Dilution Ventilation
Dilution ventilation, also known as general ventilation, is an engineering control that introduces large volumes of clean outdoor air to mix with and reduce the concentration of airborne contaminants in a workplace to acceptable limits, such as below permissible exposure limits (PELs) or lower explosive limits (LELs). It controls concentration, not emission, and is used when local exhaust ventilation is not practicable.
On a manufacturing shop floor, dilution ventilation typically involves roof or wall exhaust fans and make-up air systems to dilute contaminants like welding fumes, solvent vapors, or dust. It is applied in large, open areas where contaminant generation is low and steady, and workers are not close to sources. Design requires calculating required airflow using mass balance equations, accounting for mixing efficiency (K factor), and integrating with other controls. Continuous operation and periodic monitoring ensure concentrations stay below exposure limits.
How is required dilution ventilation rate calculated for a contaminant?
The required rate is calculated using steady-state mass balance: Q = G / C_target, where G is generation rate and C_target is target concentration. In practice, a mixing factor K > 1 is applied: Q_design = K * G / C_target. This is then converted to CFM or air changes per hour (ACH = Q / room volume).
How is dilution ventilation used for explosion prevention?
Dilution ventilation reduces vapor concentrations below the lower explosive limit (LEL), often targeting ≤10-25% of LEL. Design involves calculating vapor generation rate, setting target concentration as a percentage of LEL, and applying mass balance with a mixing factor. Continuous monitoring may be required for high-risk processes.
When does WorkSafeBC allow dilution ventilation instead of local exhaust?
WorkSafeBC allows dilution ventilation when local exhaust is not practicable, as per OHS Regulation Part 5 and Part 9 for confined spaces. Practicability is determined by physical constraints, process characteristics, and feasibility of installing LEV. Documentation must justify why LEV is not used and verify dilution ventilation keeps exposures below OELs.