This video reviews three major principles of fan performance called Fan Laws. These three Fan Laws explain how a fan’s airflow, pressure and power consumption change in response to a change in fan operating speed within certain fixed systems. A fixed system means no variables in the system are changing that would skew the calculations discussed in this video, such as closing a damper or a blast gate. The formulas discussed in this video should be used as general approximations with an understanding that certain equipment, like dust collectors, do not follow fan laws.

Let’s start by reviewing some key terms:

• Revolutions per minute (RPM) quantifies the speed a fan’s wheel rotates.
• Cubic feet per minute (CFM) is a unit of measurement for the volume of airflow a fan moves as the wheel rotates. Volume is also commonly measured in cubic meters per hour in metric units.
• Brake horsepower (BHP) is a measure of power consumption. Power consumption is commonly measured in horsepower or kilowatts.
• Static pressure (SP) is commonly measured in inches of water gauge. Pressure is also commonly measured in Pascals when using the metric system.
  • In the context of HVAC and industrial air movement, SP is the resistance to airflow. Because air has mass, it creates friction as a fan moves it through ducting or piping. That friction is resistance that a fan has to overcome to move air through a system. To overcome that resistance, a fan has to generate enough pressure to match the system’s SP.

With these concepts in mind, the first three fan laws can be defined.

The first fan law describes how airflow changes in response to a change in speed, and the change is a 1:1 ratio. This can be conceptualized by thinking of the blades on a wheel as a set of equally sized shovels, each moving an equal amount of matter with each shovel full or rotation. By changing the speed, the frequency with which each shovel moves an equal amount of matter is changed. In other words, it changes at a 1:1 ratio, which is what the first fan law shows algebraically. The change in speed, or new speed divided by old speed, is equal to the change in flow, or new flow divided by old flow.

As a numeric example, let’s say there is a fan that’s running at 1,000 RPM, moving 10,000 CFM while generating 10 inches of water gauge in SP, at 100 BHP. In that example, the speed will double, from 1,000 RPM to 2,000 RPM. So, the equation is the new speed, 2,000 RPM, divided by the previous speed, 1,000 RPM, equals the new CFM, X, divided by the old CFM, 10,000, which solves for a new flow of 20,000 CFM.

The second fan law describes how pressure changes in response to a change in speed. The equation is similar to the first fan law, but here the pressure changes as a function of the change in speed squared, meaning pressure is much more sensitive to a change in speed than flow. Mathematically, that’s the quantity of the change in speed squared equals the change in pressure.

Using the same operating point as the first example, double the speed from 1,000 RPM to 2,000 RPM to see how the 10 inch SP changes. The equation is the quantity of 2,000 RPM over 1,000 RPM, squared, equals the new pressure, X, divided by the old pressure, 10, which solves for a new SP of 40 inches of water gauge.

The third fan law describes how BHP changes in response to a change in speed. Another similar setup, but here the horsepower changes as a function of the change in speed cubed. This exponential impact on power consumption is useful in determining how much headroom a fan assembly has for speed increase, before exceeding the motor’s rated capability.

In this example, the equation is the quantity of 2,000 RPM over 1,000 RPM cubed, which equals the new BHP, X, divided by the old BHP, 100, which solves for a new power consumption of 800 BHP.

To summarize these three fan laws, flow changes proportionately to speed. SP changes as a function of the change in speed squared and BHP changes as a function of the change in speed cubed. There are other fan laws, but these three fundamentals can help quickly estimate the performance effects of changing speed, either during fan selection, or making changes in the field.

About the New York Blower Company

The New York Blower Company carries one of the most complete product lines in the air- movement industry. From axial fans to centrifugal fans, rebuild or replacement jobs, they are constantly adding to their product portfolio and increasing their capabilities. Their extensive operations include over 490,000 ft² of manufacturing space in the U.S., and over 200,000 ft² internationally. They also own the largest indoor AMCA accredited laboratory in North America.