HVAC duct sizing — choose the most widely used design approach In practical air-distribution design for buildings, several sizing methods exist (velocity reduction, equal friction, static regain, and dual methods). Which method is most commonly adopted for initial duct sizing because of its simplicity and predictable pressure-drop control?

Introduction / Context:
Duct design in heating, ventilation, and air-conditioning (HVAC) must balance airflow delivery, noise, energy use, and cost. Several textbook methods exist, but one is favored in routine building design for its practicality and speed.

Given Data / Assumptions:

• Goal: select the method most commonly used for initial duct sizing.
• Air distribution is for comfort applications at typical velocities and pressures.
• Fan total pressure and allowable friction loss per unit length guide choices.

Concept / Approach:
The equal friction method sets a uniform friction rate (pressure drop per unit length) for all main ducts. This leads to diameters that naturally taper with flow, limiting excessive velocities and simplifying balancing. It offers a good trade-off between accuracy and design time.

Step-by-Step Solution:

Verification / Alternative check:
Compare with static regain. While static regain can yield lower fan power for large systems, it needs more iteration and is less common for small to medium buildings. Velocity reduction is a rule-of-thumb start but not as systematic for pressure control.

Why Other Options Are Wrong:

Static regain method: energy-efficient for large high-velocity systems, but more complex and not the most common.Velocity reduction method: simple starting point, lacks consistent network pressure control.Dual or double method: specialized for dual-duct systems, not general sizing.Constant velocity method: oversimplifies and can mismanage pressure losses.

Common Pitfalls:
Choosing an unrealistically low friction rate (oversized ducts, high capital cost) or too high a rate (noise and energy penalty). Always confirm terminal velocities and sound criteria.

Final Answer:

equal friction method