Fans are used across electronic cooling, industrial equipment, ventilation, and thermal-management systems to move air and help maintain the required operating conditions. However, selecting a fan based only on its maximum airflow rating does not show how it may perform when the system introduces resistance from filters, grilles, ducts, heat exchangers, enclosures, or other system components. A fan curve provides a graphical representation of fan performance across a range of airflow and pressure conditions under specified test conditions, such as fan speed and air density. By interpreting the fan performance curve together with the system curve, engineers can determine the expected operating point and evaluate whether a fan is capable of delivering the required airflow at the pressure associated with the system operating point.
This post explains fan curves and P-Q curves, how to read and interpret fan performance data, how system resistance and fan speed affect the operating point, and how these curves can be used to select axial and centrifugal fans for specific application requirements.
What Is a Fan Curve?
A fan curve is a graphical representation of fan performance across a range of operating conditions. It primarily shows the relationship between airflow and fan pressure at a specified speed and under defined test conditions.
Rather than relying on a single airflow rating, engineers use a fan performance curve to evaluate fan capability across its operating range. Depending on the manufacturer’s data, the chart may also include speed, power, efficiency, and other performance information.
What Is a P-Q Curve?
A P-Q curve shows the relationship between fan pressure (P) and volumetric airflow rate (Q). It shows the pressure a fan can develop at different airflow rates under specified operating conditions.
The curve helps engineers evaluate whether a fan can provide the required airflow and pressure for an application. A P-Q curve specifically represents the pressure-flow relationship, while a broader fan performance chart may also include efficiency, power, speed, and other performance data.
Understanding Fan Performance Curves
A fan performance curve may present several operating parameters beyond the basic pressure-airflow relationship. Key parameters may include:
- Airflow: The volumetric flow rate of air, commonly expressed in CFM, m³/h, or m³/s.
- Fan Pressure: The pressure reported for the fan performance data, such as static pressure or total pressure, as specified by the manufacturer.
- Fan Speed: The rotational speed, usually expressed in revolutions per minute (RPM), at which the performance data applies.
- Power: The power required for fan operation at different airflow conditions, with the specific power quantity and units identified by the manufacturer.
- Efficiency: The relationship between fan air power and the specified input power, based on the efficiency definition and calculation method used for the performance data.
Fan performance data should always be read using the units and operating conditions specified by the manufacturer, including fan speed, air density, and applicable test or reference conditions.
How to Read a Fan Curve?
Learning how to read fan curves helps engineers determine whether a fan can meet the airflow and pressure requirements of an application. Here are the key steps:
- Check the curve conditions: Confirm the fan model, size, speed, configuration, and stated test or reference conditions.
- Identify the axes: The horizontal (X) axis typically represents volumetric airflow, while the vertical (Y) axis represents fan pressure. Check the units and whether static or total pressure is shown.
- Locate the required airflow: Find the application’s required airflow on the X-axis.
- Locate the required pressure: Determine the pressure associated with the required airflow based on the system requirements. Together, these values define the required duty point.
- Compare the duty point with the fan curve: Determine whether the fan curve reaches the required duty point under the stated conditions. The actual operating point should also be evaluated against the system curve.
- Review supporting performance data: Where provided, check the corresponding RPM, power, efficiency, and manufacturer-recommended operating range.
Following these steps provides a clearer picture of fan performance than selecting a fan based only on maximum airflow or pressure.
Fan Curve vs. System Curve
A fan curve and a system curve represent different aspects of fan-system performance. Understanding both helps determine how a fan is expected to operate when connected to a system:
- Fan Curve: Shows the pressure a fan can develop at different airflow rates for specified operating conditions, such as fan speed and air density.
- System Curve: Shows the pressure required to overcome system resistance at different airflow rates. Resistance may come from ducts, filters, grilles, heat exchangers, dampers, and other components.
- Operating Point: The intersection of the fan curve and system curve represents the expected operating point, where the pressure developed by the fan matches the pressure required by the system at the corresponding airflow.
How Fan Speed/RPM Affects the Fan Curve
Fan speed directly affects the airflow, pressure, and power characteristics of a fan. For the same fan and under conditions where the fan affinity laws apply, increasing RPM increases the corresponding airflow and pressure, while reducing RPM decreases them.
The key relationships between RPM, airflow, pressure, and power are:
- Airflow changes in proportion to speed: A 10% increase in RPM produces approximately 10% more airflow, assuming the affinity laws apply.
- Pressure changes with the square of speed: A 10% increase in RPM produces approximately 21% more pressure, assuming the affinity laws apply.
- Power changes with the cube of speed: A 10% increase in RPM requires approximately 33% more power, assuming the affinity laws apply.
As RPM changes, the fan therefore operates on a different performance curve. Engineers looking at how to set fan curves for different speeds can use manufacturer-provided curves or estimate the new performance using the fan affinity laws when the required similarity and operating conditions are satisfied.
How to Select a Fan Using a Fan Curve
Selecting a fan requires matching its performance to the airflow, pressure, and operating requirements of the application. A fan curve helps engineers compare these requirements with the fan’s available performance. Here are the key steps:
- Determine the required airflow and pressure: Establish the airflow, the application needs, and the system pressure drop the fan must overcome at that airflow.
