Like pump curve, this is another very important curve for the system designers.

**What is a System Resistance Curve?**

- This is a graph plotted with
**flow rate along X axis**and the total**resistance to the flow along Y axis**for fluid handling piping system as well as HVAC fan system.

- For the fluid handling piping system it is often termed as pump system head curve.

- For the HVAC fan system it is often termed as fan system curve.

- Each system has unique resistance or performance curve which is independent of the pump or fan.

**Total system resistance = Frictional resistance + Resistance due to static head (height difference) + Resistance due to pressure head (pressure difference) + Resistance due to velocity (typically neglected)…………………Eq.1**

**Plotting System Resistance for Closed Hydraulic System**

**Example Problem: **Plot the system resistance curve for the water system which produces **30 ft.** (water column) of frictional resistance head for a flow rate of **50 GPM.**

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**Solution:**

As a special case, for closed system, the effect of **the static head, pressure head and velocity** is considered as **ZERO. **The reason is obvious as the system is closed so no start or end point

**Only factor contributing to the system resistance here is the frictional resistance head. **

*Another rule of thumb:** The frictional resistance is proportional to the square of the flow rate.*

In the above example it is given that:

Flow rate = 50 GPM

**Total system resistance =**Frictional resistance **=**30 ft (water column)

Now let’s create four more data points from the above given data and using the rule of thumb:

Flow rate 1=50*1.2=60 GPM

So, total system resistance 1 = 30*1.2*1.2=43.2 ft.

Flow rate 2=50*0.8=40 GPM

So, total system resistance 2 = 30*0.8*0.8=19.20 ft.

Flow rate 3=50*0.6=30 GPM

So, total system resistance 1 = 30*0.6*0.6=10.8 ft.

Flow rate 1=50*0.4=20 GPM

So, total system resistance 1 = 30*0.4*0.4=4.8 ft.

Now, we have to plot the system resistance curve for the above five points (1 given + 4 derived) by putting the flow rate along the X axis and the total system resistance along the Y axis. The final system performance curve (pump system head curve to be precise) will look like below:

I the next article I will discuss how to see how to do the system curve calculation for an open system.