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Suppose you have just installed a new exhaust fan and the airflow at the outlet is much lower than the catalog value. Before blaming the fan, check how you read its performance curve. A fan curve is a graph that shows the relationship between airflow, static pressure, brake horsepower, and efficiency for a fan running at a constant speed. Once you can translate that graph into operating data, you can select the right fan, specify the right motor, and avoid a system that underperforms.
What Is a Fan Curve?
At the most basic level, a fan curve has a horizontal axis for airflow, usually cubic feet per minute (CFM) or cubic meters per hour (m³/h), and a vertical axis for static pressure, usually inches water gauge or pascals. The fan curve is drawn at a fixed fan speed. It is not a single line but a family of lines that describe different fan quantities.
Static Pressure Curve
The static pressure curve is the main line on a fan curve. At zero airflow, also called shut-off, the fan produces its maximum static pressure. As flow increases, available pressure drops. A typical curve falls from high pressure to lower pressure across the chart. When you need to overcome duct losses, you select a fan whose pressure curve remains above the system requirement at the design flow.
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Brake Horsepower Curve
The brake horsepower (BHP) curve tells you the shaft power required to move the fan at the selected speed. In most centrifugal fans, BHP increases as airflow increases, though some blade shapes produce a peak at midrange. Use this curve to size the motor; it gives you the actual mechanical load at each flow condition.
Efficiency Curve
The efficiency curve shows how well the fan converts input power into useful airflow and pressure. It starts at zero at shut-off, rises to a peak, and then falls as the fan moves into deeper overload. The best efficiency point (BEP) is the flow rate where the curve is highest. Operating near the BEP gives the lowest noise and the lowest energy cost per cubic meter of air moved.
A typical constant-speed fan curve with static pressure, BHP, and efficiency traces.
In the example above, the pressure falls steadily, the power rises, and the efficiency has a clear peak around the middle of the flow range. This is the pattern you will see on many forward-curved centrifugal fans.
How to Read a Fan Curve in Five Steps
The reading sequence is simple once you know which curves apply. Use the following steps when you have a fan curve at a known speed and need to find the operating condition.
- Find the design airflow on the horizontal axis.
- Draw a vertical line from that airflow upward until it crosses the static pressure curve.
- Read horizontally to the left from this intersection to find the static pressure the fan can produce.
- From the same airflow value on the horizontal axis, trace up to the BHP curve and then across to read the required shaft power.
- Check the efficiency curve at the same point. If the airflow is far to the left or right of the best efficiency point, choose a different fan speed or a different fan model.
The table below demonstrates how the readings look at several flow rates for a typical small fan.
| Airflow (CFM) | Static Pressure (in. w.g.) | Brake Horsepower (BHP) | Efficiency (%) |
|---|---|---|---|
| 0 | 1.5 | 0.25 | 0 |
| 1000 | 1.4 | 0.42 | 45 |
| 2000 | 1.1 | 0.58 | 62 |
| 3000 | 0.7 | 0.71 | 55 |
| 4000 | 0.2 | 0.82 | 35 |
Match the Fan Curve with the System Curve
A fan curve by itself does not tell you the operating point. The fan always works against the resistance of ducts, filters, dampers, and the process itself. The system curve expresses that resistance: pressure drop increases with the square of airflow. When you plot the system curve on the same axes as the fan curve, the intersection is the actual operating point.
The operating point is where the fan static pressure curve intersects the system curve.
If the fan curve and the system curve do not intersect near the planned flow, the actual airflow will be different from the catalog point. For example, a system curve with less resistance than expected shifts the operating point to the right, increasing flow but lowering static pressure. This is why experienced engineers always compare the two curves side by side.
Fan Laws and Variable Speed
Variable speed is the easiest way to shift a fan curve. The fan laws describe how speed changes affect performance: airflow changes directly with the speed ratio, static pressure changes with the square of the speed ratio, and power changes with the cube of the speed ratio. Reducing speed by 20% cuts power to roughly half because 0.8³ equals 0.512. This is why a variable-speed motor can save significant energy compared with a fixed-speed fan controlled by a damper.
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In practice, a variable-speed fan curve is a series of individual curves drawn for each speed. The system curve stays the same unless the ductwork changes. The new operating point is found by reading the curve at the selected speed. For blowers and air-cooling units that require speed adjustment, a motor that supports frequency conversion makes the control scheme easier to implement. Additional details on blower motor behavior are covered in our blower motor application notes.
Motor Selection and the Fan Curve
The fan curve gives you the BHP at the design point. A motor must be able to deliver at least that output continuously. Use the motor service factor or a safety margin of 10–15% to cover air density changes, belt wear, and filter loading. Read the motor nameplate carefully: the rated power is the mechanical output, not the electrical input. For single-phase motors, starting torque and running torque also affect whether the fan reaches the curve point.
At part-load conditions, the fan may operate at a lower point where power is less. A fixed-speed motor sized for the full-load point will still run, but it may be less efficient when a variable-speed drive is not used. Selecting an efficient motor close to the operating point is worth the time because it reduces energy waste and prolongs bearing and insulation life.
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For noise-sensitive environments, the motor can be as important as the fan blade. A quiet-running motor with well-balanced bearings keeps the complete fan assembly comfortable and usually provides a smoother load on the fan curve. For air cooler applications, our energy-efficient air cooler motor is designed for continuous duty at moderate static pressure and low vibration.
Common Mistakes When Reading a Fan Curve
Reading a fan curve is easy to get wrong in practice. The most frequent issues we see in the field are:
- Using the fan curve without the system curve, then wondering why the actual flow is different.
- Reading pressure or power at the wrong speed, especially after a belt or pulley change.
- Specifying a motor that only covers the design point without considering cold starts or dirty filters.
- Oversizing the motor to solve a flow problem, which increases the energy bill and can overload the fan shaft.
- Forgetting that air density and temperature affect the curve. A fan rated at sea level will produce less pressure at high altitude.
Fan Curve FAQ
What is the best operating point on a fan curve?
The best operating point is near the peak of the efficiency curve. Some manufacturers mark this as the recommended point; others leave it to the system designer. If the design flow is far from the best efficiency point, consider a different fan speed or a different impeller size.
Why does my fan run to the right of the curve?
A point to the right means actual airflow is higher and static pressure is lower than expected, usually because system resistance is lower than calculated. To close the gap, increase duct friction, close a damper, or adjust the speed.
How do I use fan curves to select a motor?
Read the BHP at the full-load airflow. Multiply by a safety factor of 1.1 to 1.2 and select the next standard motor size. Check that the motor’s breakdown torque is sufficient, especially if the fan curve has a power peak near the operating point.
What are the fan laws used for?
Fan laws predict how the fan curve shifts when speed or size changes. They are the fastest way to calculate new performance after changing the motor speed, and they help determine whether an existing motor can drive a changed fan load.
Once you have practiced on a few graphs, reading a fan curve becomes a fast mental exercise. Start with the design airflow, use the static pressure curve to check available pressure, confirm the BHP for the motor, and always compare the fan curve to the system curve. This simple sequence reduces installation surprises and helps you specify components that actually fit the duty. If you are working on a new motor selection or troubleshooting a noisy, weak, or overheating fan, a correct reading of the fan curve will point you in the right direction. For application-specific questions, contact our engineering team with your fan data and operating conditions.
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