How is natural smoke ventilation sized?
In single-storey spaces with high ceilings (warehouses, shops, exhibition halls, production halls) natural smoke ventilation collects the hot smoke rising from a fire as a layer under the roof and holds the underside of that layer at a chosen height, the smoke-free height y. To do so, the smoke leaving through the roof vents must match the smoke the plume carries into the layer. The calculator follows the steady-state approach used in BRE 368 and CIBSE Guide E: the design fire is constant, and layer temperature and depth do not change with time.
Important: this tool is a natural smoke-ventilation engineering estimate. In Turkey the Fire Protection Regulation (BYKHY), local fire-brigade rules and the approving authority's requirements (vent ratios, smoke zones, smoke curtains, controls and replacement air) must be checked separately.
1. Design fire
The fire is taken as a square surface of area Af with a heat release per unit area q. Total heat output is Q = Af·q; only the convective part heats the plume and layer (Qc = 0.6–0.8·Q), the rest is radiated. For sprinklered retail, a 10 m² fire at 500 kW/m² is a common assumption, giving a 5 MW design fire. Unsprinklered buildings need a much larger fire, and the result changes accordingly.
2. Smoke mass flow: Thomas–Hinkley
When the fire is large compared with the height of rise, the air entrained into the plume depends on the fire perimeter P and the rise height y:
M = 0.188 · P · y1.5 [kg/s] ; for a square fire P = 4√AfBecause the flow grows with y to the power 1.5, raising the smoke-free height increases smoke production quickly: moving y from 3 m to 4 m raises M by about 54%. Keep y at the value needed for escape and fire-fighting; choosing more than necessary enlarges the vent area for no benefit.
3. Layer temperature
The convective heat warms the smoke mass entering the layer: θ = Qc / (M·cp), with cp = 1.01 kJ/kgK. Heat loss from the layer to the building is ignored, so this is a high-side estimate. A hotter layer is more buoyant and needs less vent area, but a very hot layer affects sprinkler operation, glazing, steelwork and the radiation felt by occupants below. The calculator warns when θ exceeds 200 K.
4. Vent area and replacement air
Flow through the vents is driven by the buoyancy pressure across the layer depth d = H − y. Replacement air enters through low-level openings, and the pressure loss at those inlets must be included. In the BRE 368 approach the required aerodynamic area is:
AvCv = M·Tl / (ρ0·√(2·g·d·θ·T0 − M²·Tl·T0 / (AiCi·ρ0)²))When the inlet area is expressed as a ratio of the vent area (AiCi = r·AvCv) the unknown appears on both sides. The calculator solves it by fixed-point iteration, cross-checks the answer against the closed-form solution and shows a red warning if it does not converge. Smaller inlets drive the vent area up fast: in the default example, r = 0.5 instead of r = 1.5 increases AvCv by about 55%.
| Quantity | Meaning | Range in the tool |
|---|---|---|
| Cv | Vent discharge coefficient (EN 12101-2 test report) | 0.4–0.7 |
| r | Inlet-to-vent aerodynamic area ratio | 0.5–3 |
| d | Smoke layer depth | H − y |
| Av | Geometric (free) vent area = AvCv / Cv | — |
5. Number and layout of vents
The geometric area is divided by the free area of the chosen device and rounded up. Spread the vents evenly over the smoke zone; drawing too much flow through a single vent can pull clear air up through the layer (plugholing). The risk grows as the layer gets shallower, so the calculator warns when d < 1 m or d < 0.1·H.
Limits and cautions
- The Thomas–Hinkley relation is for large fires; for a small fire with a tall rise height, axisymmetric plume correlations give different results.
- Atria, balcony spill plumes, multi-storey buildings and powered smoke exhaust are outside the scope of this tool.
- Position vents with wind effects in mind and use devices tested under side wind to EN 12101-2.
- Smoke zone size and smoke curtains (EN 12101-1) limit how far the layer spreads; an oversized zone lets the layer cool and descend.
- Replacement air must open together with the vents, sit in the smoke-free zone and preferably be at low level.
Frequently Asked Questions
How is smoke vent area calculated?
First the smoke mass flow, then the layer temperature, then the vent area. For a 10 m² × 500 kW/m² fire, 80% convective, H = 8 m and y = 3 m: M ≈ 12.4 kg/s and θ ≈ 321 K; with an inlet ratio r = 1.5 the required aerodynamic area AᵥCᵥ ≈ 2.28 m², which at Cᵥ = 0.6 is ≈ 3.81 m² geometric.
Why does the inlet area matter so much?
The pressure lost at the replacement-air inlets is taken from the buoyancy pressure that drives the vents. In the same example AᵥCᵥ ≈ 2.07 m² with no inlet loss (r = ∞), ≈ 2.28 m² at r = 1.5 and ≈ 3.54 m² at r = 0.5. Adding vents does not fix undersized inlets.
Can sprinklers and natural smoke vents be used together?
Yes, it is common in warehouses and shops. Sprinklers limit fire size, so the design fire is chosen on a sprinklered basis. Vent opening time and controls should be coordinated with the sprinkler system, following the standard and the authority's accepted approach.
Is this calculation enough for approval in Turkey?
No. It is an engineering estimate. The Fire Protection Regulation (BYKHY), local fire-brigade rules and the approving authority's requirements (smoke zones, curtains, vent ratios, replacement air, controls) must be checked separately, and the final design verified against the current edition of the applicable standard.
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