The 3 Basic Fan Laws

Back in the Dark Ages of 1976, before we all had computers sitting on our desks and when engineers still used slide rules, I came to work at Eldridge as a very young sales engineer. At that time, I was given some advice by an older Eldridge engineer (my dad, Walter Leseman) that I am pleased to pass along:

“If you learn how to use the 3 Basic Fan Laws, you will know more about ventilation and fans than most anyone else you meet in the industry—these laws will keep you out of trouble!”

So here are the 3 Basic Fan Laws:

 

There is a little more to it than this with air density changes but these Basic Fan Laws can keep a field engineer or technician out of trouble.

For example, let’s say that a direct drive fan moves 15,000 CFM at 2.0” wg SP and is fitted with a 10 HP, 900 RPM motor. You want to double the airflow by modifying the fan to accommodate a 20 HP, 1800 RPM motor.

Will this work?

Answer: NO WAY!

Using Fan Law 1, you can double the CFM to get 30,000 CFM by doubling the fan speed—this part is correct.

However, Fan Law 3 tells you the Horsepower will increase by the cube of the speed ratio. The speed ratio is 2. So the cube of that ratio becomes 8. So now you need to use a motor that will provide 80 HP which in this case is a 100 HP / 1800 RPM motor because there is no such thing as an 80 HP, 1800 RPM motor available.

By the way, according to Fan Law 2, the Static Pressure (SP) increases as the square of the speed ratio. So in this case, the SP would increase 4 times to 8.0” wg SP.

In this scenario, your fan, ventilation system, and motor controls will probably not even accommodate such a modification that you are trying to accomplish.

By knowing these basic Fan Laws you can save yourself a lot of grief and avoid a big, costly mistake.

You can always contact Eldridge to figure out how to increase the air delivery and solve the ventilation problem the right way.