During the mid-1990s, Ron Michael, regional manager, Loren Cook Co., was conducting a lunch and learn at a large engineering firm on the East Coast. As talk turned to fan selection and use of Cook’s software program, Michael was interrupted by the head of the firm’s mechanical department.
“Show us how to read a fan curve,” the head of the mechanical department requested.
Being new to the field, Michael assumed every engineer — especially ones at such a large firm — knew how to read a fan curve, the graphical representation of a fan’s performance, specifically noting the relationship between the amount of airflow and the amount of static pressure produced.
“My first thought was he actually wants to see if I know how to read a fan curve,” Michael recalled. “So, I began my presentation in detail, knowing I could not screw this up.”
A few minutes into Michael’s explanation, the head of the mechanical department interrupted again.
“We don’t have that much time, based on our busy schedules, so we use the rule-of-thumb method.”
“The rule-of-thumb method?” Michael asked.
This involves placing the palm of one’s left hand on a fan-curve graph so that the index finger is aligned with the left vertical (y-) axis, along which static pressure is plotted, and the thumb is aligned with the horizontal (x-) axis, along which air-volume flow rate is plotted (more on this later), the head of the mechanical department explained. Where a vertical line extending upward from the tip of the thumb intersects the flow-versus-pressure curve is the fan’s operating point (again, more on this later) (see Figure 1).
“I didn’t quite know what to say but quickly determined that not many people in the room actually knew how to read a fan curve,” Michael reminisced.
Those people were hardly alone and would not be lacking company today. While, at first glance, a fan curve can appear complicated, it is fairly easy to interpret once one gains an understanding of the conventions and terminology used. Reading a fan curve (correctly) is a skill worth possessing, as it allows those who select and specify fans to understand the performance characteristics and capabilities of different equipment so they may determine the fan that best suits the requirements of an application. For them, this article will help to straighten out fan curves.
While there is no standard method for graphically representing fan-performance data, this article will cover what one is most likely to see.