Boiler Draught — Identifying Induced Draught A fan located at or near the base of the chimney reduces the pressure at the fuel bed below atmospheric to draw gases through the furnace. What is this type of draught called?

Introduction / Context:
Draught provides the pressure difference needed to move air and flue gases through the furnace, boiler passes, and stack. Mechanical systems use fans to create this pressure difference by either pushing or pulling the gases.

Given Data / Assumptions:

• A fan is located near the chimney base.
• Measured effect: the pressure at the grate/fuel bed is below atmospheric.
• Goal: identify the type of draught.

Concept / Approach:
If a fan pulls gases out of the boiler and maintains the furnace under slight negative pressure, it is an induced draught system. If a fan pushes air into the furnace creating positive pressure, it is forced draught. Balanced draught uses both, arranged to keep near-zero furnace pressure. Natural draught relies solely on chimney buoyancy with no fans.

Step-by-Step Solution:
Fan position and action: drawing flue gases toward the stack → suction.Furnace pressure observed: below atmospheric → negative pressure.Classify as induced draught.

Verification / Alternative check:
Manometer readings at furnace doors show slight negative pressure under induced draught operation.

Why Other Options Are Wrong:
Natural: no fan; relies on stack effect only.Forced: fan pushes air in, making furnace pressure positive.Balanced: combines both forced and induced with near-neutral furnace pressure.

Common Pitfalls:
Equating fan near chimney with “forced”; the sign of furnace pressure determines the type.

Final Answer:
induced draught