Best Air Velocity Over Coils

Best Air Velocity Over Coils: Fan-Coils (Residential) and Air Handling Units (AHUs)
1. Introduction
To achieve optimal heat transfer and minimize noise, the air velocity over coils must balance thermal performance with pressure drop and acoustic considerations. Below are recommended values and rationale, organized by application.
2. Residential Fan-Coils — Recommended Range
• Air velocity: 300–500 feet per minute (fpm) over the coil face area provides the best balance.
• Rationale: In this range the convective heat transfer coefficient (h) increases significantly compared to very low laminar flow, while fan pressure rise and noise remain manageable.
• Consequences: Below 300 fpm heat transfer falls, requiring larger coil surface area; above 500 fpm noise and pressure drop increase noticeably, and fan energy consumption rises.
3. AHU Coils — Recommended Range
• Air velocity: 400–700 fpm over AHU coils (fins) is suitable for most residential and small-to-medium commercial applications.
• Rationale: AHUs typically handle larger airflows and coil face areas, so slightly higher velocities can be used to achieve desired heat transfer without unacceptable noise.
• Consequences: Above 700 fpm noise, pressure drop, and risk of flow separation/turbulence increase; below 400 fpm thermal effectiveness decreases and coil area requirements grow.
4. Practical Notes and Recommendations
• Fan speed control: Use VFDs or multi-speed fans for optimization.
• Pressure drop and duct design: Calculate pressure drop and select a fan sized for the target face velocity.
• Noise mitigation: Use silencers, lined ductwork, or duct geometry improvements to reduce noise.
• Air–fin contact: Ensure even face velocity distribution and keep fins clean to prevent local performance losses.
• Field verification: Conduct on-site tests with an anemometer and sound measurements to confirm performance for the specific installation.