Workplace Air Quality: Dilution & Local Exhaust Ventilation Systems

Workplace Ventilation: Dilution vs. Local Exhaust

Dilution Ventilation Principles

Dilution ventilation is a general ventilation strategy often implemented in facilities to reduce local air contaminant levels by renewing the air. It involves mixing contaminated air with clean air to reduce contaminant concentrations to acceptable, lower levels throughout the workspace.

It is important to note that dilution ventilation does not control the concentration of contaminants at their source, making it generally unsuitable for highly toxic substances or when contaminant generation rates are high.

Dilution ventilation is less effective when contaminant concentrations exceed 50 ppm or steam concentrations exceed 5 mg/m³, especially in situations with scattered or mobile emission sources.

Recommended ventilation rates vary based on the environment and contaminant type. For clean air in sedentary environments, a rate of 30 m³/hour is often suggested. For environments with hot smoke and dust, a higher rate of 50 m³/hour may be necessary.

Occupational health and safety professionals are responsible for determining the appropriate ventilation strategy and selecting the correct ventilation rate for specific workplace conditions.

Theoretical Basis of Dilution Ventilation

Dilution ventilation introduces clean outside air into the workspace. Its primary function is not to prevent contaminant generation but to reduce their concentration to safe levels. This method helps prevent the accumulation of explosive or hazardous atmospheres, and its implementation is often recommended by safety guidelines to maintain a healthy working environment.

Limitations of Dilution Ventilation

Dilution ventilation has several limitations that must be considered:

• Contaminant concentrations must not be excessively high.
• Emission sources should be spread out or located away from workers.
• The resulting contaminant concentration must not exceed permissible exposure limits (PELs).
• Air intake rates vary based on the context, such as the production process and the toxicity level (low or medium) of the contaminants. It is generally not recommended for highly toxic materials.

Design Basics for Dilution Ventilation Systems

Key requirements for effective dilution ventilation system design include:

• Ensuring adequate airflow throughout the enclosure to avoid "dead zones" where air stagnates.
• Providing sufficient airflow for all contaminated areas, considering the maximum contaminant generation rate.
• Promoting natural air movement, especially in hot spots, to direct contaminants towards exhaust points.
• Properly positioning air inlets to introduce clean air effectively.
• Distributing exhaust points evenly to ensure comprehensive contaminant removal.
• Preventing the recirculation of exhausted air back into the building.
• Ensuring outdoor air intakes remain open and unobstructed, even in winter, to maintain continuous fresh air supply.

Local Exhaust Ventilation (LEV) Systems

Local Exhaust Ventilation (LEV) systems are designed to capture contaminants at or near their source, preventing them from spreading into the general workspace and reaching dangerous concentrations. This method is generally preferred for highly toxic substances or concentrated emissions.

Components of an LEV System

A typical LEV system consists of several key components:

• Hood: The capture device located at the emission source.
• Ductwork: A system of pipes for conveying aspirated (captured) air.
• Fan: Provides the necessary airflow to draw contaminants into the system.
• Filter or Scrubber: A device for cleaning the air before it is discharged, removing particulates or gases.

Basic Design Principles for LEV Systems

Effective LEV system design adheres to several fundamental principles:

• The hood must have a shape and dimensions adapted to capture the contaminant source effectively, ideally providing total or partial enclosure, or being strategically situated close to the source.
• The extraction point should be positioned away from the worker's breathing zone to maximize protection.
• Capture velocities at the hood opening typically range between 0.5 and 1 m/s, depending on the contaminant and process.
• A larger hood opening may be required to effectively enclose the source and prevent particle dispersion, especially for diffuse sources.
• Maintain high air velocity within the ductwork to prevent dust accumulation and ensure efficient transport of contaminants.
• Prevent contaminants from escaping the system or re-entering the workspace through leaks or improper discharge.

Maintenance of LEV Systems

Regular maintenance is crucial for the continued effectiveness of LEV systems:

• Regularly inspect for defects that could affect the capture rate at emission sources.
• Monitor suction flow rates and static pressure at critical points to ensure the system is operating as designed.
• Inspect hoods, ducts, and all components of the installation for wear, damage, or blockages.
• Check the fan and its components (motor, belts, blades) for proper function and signs of wear.
• Ensure filters and scrubbers are cleaned or replaced as needed to maintain air purification efficiency.