases when the water column height increases. However this static pressure at pump does not affect the flow rate of the system, pressure difference across the pump does. The maximum height between water surface and bottom of filter is specified by Eheim in the consideration of safety (Quote: Optimal operational safety), likely because if the filter is placed any lower, Eheim does not guarantee that the static pressure due to the height of that water column at the filter will not burst open the filter itself. This has got nothing to do with the max pump head specified, and the pressure difference across the pump in a properly primed setup where the filter 180cm below the water surface is still zero. To elaborate, if the structural strength of the canister filter and hoses are infinitely strong, that quote will not be there.
Shadow: If you would read up on the topic of fluid mechanics, you will realise many things create significant dynamic pressure losses in a system, which in turn affects the system's flow rate. This includes the expansion of water flow from your hose into the reservoir of the chiller, and contraction from the reservoir back to the hose to your tank.
If you guys are interested, I strongly suggest reading up on the topic of Fluid Mechanics, on how pump head, static head loss, and dynamic head loss interact and affect the flow rate of a system. This is an established fundamental principle of engineering used by the pump/fan/blower industry for decades. (I use it in my job too)
Maybe you are right, I'm not expert on fluid mechanics and too lazy to read
Haha. I enjoy any healthy discussion that makes me think. This is one of those that allowed me to apply my fundamentals on the topic on the different scenarios proposed, and work out what will actually happen. Interesting stuff.
nice to know that you are enjoying this discussion.
though you have brought up a valid points, i wouldn't dismiss 'eheim operational safety guide' as purely a safety guide. from a business/marketing perspective, it is different.
consider a typical pump case where suction height is at zero height.
maximum pump head is the height of the discharge above the pump outlet at which flow rate is reduced to zero. under this condition, maximum working pressure would produced by pump at the outlet and is equal to pressure caused by water column. also, the pump is seeing maximum differential pressure between inlet and outlet.
consider a second case where the suction height is increased by 2 meters.
assuming that the discharge height does increase by 2 meters. the differential pressure between inlet and outlet is remain the same because water columns at both inlet and outlet are increased by 2 meters.
however, the pressure at the pump outlet will be higher than previous case because the water column is now 2 meters higher. to achieve this additional 2 meter height, the pump outlet needs to produce more pressure than the previous case. this is not possible because the pump was already producing maximum pressure in the previous case. thus, the discharge height cannot increase by 2 meters.
do share why the pressure due to water column at the outlet needs not be taken into account.
thomas liew
Consider the following scenario when the pump (2m max pump head) is turned off.
Suction water column height = 2m
Outlet water column height = 2m
Flow rate = 0
when suction height and water column at the outlet height is equal, the pressure at the pump outlet is negated by the pressure at the suction side. The pump is not exerting any pressure as it is turned off, but there is already a 2m high water column at the outlet. Why? Because the water pressure at the suction side is counter-acting the pressure produced water column at the outlet height, thus supporting the water column at the outlet. (Differential pressure across the pump = zero)
Now consider the same pump turned on
Suction water column height = 2m
Outlet water column height = 4m
Flow rate = 0
Now the pressure at the pump suction end is 2mH2O, and the pressure at the pump outlet is 4mH2O. All the pump power is put into generating pump head to support this extra 2mH2O of pressure at the outlet. (Differential pressure across the pump = max pump head) Thus, there is no remaining pump power to create a flow rate.
Now consider a third scenario with the same pump turned on.
Suction water column height = 2m
Outlet water column height = 2m
Flow rate = 1000l/hr (example)
Now without any static head difference to overcome (since water column at both ends is the same height), the pump power can be put to driving the flow rate and creating just enough pump head (pressure differential) to overcome the dynamic pressure losses created by friction due to this flow rate.
If you were able to measure the static pressure at both the inlet and outlet of the pump, you will find that pressure differential across the pump for this thrid scenario is somewhere between 0 to 2mH2O, which the head needed to overcome pressure losses due to the friction caused by the flow rate.
When the pressure differential is 0, that means there are no pressure losses due to friction caused by the flow rate, and that the pump is operating at its maximum flow rate. This will occur when u place the pump by itself, without hoses on both ends, in 2m deep of water.
In a realistic scenario, when you attach hoses to your pump, throw in filter media in your canister, attach a CO2 Reactor, a chiller in line, all these will produce a pressure loss due to friction when there's a flow rate going through them. The pump has to produce a pressure head to overcome these losses. In this scenario, pressure differential across the pump will be somewhere between 0 to 2mH2O. The more head your pump needs to generate, the less flow your pump will be able to generate.
When you attach too many things in line with your pump, or if your hoses get too clogged up, and the pressure losses will build up and become so great the pressure differential across the pump reaches 2mH2O, there will be once again be no flow, and your system is officially fully clogged!
Hope I've not made things more confusing.
this does not explain why water column pressure acting at the pump outlet should not be taken into account.
thomas liew
water column pressure acting at the pump outlet should not be taken into account, because that is how pumps work. They work based on differential pressure across it. I strongly suggest reading up on fluid mechanics and how pumps work fundamentally if you are really keen to find out why.
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