Fan Operating Point: Method

Fan and System Curves

A fan does not deliver a fixed airflow. Its catalog curve gives the pressure it can produce at each airflow at one speed, falling from a maximum near shutoff to zero at free delivery. The ductwork takes a pressure that rises with the square of the airflow through it, Δp = k Q², a parabola through the point the designer calculated. The fan settles where the two curves cross: that is the only airflow at which the pressure the fan makes equals the pressure the system takes. If the ducts come out tighter than calculated the parabola steepens and the fan slides up its curve to less airflow; if looser, to more.

The fan laws move the curve with speed: airflow in proportion to speed, pressure with the square and power with the cube. Every point of a system parabola moves along that same parabola when the speed changes, which is why a speed change is the clean way to correct an airflow shortfall: the speed that lands the fan on the design point is simply the curve speed times the ratio of the design airflow to the airflow where the catalog curve crosses the system parabola. The tool scales the catalog curve to the operating speed, solves the crossing exactly in the straight segment where the sign of (fan pressure − system pressure) changes, and reports that required speed.

Power is airflow times pressure divided by efficiency. When the catalog gives power at each point the tool interpolates it at the operating airflow (scaled by the cube of the speed ratio) and the efficiency follows; otherwise an entered efficiency is used. The motor input divides by the motor and drive efficiency; the yearly energy and cost follow from the operating hours.

The left part of a centrifugal or axial fan curve, up the slope from the pressure peak toward shutoff, is the stall region: the flow separates on the blades, pulses and the fan is noisy and may surge. The tool flags an operating point left of the peak. Fan and system pressures must be on the same basis, total or static; catalog curves are usually fan static pressure, and the system loss to compare with it is the total loss less the velocity pressure at the fan outlet.