Ventilation for Confined Spaces

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Ventilation for Confined Spaces
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1910.146 requires ventilation as follows

• An employee may not enter the space until the
  forced air ventilation has eliminated any
  hazardous atmosphere
• The forced air ventilation shall be so directed
  as to ventilate the immediate areas where an
  employee is or will be present within the space
  and shall continue until all employees have left
  the space.

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• The air supply for the forced air ventilation
  shall be from a clean source and may not increase
  the hazards in the space.

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• The atmosphere within the space shall be
  periodically tested as necessary to ensure that
  the continuous forced air ventilation is
  preventing the accumulation of a hazardous
  atmosphere.

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Types of Ventilation

• Natural ventilation
• Mechanical supply ventilation (positive pressure)
• Mechanical exhausting (negative pressure)

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Natural Ventilation

• Natural air currents
• Advantages
• Quiet
• Does not require power
• Not a source of ignition
• Maintenance free
• Disadvantages
• Highly unpredictable
• Very inefficient

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Mechanical Supply

• Forcing fresh air in / contaminated air out
• Should be at a minimum rate of 20 air changes per
  hour
• Advantages
• Very effective in managing hazards
• Eliminates gas seeping back into the space
• Prevents flammable gas from being drawn across
  the fan motor
• Disadvantages
• Fan noise
• Stirring up dust

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Mechanical Exhausting

• Pulls contaminated air out / draws fresh air in
• Works best when air intake is close to work area
• Advantages
• Pulls heavier than air contaminates from low
  areas
• Disadvantages
• May not create sufficient air movement
• Fan noise
• May draw flammable gases across the fan motor
• Supply ventilation can force air into the space
  30 x the distance exhaust can draw it

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Ventilation Factors

• Job details
• Atmospheric hazard
• Size of space
• Contaminants density
• How fast its created
• How much space is available for vent. Equipment
• Airflow required
• 20 air changes per hour
• Volume required

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Ventilation Factors

• Breeze to ensure comfort
• Duct friction loss
• Can reduce air flow over 50 in 15 foot of duct
• Fan performance
• Design, Weight, Sound, Power

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Supply Ventilation Directions

• Position fan 4-6 feet from entry with intake into
  wind
• Vertical spaces
• Position hose ¾ of the distance to the bottom,
  and no more than 15 feet from the work area
• Horizontal spaces
• Lay hose on the floor with the end within 10 feet
  of the far wall
• Turn fan on before placing duct into the space
• Ensure that there is no short circuiting

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Exhaust Ventilation Directions

• Keep the duct 3 feet from the worker
• Position duct 1 foot from contaminant and 1 foot
  below the worker breathing zone
• Position so exhaust is out of wind and exhausted
  air is not pulled back into space
• If explosive, do not allow air to contact fan
  motor. Unit should be grounded
• Area around fan should be considered hazardous

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In all Cases

• Ventilation should continue while space is
  occupied
• Prevent recirculation of exhaust air
• If LEL reading is present, remove hose before the
  fan is turned off

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Lighter Than Air Gases
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Heavier Than Air Gases
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Special Situations
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Below Grade Conditions