Chimney design — The natural draught (draft) produced by a chimney primarily depends on which factors?

Introduction:
Natural draught provides the pressure head to move combustion gases without fans. It is created by density differences between hot flue gases and the cooler ambient air column surrounding the chimney.

Given Data / Assumptions:

• No mechanical draft devices (purely natural draught).
• Steady operation; uniform ambient conditions along chimney height.

Concept / Approach:
The pressure difference (draft) arises because the column of lighter hot gas inside the chimney exerts a lower hydrostatic pressure than an equal-height column of cooler, denser outside air. The draft is approximately proportional to the height and the density difference, which in turn depends on gas temperature and composition.

Step-by-Step Solution:
Express draft head ΔP ≈ g * H * (ρ_air − ρ_gas).Note dependence on H (height) and densities (set by temperatures).Conclude that both density difference and height control natural draught.

Verification / Alternative check:
Stack effect equations and design charts for chimneys explicitly use H and temperature (density) to compute draft and volumetric flow.

Why Other Options Are Wrong:

• (a) or (b) alone are incomplete; both factors act together.
• (d) Contradicts the physical basis of the stack effect.
• (e) Moisture affects density slightly but is not the primary sole factor.

Common Pitfalls:
Ignoring ambient temperature variations; neglecting frictional losses which reduce the effective draft available to equipment.

Final Answer:
Both (a) and (b)