How to Use a Ductulator: Duct Sizing Chart and Worked Example

Airflow is normally shown in CFM in IP charts or L/s in SI tools. It comes from the downstream terminals connected to the segment. Velocity is airflow divided by free internal area and is usually shown in fpm or m/s. Friction rate describes the pressure loss of straight duct per standard length, commonly inches of water gauge per 100 ft or pascals per metre. Duct size is the round diameter or an equivalent size used to select a rectangular duct.

On a manual wheel, align the known airflow with the selected friction rate. Read the round diameter near the size scale and the corresponding velocity. If velocity exceeds the project criterion, move to a larger diameter and accept the lower friction. If space restricts the duct, a smaller size may be possible, but the higher velocity and pressure loss must be carried through the fan and acoustic review.

Digital ductulators perform the same relationships and often add rectangular dimensions, aspect-ratio limits and unit conversion. They are faster to iterate, but they still require accurate airflow and criteria. A precise answer from the wrong terminal total is still wrong.

Assume a supply segment carries 1,000 CFM and the preliminary equal-friction target is 0.08 in. w.g. per 100 ft. A ductulator gives a round size in the neighbourhood of 14 inches, depending on the roughness and calculation basis used by the chart. A 14-inch round duct has about 1.07 ft² of area, so its velocity is approximately 935 fpm. The straight-duct loss over 60 ft is 0.048 in. w.g. at the selected friction rate.

If ceiling coordination requires a rectangular duct, do not choose dimensions from gross area alone. An equivalent-duct relationship accounts for the greater wetted perimeter of a rectangle. A very flat duct uses more sheet metal and may have greater pressure loss than a compact shape. Limit aspect ratio where practical and use actual free dimensions after liner.

Now add fittings. Two elbows, a tee branch, transition and balancing damper can easily create several times the 0.048 in. w.g. straight-duct loss. The ductulator has sized one segment; the pressure calculation must follow the whole path from fan to terminal.

Repeat the sizing process for every connected segment as airflow accumulates toward the fan. Then calculate the pressure loss to each terminal and identify the critical path. The longest path is not always critical: larger ducts and fewer fittings can make a longer route easier than a short route with high-loss branches and devices.

Review supply, return, exhaust and outdoor-air systems separately. They can use different criteria and include different terminal devices. Confirm that diffuser neck velocity, grille face velocity and equipment connection velocity are acceptable in addition to the duct velocity shown by the chart.

Finally coordinate dimensions with structure, ceiling, access and fire protection. A mathematically efficient round size may not fit the available zone, while an excessively flat rectangle may create fabrication and pressure problems. Good sizing balances space, energy, sound, constructability and available fan pressure.

The most common error is reading the correct size for the wrong airflow. Trace the connected system before using the chart and confirm whether the segment carries one terminal, one branch or the sum of several branches. Return and exhaust airflow can differ from supply because of outdoor air, transfer and pressurization, so do not copy sizes between systems without rebuilding the flow balance.

Chart interpolation is another source of error. A manual wheel is intentionally approximate; selecting a standard construction size and recalculating its exact velocity and friction is more defensible than reporting false precision. Confirm whether the chart assumes standard air, a specific roughness and round equivalent diameter. High temperature, altitude or unusual duct material can require property or roughness corrections.

Rectangular conversion deserves its own check. Equivalent diameter preserves approximate friction behaviour, not velocity or area by itself. After choosing width and height, calculate actual free area, velocity and aspect ratio. Then coordinate liner, turning vanes, access doors and reinforcement because each can affect the real airflow path or pressure loss.

A completed duct schedule should show airflow, dimensions, velocity and friction rate for each segment. The pressure report should separately show fittings and devices on each terminal path. Together those outputs make the ductulator result auditable and allow reviewers to see whether the selected fan pressure, acoustics and ceiling coordination are consistent.