What is the minimum air flow required during a purge in a prefire cycle of a water tube boiler?

The minimum airflow required during a purge in a prefire cycle of a water tube boiler is set at 70%. This airflow is crucial to ensure that any combustible gases are safely cleared from the combustion chamber before ignition. The purging process prevents the accumulation of flammable gases that could lead to dangerous explosions or backfires when the burner is ignited.

Maintaining this 70% airflow standard allows for adequate ventilation to effectively remove residual gases while providing enough airflow for proper combustion once the burner is activated. It's a balance between safety and ensuring efficient boiler operation. This 70% requirement is rooted in industry safety standards to protect both the equipment and personnel involved in boiler operations.