The Fan Curve: Definition
A fan curve is a graph that displays the relationship between three primary parameters: static pressure, air volume (CFM), and brake horsepower (BHP). The fan selected for a particular application needs to deliver a specific air volume, overcome total system losses, and properly match with a correctly sized 3-phase AC motor.
Static Pressure (“w.c)
If a fan is unable to generate sufficient static pressure to counter all pressure losses (including those from hoods, ducting, abort gates, dirty filters, and return air), workstations with the highest losses may lack the required suction. Certain workstations, such as wide belt sanders, may need as much as 10 inches (w.c) hood pressure for optimal ventilation. Considering additional duct losses (for example, 4 inches w.c) and dirty filter losses (another 4 inches w.c), the fan must be capable of providing 18 inches w.c. This demonstrates the importance of selecting the appropriate 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 the minimum amount of power needed to operate the fan, as measured or calculated by the fan's manufacturer. It represents the mechanical power necessary to operate the fan. To calculate the necessary electric motor power in horsepower, it's essential to know the motor efficiency and the belt drive efficiency, if a belt drive is 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
In the static pressure vs. air volume curve, the point of intersection between the static pressure curve and the system curve is termed the "fan operating point." The system curve outlines the required static pressure to move a specific air volume for a particular duct system with connected workstations. This represents the theoretical static pressure and air volume that your fan should deliver. However, the actual installation operating point will typically be slightly lower than the calculated operating point due to a "system error" (or if an engineered inlet is used, but the fan curves are plotted without considering it). This generally indicates that the fan's efficiency when installed is lower compared to the ideal conditions measured in the lab.
Pressure vs. Brake Horsepower
After determining the operating point, you can draw a vertical line from it to intersect the brake horsepower curve and note the corresponding 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've explained how to interpret the curve, let's delve into practical examples of changes in the fan's operating points. Note: these examples do not involve an Ecogate system; that is covered in a separate article.
Initially, a factory required a total air volume of 50,000 CFM for ten machines. Over time, they decommissioned five machines, reducing the total required air volume to 25,000 CFM. The factory also sealed off the unused pipes.
Operating Point Shift
By modifying the duct system (either closing or adding drops), the system curve changes, leading to increased system resistance.
When duct branches are sealed off, the required air volume decreases, but the operating point must still lie on the fan curve. Consequently, the operating point shifts to the left, indicating lower CFM but higher static pressure. While this is generally acceptable from the fan's perspective, it's crucial not to move too far left into the fan's unstable area. In this region, the fan may vibrate, which could reduce its lifespan or even lead to disintegration.
Since the air volume in the branches and main duct has been reduced, it's essential to verify if the air velocity in these areas remains above the safe minimum transport air velocity for the transported dust.
In this case, the initial system was the same as in Example A. However, over time, a few machines were added, which increased the total air volume to 70,000 CFM (an increase of 20,000 CFM). This increased air volume resulted in a different system curve. As before, the operating point must still be on the fan curve. Therefore, the operating point shifts to the right, to the point labeled (1). Here, you can see that the pressure generated by the fan is significantly lower, and it's likely that this pressure won't overcome all pressure losses, leading to insufficient airflow.
Example B Operating Point shift
With additional CFM and shift again with manual blast gates
The factory could install manual blast gates on a few machines that aren't frequently used. By closing some of the ducts, they can decrease the required air volume and shift back along the fan curve to the area of higher pressure, labeled (2) on the chart. However, this approach can introduce other problems: even if some or all manually operated gates are closed, there's still a need to maintain minimum transport air velocities in all branches. This is impossible with manually operated gates and is not allowed by NFPA 664.
Another issue with manually operated gates is that they're often left open by operators, causing the system curve to remain at point (2), where the fan can't generate enough pressure to overcome all losses, resulting in insufficient airflow. The airflow and pressure managed by the Ecogate control system present a more efficient and automated solution, as they maintain the minimum transport air velocity in the entire duct system, ensuring the fan always operates at the correct point.
Additional Resources: Fan Engineering