Louver Free Area, Velocity, and Water Penetration

Free area ratio is the number that turns a louver from a hole into a component: a conventional stationary louver passes roughly 35 to 50 percent of its gross face, so sizing on gross area understates the real velocity by a factor of two or more -- and the resulting velocity is double what was intended, which is how rain gets into a mechanical room that was designed correctly on paper. The limit that governs an INTAKE is water penetration, and it is a tested property rather than a rule of thumb. AMCA 500-L establishes the velocity at which a specific louver begins to pass water, and drainable-blade designs carry that point far higher than conventional ones. That is why a drainable louver can be smaller for the same airflow, and why substituting a cheaper louver of the same size late in a job is a performance change rather than a purchasing decision. RELIEF and exhaust louvers are a different problem: water penetration matters less because air is leaving, so they are limited instead by pressure drop and by the noise a high free-area velocity generates -- and a louver sized to an intake's velocity limit will be unnecessarily large for a relief application. Both the free area ratio and the penetration velocity are ENTERED from the specific louver's published AMCA data, because they are properties of its blade design and depth and cannot be assumed from its size. The gross-area error is computed here rather than described: sizing to a velocity limit on gross area runs the free area at that limit divided by the free area ratio, which is the number that puts water in the room. This does not compute the pressure drop, which rises with the square of free-area velocity and needs the louver's own curve; it does not address the sand trap, bird screen or insect screen that further reduce free area, the wind-driven rain performance which is a separate AMCA 550 test, or the drain and gutter provisions a wet climate needs. The louver manufacturer's AMCA-certified data and the mechanical engineer of record govern.

free area = gross area x the free area ratio; free-area velocity = cfm / free area; the free area a limit requires = cfm / that limit, and the gross louver it implies is that over the free area ratio; sizing to a limit on GROSS area instead runs the free area at the limit divided by the free area ratio.

Louver free area and face velocity as AMCA practice writes it, against the beginning point of water penetration established by AMCA 500-L testing for the SPECIFIC louver. The free area ratio and the penetration velocity are ENTERED from the louver's published certified data. It does not compute pressure drop, account for sand traps, bird or insect screens that further reduce free area, address wind-driven rain performance (a separate AMCA 550 test), or size drains and gutters.

Two areas and a velocity; no certified rating table is reproduced.

Estimate. AHJ and licensed professional govern.

Field names used by the API: width_ft, height_ft, free_area_ratio, airflow_cfm, water_penetration_fpm, allowable_velocity_fpm, application, gross_area_ft2, free_area_ft2, free_velocity_fpm, gross_velocity_fpm, gross_sized_free_velocity_fpm, free_area_needed_ft2

• The free area ratio is entered it is a property of the blade design and depththe louver's AMCA-certified data
• The penetration velocity is tested AMCA 500-L establishes it per louver, not per typethe louver manufacturer
• Screens are not accounted for bird, insect and sand screens further reduce free areathe mechanical engineer of record