HISTORICAL BACKGROUND

Report
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
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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
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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
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Material to be removed
Degree of removal required
Concentration of material
Conditions of air stream
Economics
Types of Air Cleaners
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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
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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
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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.

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