Video – “Captor Hoods – Basic Principles and Design Theory/Equations”

Understanding How Captor Hoods Actually Work

Captor hoods are one of the most commonly used forms of Local Exhaust Ventilation (LEV) hood found throughout industry.

However, despite their widespread use, captor hoods are often misunderstood. Many LEV systems fail to achieve effective contaminant control because insufficient attention has been given to hood design, hood positioning and the relationship between airflow and capture distance.

This LEVCentral video presentation explains the fundamental theory behind captor hood performance and explores the engineering principles that determine whether a hood will successfully capture airborne contaminants.

Presented by Bill Cassells, the video examines the relationship between:

Hood geometry
Airflow
Face velocity
Capture velocity
Effective captor distance
Fletcher’s Equations
Hood positioning
Contaminant behaviour

The presentation provides a practical introduction to the mathematics used to estimate captor hood performance and explains why simply increasing airflow is not always the most effective solution.

This resource is particularly relevant to:

LEV Designers/Commissioners/Testers
Occupational Hygienists
Safety Professionals
Engineering Students
Duty Holders

Captor Hoods – Basic Theory and Design Equations

Video Resource

Presenter: Bill Cassells

Produced By: OXYL8 / LEVCentral

Resource Type: Educational Video

Subject: Captor Hood Design and Performance

Status: Current Resource

Last reviewed by LEVCentral: June 2026

LEVCentral Expert Commentary

One of the most common misconceptions in LEV is the belief that increasing airflow automatically improves contaminant capture.

In reality, captor hood performance is governed by a combination of factors including:

Hood shape
Hood size
Air volume
Distance from source
Process characteristics
Worker interaction

A poorly positioned hood with a large airflow may perform significantly worse than a well-designed hood operating at a lower airflow rate.

This video explores the engineering principles that underpin successful source-capture ventilation and explains why effective captor distance is often the critical design parameter.

A key theme throughout the presentation is that:

Airflow is only valuable when it successfully captures contaminants before they enter the worker’s breathing zone.

The video also introduces Fletcher’s Equations, which remain one of the most widely used methods of estimating captor hood performance and effective capture distances.

For many LEV engineers, understanding these relationships represents an important step from simply measuring systems to genuinely understanding how they work.

Key Learning Points

Captor Hoods Have a Limited Effective Range

The influence of a captor hood decreases rapidly with distance from the hood opening.

Airflow Alone Does Not Guarantee Capture

Successful contaminant control depends on capture effectiveness rather than airflow volume alone.

Hood Position Is Critical

Small changes in hood location can have a significant impact on performance.

Fletcher’s Equations Help Predict Performance

The equations provide a practical method for estimating capture velocity and effective captor distance.

Understanding Theory Improves LEV Design

Engineers who understand the principles behind captor hood performance are better equipped to design, commission and assess LEV systems.

Topics Covered in the Video

What Is a Captor Hood?

Introduction to source-capture ventilation and common hood types.

Capture Velocity

Understanding the air movement required to draw contaminants into the hood.

Effective Captor Distance

How hood performance changes as distance from the source increases.

Fletcher’s Equations

Introduction to the mathematical relationships used to estimate hood performance.

Practical LEV Design Considerations

Applying theory to real-world LEV systems.

Common Design Mistakes

Examples of why some captor hoods fail to achieve effective contaminant control.

Why This Resource Matters

Many LEV practitioners routinely measure airflow, pressure and velocity without fully understanding the theoretical principles behind hood performance.

This resource helps bridge the gap between:

Theory and practice
Design and testing
Measurement and interpretation

It provides an accessible introduction to the engineering principles that underpin effective source-capture ventilation and supports a deeper understanding of LEV system performance.

Relationship to Other LEVCentral Resources

This video complements:

Together these resources provide a strong foundation in captor hood theory, design and practical application.

Recommended Learning

Thought Leadership

Why Airflow Is Not Enough
Designing for Capture Rather Than Compliance
Understanding Effective Captor Distance
Applying Engineering Principles to LEV Performance

LEVCentral Observation

Captor hoods remain one of the most frequently used and most frequently misunderstood LEV hood types.

Effective design requires more than selecting a fan and calculating an airflow rate. It requires an understanding of how contaminants behave, how air moves around the hood and how capture effectiveness changes with distance.

This video provides an accessible introduction to those principles and helps explain why successful LEV design is ultimately about controlling contaminants at source rather than simply moving air.