Fan Laws

June 19, 2023 by

Mechanical Systems – Fan Laws

Fan Laws

ARCHITECTURAL ENGINEERING PE EXAM SPECIFICATIONS

Understanding Fan Laws: Exploring the Relationship Between Airflow, Speed, and Power

Fans are a universal part of our lives, whether they are cooling our homes, circulating air in industrial settings, or assisting in various mechanical processes. Have you ever wondered how changes in fan speed affect airflow or power consumption? That’s where the concept of fan laws comes into play. In this blog post, we will delve into the fascinating world of fan laws and unravel the relationships between airflow, fan speed, and power.
What are Fan Laws?
Fan laws are mathematical formulas that describe the relationships between various parameters of a fan system, such as airflow, fan speed, pressure, and power. These laws help us understand how altering one parameter affects others, enabling us to optimize fan performance and energy efficiency.
Fan Law #1: The Affinity Law (Flow)
The Affinity Law states that the airflow (or volume flow rate) of a fan is directly proportional to its speed. In other words, if the fan speed is doubled, the airflow will also double, assuming all other factors remain constant. Similarly, reducing the fan speed by half will result in half the airflow.
q1 ÷ q2 = n1 ÷ n2
where q = flow rate (cfm) and n = wheel velocity (RPM)
Fan Law #2: The Affinity Law (Pressure)
The Affinity Law also applies to fan pressure. It states that the pressure generated by a fan is proportional to the square of its speed. Therefore, if the fan speed is doubled, the pressure will increase fourfold. Conversely, reducing the fan speed by half will result in a quarter of the pressure.
dp1 ÷ dp2 = (n1 ÷ n2)2
where dp = head or pressure (in, ft, psi) and n = wheel velocity (RPM)
Fan Law #3: The Affinity Law (Power)
The Affinity Law extends to fan power consumption as well. Power is directly proportional to the cube of the fan speed. Consequently, doubling the fan speed will increase the power requirements by a factor of eight. Conversely, reducing the fan speed by half will result in only an eighth of the power consumption.
P1 ÷ P2 = (n1 ÷ n2)3
where P = Power (watts, bhp) and n = wheel velocity (RPM)
Fan Law #4: The Static Pressure Law
The Static Pressure Law states that the total pressure generated by a fan system is the sum of the static pressure and velocity pressure. When the fan speed changes, the static pressure remains constant while the velocity pressure varies based on the square of the speed.
Application and Benefits of Fan Laws
Understanding fan laws is crucial in several scenarios. For instance, when retrofitting or upgrading a fan system, knowing the relationship between airflow, speed, and power helps optimize performance and energy efficiency. Engineers can determine the most suitable fan speed to achieve the desired airflow while minimizing power consumption.
Additionally, fan laws aid in troubleshooting and diagnosing issues within a fan system. By analyzing the changes in airflow, pressure, and power, engineers can identify potential problems and make informed decisions about maintenance or repairs.
Fan laws provide valuable insights into the relationships between airflow, fan speed, and power consumption. By leveraging these laws, engineers and system designers can optimize fan performance, improve energy efficiency, and troubleshoot any issues that may arise. Understanding fan laws is not only relevant in the realm of engineering but also for anyone interested in the mechanics behind fans and their applications in various industries.

Let us know if there is anything we can do to help you prepare for the exam.

Fan Laws

Architectural Engineering PE Exam Resources
NCEES
Contact Us

Copyright©  All Rights Reserved

EngineeringDesignResources.com prohibits the use or reproduction of this material by any means, graphic, electronic, or mechanical, except in the case of brief quotations embodied in critical articles and reviews. This includes photocopying, recording, taping, or by any information storage retrieval system.

Due to the dynamic nature of the Internet, web addresses or links in these materials may have changed.

Any resemblance in the images in this material to actual people or locations is merely coincidental. EngineeringDesignResources.com prohibits reprinting, copying, changing, reproducing, publishing, uploading, posting, transmitting, or using in any other manner images in this material.

Filed Under: Architectural Engineering PE Exam, Mechanical Systems, PE Exam