What type of draft occurs when the boiler operates slightly below atmospheric pressure?

The correct answer, which identifies the type of draft that occurs when the boiler operates slightly below atmospheric pressure, is known as a balanced draft. In a balanced draft system, the airflow into and out of the boiler is controlled to achieve a state where the pressure inside the combustion chamber is nearly equal to atmospheric pressure but may slightly dip below it. This control allows for efficient combustion by optimizing the amount of air supplied and exhaust gases removed.

A balanced draft setup typically uses both forced draft fans to push air into the boiler and induced draft fans to pull flue gases out, creating an overall balance that helps maintain efficient operation.

In contrast, the terms associated with negative and positive drafts refer to systems where pressure is distinctly below or above atmospheric pressure respectively. A negative draft indicates a more significant pressure drop that could lead to insufficient combustion air, while positive draft denotes an increased pressure in the combustion area, which is often not desired in typical operation scenarios. Induced draft specifically refers to the mechanism used to create a draft that removes flue gases, rather than the overall balance of pressures in the system.

Thus, the identification of a balanced draft accurately reflects the operational characteristics and conditions of the boiler in this scenario.