The Fan Curve: Definition
A fan curve displays the relationship between 3 primary parameters: static pressure (“wc), air volume (CFM) and brake horsepower (BHP). The fan selected for a particular application will need to deliver a particular air volume, overcome total system losses and match correctly with a properly sized 3-phase AC motor.
Static Pressure (“w.c) If a fan cannot generate enough static pressure to overcome all pressure losses (hoods, ducting, abort gates, dirty filters and return air) the workstations that carry the most losses will not have adequate suction. Some workstations, such as wide belt sanders, require as high as 10” wc hood pressure for proper ventilation. If you add duct losses for example 4”, and a dirty filter loses 4”, the fan must be capable of providing 18” wc. This is why it’s important to select the correct fan.
Air Volume (CFM)
Each piece of equipment that is connected to the system has a certain design air volume. The design air volume should be defined by manufacture of the workstation by specifying drop air velocity and duct diameter(s). European CNC machines and workstations producing high volume of wood chips typically require high drop air velocity (5,900 FPM). Smaller machines require drop air velocity in the range of 4,500 FPM. The fan selected will need to have an air volume value capacity equal to or greater than the total air volume of all workstations connected (total design air volume).
Brake Horsepower (BHP) and Horsepower (HP)
Brake Horsepower is defined as the minimum amount of power needed to operate the fan. This value is measured or calculated by the fan manufacturer. The brake horsepower is the mechanical power necessary to operate the fan. To calculate the necessary motor electric power in horsepower we need to know motor efficiency and belt drive efficiency (if used).
For more information on how to size your fan motors, click here.
Sample Fan Curve
The fan curve usually has a specified operating point stated by the fan manufacturer. This operating point will contain all three important values discussed above. When designing a complete duct system, you need to select the fan that can overcome the total system pressure losses while delivering adequate air volume (CFM).
How to Read the Fan Curve
Now we have a general idea of what a fan curve is and information that it could provide. Let’s dive into how to read it.
Fan System Operating Point