Mechanical Draught — Effect of Smoke on Draught Level How does the level of mechanically produced draught (fan-based) vary with the amount of smoke in the gases?

Introduction / Context:
Draught can be natural (chimney) or mechanical (fans). Unlike natural draught, which depends on gas density and temperature differences in the chimney, mechanical draught is generated by equipment and set by fan characteristics and control settings.

Given Data / Assumptions:

• Induced/forced/balanced draught systems using fans.
• Fan speed and control position held constant.
• Smoke concentration varies due to firing conditions.

Concept / Approach:
The draught produced by a fan is primarily a function of fan speed, blade geometry, and system resistance curve. While increased smoke can increase particulate loading and slightly change resistance, the generated draught set point itself is not directly determined by smoke amount; the fan maintains the target pressure difference. Therefore, in the usual theoretical treatment, mechanical draught is considered independent of smoke quantity.

Step-by-Step Solution:
Define mechanical draught: pressure difference created by a fan.Hold fan operating point fixed (speed/damper).Recognize smoke is a by-product; it does not directly set draught level.

Verification / Alternative check:
Plant practice: the same draught setpoint is maintained by fan controls regardless of smoke (combustion tuning addresses smoke separately).

Why Other Options Are Wrong:
increases / decreases: these imply a direct dependence that does not exist under constant fan setting; only system losses shift slightly.

Common Pitfalls:
Confusing draught magnitude with total pressure losses; smoke can affect fouling, but the fan still sets draught.

Final Answer:
does not affect