Air flow rates for fans are typically specified at free air conditions or zero static pressure. In reality, a fan never operates at free air so this point should not be used for selecting a fan. A fan will operate at the point where the system resistance curve intersects the fan curve. A fan should be selected so that the intersection of these curves occurs as close to the peak efficiency region as possible. The peak efficiency region varies from fan to fan but is typically to the right of the knee of the curve (as shown). As the operating point shifts to the shut off point, the fan will become less efficient resulting in increased power consumption and noise.