Download
local exhaust ventilation n.
Skip this Video
Loading SlideShow in 5 Seconds..
LOCAL EXHAUST VENTILATION PowerPoint Presentation
Download Presentation
LOCAL EXHAUST VENTILATION

LOCAL EXHAUST VENTILATION

888 Views Download Presentation
Download Presentation

LOCAL EXHAUST VENTILATION

- - - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript

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

  2. Components of LEV System • hood • ductwork • air mover (fan) • air cleaner • [makeup air system]

  3. Types of Local Exhaust Hoods • Enclosing • Chemical fume hoods, glove boxes, biological safety cabinets • Exterior • Open surface tanks • Others

  4. Examples (Source: Princeton University EH&S Department)

  5. Examples

  6. 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

  7. LEV Hood Design • purpose: keep contaminant out of B.Z. • considerations: minimize interference minimize pressure drop minimize exhaust volume • terminology: face slot duct

  8. Ductwork • carries contaminant from hood to discharge • straight duct • elbows • entries • contraction/expansions • clean-out doors

  9. 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

  10. 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

  11. Types of Air Cleaners • absorbers/adsorbers • filters • cyclones • electrostatic units • combustion units • wet scrubbers • combination units

  12. 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.

  13. Adsorbers • Contaminant-in-air passes through bed of solid • Contaminant adheres to surface • Examples: activated carbon; silica gel • Typical uses: organic vapors

  14. 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

  15. Cyclones • Centrifugal force used to separate particles • Good for large particles only

  16. Electrostatic Precipitators • Voltage applied • Charged particles are drawn to plate • Collector plates need to be cleaned • Good for very small particles

  17. 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

  18. Wet Scrubbers • Particles contact water and are “washed” from the airstream • Minimizes secondary dust problem in disposal • Good for dusts

  19. 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

  20. Axial Flow Fans • Air enters & leaves fan moving in same direction • Types • Propeller • Tube-axial • Vane-axial

  21. Centrifugal Flow Fans • Air exits 90 degrees to angle of entry; is “thrown” by force • Radial (paddle wheel) • Forward curved • Backward curved

  22. 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

  23. 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.