What are the 3 Fan Laws?

The 3 Fan Laws help us understand how different variables in an air-side system affect one another. While the math behind them may seem complex, you don’t need to know them to practically apply these 3 laws.

The basic takeaway
Fan Law #1 shows us that if the fan speed changes, the airflow volume will change at the same rate. Fan Law #2 shows us that if the fan speed changes, the static pressure will change at the square of the change of the fan speed. Fan Law #3 shows us that if the fan speed changes, the power will increase at the cube (to the third power) of the change in fan speed.

Minor changes to variables in your HVAC system can cause bigger changes to other variables in your system, which is important for maintaining the system with normal operating parameters based on its design.

Want to go even deeper? We can break everything down for you so it makes more sense.

CFM2 = CFM1 x (RPM2 / RPM1)

Okay, but what does that mean?

CFM2 – The new airflow volume in “cubic feet per minute.” CFM1 – What the airflow volume was before in “cubic feet per minute.” RPM2 – The new speed of the fan motor in “revolutions per minute.” RPM1 – What the speed of the fan was before in “revolutions per minute.”

What this shows us is that the change in airflow volume in a system is exactly proportional to a change in fan speed.

For example, if you need to increase your airflow volume by a certain percentage, you’ll have to increase your fan speed by that same percentage to achieve the desired airflow volume.

SP2 = SP1 x ( (RPM2 / RPM1) ^ 2 )

Here we see a relationship between the static pressure changes in a system and the fan speed. But what do all of these variables mean?

SP2 – The new static pressure in the system. SP1 – The old static pressure in the system. RPM2 – The new speed of the fan in “revolutions per minute.” RPM1 – What the speed of the fan was before in “revolutions per minute.”

This shows us that the static pressure in a system will change in proportion to the square of the change in fan speed. In other words, if your fan speed increases by 5% (1.05), the static pressure will need to increase by the square of that change: (1.05 x 1.05 = 1.10) or a 10% increase.

For example, a static pressure of 2.0″ wc would increase to 2.20″ wc with just a 5% increase in the fan speed.

HP2 = HP1 x ( (RPM2 / RPM1) ^ 3 )

Here we see a relationship between the power used by the fan motor and the fan speed. In other words, the energy being used is related to the speed of the fan in this formula. But what do all of these variables mean?

HP2 – The new amount of power being drawn to the fan motor in horsepower, but could be represented in other units. HP1 – What the power being drawn to the fan motor was before. RPM2 – The new speed of the fan in “revolutions per minute.” RPM1 – What the speed of the fan was before in “revolutions per minute.”

This shows us that the power needed to meet the demand will change in proportion to the cube of the change in the fan speed. In other words, if your fan speed increases by 5% (1.05), the power will need to increase at the cube of that 5% change: (1.05 x 1.05 x 1.05 = ~1.16) or 16%.

For example, if a variable frequency drive is currently allowing the fan motor to operate at 10 horsepower, a 5% increase in fan speed would require 15% more horsepower to keep up with the load: 11.5 horsepower.

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