Boiler Draught — Identify the Incorrect Statement Which one of the following statements about boiler draught systems is WRONG?

Introduction / Context:
Draught is the slight pressure difference that draws combustion air into the furnace and expels flue gases. It may be produced naturally by a chimney (buoyancy) or mechanically by fans (forced, induced, or balanced draught). Understanding what draught does—and does not do—helps in choosing efficient and safe systems.

Given Data / Assumptions:

• Comparing natural and mechanical draught arrangements for conventional boilers.
• Considering typical impacts on fuel use, stack height, and system design.

Concept / Approach:
Natural draught depends on temperature difference and stack height, and is relatively weak and difficult to control; it often requires higher excess air and results in higher stack temperatures, which can increase fuel consumption. Mechanical draught uses fans to control and increase draught, allowing better air–fuel control, lower excess air, and reduced stack temperature, thereby reducing specific fuel consumption at a given output.

Step-by-Step Solution:
Evaluate (a): Mechanical draught reduces reliance on tall chimneys because fans provide the needed pressure head. This is correct.Evaluate (b): Claiming natural draught reduces fuel consumption is generally incorrect. Natural draught frequently leads to less efficient operation than well-designed mechanical systems.Evaluate (c): Balanced draught combines forced-draught (FD) and induced-draught (ID) fans to maintain near-atmospheric furnace pressure while ensuring adequate flow; also correct.Therefore, the wrong statement is (b).

Verification / Alternative check:
Plant performance data consistently show improved efficiency with controlled mechanical draught due to reduced excess air and improved heat recovery (economiser/air-preheater effectiveness).

Why Other Options Are Wrong:

• (a) and (c) are standard practices; “all of the above” cannot be correct since at least two are true.

Common Pitfalls:
Equating “free” natural draught with lower fuel use; in reality, poor control and higher stack losses can negate any perceived benefits.

Final Answer:
The natural draught reduces the fuel consumption.