Face velocity
The speed of air passing through a coil or filter, found by dividing airflow by the face area it passes through. Pressure drop across the coil or filter rises roughly with the square of face velocity, so it is the variable that actually drives resistance, not airflow alone.
What is face velocity on a coil or filter?
Face velocity is airflow divided by the face area of a coil or filter, and it is the speed the air actually passes through that component at. Pressure drop rises roughly with the square of face velocity, which is why airflow alone does not predict resistance.
Face velocity: how it works
The same airflow spread across a larger face area moves more slowly through it, and a slower face velocity produces less pressure drop and, for a wet cooling coil, more contact time for moisture removal. A manufacturer's rated pressure drop applies at the tested face velocity specifically, not at any airflow the coil might see once installed.
Where you meet the term
- In manufacturer coil and filter performance data, published against a stated face velocity or airflow and face area.
- In a static pressure budget, where reading a coil's rated drop at the wrong face velocity understates or overstates the real figure.
- In a bypass factor calculation, where face velocity is the variable that changes how much air actually contacts a coil's wetted surface.
How to check it
- Confirm the coil or filter's actual face area. Face velocity cannot be read from an airflow figure alone. The component's actual face area, and whether it matches what the manufacturer tested, both matter. Technician
- Compare against the manufacturer's tested condition. A pressure drop figure applies at one tested face velocity. Reading it against a system running a different airflow through the same face area needs the manufacturer's performance curve, not the single tested figure. Technician
Face velocity: what it is not
- Pressure drop does not scale in proportion to airflow. It scales closer to the square of face velocity, so a modest airflow increase can raise pressure drop by far more than the airflow increase alone would suggest.
- A larger coil is not automatically a lower-resistance coil at a different airflow. Face velocity, not coil size alone, is what the pressure drop figure actually depends on.
Face velocity questions
Does a bigger filter always mean lower face velocity?
Only if airflow stays the same. A larger filter at the same airflow lowers face velocity and pressure drop; the same filter at higher airflow raises both, regardless of the filter's physical size.
Why does a wet cooling coil have a different face velocity effect than a dry one?
A wet coil's condensate film adds resistance beyond dry airflow friction, and that added resistance also scales with velocity, which is why manufacturers publish separate wet and dry pressure drop figures at the same face velocity.
Is face velocity the same thing as duct velocity?
No. Duct velocity is airflow divided by duct cross-sectional area, while face velocity is specific to a coil or filter's face area. The two are related but calculated at different points in the system.
Where this definition comes from
Written from standards body publication material held in the HVAC Bench evidence record. Where a document limits a definition to certain models, that limit is repeated here rather than generalised across a brand.
- ASHRAE Handbook, FundamentalsASHRAE reference volume carrying the psychrometric relationships, the climatic design condition tables and their annual percentile basis, duct and pipe fitting loss coefficients, and the bin and degree-day methods used for seasonal energy estimation.