The most common fan selection mistake: choosing by the catalog "maximum airflow" number. That number is free air delivery (zero resistance) — you will never achieve it inside a cabinet. What really determines cooling performance is the operating point on the P-Q curve: where the system resistance curve intersects the fan curve. Browse more products: AC axial fans.
Three Key Concepts
| Term | Definition | Typical Value |
|---|---|---|
| Free Air Delivery (FAD) | Maximum airflow at zero static pressure | Catalog rated value |
| Max static pressure | Maximum pressure at zero airflow | 50-150Pa for axial |
| Operating point | Intersection of system resistance curve and P-Q curve | Actual airflow |
Why Free Air Delivery Is Misleading
Example: JF12038 rated free air delivery 95 CFM
- Cabinet with filter, system static pressure ~50Pa
- Read P-Q curve: at 50Pa, airflow is only ~60 CFM
- → Actual cooling airflow is only 63% of rated
- → Selecting by 95CFM = 37% cooling shortfall
Not sure about your cabinet static pressure? Send us your cabinet type and filter configuration — we'll help you 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 (FAD)?
Free Air Delivery is the maximum airflow when the fan outlet has zero resistance, blowing directly into open space. It is the theoretical maximum — you will never achieve this in a real cabinet.
Why shouldn't I select by free air delivery?
Cabinets have filters, openings, and internal component resistance. Real operating static pressure is typically 30-80Pa. At 50Pa, an axial fan may only deliver 60-70% of its free air delivery. Selecting by FAD means you've underestimated cooling by 30-40%.
How do I calculate system resistance?
Total static pressure = filter resistance (30-100Pa) + opening resistance (20-50Pa) + internal component resistance (20-50Pa). If no data: estimate 80-100Pa for filtered cabinets, 30-50Pa for unfiltered.
How do I find the operating point on the P-Q curve?
Draw a system resistance curve (approximate straight line from the origin) on the P-Q graph. The intersection of the two lines is the actual operating point — that airflow is what you will really get.
Does JIEFENGFAN provide P-Q curves?
Yes. Every model has measured P-Q performance curves (separate for 12V / 24V / 220V). Email us or contact online support to request the PDF. Tell us your target airflow and static pressure and we'll find the right model on the curve for you.