System Curve
The system curve of a hydronic system is a plot of the pressure loss vs. the flowrate of the piping
system itself. This curve describes the performance of the piping network itself. By plotting this
curve onto the pump P-Q curve, we have the following:
P
pump curve
operating point
system curve
Q
Note that the characteristics of the system curve is the opposite of the pump curve, whereby
water will have a higher pressure loss in the piping system when the flowrate is higher.
The intersection of these two lines will give the operating point
of the system.
What we have done in the Application Example No. 1 and 2 is to calculate the desired design
operating point of the mini chiller with that particular piping network. In the first example, we have
found that the pump has a much higher capacity compared with the requirement (System Curve
1)
P
system curve 2
20 system curve 1
dP
pump curve
2.5 Q
calculated operating point
which does not lie on the pump curve
In order to "balance" back the pump operation, we need to increase the piping friction loss
by dP (e.g. by adjusting the globe valve) so as to obtain System Curve 2. If this is not done,
the pump will deliver a much higher flowrate compared with the design value.
Section 5 Page 77
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