LOCAL EXHAUST VENTILATION …study of the principles associated with proper application of local exhaust ventilation (LEV) for control of airborne contaminants Components of LEV System • • • • • hood ductwork air mover (fan) air cleaner [makeup air system] Types of Local Exhaust Hoods • Enclosing – Chemical fume hoods, glove boxes, biological safety cabinets • Exterior – Open surface tanks • Others Examples (Source: Princeton University EH&S Department) Examples Air Flow into Hood • Velocity contours have been determined • Best described by equation: Q = V(10X2 + A) • Effect of flanging • Formulae for various hood types LEV Hood Design • purpose: keep contaminant out of B.Z. • considerations: minimize interference minimize pressure drop minimize exhaust volume • terminology: face slot duct Ductwork • carries contaminant from hood to discharge • straight duct • elbows • entries • contraction/expansions • clean-out doors Duct Considerations • • • • • • • resultant air velocity in duct maintain minimum transport velocity minimize friction losses shape is a factor (round is preferred) diameter (determined by Q; friction loss) length (layout of process) material of construction Air Cleaning Equipment • Purposes: remove contaminant before discharge; recover valuable materials • Selection depends on – – – – – Material to be removed Degree of removal required Concentration of material Conditions of air stream Economics Types of Air Cleaners • • • • • • • absorbers/adsorbers filters cyclones electrostatic units combustion units wet scrubbers combination units Absorbers • Contaminant-in-air contacts liquid • Liquid dissolves or reacts with contaminant and retains it • Use packed towers/packed beds • Typical uses: acid gases, chlorine, etc. Adsorbers • Contaminant-in-air passes through bed of solid • Contaminant adheres to surface • Examples: activated carbon; silica gel • Typical uses: organic vapors Filters • Contaminated air passes through fabric, which collects particles • Incorporated into “bag houses” • Various materials used as filters • Can be made very efficient • Surface must be replenished/replaced Cyclones • Centrifugal force used to separate particles • Good for large particles only Electrostatic Precipitators • • • • Voltage applied Charged particles are drawn to plate Collector plates need to be cleaned Good for very small particles Combustion Processes • For combustible contaminants • Contaminant converted to harmless form • Thermal oxidation – Contaminant-in-air passes over flame • Direct combustion – Contaminant-in-air used as fuel • Catalytic oxidation – Contaminant-in-air passes over catalyst Wet Scrubbers • Particles contact water and are “washed” from the airstream • Minimizes secondary dust problem in disposal • Good for dusts Air Movers (Fans) • fan is the “moving force” for the system • location • many types available depending on the nature of contaminant, volume of air being moved and pressure drop through system Axial Flow Fans • Air enters & leaves fan moving in same direction • Types – Propeller – Tube-axial – Vane-axial Centrifugal Flow Fans • Air exits 90 degrees to angle of entry; is “thrown” by force • Radial (paddle wheel) • Forward curved • Backward curved Fan Selection Considerations • • • • • • Total quantity of air being moved Pressure requirements Presence of particulates? Explosive/flammable materials? Noise generated by air mover Others unique to the application Principles of LEV • Enclose source as much as practicable. • Capture/control contaminant with adequate velocity. • Keep contaminant out of breathing zone. • Discharge air away from fresh air inlets. • Provide adequate make-up air.