Local exhaust systems capture air contaminants directly at their point of generation, removing them before they distribute throughout a space. Also called source capture or spot exhaust, this approach offers superior contaminant removal efficiency compared to general ventilation methods. Local exhaust reduces required overall ventilation rates, lowering energy consumption and equipment costs.
System Components and Design
Local exhaust systems comprise a capture hood positioned near the emission source, connecting ductwork, and dedicated exhaust fans. Capture hoods must be properly sized and positioned to achieve 50 to 100 feet per minute (FPM) face velocity across the opening. Examples include laboratory fume hoods (typically 80 to 150 linear feet per minute sash velocity), kitchen ventilation hoods over cooking equipment, and welding fume capture systems. Makeup air must be supplied to replace exhausted air and prevent negative room pressurization.
Applications and Efficiency
Local exhaust effectively controls chemical vapors, welding fumes, dust, heat, and cooking odors. Kitchens, laboratories, manufacturing facilities, and healthcare settings depend on local exhaust for occupant protection. Well-designed local exhaust systems remove 80 to 95 percent of contaminants at the source compared to 60 to 70 percent effectiveness for dilution ventilation. Variable air volume (VAV) local exhaust systems adjust fan speed based on actual use, reducing energy consumption. Building codes require local exhaust in specific applications to protect occupant health and meet ASHRAE standards.