SHAPA – Dust Hood Selection

LEVCentral Expert Commentary

If there is one component that determines whether an LEV system will succeed or fail, it is the hood.

The hood is the interface between the contaminant source and the extraction system. It is here that contaminants must first be captured before they disperse into the workplace. No amount of additional fan power, larger ductwork or higher airflow can fully compensate for a poorly designed or incorrectly positioned hood.

This excellent SHAPA Technical Bulletin explains the principles of dust hood selection and demonstrates why hood design should always begin with a thorough understanding of the process, the contaminant and the way operators interact with the equipment. The paper shows that selecting the correct hood is not simply a matter of choosing a convenient shape, but of understanding how contaminants are generated and how they move through the surrounding air.

The guidance identifies three fundamental hood types:

• Enclosing (Containment) Hoods
• Receiving Hoods
• Capturing Hoods

Each has distinct advantages and limitations depending upon the process being controlled. In general, enclosing hoods provide the highest level of control because they contain the contaminant within the hood, whereas receiving and capturing hoods rely increasingly on airflow and correct positioning to achieve effective contaminant capture.

Perhaps the most valuable message within this document is that good hood design reduces the quantity of air required. By capturing contaminants efficiently at source, engineers can specify smaller ductwork, lower-capacity fans and more compact air cleaning equipment, resulting in lower capital costs, lower energy consumption and improved long-term system performance.

View SHAPA Guide

Key Learning Points

• Hood design is the single most important element of many LEV systems.
• Effective hood selection begins with understanding:
  • The contaminant.
  • The process.
  • The source of emission.
  • Operator interaction.
• Three principal hood types are described:
  • Enclosing hoods.
  • Receiving hoods.
  • Capturing hoods.
• Correct hood positioning is often more important than increasing airflow.
• Poor hood design may result in:
  • Inadequate contaminant capture.
  • Excessive airflow requirements.
  • Higher energy consumption.
  • Increased capital cost.
  • Reduced operator acceptance.
• Well-designed hoods minimise the volume of air required while maximising contaminant capture.
• Hood selection directly influences duct sizing, fan selection, filter design and overall system efficiency.

Source Document Information

Organisation: SHAPA (Solids Handling & Processing Association)

Document: Dust Hood Selection (Technical Bulletin No. 19)

Author: Nigel Hubbard, Dustcheck Ltd

Document Type: Technical Guidance

Primary Topics: Hood Design, LEV Design, Dust Control, Capture Efficiency

Audience: LEV Designers, Commissioning Engineers, Dust Control Engineers, Occupational Hygienists, Process Engineers and Duty Holders.

Further Resources

• ACGIH Industrial Ventilation – A Manual of Recommended Practice for Design
• HSG258 – Controlling Airborne Contaminants at Work
• SHAPA – Ten Key Steps for Comparing Dust Extraction System Proposals
• SHAPA – Cartridge Filter Technology
• SHAPA – Dust Extraction – Five Ways to Achieve Long Life with Minimal Expense

Recommended Learning

• M200 Basic Principles in Occupational Hygiene
• 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

There is a tendency to think of fans, filters and ductwork as the most important components of an LEV system because they are the largest and most visible items of plant. In reality, the hood is where success or failure is decided.

An efficient hood captures contaminants before they can disperse into the workplace, allowing the remainder of the system to operate efficiently. Conversely, a poorly designed hood cannot usually be rescued by installing a larger fan or increasing airflow. In many cases this simply increases energy consumption without achieving effective control.

This is why experienced LEV designers often begin with the hood and work backwards through the system. Once the contaminant is being captured effectively, the remainder of the design becomes an exercise in transporting, cleaning and safely discharging the contaminated air.

Understanding hood design is therefore one of the defining skills of a competent LEV engineer and remains at the heart of successful ventilation system design.