Ventilation for Confined Spaces

1910.146 requires ventilation as follows:

An employee may not enter the space until the forced air ventilation has eliminated any hazardous atmosphereThe 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.

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

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.

Types of Ventilation

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

Natural Ventilation

Natural air currentsAdvantages Quiet Does not require power Not a source of ignition Maintenance free

Disadvantages Highly unpredictable Very inefficient

Mechanical SupplyForcing fresh air in / contaminated air outShould be at a minimum rate of 20 air changes per hourAdvantages 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

Mechanical ExhaustingPulls contaminated air out / draws fresh air inWorks best when air intake is close to work areaAdvantages 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

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

Ventilation Factors

Breeze to ensure comfortDuct friction loss Can reduce air flow over 50% in 15 foot of

duct

Fan performance Design, Weight, Sound, Power

Supply Ventilation Directions

Position fan 4-6 feet from entry with intake into windVertical 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 spaceEnsure that there is no “short circuiting”

Exhaust Ventilation Directions

Keep the duct 3 feet from the workerPosition duct 1 foot from contaminant and 1 foot below the worker breathing zonePosition so exhaust is out of wind and exhausted air is not pulled back into spaceIf explosive, do not allow air to contact fan motor. Unit should be groundedArea around fan should be considered hazardous

In all Cases

Ventilation should continue while space is occupiedPrevent recirculation of exhaust airIf LEL reading is present, remove hose before the fan is turned off

Lighter Than Air Gases

Heavier Than Air Gases

Special Situations

Below Grade Conditions