A Double Cone Ventilator is a static roof-mounted extract terminal made of two coaxial cones — one inverted over the other — that pulls air out of a building using wind and the stack effect, with no moving parts. The defining component is the upper inverted cone, which creates a venturi gap above the lower cone so wind passing across the roof accelerates through the annular slot and drags warm interior air upward by Bernoulli suction. It exists to ventilate animal sheds, workshops, and process buildings where powered fans are impractical. A well-sized unit moves 200–800 m³/h with zero electricity.
Double Cone Ventilator Interactive Calculator
Vary throat diameter, venturi gap ratio, cone angle, and strut size to check the key double-cone geometry and see the suction zone respond.
Equation Used
The calculator sizes the double cone ventilator gap from the throat diameter. The article guidance is that the vertical offset should be 0.15 to 0.25 times the throat diameter; smaller gaps can choke flow, while larger gaps weaken the venturi effect and invite driven rain.
FIRGELLI Automations - Interactive Mechanism Calculators.
- Geometry-only sizing based on the article guidance; actual airflow depends on wind, stack temperature difference, and installation exposure.
- Venturi gap is the vertical offset between the lower cone rim and underside of the upper cone.
- Cone included angle guidance is 60 to 90 deg; strut diameter should stay under 3% of throat diameter.
The Double Cone Ventilator in Action
The mechanism is dead simple but the physics is layered. Warm air inside the building rises by buoyancy — the stack effect — and approaches the throat of the lower cone. Wind blowing across the roof hits the outer rim of the upper inverted cone and is forced to accelerate through the narrow annular gap between the two cones. As that wind speeds up, its static pressure drops (Bernoulli), and the low-pressure zone at the gap sucks the rising interior air out through the throat. You get two extraction drivers stacked on the same terminal: thermal lift from below, wind suction from above.
Geometry is where most field installs go wrong. The gap between the lower cone's top edge and the underside of the upper cone must sit between 0.15 and 0.25 of the throat diameter — too tight and you choke flow, too wide and the venturi effect collapses and rain blows straight in. The included angle of each cone is typically 60° to 90°; flatter cones look pretty but stall the venturi at low wind speeds. If you notice the unit performs fine in summer storms but does nothing on a still humid day, your throat is undersized for the buoyant flow demand — the stack effect alone cannot pull enough air through it.
Common failure modes are not mechanical, they are aerodynamic. Birds nesting in the throat cut flow by 40% overnight. Ice bridging the gap in winter turns the unit into a static cap. And if the operator mounts the ventilator in a roof aerodynamic shadow — downwind of a parapet, taller building, or silo within 8 throat diameters — wind suction drops to near zero and you are running on stack effect alone.
Key Components
- Lower Cone (throat cone): Sits over the roof penetration with its narrow end down. Diameter at the throat sets the volumetric flow capacity — typically 200 mm to 600 mm for poultry sheds and small workshops. The top rim is the high-velocity edge of the venturi gap.
- Upper Cone (deflector cone): Inverted over the lower cone, narrow end up. Sheds rain, deflects wind across the gap, and creates the low-pressure zone that drives extraction. Vertical offset above the lower cone must be 0.15–0.25 × throat diameter, held within ±2 mm by the support struts.
- Support Struts: Three or four stainless rods (typically 6 mm diameter) hold the upper cone at the correct offset. They must be slim enough not to disturb the wind flow into the gap — strut diameter under 3% of throat diameter is the rule we follow.
- Roof Flange / Storm Collar: Seals the unit to the roof penetration and prevents driven rain from tracking down the outside of the duct. Galvanised or 304 stainless for corrosive environments like piggeries and salt-air coastal sites.
- Internal Bird Mesh: 16 mm aperture stainless mesh across the throat. Coarser mesh than that lets sparrows through; finer mesh blocks more than 8% of the throat area and measurably degrades extraction at low wind speeds.
Who Uses the Double Cone Ventilator
You see Double Cone Ventilators wherever a building needs continuous air change without a power bill — agricultural buildings, light industrial workshops, mine adits, and process buildings handling moisture or odour. They are the workhorse of natural ventilation because they ride two physical drivers at once and forgive a wide range of weather. A reader sizing one for the first time usually asks why not just use a standard cowl. The answer is throughput per unit cost — the double cone pulls 30–60% more air than a single static cowl of the same throat diameter at any given wind speed, because the venturi gap multiplies the suction term.
- Poultry farming: Big Dutchman and Munters install double cone roof terminals on broiler sheds across the EU to back up tunnel ventilation when fans are off during low-load nights.
- Mining: Adit ventilators on small underground gold workings in Bendigo, Australia — passive double cones over decline raises clear blasting fumes overnight.
- Light industrial workshops: Welding shops in the UK use Colt International natural ventilators of the double-cone type over weld bays to extract fume without ducted extraction fans.
- Grain storage: Roof-mounted on bulk grain bins at GrainCorp sites in NSW to evacuate moisture and prevent condensation on the underside of the roof sheet.
