A gasifier shell radiates heat by infrared, and at 400 °C a bare steel surface sends out about 9 kW per square metre, so the reactor needs insulation or shields with air gaps and a clear space of 15 cm or more from fuel, rubber and plastic. Distance alone is a weak defence against a large hot surface: 15 cm from a 57 cm drum only cuts the radiated flux to about two thirds, whereas each extra sheet of shield metal with an air gap cuts it further.
Part of Vehicle Mounting
Heat shielding protects the vehicle body, fuel and cab from the radiant and convective heat of a gasifier reactor.
What The Carrier Asks For
The reactor reaches 800-1,400 °C inside. The carrier wraps it in ceramic-fibre blanket (salvaged industrial insulation) or multiple layers of sheet metal with air gaps, keeps all flammable material (wood fuel, rubber, plastic) at least 15 cm from the reactor surface, and shields the vehicle body from radiant heat with a metal barrier.
How Much Heat A Hot Shell Sends Out
Net radiation from a surface at temperature T to surroundings at 293 K is εσ(T⁴ − 293⁴), with σ = 5.67 × 10⁻⁸ W/m²K⁴ and an emissivity ε of 0.8 for oxidised steel (an assumption).
| Shell surface | σ(T⁴ − 293⁴) | With ε = 0.8 |
|---|---|---|
| 200 °C | 2.4 kW/m² | 1.9 kW/m² |
| 300 °C | 5.7 kW/m² | 4.6 kW/m² |
| 400 °C | 11.2 kW/m² | 9.0 kW/m² |
| 500 °C | 19.8 kW/m² | 15.9 kW/m² |
| 600 °C | 32.5 kW/m² | 26.0 kW/m² |
The shell temperature depends on how well the reactor is insulated, which I cannot specify; measure it rather than assume it. A steel surface that shows a dull red glow in shade is above about 500 °C (recalled), so shield accordingly. For comparison, wood needs about 12 kW/m² to ignite from a pilot flame (recalled), and lower fluxes over hours dry and char it until it ignites more easily.
Distance Helps Less Than You Think
For a long cylinder of radius R, the radiated flux at distance r from its axis falls roughly as R ÷ r. A 200-litre drum is about 57 cm across, so R = 0.285 m. At the carrier’s 15 cm gap, r = 0.435 m, and the factor is 0.285 ÷ 0.435 = 0.66.
- Shell at 400 °C: 9.0 × 0.66 = 5.9 kW/m² at the 15 cm line.
- Shell at 500 °C: 15.9 × 0.66 = 10.4 kW/m², close to the ignition flux of wood.
Solving for the shell temperature at which the flux at the 15 cm line reaches 12 kW/m² gives about 525 °C, so up to roughly that temperature wood stays under the ignition flux, though long exposure below it still dries and chars it. The rule does not protect against a large flat hot surface at all, because flux from an extended flat source barely falls with distance.
What A Shield Does
A thin metal sheet with an air gap, between two surfaces of equal emissivity, halves the radiant exchange; n sheets cut it to 1 ÷ (n + 1). So two sheets reduce a 9 kW/m² flux to 3 kW/m². The sheets must be spaced off the shell (not touching, or they conduct), vented so hot air can leave, and fixed so vibration does not rattle them against the reactor. Ceramic-fibre blanket on the shell reduces the shell temperature itself, which cuts the radiation at the source.
Method
- Measure or estimate the shell temperature; add insulation if it exceeds 400 °C.
- Put at least two spaced metal shields on any side facing the cab, the petrol tank and lines, wiring, tyres or the load.
- Leave air gaps open at the top and bottom.
- Keep wood fuel in a separate bin beyond the shield, not against the reactor.
- Run the unit outdoors and feel for heat on the shield side with the back of a hand held at a distance, not touching.
Common Mistakes
- Trusting 15 cm. It does not guard against a hotter shell.
- Shields welded straight to the shell. They conduct heat through.
- Forgetting the petrol system. The petrol tank and lines are still there for starting.
Safety
Fire, burns and carbon monoxide travel together on a gasifier vehicle
The reactor’s surface can ignite wood, rubber and plastic that are close to it, and the gas inside is about 18-22% carbon monoxide by volume. Never park a vehicle with a hot gasifier in a garage, shed or lean-to, and never fuel it with petrol while the unit is hot. Do not touch the shell. See Carbon Monoxide Risks.
What’s Next
- Learn next: Gas Cleaning for Mobile Use — the gas line’s cooler and filters
- Combine with: Vibration Isolation — keeping rubber mounts away from the heat