A vehicle cyclone is a 15-20 cm steel cylinder with a tangential inlet, mounted vertically over a collection pot, that spins dust and char out of the hot gas before the cooler. A body of 15 cm built to the Stairmand high-efficiency proportions has a 7.5 × 3 cm inlet and runs at 13.0-20.7 m/s on 50-80 normal cubic metres an hour of 300 °C gas, which is the usual working speed, and the pot needs emptying every 100-200 km.
Part of Gas Cleaning for Mobile Use
The compact cyclone is the first stage on a wood gas vehicle: a spinning chamber that throws dust and char from the hot gas into a pot.
What The Carrier Specifies, And What It Misses
The carrier’s cyclone is a 15-20 cm diameter steel cylinder with a tangential gas inlet. Gas enters, spins, heavy particles hit the wall and fall into a collection chamber, and clean gas leaves from the top centre. It is mounted vertically so gravity helps the collection, and the pot is emptied every 100-200 km.
What it lacks is proportions. A cyclone’s performance depends on the inlet speed and the ratios of its parts, not on the diameter alone. The proportions below are the Stairmand high-efficiency design (recalled from chemical-engineering references), with D the body diameter.
| Part | Ratio | For D = 15 cm |
|---|---|---|
| Inlet height | 0.5 D | 7.5 cm |
| Inlet width | 0.2 D | 3.0 cm |
| Gas outlet pipe diameter | 0.5 D | 7.5 cm |
| Outlet pipe length into body | 0.5 D | 7.5 cm |
| Cylinder section | 1.5 D | 22.5 cm |
| Cone section | 2.5 D | 37.5 cm |
| Dust outlet diameter | 0.375 D | 5.6 cm |
| Overall height | 4 D | 60 cm |
Inlet Speed Is The Design Variable
Gas at 300 °C is 573 ÷ 273 = 2.1 times its normal volume. The inlet area is 0.5 D × 0.2 D = 0.1 D², and normal practice is an inlet speed of roughly 15-20 m/s (recalled).
- D = 15 cm: area 0.1 × 0.15² = 0.00225 m².
- Flow 50-80 Nm³/h × 2.1 = 105-168 m³/h = 0.0292-0.0467 m³/s.
- Speed: 0.0292 ÷ 0.00225 = 13.0 m/s to 0.0467 ÷ 0.00225 = 20.7 m/s.
- D = 20 cm: area 0.004 m², speed 7.3-11.7 m/s, slow and ineffective at this flow.
- D = 12 cm: area 0.00144 m², flow for a small 1.25 L engine at 34-51 Nm³/h (0.0198-0.0298 m³/s hot): 13.7-20.7 m/s, correct.
- D = 20 cm for a 3.25 L engine at 85-135 Nm³/h (0.0496-0.0788 m³/s hot): 12.4-19.7 m/s, correct.
So match the diameter to the engine: about 12 cm for a small car, 15 cm for the middle row, 20 cm for the big truck row. Full-throttle gas flows come from Downdraft Gasifier Sizing for Vehicles. At part load the speed drops with the gas flow, and the cyclone catches less, which is acceptable because the dust load also falls.
A Correction On What It Catches
The parent and the carrier both describe tar droplets being flung to the wall. In gas at 300 °C the tar is vapour, so a cyclone here collects dust and char; the tar is removed in the cooler and filters (Tar Removal (Cyclone & Cooling)). The parent’s figure that it removes particles above 10 microns is the useful claim.
Method
- Fabricate to the proportions, with a smooth interior and continuous welds.
- Insulate the body so it stays above the tar condensation range, which keeps dust dry.
- Mount it vertically on the isolated frame with a sealed pot and a quick, gas-tight cleanout.
- Empty the pot on schedule and check the contents: char dust is normal; wet tarry sludge means the gas is cooling too early.
Common Mistakes
- Choosing the diameter by what fits. Inlet speed decides.
- A leaking pot seal. Air leaks in at the dust outlet and upsets the separation.
- Emptying it hot. Dust and char re-ignite when air arrives.
Safety
The pot holds hot char and carbon monoxide
The gas passing through is about 18-22% carbon monoxide by volume, and the char in the pot can ignite on contact with air. Empty it outdoors, with the unit cold and the engine off, into a metal bucket. Never do it in a garage or shed, and do not drive with a loose pot. See Carbon Monoxide Risks.
What’s Next
- Learn next: Radiator Setup — the stage after the cyclone
- Combine with: Fabric Filter Housing — the final stage