To calculate ventilation for a confined space, consider these key points:

1. Hazard Assessment: Before entry, a competent person must assess the space for potential hazards, including atmospheric hazards like oxygen deficiency or the presence of toxic, flammable, or explosive substances [1]. Testing the atmosphere is crucial to identify these hazards 3, 10.
2. Ventilation Requirements:

• Adequately ventilate confined spaces to prevent the accumulation of toxic materials or oxygen deficiency, ensuring the air replacing what's withdrawn is clean and respirable [2].
• If a hazardous atmosphere is identified, the space must be ventilated or purged before entry 3, 4.
• Continuous ventilation is required to maintain a safe atmosphere while the space is occupied [3].

3. Mechanical Ventilation:

• If mechanical ventilation is needed, ensure it's provided and operated as necessary to maintain a safe atmosphere [4].
• The ventilation system should include an alert system to warn workers of system failure, allowing them to evacuate safely [4].
• Workers must be trained in evacuation procedures in case of ventilation system failure [4].

4. Airflow and Velocity:

• For fixed enclosures, ensure a rate of airflow sufficient to maintain a velocity away from the welder of not less than 100 linear feet per minute [2].

5. Alternative Measures:

• If ventilation is impractical or ineffective in eliminating a hazardous atmosphere, use appropriate personal protective equipment (PPE) such as supplied air respirators 1, 3.

6. Oxygen Use: Never use oxygen for ventilation purposes 1, 16.
7. Continuous Monitoring: If a safe atmosphere cannot be maintained through ventilation alone, continuous atmospheric monitoring by a competent person is necessary [3].
8. Emergency Procedures: Develop and implement emergency procedures, including immediate evacuation, if the atmosphere becomes hazardous [4].

Keep in mind that if hazards remain even after taking precautions, the confined space may need to be treated as a permit-required confined space [5].