Size Louver

A louver is sized twice: once by the engineer, in free area, and once by the elevation, in face area. The gap between those two numbers is the free-area fraction of the specific product, and it is the number everyone remembers instead of reading. The remembered figure sizes a louver that whistles, drinks rain, or does not fit the opening the facade already committed to.

Inputs the command needs

1. The airflow, per louver, with the engineer named as its source, and whether the louver is intake or exhaust, because the allowable velocities differ and both come from the data or the engineer.
2. The louver, by manufacturer and model, with its data sheet: free-area fraction at the size in question, maximum recommended face velocity or the beginning-of-water-penetration velocity, and the frame dimensions.
3. The screen, if any. Bird and insect screens reduce free area by an amount the data sheet states; an insect screen is not a bird screen.
4. The opening as drawn, if the elevation already has one, with what surrounds it.
5. Any criterion from the spec or the engineer: a velocity limit, a rain class, a pressure-drop budget.

Workflow

Step 1 — Read the inputs back and flag what is missing

List each figure with its source. Anything without a source is not supplied and stays that way. Never substitute a typical velocity or a typical free-area percentage; free area varies by product and by size, and the data sheet's figure at the tested size is the only one that counts.

Step 2 — Work out the free area

required free area = airflow / maximum face velocity     <arithmetic shown>

Both terms with their sources beside them. If a screen derating applies, apply it here, from the data sheet's figure, shown.

Step 3 — Work out the face area

required face area = required free area / free-area fraction   <arithmetic shown>

State the fraction's source and the size it was tested at. If the required louver is much smaller than the tested size, say the fraction will be lower at small sizes and the data sheet or the manufacturer has to confirm it; do not extrapolate.

Step 4 — Fit it to the opening

face required     <width x height options that satisfy the area>
frame and mullions <from the data sheet, subtracted from the glass line>
opening as drawn  <as given>
difference        <fits / short by, in the user's units>

A louver bank has mullions and a frame, and the free area lives inside them. If the opening as drawn is short, report by how much and note the two moves: a bigger opening, or a product with a higher fraction, both of which are decisions for the engineer and the elevation, not for this command.

Step 5 — Report and hand off

The table, then: every not supplied figure counted, the intake-exhaust distinction confirmed, and the hand-offs named. Clearances to grade, property lines, other openings, and contaminant sources are code text you quote; whether the pressure drop works is the engineer's; where the louver sits on the elevation is facade-module.

Step 6 — Save, if asked

Ask whether to write the sizing to a file and where.

Rules

• Never supply a velocity, a fraction, or a derating. Every term carries its source, and the source is a data sheet or the engineer.
• Show the arithmetic at every step, in the user's units, never converted.
• Free area varies by size: state the tested size next to the fraction, every run.
• Intake and exhaust are different problems. Confirm which this is.
• Code clearances are quoted by the user or left as named open questions.
• Never pick the louver. This sizes the face the airflow needs; the product is the spec's decision.

Anti-patterns

• Using a remembered "about 50%" free area. The range across real products is wide, and the remembered figure is wrong in the direction that whistles.
• Sizing at the data sheet's velocity for a louver in a rain class the spec does not allow.
• Ignoring the screen because it is a screen.
• Extrapolating a big louver's fraction down to a small louver.
• Treating the opening as drawn as the requirement instead of the constraint.
• Quoting a code clearance from memory instead of asking for the text.