- Public toilets and remote washrooms: Highway service-area ablution blocks fitted with 300 mm double cone vents in place of mains-powered extract fans to cut electrical load and maintenance call-outs.
- Tunnel and culvert ventilation: Drainage culverts under rail embankments fit small double cone terminals to vent methane buildup from organic sediment without any active equipment.
The Formula Behind the Double Cone Ventilator
The combined extraction flow through a double cone ventilator comes from the sum of two pressure drivers — the stack effect from temperature difference inside vs outside, and the wind-driven venturi suction across the gap. At the low end of the typical operating range (still air, 2 K indoor-outdoor temperature difference) you are running almost entirely on stack effect and pulling maybe 80–120 m³/h through a 300 mm unit. At the nominal range (5 m/s wind, 8 K ΔT) the wind term dominates and flow climbs to 350–450 m³/h. At the high end (12 m/s wind in a storm) the unit can hit 700+ m³/h, but you are now into the regime where rain ingress through the gap becomes the limiting design factor, not flow.
Variables
| Symbol | Meaning | Unit (SI) | Unit (Imperial) |
|---|---|---|---|
| Q | Volumetric extraction flow rate | m³/s | CFM |
| Cd | Discharge coefficient of the cone-pair throat (0.55–0.70 for typical geometry) | dimensionless | dimensionless |
| Athroat | Cross-sectional area of the lower cone throat | m² | ft² |
| ρ | Air density (≈ 1.2 at 20 °C) | kg/m³ | lb/ft³ |
| g | Gravitational acceleration | 9.81 m/s² | 32.2 ft/s² |
| h | Stack height — vertical distance from inlet to throat | m | ft |
| ΔT | Indoor minus outdoor air temperature | K | °R |
| T | Outdoor absolute temperature | K | °R |
| Cp | Wind pressure coefficient at the cone pair (0.4–0.6 typical) | dimensionless | dimensionless |
| v | Wind speed at roof level | m/s | ft/s |
Worked Example: Double Cone Ventilator in a free-range layer hen barn in Lancashire
A 400-bird free-range layer barn near Preston, Lancashire fits a 400 mm throat Double Cone Ventilator on the ridge. Stack height from floor inlets to ventilator throat is 4.5 m. Indoor temperature 18 °C, outdoor 10 °C (ΔT = 8 K, T = 283 K). Cd = 0.62, Cp = 0.5, ρ = 1.2 kg/m³. The owner needs to know the extraction rate at calm dawn (1 m/s), nominal afternoon breeze (5 m/s), and a stiff Irish Sea westerly (12 m/s).
Given
- Dthroat = 0.400 m
- Athroat = 0.1257 m²
- Cd = 0.62 —
- h = 4.5 m
- ΔT = 8 K
- T = 283 K
- Cp = 0.5 —
- ρ = 1.2 kg/m³
Solution
Step 1 — compute the stack pressure driver, which is the same regardless of wind:
Step 2 — at nominal 5 m/s wind, compute the wind pressure driver:
Step 3 — total driving pressure and nominal flow:
That is enough air change for roughly 360 hens at the UK Defra recommended minimum winter ventilation rate of 3 m³/h per bird — close to fully covering the 400-bird stocking with margin from the wind variability.
At the low end, 1 m/s dawn calm, the wind term collapses to ΔPwind = 0.30 Pa and total driver falls to 1.80 Pa:
That covers about 160 birds — borderline on a still summer dawn with full stocking, which is why production barns near the limit normally pair the static ventilator with a small backup extract fan for windless conditions.
At the high end, 12 m/s westerly storm, ΔPwind = 43.2 Pa:
In theory the unit pulls more than twice the design air change. In practice, above 10 m/s wind the venturi gap starts admitting horizontal rain in coastal sites — you would typically fit a storm baffle or accept the spray, not try to harvest the extra flow.
Result
Nominal extraction at 5 m/s wind and 8 K ΔT comes out at roughly 1090 m³/h through the 400 mm unit. That equates to a complete air change every 4–5 minutes for the barn volume, easily holding ammonia below the 25 ppm welfare threshold. At the low end (1 m/s) flow drops to 486 m³/h — the bird shed feels still and humid by mid-morning if calm persists; at the high end (12 m/s) the theoretical 2425 m³/h is throttled in real life by rain ingress and operator-fitted storm baffles. If you measure 600 m³/h instead of the predicted 1090, check three things in this order: (1) bird mesh blocked with feathers and dust — common at the 6-month mark and cuts flow 30–40%, (2) stack-effect inlet area at floor level undersized so the building is suction-limited not stack-limited, and (3) the ventilator sitting in the wind shadow of an adjacent silo or feed bin within 8 throat diameters, which kills the Cp term entirely.
