Relationship between pump curve and pipe network curve

Hello everyone,

I have a small understanding problem regarding the relationship between the pump characteristic curve and the pipe network characteristic curve. If I increase the head at the pump, I expect a lower flow rate in the heating circuit, which is logical. The pipe network characteristic curve increases with increasing flow rate, since it shows the pressure loss there, i.e. more flow leads to more pressure loss in the system, also logical. If I now have a flow meter in the system, it should indicate a lower flow rate when the pump head is increased, right? In fact, exactly the opposite happens, i.e. a higher head causes a larger flow rate with otherwise the same parameters.

Maybe someone can help me to resolve this mental block.

Thanks a lot in advance!

More pressure at the pump (because that determines the delivery head) results in a higher volume flow, according to the characteristic curve. If you provide more pressure, more pressure loss can also be compensated. Anything else is illogical, otherwise you could keep reducing the pump pressure more and more and more water would flow, virtually for free, because a pump at the lowest setting hardly requires any energy.

from on the go

Hello,

thank you very much for the answer. Then my misunderstanding is rather with the pump characteristic curve, for example:



I can imagine the raw network curve, more flow rate -> more pressure losses or resistance in the pipeline network, but how does that relate to the pump characteristic curve?

First of all, there is no direct connection. The pipe characteristic curve indicates how much pressure must be "supplied" for a certain volume flow (the more flow, the more pressure is logically needed); the pump characteristic curve shows how much pressure the pump can build up at maximum at which volume flow, the more liquid it pumps, the "harder" it is for the pump to build up pressure (simplified). At one point, the pipe network and the pump meet, and the whole thing then stabilizes at that value, the pump can then no longer build up pressure because of the existing volume flow (which the pipe network would need for more flow), but if less were to flow, the pump could build up more pressure so that the volume flow would increase. from unterwegs

Hello, thanks again for the reply. So I can imagine the two characteristic curves separately. For me, however, it doesn't yet make sense if I increase the pump head (-> less electricity through the pump), why the operating point of the system characteristic curve then adjusts so that more volume flow arrives in the pipe network. Logically, that of course makes sense as you wrote, but the link to the characteristic curve is missing for me. The system characteristic curve would become flatter, for example, if less pressure loss occurs, such as fewer radiators being on. But that doesn't help me understand the volume flow.

For me, it doesn't quite make sense yet when I increase the pump head (-> less power through the pump), why the operating point of the system characteristic curve then adjusts so that more volume flow arrives in the pipe network.

Increasing the pump head means increasing the pump pressure (<- more power for the pump = more pressure at first), so according to the system characteristic curve, it settles at a higher volume flow. If you adjust the pump e.g. via the power supply, you basically shift the pump characteristic curve slightly upwards parallel to the x-axis (the pump generates more pressure at the same volume flow with higher power input). The intersection with the system characteristic curve moves further to the right, so the whole thing settles at a higher volume flow. The system characteristic curve itself would not change in this case. If you close a radiator, the pressure loss should decrease because the water has to pass through bends, valves, etc.; this means that at the same pressure, the volume flow increases. You can imagine this as the system characteristic curve being stretched somewhat to the right (not shifted, since the point "no pressure-no volume flow" at the origin remains). Also in this case, the intersection moves to the right, and the system settles at a higher volume flow with no other changes. By the way, this is a problem in house construction: if too many valves close (due to individual room control and poorly adjusted heating systems), the rest of the water rushes through quite quickly with uncontrolled pumps, and the delta T (difference between supply and return flow) becomes too small, which then leads to more frequent switching cycles of the heat source of the system. But now I wonder what exactly you want to understand about the volume flow. For more detailed information, you can search the internet for physical expertise; you will surely find something.