FAN PERFORMANCE CURVES MADE SIMPLE
Here's how you can use performance data, collected at nyb's testing facility, to generate a fan curve for your application!
In this video, we review fan performance curves to understand what data they give, and how to interpret that data.
A fan performance curve is a graph that shows all possible combinations of airflow, pressure, and power consumption of a fan operating at a given speed, in a system with a given resistance. Airflow is plotted along the x axis at the bottom of the curve, often quantified as Cubic Feet per Minute. Static pressure is plotted along the y axis on the left side of the curve, commonly quantified as inches of water gauge. Power consumption is plotted along the secondary y axis on the right side of the curve.
You can reference our previous video about fan laws for common metric units of messurement for each of these axes. (https://youtu.be/KJDHS70jEY4)
Each fan has a unique curve showing a spectrum of potential operating points, called a “fan curve”. Each potential operating point shows the amount of air flow and pressure a fan generates at that point. Changing the fan speed shifts the fan curve, either up with a speed increase, or down with a speed decrease.
Where a fan will operate on its curve depends on the system the fan is installed in. This is because a system has its own unique resistance to airflow. That resistance is friction, produced as a gas stream moves, or drags, through ducting or piping, and other equipment in the system. This resistance, quantified as static pressure, is plotted on the “system resistance curve” showing its resistance to each quantity of airflow.
So when you ask for a fan that can produce 20,000 CFM at 5 inches of static pressure, that means you're planning to install a fan in a system that will resist the fan’s 20,000 CFM with 5” of static pressure. This means that to deliver 20,000 CFM in that system, the fan has to generate 5” of static pressure to overcome the system’s resistance to its airflow.
By knowing the amount of system resistance at a particular amount of airflow, the system’s resistance to other quantities of airflow can also be determined using the fan laws, provided no other variables change in the system, such as closing a damper. The intersection of the fan curve and system curve is the point where that unique fan will operate in that unique system.
A fan curve also shows the amount of power a fan uses when running at a given operating point. This data is represented as another curve, called the brake horsepower curve. To determine power consumption at a given operating point, follow the intersection of the fan curve and system curve to the corresponding point on the brake horsepower curve, then straight across to the secondary y axis on the right. At this operating point, the fan consumes 20 brake horsepower.
So, to summarize our example, our fan is moving 20,000 cubic feet per minute, while generating 5” of static pressure, which it needs to do to overcome the system’s resistance and deliver the airflow. And to do all of that, the fan consumes 20 brake horsepower.
To put these concepts into practice, visit our website and access our fan selection software.