How to Read a Fan P-Q Performance Curve
The most common fan-selection mistake: choosing by the "maximum airflow" on the datasheet. That number is free air delivery (zero resistance) and is never reached once the fan is installed in a cabinet. What actually determines cooling is the operating point on the P-Q curve — the intersection of system resistance and the fan curve. More products: AC axial fans.
Three Key Concepts
| Term | Definition | Typical Value |
|---|---|---|
| Free Air Delivery (FAD) | Maximum airflow at zero static pressure | Datasheet rated value |
| Max Static Pressure | Maximum pressure at zero airflow | Axial: 50-150 Pa |
| Operating Point | Intersection of system resistance curve and P-Q curve | Actual airflow |
Why Free Air Delivery Misleads You
Example: JF12038 rated free air delivery = 95 CFM
Cabinet with filter, system static pressure ≈ 50 Pa
Check P-Q curve: at 50 Pa, airflow drops to about 60 CFM
→ Actual cooling airflow is only 63% of the rated number
→ Selecting by 95 CFM means you under-count cooling by 37%
Not sure about your cabinet static pressure? Send us the cabinet type and filter setup, and we will find the operating point on the P-Q curve for free, or contact us for selection support.
Frequently Asked Questions
What is free air delivery?
Free Air Delivery (FAD) is the maximum airflow when the fan outlet has zero resistance and blows directly into open space. It is the theoretical maximum the fan can reach, and it is never achieved once the fan is installed in a cabinet.
Why shouldn't I select fans by free air delivery?
Cabinets have filters, openings, and internal component resistance. The actual operating point static pressure is typically 30-80 Pa. At 50 Pa static pressure, an axial fan may deliver only 60%-70% of its free air delivery. Selecting by FAD means you under-size cooling by 30%-40%.
How do I calculate system resistance?
Total static pressure = filter resistance (30-100 Pa) + opening resistance (20-50 Pa) + internal component resistance (20-50 Pa). If you have no data, estimate 80-100 Pa for filtered cabinets and 30-50 Pa for unfiltered ones.
How do I find the operating point on the P-Q curve?
Draw the system resistance curve (an approximate straight line starting from the origin) on the P-Q curve. The intersection of the two curves is the actual operating point. The airflow at that point is what you actually get.
Does JIEFENGFAN provide P-Q curves?
Yes. Every JIEFENGFAN model comes with a measured P-Q performance curve (one for 12V, 24V, and 220V). Email or online chat to request the PDF. Send us your target airflow and static pressure, and we will find the right model on the curve for you.