Noise control for air-cooled equipment: Cooling-fan noise arises largely from blade-passage turbulence. Which design change(s) reduce fan noise by allowing lower tip speed for the same airflow?

Introduction / Context:
Fans in air-cooled heat exchangers and cooling towers can produce significant broadband noise from turbulent flow and tonal noise at the blade-pass frequency. Acoustic mitigation often targets reduced tip speed and improved aerodynamic loading to lower sound generation at the source.

Given Data / Assumptions:

• Airflow requirement is fixed by process duty.
• Primary noise mechanism: turbulence and blade-pass tones.
• Design variables: fan diameter and blade count.

Concept / Approach:
For a given volumetric flow, increasing fan diameter allows the same flow at lower rotational speed, cutting tip speed and reducing noise (sound power often scales strongly with tip speed). Adding blades increases solidity, enabling similar flow at reduced speed or lower loading per blade, which also lowers noise. Together, these changes shift operation to a quieter regime without sacrificing cooling capacity.

Step-by-Step Solution:
Hold airflow constant; seek to reduce tip speed V_tip = π * D * N_rot.Increase diameter D → lower N_rot for same flow; V_tip decreases.Increase blade count → maintain flow at lower N_rot and reduced per-blade loading; turbulence and tones decline.

Verification / Alternative check:
Manufacturers’ acoustic curves show sound power reductions with larger, slower fans and optimized blade counts/pitch. Field retrofits often employ variable-speed drives plus larger-diameter impellers to meet noise limits.

Why Other Options Are Wrong:
Only (a) or only (b): Each helps, but the combined choice acknowledges both effective strategies.Neither: Contradicts well-established fan acoustics principles.

Common Pitfalls:

• Reducing speed without compensating diameter or blade count can undersupply airflow.
• Ignoring structural and motor constraints when increasing diameter or blade number.

Final Answer:
both (a) & (b).