The Catalogue Says 1,200 m³/h — Why Is the Field Number Different? Fan Curves and System Resistance
Updated: Sep 4
The flow figure on a fan's nameplate describes "free air" operation — no ductwork, no filter, no resistance of any kind. The moment that fan is connected to a hose, a duct, a filter and a hood, it stops delivering that number. What it actually delivers is set by where the fan's own performance curve intersects the resistance curve of the system it's connected to.
Reading a Fan Curve
Flow sits on the horizontal axis, pressure the fan can generate on the vertical. At one end of the curve (bottom right) pressure is zero and flow is at its maximum — this is usually the catalogue number. At the other end (top left), flow approaches zero while the fan produces its maximum pressure. In real operation the fan sits somewhere between these two extremes, and it cannot decide where on its own — the system's resistance decides that for it.
What Builds System Resistance?
Duct friction (rising with length, diameter, internal roughness, and roughly the square of velocity), elbows and transitions, hood entry loss, and filter resistance together form the system's resistance curve. That curve also rises roughly with the square of flow, and where it crosses the fan curve is the actual operating point.
Why the Operating Point Shifts as the Filter Loads?
A clean filter offers little resistance; as dust accumulates, resistance climbs, the system curve shifts upward, and the intersection with the fan curve moves left — toward lower flow. So the airflow a system delivers on commissioning day and the airflow it delivers a few weeks later are not the same figure, and that difference isn't a fault — it's the filter loading normally. Differential pressure switches exist precisely to track that shift.
Verifying It in the Field
Fan selection should be based on the estimated system resistance, not the catalogue flow figure. At commissioning, actual flow and pressure should be measured and compared against the design target — a running fan by itself does not confirm that the intended performance has been achieved.
Conclusion
The catalogue flow rate is a starting reference, not a guarantee. Actual performance depends on the installed system's total resistance and the filter's condition at any given moment; correct fan selection accounts for both together.
Frequently Asked Questions
How much does a fan rated at 1,200 m³/h actually deliver? — Once connected to a system, it delivers whatever the fan curve shows at the calculated resistance point, which is usually below the nameplate figure. Why does flow drop as the filter gets dirty? — Because filter resistance is part of total system resistance and it shifts the operating point along the fan curve.
References:
● ACGIH — Industrial Ventilation: A Manual of Recommended Practice for Design (fan performance and system resistance chapters).
● ISO 5801 — Industrial fans — Performance testing using standardised airways.
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