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The Three Fan Laws

the three fan laws

The three fan laws are critical mathematical formulas that help HVAC technicians and designers understand how airflow, static pressure, and horsepower change under various conditions in heating, ventilation, and air conditioning systems. 

These laws describe how changes in fan speed or static pressure can affect airflow and the energy required to maintain it.

Understanding these laws allows for more precise sizing and selection of equipment and ductwork and enables technicians to troubleshoot and optimize HVAC systems effectively.

Fan Law 1: Relationship Between CFM and RPM

  • Law: CFM is directly proportional to the fan's RPM.

  • Formula: CFM2 = CFM1 × (RPM2 ÷ RPM1)

  • Practical Meaning: Increasing or decreasing the fan's RPM will proportionally increase or decrease the airflow (CFM). A 10% change in RPM results in a 10% change in CFM.

  • Field Application: To adjust airflow, technicians can change the fan's speed. For example, decreasing a fan's RPM from 1100 to 990 will decrease airflow from 1000 CFM to 900 CFM.

Fan Law 2: Static Pressure and CFM Relationship

  • Law: Total Static Pressure changes with the square of CFM (or RPM).

  • Formula: SP2 = SP1 × (CFM2 ÷ CFM1)²

  • Practical Meaning: A slight increase in airflow (CFM) leads to a disproportionately larger increase in static pressure. A 10% increase in CFM results in a 21% increase in static pressure.

  • Field Application: When increasing airflow, it's crucial to consider the impact on static pressure across system components. For instance, increasing airflow from 1000 CFM to 1200 CFM can significantly increase the pressure drop across a filter, affecting its efficiency.

Fan Law 3: Horsepower and CFM Relationship

  • Law: Horsepower required by the fan changes with the cube of CFM (or RPM).

  • Formula: HP2 = HP1 × (CFM2 ÷ CFM1)³

  • Practical Meaning: A slight increase in airflow requires a disproportionately larger increase in horsepower. A 10% increase in CFM results in a 33% increase in horsepower.

  • Field Application: Before increasing airflow, it's essential to ensure the motor can handle the increased load. An increase from 1000 CFM to 1200 CFM significantly increases the horsepower required, potentially overloading the motor.

Key Takeaways:

  • These laws highlight the nonlinear relationships between airflow, static pressure, and horsepower in HVAC systems.

  • Adjustments to fan speed (RPM) directly affect airflow and indirectly influence system static pressure and horsepower requirements.

  • HVAC technicians must consider these relationships when modifying system performance to avoid unintended consequences, such as overloading motors or creating excessive static pressure.

fan and pump law cheat sheet

You can download your fan and pump law PDF here:

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