Duct Size Calculator and HVAC Fan Curve

The tree conserves airflow at each junction. Equal-friction sizing solves a round diameter for each design flow, then rounds upward. For a rectangle, Huebscher equivalence is

The diameter and both sides use the same length unit. The selected rectangle is evaluated as an equivalent round duct for straight friction, while fitting losses use its actual area.

For every segment,

Flow validity uses the actual velocity and hydraulic diameter: $Re=VD_h/\nu$, with $D_h=2ab/(a+b)$ for rectangles and $D_h=D$ for round ducts. Equivalent-round Reynolds numbers are retained only for the friction approximation. Viscosity entered in mm²/s is explicitly converted to m²/s.

Colebrook is iterated ten times. The solver uses 64/Re below 2300 and a transition blend to 4000 only to continue the resistance curves; design sizing requires turbulent flow.

Balancing adds nonnegative terminal K-values so all requested paths have equal loss. At other flows, those K-values remain fixed. The tree solver updates friction factors and parallel-path flow fractions until they converge; a pressure-residual condition verifies equilibrium. Bisection finds the fan–system intersection.

The three fan points use linear interpolation. Speed scales flow by the speed ratio and pressure by its square; density scales pressure proportionally. Fan total pressure equals fan static pressure plus fan outlet velocity pressure. Shared equipment loss scales with total flow squared. Air power is total pressure times airflow, before fan, drive and motor losses.

Compressibility screening follows the approximate pressure-drop guidance in ASHRAE Fundamentals (2017), Chapter 3, Compressible Conduit Flow. For each design/operating state, a conservative static-pressure variation bound is the total-pressure budget plus the largest duct or fan outlet velocity pressure:

The bound follows the static/total-pressure relation; it is intentionally more restrictive than checking total-pressure loss alone. The minimum absolute pressure is a user-supplied lower bound over the modeled circuit. The screen does not modify the constant-density solution.

• One shared trunk T feeds direct branch A and subtrunk S; S feeds terminal branches B and C. All terminals discharge to the same reference pressure. The model does not support arbitrary topology.
• Air is incompressible with uniform density and kinematic viscosity supplied by the user. Defaults represent ordinary room-temperature air. The entered minimum absolute pressure must be a valid lower bound throughout the modeled circuit, including fan suction; its 100 kPa default is a near-sea-level estimate.
• All ducts are rigid, internally clear, constant-section segments, with one common roughness and one cross-section family. Rectangular aspect ratios are limited to 1–4.
• Nonnegative fitting K-values refer to actual local segment velocity and are fixed with flow. Convert coefficients referenced to a different section using the square of the velocity ratio. Junction interaction, negative individual loss coefficients and fan system effects require separate engineering judgment.
• Terminal K includes the outlet kinetic-energy loss unless the selected diffuser data already includes it. Shared equipment loss includes only components outside the chosen rated fan boundary.
• Default lengths, K-values and three blower points are editable illustrative design inputs, not manufacturer or standards data. Replace them for an actual selection. The default 0.09 mm roughness follows ASHRAE's average rigid-duct friction chart.
• The drawing is schematic with symbolic tracer motion. Its Schematic units selector switches every drawing caption together between metric (mm, L/s, Pa) and US (in, cfm, conventional in. w.c.). This selector is independent of the viewer’s field-unit switch. Size rounding does not guarantee a commercially available size. Fan speed scaling does not assess motor power or permitted wheel speed.

Use for preliminary sizing and comparison of turbulent, low-speed air flow in rigid round or rectangular ducts. Error conditions reject design or operating Reynolds numbers below 4000, using actual duct velocity and hydraulic diameter. The equivalent-round Reynolds number is not the rectangular-flow validity criterion. The transition blend only continues the system curve numerically; it does not validate transitional flow.

Only use rated data from a stable decreasing portion of one blower curve. The tool requires an intersection within the entered range and reports convergence residuals. A three-point linear curve cannot resolve stall, surge, controls, or multiple operating points. The pressure margin at the requested flow is only meaningful within the rated range.

The requested-split pressure budget assumes the calculated balancing dampers are fitted. With balancing disabled, delivered outlet flows generally differ from the requests; use the delivered-flow table. The passive network is re-solved with fixed geometry as blower flow changes. The 95% delivery advisory and velocity threshold are review aids, not compliance criteria.

No ACCA Manual D text or ASHRAE fitting database is reproduced. Flexible duct compression, leakage, changing filter loading, stack and wind effects, acoustics, heat transfer, duct construction and code compliance are outside the model. Check against the standard in force and the actual fan installation category. Installation, balancing and commissioning belong to a qualified person.

The constant-density approximation has two independent screens. Every design and operating duct velocity, and the fan outlet velocity at both flows, must be at most 60 m/s. The pressure-variation bound at each flow must also be at most 10% of the entered minimum absolute system pressure. Both screens block results when exceeded; neither is a universal compressibility boundary.

Enter the lowest expected absolute static pressure anywhere in the modeled circuit, including fan suction, in the Minimum absolute system pressure field. Use site barometric pressure minus the largest expected suction; never enter gauge pressure. The default 100 kPa is a near-sea-level estimate, not a site measurement. Air density and viscosity must represent the same conditions.

ASHRAE Fundamentals (2017), Chapter 3, Compressible Conduit Flow, gives an approximate 10% initial-pressure-drop limit for incompressible conduit analysis. This tool conservatively compares the total-pressure budget plus the largest duct/fan outlet velocity pressure with 10% of the supplied minimum absolute static pressure. The added velocity-pressure allowance covers static-pressure changes caused by section velocity changes. This is an applicability screen, not a compressible-flow correction; use a compressible model outside it. NASA Glenn discusses the separate dependence on Mach number.