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
On a fan curve, the x-axis is the air volume value in Cubic Feet per Minute (CFM). There are two y-axes – pressure on the left (“wc) and brake horsepower (BHP) on the right. We will discuss the pressure vs. volume curve and pressure vs. brake horsepower curve separately.
Static Pressure vs. Air Volume
On the Static pressure vs. Air volume curve, the intersection between the static pressure (SP) curve and the system curve is known as the “fan operating point”. The system curve (or system line) describes what static pressure is required for a particular duct system with workstations connected to move particular air volume. This is the theoretical static pressure and air volume that your fan should provide. The actual installation operating point will typically be slightly below the calculated operating point due to co called “system error” (or if an engineered inlet is used but the fan curves are plotted without the engineered inlet). It generally means fan efficiency as installed is lower, compared to ideal conditions as measured in the lab.
Pressure vs. Brake Horsepower
Once you have identified the operating point, you can draw a line straight up to the brake horsepower curve and reflect the value on the right y-axis.
How to Interpret the Fan Curve
The fan curve describes how the fan performs between two points. On the left side the air volume is zero - i.e. no air flow and the fan is providing just pressure. On the right side the fan pressure is zero. Neither of these points is useful. Where the fan is providing useful work is between these two points. On the left side of the fan peak pressure there are two values of air volume for the same pressure - the fan air volume is actually oscillating - the fan cannot be used at that area of the fan curve.
Application of Fan Curve
Now that we understand how to read the curve, let’s see practical examples of changes of the fan’s operating points. Note: in these examples, an Ecogate system is not used, that is covered in a different article.
Example A. A factory had a total air volume of 50,000 CFM for ten machines. Over the time, they eliminated 5 machines and brought down the total required air volume to 25,000 CFM. The factory also capped off the unused pipes.
Operating Point Shift
By closing or adding drops to the duct system, the system curve is changed. The duct system now has higher resistance.
As duct branches are capped, required air volume is reduced, but operating point still must be on the fan curve. Therefore the operating point is moved to the left -> lower CFM but higher static pressure. This is generally acceptable from the point of view of the fan, but you cannot go too much to the left because that would be an unstable area of the fan. In that area, the fan will vibrate – that will decrease the life of the fan or the fan could even disintegrate.
Because you just reduced air volume in branches and in the main duct you need to carefully check if the air velocity in the branches and in the main duct are above safe minimum transport air velocity for transported dust.
Example B. The initial system was the same as in example A. However, over time, they added a few machines which brought the total air volume up to 70,000 CFM (a 20,000 CFM increase). The required air volume increased, the system curve is different. Again – the operating point still must be on the fan curve. Therefore the operating point is moved to the right to the point labeled (1). As you see the pressure generated by the fan is significantly lower on the right side – and with high probability this pressure will not able to overcome all pressure loses, therefore air flow will be too low.
Example B Operating Point shift
With additional CFM and shift again with manual blast gates
The factory can install manual blast gates on a few machines that are not used very often to close some of the ducts to decrease required air volume and move on the fan curve back to the area with higher pressure labeled (2) on the chart. This approach can bring other issues: even if some or all manually operated gates are closed there is still a requirement to maintain minimum transport air velocities in all branches – this is impossible with manually operated gates and it is not allowed by NFPA 664.
Another issue with manually operated gates is that they are typically left open by operators and the system curve will stay at the point (2) where the fan is not capable to generate enough pressure to overcome all losses resulting in insufficient airflow. The airflow and pressure managed by the Ecogate control system is a more efficient and automated solution in that the minimum transport air velocity is maintained in the entire duct system and the fan is always in the proper operating point.
Additional Resources: Fan Engineering