The Flow Calculator provides a tool for you to complete partial calculations to choose the cross section or to estimate the flow rate. This tool is not an HVAC analysis software. Although the software runs its main calculations of an HVAC system at the diagram stage, you can work with the flow rates during the routing stage.
The values are not connected to the run values. The Flow Calculator estimates for parts, not for the duct run.
To estimate sizes, the process must account for the frictional and dynamic resistance to air flow. Fluid flow is used to correlate pressure changes with flow rates and the nature of the duct. For a given duct, the pressuredrop for a given flow rate is needed.
The inputs that you record on the Calculate Flow dialog box include duct flow rate, maximum allowed velocity, and pressure loss. If you type flow rate and velocity, the output is diameter (circular) and pressure loss per length. If you type flow rate and pressure loss per length, the output is diameter (circular) and velocity. If you input flow rate and diameter (circular), the output is velocity and pressure loss per length. Also, the software estimates the equivalent rectangular cross section based on the input aspect ratio.
You can display the Calculate Flow dialog box by clicking Tools > Flow Calculator.
Assumptions

The software processes the calculations based on air flowing in the duct at standard atmospheric conditions (temperature at 70° F, and air pressure at 14.7 psi).

The Flow Calculator estimates diameter, pressure loss, flow rate, and velocity only in uniform straight features. The software does not estimate on the basis of fittings, size changes, direction changes, or obstructions. For additional information on pressure loss in fittings, refer to a handbook with industry standards.

The Flow Calculator is based on the equal friction method of duct design. The equal friction method is commonly used in the industry in systems with low to moderate velocities, based on a friction drop per 100 feet of duct. For this reason, the duct length estimates in the software are based on 100 feet length (or 1 meter in metric calculations).

Friction factor requires a duct roughness based on duct material. The Flow Calculator uses a mediumsmooth material with a roughness factor of 0.0003 feet (like galvanized steel). You can perform suggested estimations for galvanized steel and apply correction factors to the results for other material types.

The software performs estimates without the presence of a fan.

All estimates are based on straight sections with no loss coefficients available for fittings.

The software uses imperial units. Before calculating results, the software converts values that you type to the proper imperial units using conversion factors. You can review the results in the units sets by clicking Tools > Options and selecting the Units of Measure tab.

The width ratio (or aspect ratio) refers to the ratio of depth to width for a rectangular cross section.