Which draft can be used in some boiler designs in addition to forced, induced, and balanced drafts?

• A. Atmospheric draft.
• B. Positive draft.
• C. Natural draft.
• D. Vacuum draft.

Answer: (C)

Draft in boilers is the movement of air and combustion products driven by pressure differences. In some boiler designs, natural draft is used, which relies on the buoyancy of hot combustion gases to pull air into the furnace and push exhaust gases up the stack without any fans. The hot gases are less dense than the surrounding cooler air, so they rise through the stack, creating a flow that continues as long as the flame keeps the gases hot. The strength of natural draft depends on stack height, the temperature of the flue gases, and ambient conditions; it works best in smaller, simpler boilers or where dependable electricity for fans isn’t available. It’s less controllable than fan-assisted methods and more sensitive to weather and installation specifics. While atmospheric draft is a related idea, natural draft is the commonly used term for this approach, and it fits the scenario of adding a non-fan method alongside forced, induced, or balanced drafts.