The historical double-cone ventilator illustrated by Hiscox uses two opposed, horizontally aligned cones. An uptake feeds the space between them, and smoke leaves around the leeward cone edge. The animation shows this static arrangement and an illustrative extraction path. Its calculator compares explicitly defined cone and opening dimensions; it does not predict ventilation capacity.
Double Cone Ventilator Interactive Calculator
Compare opposed horizontal cones, radial edge clearance and support diameter. Animated markers explain an extraction path; the outputs describe geometry, not airflow.
Equation Used
- Opposed cone arrangement based on Hiscox figure 692.
- Cone-base diameter defined as D−2g; dimensions not supplied by historical plate.
- Four support rods and 0.5D base separation are illustrative.
- Flow markers show direction only, not CFD.
Illustrated geometry only. No airflow, pressure, rain rejection or ventilation rating.
Opposed cones and a central uptake
The section shows the two cone tips pointing outward, a casing around their bases, and an uptake entering the central chamber from below. The right side is labeled leeward for the illustrated left-to-right wind direction. Animated markers show a possible extraction path around that cone edge. They are direction symbols, not computed streamlines or a pressure field.
The original reference provides no dimensions. For this geometric study, the cone base diameter is D−2g, where D is the casing bore and g is the radial clearance between the cone edge and casing. The axial distance between cone bases is illustrated as 0.5D. Four support rods are shown schematically. These dimensions define the geometric example; the historical source supplies no product dimensions or test results.
The face view distinguishes the central cone from the surrounding annular opening. Changing cone angle changes its axial projection; changing clearance changes both the cone base and gross opening area. Nothing in the ventilator rotates.
What this comparison is useful for
Use the controls to understand the relation between cone angle, cone length, edge clearance and available opening area. The diagram can support an early layout discussion or historical mechanism explanation.
Actual ventilation selection needs pressure and flow data for the complete terminal and building, together with wind exposure, inlet losses, weather protection and the required duty. This geometry calculator supplies none of those performance ratings.
Defined cone and opening geometry
Let D be casing bore, r the radial gap ratio, α the full included cone angle, and ds the support rod diameter. All entered lengths are in millimeters.
Gap g=rD. Cone base diameter Dc=D−2g. Cone projection h=Dc/[2 tan(α/2)]. Gross annular area A=π(D²−Dc²)/4; divide square millimeters by 100 to display square centimeters. Support diameter ratio=100ds/D.
Gross opening area excludes rod blockage, screens, thickness and any discharge coefficient. The rod ratio is descriptive, not a pass/fail limit. Cone angle is not an airflow coefficient.
Example: a defined 400 mm casing
For D=400 mm, gap ratio 0.20, included angle 75° and rod diameter 6 mm, the radial gap is 80 mm and cone base diameter 240 mm. The cone projects approximately 156.4 mm. Gross annular area is 804.25 cm² and rod/casing diameter ratio 1.5%.
Changing the included angle to 100° shortens the cone to about 100.7 mm without changing the annular area. Increasing the gap changes the opening area and cone diameter. Increasing rod diameter changes its drawing and ratio, but this calculator does not subtract supports from gross area.
What the animation does and does not establish
A wider radial gap increases the gross opening in this defined geometry. A sharper cone projects farther for a given base diameter. Neither observation proves that a particular combination provides more extraction, rejects rain or meets a building ventilation requirement.
The horizontal arrangement uses the stated example dimensions. Selecting a working ventilator requires tested airflow, pressure and weather-performance data for the intended installation.
Double-cone ventilator questions
Why are the cones horizontal?
That is the opposed arrangement shown in the historical figure used for this page. The uptake enters the space between them.
What moves in the animation?
Only explanatory flow markers. The casing, cones and supports are static.
Does the calculator estimate airflow?
No. It reports geometry, not pressure, suction, air changes or a flow rating.
Is the gap the distance between the cones?
No. It is the radial edge clearance between each cone base and the casing. The illustrated axial separation is a stated assumption.
Historical construction reference
Gardner D. Hiscox, Mechanical Movements, Powers and Devices, printed page 185, figure 692 (PDF page 195). The plate shows opposed cones with an uptake between them and describes discharge around the leeward edge; it supplies no performance curve or manufacturing dimensions.
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