Fabric Air Duct (Textile Air Dispersion) Sizing: Diameter, Velocity, Fabric Area, Mass & Hangers

The formula

This is the expression the tool evaluates. Every term is named underneath, with the unit it must be supplied in.

How the result is derived

Step by step, from the values you type to the figure on screen.

1. The 7 inputs are read from the form on every keystroke: Airflow at the Inlet, Design Inlet Velocity, Size Step Offered, Duct Length, Seams & End Allowance, Fabric Weight and Hanger Spacing.
2. Each value is checked against the accepted range in the input table below. A value outside its range stops the calculation rather than producing a misleading figure — the results blank out and a message appears.
3. The validated values are substituted into the expression above, which resolves Duct Diameter together with every supporting figure in one pass — no value is carried over from a previous entry.
4. The supporting outputs — Computed Diameter, Inlet Velocity in the Chosen Size, Air Released per Metre (Even Spread), Fabric incl. Seams, Fabric Mass and Suspension Points — come from the same pass, so they always describe the same case as the headline figure.
5. Results are rounded for display only. The full-precision value is used throughout the chain, so reading a rounded intermediate figure back into the tool by hand can shift the last digit.

What each input means

Where to read each value on the floor, the unit it must be in, and the range the tool accepts.

What the tool returns

The headline figure and every supporting value it is built from.

Worked example

Substituting

Rounding 768 mm up to 800 mm drops the inlet velocity from 6.0 to 5.5 m/s, and adds 4% to the fabric.

How to use it

1. Work through the input groups in order — Air and Duct. The defaults are a realistic case, so you can change one value at a time and watch what moves.
2. There is no calculate button. Every figure recalculates as you type or drag, which is what makes this usable for a what-if sweep rather than a single answer.
3. Read Duct Diameter in the dark results panel — that is the headline figure, expressed in mm.
4. Check the supporting rows underneath (Computed Diameter, Inlet Velocity in the Chosen Size, Air Released per Metre (Even Spread), Fabric incl. Seams, Fabric Mass and Suspension Points) before acting on the headline — they are where an implausible input usually shows itself first.
5. Reset to defaults returns every field to the reference case, which is the quickest way to check whether a surprising result came from the tool or from an input you had changed earlier.

Where this is used

• Process planning — establishing Duct Diameter before a trial is booked, so machine time and material in Nonwovens & Technical Textiles are committed against a calculated figure rather than an estimate.
• Costing and quotation — Duct Diameter is an input to the cost sheet, and quoting from a worked number rather than a remembered one is what keeps a margin intact.
• Troubleshooting — when the floor result drifts from plan, entering the measured values (starting with Airflow at the Inlet) shows how much of the gap in Duct Diameter each variable explains.
• Teaching and study — the accepted ranges bracket normal Nonwovens & Technical Textiles practice, so moving one variable at a time shows the shape of the relationship rather than a single answer.

Reading the result

Typical bands and what each one is telling you.

Assumptions and limits

• The diameter follows from continuity: airflow equals area times velocity. The design velocity, size step, seams allowance and hanger spacing are yours or the maker's; the tool has no typical value for any of them. Air released per metre assumes the duct lets the air out evenly along its length, which is how most fabric ducts are specified; nozzles and orifices that throw air further are the maker's design. Mass is the fabric only; rails, cables and end caps are extra. **This is a sizing estimate: pressure, throw and comfort are checked by the duct maker's software for the room.**
• Every input is bounded to the range normal practice occupies (Airflow at the Inlet 100 to 500000 m³/h, Design Inlet Velocity 1 to 20 m/s and Size Step Offered 1 to 200 mm, and so on for the rest). Those bounds are guard rails against typing errors, not a claim that the formula fails one unit outside them.
• The calculation is deterministic: the same inputs always give the same result. It carries no allowance for machine condition, operator skill, ambient conditions or lot-to-lot material variation unless an input above explicitly represents one.
• Nothing is sent anywhere. The maths runs in your browser, so the numbers you type never leave the page.

Standards and further reading

• The duct maker's selection software and data: permeability, orifice patterns, throw and pressure for each size.
• Fire performance rules for building materials where the duct is installed, such as EN 13501-1 in Europe or UL 2518 in the USA for air dispersion systems.

Questions people ask