A fan curve is not a specification sheet. It is a map of every duty the fan can deliver, and the only point on it that matters is the one where the fan meets your system. Reading it properly takes five minutes and saves the two most common ventilation failures: a fan that will not deliver the design flow, and a fan that delivers it while running rough and drawing more power than the motor is rated for.
The two lines you are looking at
The fan curve plots pressure against volume flow. It slopes downward: the more resistance the fan has to push against, the less air it moves. The system curve plots the resistance your ductwork imposes against flow, and it slopes upward, roughly as the square of the flow. Double the flow through a fixed duct and you need about four times the pressure.
The operating point is where the two lines cross. Not where you would like it to be, and not where the catalogue extract puts it. The fan will run where the curves intersect, every time.
Why the crossing point moves
The system curve is not fixed. A filter loads up and the curve steepens, moving the operating point up and to the left: less air, more pressure. A damper closes and the same thing happens, harder. A duct run gets extended during a fit-out that nobody told the ventilation engineer about, and the curve steepens again.
This is why you should never select a fan whose design point sits at the very edge of what it can do. Leave headroom, because the system you commissioned is not the system that will be running in three years.
Where on the curve to sit
Peak total efficiency on a backward-curved centrifugal fan generally falls somewhere between 55 and 70 per cent of free delivery. Selecting near that region gives you the lowest absorbed power for the duty and usually the lowest noise as well, because a fan working near its best efficiency point is not fighting itself.
Selecting far to the right, close to free delivery, gives a fan that is loud and drawing high power for very little pressure. Selecting far to the left puts an axial fan into its stall region, where flow separates from the blade, the fan runs rough and the curve becomes unpredictable. On a centrifugal fan the left-hand region is less dramatic but still inefficient.
Check the power curve too
Most curves show absorbed power on a second axis. On a forward-curved impeller, power rises steeply as flow increases, which means an overly clean system, or a filter that has not been fitted yet at commissioning, can overload the motor. Backward-curved blading is non-overloading: power peaks and then flattens. If the system resistance in your project is genuinely uncertain, that difference is worth more than a few points of efficiency.
Density is not a detail
Published curves are for standard air, usually 1.2 kg/m³. Hot air is thinner and the fan develops less pressure. Altitude does the same. If your extract is at 200 °C, or the site is at 1 500 m, the curve has to be corrected before the operating point means anything. Ask for the corrected curve rather than applying a rule of thumb to a printed one.
What to ask a supplier for
Ask for the measured curve for the specific impeller diameter and speed being offered, with your duty point marked on it, the absorbed power at that point, and a note of how far it sits from peak efficiency and from the stall region. A supplier who can produce that in a day has measured the fan. One who cannot is quoting from a family curve.