LOCAL EXHAUST VENTILATION …study of the principles associated with proper application of local exhaust ventilation (LEV) for control of airborne contaminants.

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.