Internally Lined Ducts: Recalculate Clear Area Before Pressure Loss
A duct pressure-loss sheet can be mathematically correct and still describe the wrong opening. This happens when a lined duct’s metal dimensions are entered as its clear air dimensions. Internal lining reduces area, increases velocity at the same flow and changes hydraulic diameter. Its surface characteristics are an additional input. Correct the geometry before deciding which friction data to use.
1. Establish what the drawing dimensions mean
A notation such as “800 × 500, lined” is incomplete unless the drawing convention states whether 800 × 500 mm describes the clear air opening or the sheet-metal casing. If those are already clear internal dimensions, subtracting the liner again creates a second error. Check the drawing notes, fabrication detail and model parameters together; do not infer the convention from a typical office practice.
For the calculations below, W and H mean the inside dimensions of the metal shell before lining. Neglect sheet thickness when discussing the resulting overall casing size. With uniform lining thickness t on all four faces, clear width a = W − 2t, clear height b = H − 2t and clear area A = ab. For different thicknesses on opposite faces, subtract each actual thickness separately.
Internal liner and external wrap occupy different sides of the air boundary. Johns Manville’s insulation overview distinguishes lined metal ducts, externally wrapped ducts and double-wall construction. That distinction belongs in the calculation input, not only in the insulation schedule.
2. Worked example: quantify the lost area
Assume an original straight-duct example with a metal-shell inside opening of 800 × 500 mm, uniform 25 mm internal lining on four faces, airflow Q = 1.60 m³/s and air density ρ = 1.20 kg/m³. Assume steady flow, uniform nominal lining thickness and no other obstruction. These are illustrative inputs, not recommended operating limits.
Unlined opening: A₀ = 0.800 × 0.500 = 0.4000 m².
Lined clear opening: a = 0.750 m and b = 0.450 m; A₁ = 0.3375 m².
Area reduction = (0.4000 − 0.3375)/0.4000 = 15.625%.
Unlined mean velocity: V₀ = 1.60/0.4000 = 4.00 m/s.
Lined mean velocity: V₁ = 1.60/0.3375 = 4.741 m/s, an 18.52% increase.
Percentages do not transfer directly between area and velocity because velocity varies inversely with area. A 15.6% area reduction does not produce a 15.6% velocity increase. Retaining extra calculation precision through the following steps avoids a misleading mismatch between the pressure-loss results.
3. Recalculate velocity pressure and hydraulic diameter
For this low-speed, constant-density example, velocity pressure pᵥ = ρV²/2. The unlined value is 9.60 Pa; the lined value is 13.485 Pa. The same airflow now produces approximately 40.47% higher velocity pressure. That increase affects any fitting calculation whose loss coefficient is referenced to this section’s velocity pressure.
For a rectangular clear opening, hydraulic diameter Dₕ = 4A/P = 2ab/(a + b), where P is the wetted perimeter. Here Dₕ falls from 0.6154 m to 0.5625 m. Hydraulic diameter is used with the actual section velocity in the friction calculation below. It is not a replacement circular area through which the same airflow is passed.
ASHRAE’s Duct Design chapter presents Darcy–Weisbach and Colebrook methods, discusses hydraulic diameter for noncircular ducts, and distinguishes roughness categories. Use a method consistent with the selected duct construction rather than transferring an unlined-metal chart without checking its assumptions.
Original example at 1.60 m³/s and 1.20 kg/m³. The 800 × 500 mm dimensions refer to the inside of the metal shell before lining; no second subtraction is appropriate when drawing dimensions already mean clear air.
4. Separate the geometry penalty from the surface input
Take a straight length L = 18.0 m. For a deliberately controlled comparison, first hold the Darcy friction factor f at an assumed 0.020 in both cases. This isolates geometry; it does not assert that the two actual surfaces have equal friction factors. Use Δp = f(L/Dₕ)(ρV²/2). The factor is Darcy, not the Fanning factor, which uses a different numerical convention.
Unlined geometry: Δp₀ = 0.020 × (18/0.6154) × 9.60 = 5.62 Pa.
Lined geometry with the same assumed f: Δp₁ = 0.020 × (18/0.5625) × 13.485 = 8.63 Pa.
Geometry alone raises the calculated straight-run loss by approximately 53.67%.
Sensitivity case only: if the appropriate lined-duct Darcy factor were 0.028 at this duty, the result would become 12.08 Pa.
The last value is a hypothetical friction-factor sensitivity, not a property assigned to every liner. Actual f depends on Reynolds number, effective surface roughness and construction. Use product-specific test data or an appropriate documented roughness model. Where manufacturer data already supplies loss per length for the clear opening, do not apply a second roughness multiplier to the same loss.
Johns Manville’s technical webinar answers point designers to friction-loss methods for insulated and uninsulated ducts. A liner’s published maximum air velocity is a product rating; it does not tell you its pressure loss or establish the acoustic outcome for the project.
5. Carry the clear geometry through fittings and transitions
At a fitting, use Δp = Kpᵥ with the velocity reference defined for that particular coefficient. If an illustrative K of 0.60 were applicable to this section, its calculated loss would change from 5.76 to 8.09 Pa at the two velocities. This comparison holds K fixed only to show the pressure-basis effect. A real lined fitting may have changed area ratios, aspect ratios or effective turning geometry, so K itself needs review.
Check the change between lined and unlined sections. Equal sheet-metal dimensions do not imply equal clear openings across the join. Conversely, enlarging the metal shell to preserve clear area changes the coordinated physical envelope. Track both sizes so an aerodynamic correction does not create an unrecorded space clash.
To retain an 800 × 500 mm clear opening with the assumed 25 mm liner, the metal-shell inside dimensions become 850 × 550 mm. That restores the target area but does not automatically restore the unlined pressure loss, because the airstream surface has changed. Increasing width alone while holding height fixed is another geometry entirely and requires recalculation.
6. Issue a traceable lined-duct calculation
Label scheduled dimensions explicitly as clear-air or metal-shell dimensions.
List liner location, thickness and the clear dimensions used for each section.
Recalculate area, mean velocity, hydraulic diameter and straight-run loss at the design airflow.
Record the friction method and source of surface data; review every affected fitting’s reference velocity and geometry.
Update the critical path, fan duty and physical coordination together, then reconcile the calculation against the fabrication drawing.
The pressure differences above belong to one illustrative straight run. They are not a percentage increase for the whole system. Rebuild the actual path before changing a fan selection. The paid HVAC Duct Design and Sizing Handbook is a related reference for the wider workflow; keep the clear-air dimensions visible in every project calculation.
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