VARAIBLE FREQUENCY DRIVER CONTOLLING PUMPS

[etchyHA]

hi guys,,
the vfd controlling pump velocity acts when the load decreases the pressure drop increases giving signals to vfd to reduce frequency hence velocity so the flow rate decreases my question is what happens to the head of pump? the pump head suppose to over come the all system friction so when the flow rate is decreases the head is decreases also or what ?

 

[quark]

This issue was beaten to death many times and a visit to past threads in this as well as pump forum gives you good insight. Further, I suggest you to use line breaks so that the question becomes simple and we can understand it easily.

Coming to your question, when the speed decreases the flowrate decreases and does the head, but not in same proportions. Remember the affinity laws

Q1/Q2 = N1/N2 (Q is flowrate and N is speed)
H1/H2 = (N1/N2)² (H is head)
P1/P2 = (N1/N2)³ (P is power)

For a closed loop (without the pressure break point) this doesn't create a problem.

In a closed loop, the system resistance is predominantly dynamic. So, Darcy's equation indicates frictional loss is proportional to flowrate. The affinity laws indicate the head of pump in proportional to flowrate (by solving the first two equations) so they both cancel each other.

However, in open systems this is the most critical aspect.