Using Fan Law Equations to Size a Fan and Fan Motor

Change is inevitable. When you’re dealing with heavy-duty industrial machinery like fans, you don’t want to leave the impact of change to chance. Fortunately, there are physics that govern what happens to the fan curve when you change one component like speed. There are three fan law equations that can help you predict the effect of possible or likely change so that you can size your system to accommodate it.

The Three Fan Law Equations


The most simple and straightforward of the fan law equations represents how volume relates to speed. In this case, the volume change ratio (V1/V2) is equal to the speed change ratio (n1/n2). This means that as speed increases or decreases, the volume will increase or decrease proportionally.


The second of the fan law equations represents how pressure relates to speed. Here, we find that the pressure change ratio (P2/P1) is equal to the square of the speed change ratio (n1/n2)2. This means that a small change in speed creates a proportionally larger change in pressure.


Finally, the third fan law equation dictates how horsepower changes relative to how speed changes. The horsepower change ratio (HP1/HP2) is equal to the cube of the speed change ratio (n1/n2)3 so horsepower changes a lot relative to just a little speed change. That’s why it’s so important to think about future horsepower needs when you’re sizing your fan motor.

Planning Ahead Based on Fan Law Equations

We know the future isn’t set in stone. But that doesn’t mean you can’t be ready for it. The simple fan law equations above make it possible for you to design your fan system to meet today’s needs while allowing for future needs if they arise. Based on the calculations, increasing speed can allow you to gain volume or pressure when you need it, but you need to be sure your motor has enough horsepower to handle the increase. The third equation will tell you how much horsepower you should have available.

Hear it from an Application Engineer

Chet White, Senior Application Engineer and Sales Manager, explains the fan law equations and provides a practical example in this 3-minute video.

To determine the right specifications for your centrifugal fan application, reach out and connect with one of our application engineers to discuss the details of your project.

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