Plant Operation Practice Test

Which type of draft is considered the oldest form used in boilers?

1. Natural draft

2. Forced draft

3. Induced draft

4. Mechanical draft

The correct answer is: Natural draft

Natural draft is considered the oldest form of draft used in boilers due to its reliance on the basic principles of physics and the natural buoyancy of hot gases. In a natural draft system, the movement of air is driven solely by the temperature difference between the hot gases produced by combustion and the cooler air outside the boiler. This temperature difference creates a pressure difference, allowing for the upward flow of exhaust gases through the flue. Historically, this method has been easy to implement, as it does not require additional mechanical equipment or energy sources, making it a practical choice for early steam boilers. Since ancient times, many heating systems utilized the natural draft principle as it draws air into the combustion zone and expels combustion products without the need for fans or blowers. In contrast, forced draft relies on fans to push air into the combustion chamber, while induced draft uses fans to create a vacuum that pulls flue gases out of the system. Mechanical draft involves various fan-driven approaches to either push air into or pull flue gases out of a boiler. These methods developed later to improve efficiency and control of the combustion process, especially as the energy needs grew and stricter regulations were put in place regarding emissions and efficiency standards.