What is the minimum airflow during a purge on a prefiring cycle on an automatic system for a water-tube boiler?

What is the minimum airflow during a purge on a prefiring cycle on an automatic system for a water-tube boiler?

• A. 50%
• B. 60%
• C. 70%
• D. 80%

Answer: (C)

Before ignition, a purge is used to clear any residual fuel vapors from the furnace so accidental ignition can’t occur. The purge air must be enough to dilute and sweep away combustible vapors quickly, bringing their concentration well below the lower explosive limit before the burner is allowed to fire. Seventy percent of the normal firing air is the minimum flow that reliably achieves this safe clearance within the purge period. If the flow drops below this, vapors may linger and create a risk of ignition when the ignition source comes on. Using more than this minimum is safe but not required for the purge to be effective; the 70% value is chosen as a practical lower bound that balances safety with purge time and energy usage.

Before ignition, a purge is used to clear any residual fuel vapors from the furnace so accidental ignition can’t occur. The purge air must be enough to dilute and sweep away combustible vapors quickly, bringing their concentration well below the lower explosive limit before the burner is allowed to fire.

Seventy percent of the normal firing air is the minimum flow that reliably achieves this safe clearance within the purge period. If the flow drops below this, vapors may linger and create a risk of ignition when the ignition source comes on. Using more than this minimum is safe but not required for the purge to be effective; the 70% value is chosen as a practical lower bound that balances safety with purge time and energy usage.