Fan Curve 101

Understanding Fan Operations

Fans do more than just spin; they play a crucial role in effective dust, mist, and fume extraction from all your workstations, a process defined as the total system design air volume. To achieve this, the fan must generate enough pressure to counteract all pressure losses within the dust, mist, or fume collection system.

Defining Total System Design Air Volume

The Total System Design Air Volume is the aggregate air volume of all workstations connected to the dust, fume, or mist collection system, measured in Cubic Feet per Minute (CFM). The workstation manufacturer typically specifies the required drop air velocity for proper dust (mist, fume) extraction and the diameter of the drop. The Air Volume (CFM) is then calculated by multiplying the Drop Area in square feet by the Air Velocity (in Feet per Minute, FPM).

Deciphering Total System Pressure Losses

Total system pressure losses are determined by adding all losses, including those from workstation hoods, duct system, abort gate, dust collector filter, and return air (if used). Typical losses range from 4” to 11” wc for hoods, 4” to 14” wc for ducts, 0.5” to 2” wc for safety abort gates, 1.5” to 5” wc for dust collector filters, and 0.5” to 1.5” for return air.

Understanding the Fan Curve

The fan curve illustrates the relationship between Air Volume and Pressure, the two most crucial fan values. Additional data usually include fan power (in BHP) and fan efficiency. A fan can only operate within specific combinations of air volume and pressure, denoted by the fan curve.

Two areas are particularly impractical for fan operation: the unstable fan area and the area beneath total system pressure losses.

The Unstable Fan Area

Operating the fan in this area, characterized by low air volume and high pressure, results in mechanical instability, noise, and vibration, potentially leading to rapid wear or mechanical disintegration of the fan, and risking violation of the manufacturer’s warranty.

The Area Under Total Pressure Losses

As the fan must overcome the total pressure losses of a dust, mist, or fume collecting system for effective airflow, a fan generating pressure lower than the total losses is unsuitable for that particular system.

Figure 1: The fan should not operate in the unstable area on the left (A), or at the bottom of the chart where pressure is under total pressure losses for a given system.

What is the System Curve?

The System Curve represents the air volume that a particular duct system can pull at a certain pressure, or the required pressure to pull a certain air volume. The fan cannot operate in the pattern-filled area, while the remaining part of the fan curve showcases practical combinations of air volume and fan pressure that can be used for a specific fan.

Fan Curve for Fan as Installed (System Effect)

The fan curve, generated by the manufacturer’s software or measured in a lab, provides an idealized view of fan performance. But in real-world installations, where the fan is connected to a duct system, its efficiency will likely be lower. This is mainly due to non-ideal air distribution at the fan inlet, which can be caused by duct elbows being too close to the fan inlet and outlet. This effect is more pronounced when elbows are located closer than three duct diameters from the fan inlet and outlet.

This leads to the "as installed" fan curve, which represents the actual operational behavior of the fan when installed, being below the lab-measured fan curve, indicating decreased air volume and pressure. Refer to Figure 2 for a graphical representation of this concept.

Figure 2: The red fan curve "as installed" represents combinations of fan pressure and air volume for the given fan that can be used in actual installations.

Fan Inlet Box (Engineered Inlet)

A Fan Inlet Box is a space-saving option that allows for direct connection of the vertical duct to the fan inlet, albeit at the cost of decreased fan efficiency. If the fan comes with a Fan Inlet Box, the fan curve, which is below the standard fan curve, incorporates the system effect of the Fan Inlet Box. While the inlet box saves space, it increases pressure losses by 1” to 2” w.c., which should be included in your calculations.

Fan Curve: Key Takeaways

The fan curve essentially determines permissible combinations of fan pressure and fan air volume for a given fan, usually indicating that lower fan pressure corresponds to higher air volume. This suggests that fan motor power is divided between fan pressure and air volume. For instance, if duct losses can be minimized through better design, the same fan and motor can handle a higher air volume (i.e., the same fan can cater to more workstations). Better duct design implies hoods with lower losses, a shorter duct system, and a minimal number of elbows.

To learn more about the fan curve and changes in the fan operating point, refer to this article.

Author: Ales Litomisky