Basic LEV Maths – Fan Laws

A Simple Introduction to One of the Most Important Mathematical Principles in LEV

 

LEVCentral Expert Commentary

One subject that many new LEV engineers find intimidating is fan laws. Yet they are amongst the simplest—and most useful—mathematical relationships used in Local Exhaust Ventilation.

This short training guide was originally produced as part of the OXYL8 P601 and P602 training programme to explain fan laws in plain English, using straightforward worked examples rather than complex engineering formulae. It assumes very little prior mathematical knowledge and demonstrates how relatively small changes in fan speed (RPM) can produce surprisingly large changes in flowrate, static pressure and power consumption.

Rather than presenting fan laws as abstract equations, the guide walks through each relationship step-by-step before applying it to practical LEV examples similar to those encountered during commissioning, fault-finding and BOHS examinations. For many students, this approach makes fan laws far easier to understand than traditional engineering textbooks.

Although deliberately simplified, the guide introduces concepts that every LEV engineer should understand before progressing to more advanced topics such as fan selection, system curves, pressure loss calculations and commissioning.

View the Training Guide

Key Learning Points

The guide explains:

• What fan laws are.
• Why fan laws are considered empirical relationships.
• The relationship between:
  • Fan speed (RPM).
  • Airflow (Q).
  • Static pressure (SP).
  • Power (kW).
• Why:
  • Flow varies directly with fan speed.
  • Static pressure varies with the square of fan speed.
  • Power varies with the cube of fan speed.
• Simple methods for performing fan law calculations.
• Worked examples using realistic LEV system data.
• Practice questions with solutions.
• How fan laws apply during LEV commissioning and fault finding.

Source Information

Author: Bill Cassells

Organisation: OXYL8 Ltd / LEVCentral

Resource Type: Training Guide

Primary Topics: Fan Laws, LEV Mathematics, Fan Performance, Commissioning, LEV Design, BOHS Training

Audience: LEV Engineers, Commissioning Engineers, Occupational Hygienists, BOHS P601 Candidates, BOHS P602 Candidates and anyone wishing to understand basic fan performance calculations.

LEVCentral Perspective

One of the biggest barriers to learning LEV mathematics is not the mathematics itself—it’s the way it is often taught.

Traditional engineering texts tend to introduce fan laws using algebraic notation and theoretical derivations. Whilst technically correct, this can make the subject appear much more difficult than it really is.

This guide deliberately takes a different approach.

Using everyday examples, percentages and straightforward arithmetic, it demonstrates that most fan law calculations can be completed quickly without advanced mathematics. Once the underlying relationships are understood, engineers are far better equipped to interpret changes in system performance during commissioning and troubleshooting.

The guide also highlights one of the most important practical lessons in LEV:

A modest increase in airflow can require a very large increase in motor power.

Many engineers are surprised to discover that increasing airflow by only 20% can increase power consumption by more than 70%. Understanding this cubic relationship helps explain why “simply speeding up the fan” is rarely an efficient solution to poor LEV performance.

Further Resources

• Basic LEV Maths – Module 1 (Pressure, Velocity Pressure and Basic LEV Mathematics)
• BOHS – Introductory Guide to Mathematics for Occupational Hygiene 1: LEV Mathematics
• Fan Handbook – Selection, Application, and Design by Frank P. Bleier
• Woods Guide to Fan Engineering
• ACGIH Industrial Ventilation: A Manual of Recommended Practice for Design
• Fan Laws and System Curves (New York Blower Engineering Letter)
• HSG258 – Controlling Airborne Contaminants at Work

Recommended Learning

• M505 Control of Hazardous Substances
• P600 Methods for Testing Performance of LEV
• P601 LEV Thorough Examination & Testing
• P602 LEV Basic Design Principles
• P604 LEV Commissioning & Performance Evaluation

Thought Leadership

Every experienced LEV engineer eventually realises that fan laws are not simply examination material—they explain much of what happens in real ventilation systems.

They help predict the effect of changing pulley sizes, installing variable speed drives, adding additional hoods, replacing filters or modifying ductwork. More importantly, they explain why apparently small changes to a ventilation system can produce unexpectedly large consequences elsewhere.

From a LEVCentral perspective, fan laws are one of the essential building blocks of LEV competence. Once understood, they make many aspects of commissioning, fault diagnosis and system optimisation far easier to interpret.

This guide was written with exactly that objective in mind: to remove the fear from LEV mathematics and show that the subject is both logical and highly practical.