Controlling Airborne Hazards

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Controlling Airborne Hazards

• Industrial Hygiene
• IENG 341
• Carter J. Kerk
• Industrial Engineering Department
• SD School of Mines
• Spring 2008

2
Reading Assignment

• Read Nims, Chapter 7
• Critical Thinking Questions, p. 186-7
• Due ?

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Outline Controlling Airborne Hazards

• Hazard Control Options
• Local Exhaust Ventilation Systems (LEVS)
• Dilution Ventilation for Contaminant Control

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Hazard Control Options

• Remember
• Identify (Chapters 4 5)
• Evaluate (Chapters 4 5)
• Control (This Chapter!)
• Engineering Controls
• Administrative Controls
• Personal Protective Equipment (PPE)

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Engineering Controls

• Equipment design changes
• Automation, enclosures, etc.
• Reduce or prevent release of contaminants
• Substitution
• Use a less hazardous material
• Work practice changes
• Fundamentals of work methods engineering
• Including personal hygiene
• Improved housekeeping and maintenance
• Constant vigilance!

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Administrative Controls

• Use only after engineering controls!
• Control or limit human access
• Worker rotation
• This is a great tool, but remember it now exposes
  even more workers
• Worker training

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Worker Training

• Can be effective if consistent and appropriate
• Specialized
• Special work practices/procedures
• Emergency situations (spills, uncontrolled
  releases of contaminants, etc.)
• Training effect can be noticeable immediately
  following training, but these results decrease
  over time without further training (Hawthorne
  Effect?)

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Personal Protective Equipment (PPE)

• Last resort!
• We will cover this in Chapter 12

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Local Exhaust Ventilation Systems (LEVS)

• Designed to capture contaminants at the point of
  release
• Compared to Dilution Ventilation (what is DV?),
  LEVS is
• more effective at removing point source releases
• is more energy efficient
• Has less impact on the overall HVAC

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LEVS Basic Components

1. Intake, or hood captures or draws contaminant
   into the system
2. Ducts
3. Air cleaner
4. Fan
5. Outlet, or exhaust

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Three Main Types of Hoods
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Hood Entry Loss

• When air enters the hood
• Some turbulence is created
• Loss of kinetic energy of the air
• A flange around the opening
• Reduces the entry loss
• Causes more air to enter
• Vent Manual
• ACGIH Industrial Ventilation, A Manual of
  Recommended Practice
• Tables of data concerning ventilation systems and
  components

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Hood Capture Velocity

• Q A V
• Q volumetric flow rate, cfm
• A cross-sectional area of air flow, ft2
• V velocity of air, fpm
• Flow rate must be high enough to capture the
  contaminants
• Gases / Vapors 1000 fpm
• Wood, cotton dust 2000-2500 fpm
• Lead, cement dust - gt4500 fpm

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Hood Placement Tips

• Enclose the process as much as possible without
  interfering with the worker
• Draw contaminants away from the breathing zone
• Take advantage of natural inertia from the
  process, e.g., heat, grinding spray
• Locate as close as possible to release point
• More than two feet from source will likely be
  ineffective

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Ducts

• Friction losses between air and sides of duct
• Other loss sources
• Duct length
• Turns, tapers, multiple duct joints
• Smaller diameter ducts have higher losses than
  larger diameters
• Calculate losses or look up estimates in tables
  in the Vent Manual

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Duct Air Pressure

• TP SP VP
• TP Total Pressure, inches of water gage
• SP Static Pressure
• Air moving inside ducts exerts pressure in all
  directions, this is static pressure
• VP Velocity Pressure
• V 4005 (VP)1/2
• Bernoullis theorem
• V air velocity, fpm
• Adequate pressure and velocity must be maintained
  through duct system to keep airborne particles
  airborne!

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Air Cleaners

• Filters, collectors, settling chambers,
  precipitators
• Remove dusts, fumes, fibers, radioactive
  particles, gases, vapors

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Selection of the Filter

• Consider the properties of the contaminant
• Size, density, toxicity, solubility,
  concentration
• How clean must the exiting air be?
• May be regulated by federal, state, or local
  limits
• Costs
• First cost, operating and maintenance costs,
  energy costs, space considerations, disposal of
  collected contaminants

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Fans

• Placed near the end of the system
• Their selection and performance is critical to
  the system
• Two types of fans
• Centrifugal (or squirrel-cage)
• Wheel type, blades (straight, forward-curved,
  backward-curved
• Axial-Flow fans
• Like aircraft propeller
• Can move large volumes of air, but not a
  pressures generally required in LEVS

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Exhaust

• Allow release of air with minimal added pressure
• Straight-path is most efficient
• Rain cover over top impedes airflow
• Dont let aesthetic designs over-rule efficient
  designs

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Evaluating LEV Performance

• Is it designed, installed, and operating
  properly??
• Face Velocity
• Get an average face velocity
• Q VA
• Pitot Tube
• Measure pressure in a duct
• U-shaped liquid-filled tube comprising a
  manometer (see Figure 7-6)
• Look for discoloration at duct seams indicating
  leaks or accumulations

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Example Duct Flow

• Designed velocity is 2000 fpm. Using a pitot
  traverse, the average pressure is 0.11 in w.g.
• Actual duct velocity 4005 (0.11)1/2 1328 fpm
• Measure velocity is much lower than designed
  velocity.
• Potential solutions?

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OSHA Standards

• OSHA has minimum ventilation rates for many
  processes
• Abrasive blasting, welding in confined spaces,
  grinding, polishing, buffing, open surface tanks,
  sawmills, spray finishing
• 29 CFR 1910.94 Ventilation
• 29 CFR 1926.57 Ventilation

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Dilution Ventilation

• Uses
• Non-toxics (or very low toxics)
• Nuisance contaminants
• Combustible vapors (keep below LEL)
• LEVS not feasible
• Move contaminated air away from breathing zone
  (see Fig 7-7)

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Figure 7-7. If relying on dilution ventilation
to control contaminants, placement of the inlet
and patterns of air movement relative to the
location of the worker are critical so that
contaminated air is drawn away from the breathing
zone of the worker.
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Dilution Flow Rate for Low Toxics

• Qdil (387)(pounds)(106)(Kmix) /
  (MW)(t)(Ca)(d)
• Qdil flow rate of dilution air, cfm
• 387 volume occupied by one pound of air at STP,
  ft3
• STP 70F, sea level
• Pounds amount of contaminant evaporated, lbs
• Kmix mixing factor, accounts for air movement
  and mixing
• 1.0, perfect and rare 1.5 optimum, 1.5 2.0
  good 2.5 3.0 conservative
• MW molecular weight of contaminant
• t time for known amount of contaminant to
  evaporate, minutes
• Ca acceptable concentration, ppm
• d air density correction factor 1 _at_ STP
  (conservative), 0.66 at extreme altitude and high
  temperature

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Reading Assignment

• Read Nims, Chapter 7
• Critical Thinking Questions, p. 186-7
• Due ?