Reading a Centrifugal Compressor Performance Curve

The Anatomy of the Curve

A centrifugal compressor performance curve is the map of its aerodynamic capabilities. Just like a pump curve, it dictates exactly what the machine can and cannot do.

However, compressor curves are significantly more complex than pump curves because gas is highly compressible.

The X-Axis: Actual Inlet Flow (ACFM or $Am^3/h$)

Compressor manufacturers always plot the x-axis using Actual Volumetric Flow at the suction inlet conditions, not Standard Flow (Sm3/h).

The physical size of the impeller is fixed. It can only scoop a specific physical volume of gas per revolution. It does not know or care how many "Standard Moles" are crammed into that volume. Therefore, if the suction pressure or temperature changes, the actual volumetric flow changes, and the compressor's operating point will drastically shift on the map.

The Y-Axis: Polytropic Head

Instead of pressure, the y-axis is plotted in Polytropic Head ($H_p$) (in meters or feet). Just like a pump, the centrifugal compressor generates a fixed amount of Head (energy per unit mass) at a given speed, regardless of gas density.

The Polytropic Head is the thermodynamic work required to compress the gas, accounting for the path of compression:

$$ H_p = Z_{avg} \cdot \frac{R \cdot T_1}{MW} \cdot \frac{n}{n-1} \cdot \left[ \left(\frac{P_2}{P_1}\right)^{\frac{n-1}{n}} - 1 \right] $$

Key Boundaries on the Map

1. The Speed Lines (RPM)

Unlike pumps which often run at a fixed speed, massive compressors are usually driven by gas turbines or VFD electric motors. The performance map displays a family of horizontal curves, each representing a different rotational speed (e.g., 80%, 90%, 100%, 105% of maximum continuous speed).

Increasing the speed raises the entire curve, allowing the compressor to generate more head and flow.

2. The Surge Line (Left Boundary)

As discussed in our Surge article, this line connects the peak head points of all the speed lines. Operating to the left of this line causes aerodynamic stall, violent flow reversal, and mechanical destruction.

3. Stonewall / Choke (Right Boundary)

On the far right side of the curve, the line drops vertically downward. This is the Stonewall or Choke limit.

At this massive flow rate, the velocity of the gas entering the "eye" of the impeller reaches Mach 1 (the speed of sound). Because pressure waves cannot travel faster than the speed of sound, the fluid literally cannot flow any faster through the physical cross-sectional area of the impeller, regardless of how much downstream pressure drops. The flow is choked.

Operating in Stonewall is not immediately mechanically destructive like Surge, but it causes severe inefficiency, heavy vibration, and a complete inability to generate discharge pressure.