Mechanical forced air ventilation commonly is used to ventilate permit spaces.
This may involve the use of grounded air movers that are powered by compressed air or electric fans or blowers. A ventilation unit works by drawing air from one area, compressing it and directing it into the space. When these types of equipment are used in flammable or combustible atmospheres, the equipment must be rated as explosion-proof by nationally recognized testing labs. Forced air ventilation pushes fresh clean air into the space and purges contaminated air. This commonly is referred to as "positive pressure ventilation."
Forced air ventilation must ventilate the immediate areas where entrants are present within the space.
Ventilations also must be continued until all employees have left the space.
The air used to ventilate the permit space must be from clean source. Ventilation must not increase hazards in the space. In addition, the atmosphere within the space must be tested periodically to ensure that the continuous forced air ventilation is preventing the formation of a hazardous atmosphere.
Understanding purge times is essential element of forced air ventilation. In order to calculate purge time, one first must know the cubic footage of the permit space. This number then is divided by the cubic foot per minute (cfm) displacement of the blower. This results in
the air changes per hour (ACH). Federal OSHA currently does not regulate ACH. Consult your state’s requirements, company’s written confined space entry program, or ask your supervisor should you have questions regarding ACH requirements.
In addition to allowing for an adequate number of air changes, consider continual ventilation of the space during the entire entry period. Never assume that ventilation was effective. Always use an atmospheric monitor to test the air within a permit space prior to entry and continually throughout the entry period.
