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.
