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.