- Locate the duty point: Find the required airflow on the X-axis and pressure on the Y-axis to identify the target duty point.
- Find a suitable fan curve: Compare available fan curves and identify a fan that can provide the required airflow at the required pressure while operating appropriately with the system curve.
- Check the operating region: Confirm that the duty point falls within the manufacturer’s recommended operating range and avoids unstable or unsuitable regions where applicable.
- Evaluate efficiency: Check the efficiency at or near the expected operating point.
- Check power and speed: Review the required power and corresponding RPM at the expected operating point to confirm they are suitable for the application.
- Verify the curve conditions: Confirm that the performance data apply to the selected fan configuration, speed, air density, and stated test or reference conditions
After identifying a suitable fan from the performance curve, verify its physical, electrical, environmental, and installation requirements before final selection.
Axial vs. Centrifugal Fan Performance Curves
Axial and centrifugal fans move air differently, resulting in different pressure-airflow characteristics. However, actual curve shape and performance depend on fan design, size, speed, and configuration. Here are the key characteristics to consider:
Axial Fan Performance Curves
An axial fan performance curve represents the performance of a fan in which airflow is primarily parallel to the fan's rotational axis. Key characteristics include:
- Airflow and Pressure: Axial fans are commonly used for applications requiring relatively high airflow, with the achievable pressure depending on the specific fan design and operating conditions.
- Curve Shape: The P-Q relationship varies by axial fan design, and some axial fan designs may exhibit operating regions associated with aerodynamic stall or unstable flow.
- Design Variation: Curve shape and pressure capability can differ among axial fan designs, such as propeller, tubeaxial, and vaneaxial fans.
Centrifugal Fan Performance Curves
A centrifugal fan performance curve represents the performance of a fan where air enters generally along the impeller axis and is discharged radially from the impeller. Key characteristics include:
- Airflow and Pressure: Centrifugal fans are commonly used in applications where higher pressure capability may be required to overcome system resistance, depending on the specific fan design.
- Curve Characteristics: Pressure, efficiency, and power characteristics vary with fan design, operating speed, and impeller geometry.
- Impeller Design: Curve shape and performance characteristics can differ among forward-curved, backward-curved or backward-inclined, and radial impeller designs.
Common Mistakes When Reading Fan Curves
Even when a fan curve provides detailed performance data, incorrect interpretation can lead to the fan operating at a different point than the application requires. Common mistakes to avoid include:
- Selecting by Maximum Airflow: Maximum or free-air airflow represents performance under conditions with little or no external system resistance and does not indicate the airflow a fan will deliver at the application's operating pressure. Fan performance should be evaluated at the required airflow and pressure.
- Confusing Pressure Values: Static pressure and total pressure are different quantities. Always verify which pressure value the curve represents before comparing fan performance with system requirements.
- Ignoring Curve Conditions: Performance data may apply to a specific fan speed, air density, configuration, and stated test or reference conditions. Differences in these conditions can affect the applicable fan performance.
- Overlooking the System Curve: Evaluating the fan curve without considering system resistance can result in an incorrect estimate of the actual operating point.
- Ignoring Efficiency and Power: A fan may satisfy the required airflow and pressure, but its efficiency and power demand at that operating point should also be evaluated.
- Applying Affinity Laws Incorrectly: Fan affinity laws should be used only when the required similarity conditions are reasonably satisfied and should not replace manufacturer performance data when verified curves are available.
Turn Fan Curve Data into the Right Fan Selection with Sofasco
A fan curve is most useful when it is applied to the actual airflow and pressure requirements of the system. By comparing P-Q performance with system resistance, efficiency, power, and operating conditions, engineers can evaluate how a fan is expected to perform at the required duty point.
Need help evaluating fan performance for your application? Contact Sofasco to discuss your airflow, pressure, and operating requirements and evaluate a suitable fan solution for your application.
Frequently Asked Questions
Q1. Why can actual fan performance differ from the published fan curve?
Published fan curves are based on specified test conditions. Installation effects such as inlet restrictions, obstructions, turbulence, and differences in air density can cause actual fan performance to differ from the published data.
Q2. What does a steep or flat P-Q curve indicate?
A steep P-Q curve indicates a larger change in pressure for a given change in airflow, while a flatter curve indicates a smaller pressure change.
Q3. Do temperature and altitude affect fan curve performance?
Yes. Temperature and altitude can change air density, which can affect fan pressure and power requirements. Density corrections or manufacturer data may be needed when conditions differ from those specified for the curve.
Q4. What is the operating point on a fan curve?
The operating point is where the fan performance curve intersects the system resistance curve. At this point, the pressure produced by the fan matches the pressure resistance of the system, establishing the airflow the fan will actually deliver in that system.
Q5. What is the difference between a fan curve and a system curve?
A fan curve represents the relationship between airflow and the pressure produced by a fan under defined operating and test conditions. A system curve represents the pressure resistance of the airflow system at different flow rates. Their intersection determines the system operating point.
Q6. What is free-air airflow on a fan curve?
Free-air or free-delivery airflow refers to fan operation at zero fan static pressure. It represents an endpoint of a static-pressure performance curve and should not be assumed to equal the airflow the fan will deliver after it is installed in a system with airflow resistance.
