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[Fluid Flow]
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Head-Loss

Five air-side head-loss calculators: duct entrances and exits, louvres, perforated plates and wire screens — loss coefficients and pressure drop for ventilation and cooling ducts.

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Every K = 1 component costs one velocity head — about 60 Pa at 10 m/s in air — and fan power rises with the cube of the flow you push through it.

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Results are indicative

Figures assume typical conditions and the stated method. For measured, guaranteed numbers on your plant, our engineers run site surveys, heat loss audits, and full process models.

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Fans are sized by summing pressure drops, and most of a duct run's drop comes from components, not straight duct. Each of these five tools returns a loss coefficient K and the resulting pressure drop for a common air-side component.

What's in this suite

  • Duct Entrance — bellmouth entry loss as a function of rounding radius: from K=0.5 for a sharp edge down to about 0.03 for a well-rounded bellmouth (r/Dh​≈0.2).
  • Duct Exit — abrupt discharge into a plenum or atmosphere; all kinetic energy is lost, K=1.0 (the Borda–Carnot limit).
  • Louvre — angled blinds in a duct, with a geometric base term plus a Reynolds-number turbulence correction.
  • Thin Perforated Plate — thin-plate orifice-array loss from the free-area ratio (t/d<0.015, Re>105).
  • Wire Screen — woven-mesh loss from the free-area ratio (Re>103).

Method

Every component follows the velocity-head form

Δp=K⋅21​ρV2

where V is the mean duct velocity at the component and ρ the air density. The coefficient K comes from fits to Idel'chik and ASHRAE tabulated data (entrance polynomial accurate to about ±10%), or from free-area-ratio expressions for plates and screens, such as

K=1.3(1−FAR)+(FAR1−FAR​)2

for wire screens — a viscous-drag term plus a sudden-expansion term.

Assumptions

Incompressible flow (air at ordinary ventilation velocities), fully turbulent regime within each tool's stated Reynolds limits, uniform approach velocity. Components in series simply add their K values when referenced to the same velocity.

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Frequently asked questions

K expresses a component's pressure drop in velocity heads: Δp = K·½ρV². A K of 1 means the component destroys exactly the kinetic energy of the approaching flow. It is dimensionless, so the same K applies at any flow rate — the pressure drop then scales with velocity squared.

The mean velocity in the duct at the component plane — volumetric flow rate divided by duct cross-sectional area. If the duct changes size along the run, evaluate each component at its own local velocity before summing pressure drops.

Compute each component's Δp at its local velocity and add them, together with straight-duct friction. Where the duct area is constant you can equivalently sum the K values first. The result is the static pressure the fan must develop at that flow rate.

The perforated-plate tool assumes a thin plate (thickness under 1.5% of hole diameter) at Reynolds numbers above 10⁵; the screen correlation applies above 10³; the entrance polynomial is a fit to Idel'chik/ASHRAE data accurate to roughly ±10%. Outside those ranges treat results as indicative only.