Double Cone Ventilator vs Alternatives
Static double cone ventilators compete against powered roof fans and simpler single-cowl vents. The right choice depends on whether you need guaranteed flow at any wind speed, capital cost limits, and whether the building has electrical infrastructure to support fans. Here is how the three options compare on the dimensions readers actually search for.
| Property | Double Cone Ventilator | Powered Roof Extract Fan | Single Static Cowl |
|---|---|---|---|
| Flow at 5 m/s wind, 400 mm throat | ~1090 m³/h | 1500–4000 m³/h (constant) | ~650 m³/h |
| Flow at 0 m/s wind (stack only) | ~150 m³/h | Same as rated (constant) | ~120 m³/h |
| Electrical power draw | 0 W | 60–400 W continuous | 0 W |
| Capital cost (400 mm class) | £180–£350 | £450–£1200 plus wiring | £90–£180 |
| Service life | 20+ years, no moving parts | 5–10 years on motor bearings | 20+ years |
| Maintenance interval | Annual mesh clean | Bearing/grease 6 months, motor replace 5–10 yr | Annual visual |
| Rain rejection at storm wind | Good below 10 m/s, baffle needed above | Excellent (sealed housing) | Excellent |
| Best application fit | Agricultural sheds, workshops, mine adits | Process buildings needing guaranteed flow | Domestic and small outbuildings |
Frequently Asked Questions About Double Cone Ventilator
The wind term in the driving-pressure equation scales with v2, so it disappears almost completely below 1 m/s. Once the wind dies you are running on stack effect alone, and stack effect needs both vertical height and indoor-outdoor temperature difference. On a still humid summer morning ΔT can drop to 1–2 K and the stack pressure falls below 0.5 Pa.
Two practical fixes — increase stack height by raising the duct above the ridge by another 0.5–1 m, or accept that you need a small assist fan for calm conditions. Most production barns under welfare regulation fit a low-wattage assist fan precisely for this regime.
Work backwards from your target Q. For a 400 m³ barn needing 6 air changes per hour, Q = 2400 m³/h = 0.667 m³/s. Use the formula at your design wind condition (commonly the local 10th-percentile wind speed, not the average — you size for the calm end of normal weather, not the average) and rearrange for Athroat.
Rule of thumb that lands close to right for UK and northern European climates: throat area in m² ≈ building floor area in m² × 0.0015 for general livestock, × 0.0025 for high-moisture process buildings.
Rotating turbine cowls (the spinning ball type) outperform double cones at very low wind speeds because they convert any breeze into rotational pumping — you get measurable flow at 0.5 m/s where the double cone's venturi has barely woken up. But turbine cowls have bearings, and bearings seize. After 4–6 years in a dusty workshop a rotating turbine becomes a static cowl with worse flow characteristics than a properly sized double cone.
For a workshop with people inside daily who will notice and replace failed parts, turbine cowls are fine. For a remote or unattended building, choose the double cone — no moving parts, 20+ year life, predictable failure modes.
The sweet spot is a vertical gap of 0.20 × throat diameter, with acceptable performance from 0.15 to 0.25. Below 0.15 the gap chokes and Q drops sharply because the venturi exit area becomes the flow-limiting cross-section, not the throat. Above 0.25 the wind sheets across the top of the lower cone without coupling into the gap and the suction term collapses by 30–50%.
Hold the offset to ±2 mm on a 400 mm throat. The cheap imported units we have stripped down often arrive with one strut 5–8 mm short of the others — symptom is asymmetric flow that swirls visibly in smoke testing.
The formula assumes the inlet path into the building is not the limiting restriction — that the cone is the bottleneck. In real buildings, floor-level inlets, eave gaps, or door undercuts are often smaller than the cone throat, so the system becomes inlet-limited. Total inlet area at low level should be at least 1.5× the throat area for the cone to operate near its theoretical Q.
Quick diagnostic — open a door wide and watch the flow. If extraction visibly jumps you have an inlet-area problem, not a ventilator problem.
It matters more than most installers think. The wind pressure coefficient Cp in the formula is set by where the unit sits in the building's aerodynamic field. Up on the windward slope, Cp can be slightly negative (pressurised) and the unit fights the stack effect. On the lee side of the ridge or right on the ridge crest, Cp sits at +0.4 to +0.6 and the unit performs as designed.
Keep the ventilator at least 600 mm above the ridge line and at least 8× throat diameters away from any taller obstruction — adjacent silos, feed bins, or another building. Inside that exclusion zone you can lose half the rated wind-driven flow.
No, and this is a common misconception with DIY-built units. The upper cone diameter is set by the venturi geometry — making it wider increases the gap exit area faster than it increases the captured wind, so the local velocity through the gap drops and Bernoulli suction falls with the square of that velocity. The optimum upper cone diameter is 1.4–1.6× the lower cone throat diameter.
If you have a homebrew unit underperforming, measure both cone diameters before changing anything else — units bought from non-specialist roofing suppliers sometimes ship with mismatched cone pairs that simply cannot work.
References & Further Reading
- Wikipedia contributors. Stack effect. Wikipedia
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