ABSA 5th Class Practice Exam

The correct answer focuses on the principle of convection, which is essential to understanding how draft is generated in a boiler. When cooler air surrounds the boiler, it creates a difference in temperature between the inside of the boiler and the surrounding environment. The warmer air inside the boiler tends to rise, creating a low-pressure area. As a result, the cooler air from outside is drawn into the boiler to replace the rising hot air. This natural movement of air, driven by temperature differences, enhances the draft that is essential for efficient combustion and proper operation of the boiler.

In contrast, warming air inside the boiler contributes to a general heating process rather than generating draft on its own. While using a fan to circulate air may aid in air movement, it is not a natural cause of draft. Increasing the fuel supply does not inherently create draft either; it mainly increases combustion but does not address the airflow dynamics that are critical for establishing draft. Therefore, the cooler surrounding air is key to enhancing the natural draft in the boiler system.