The final filter on a wood gas vehicle is a sealed steel canister packed with cotton cloth or dry sawdust around a replaceable element, sized to give about 0.64-1.9 m² of cloth for 60 normal cubic metres an hour and changed every 200-500 km by the carrier’s rule. A clear water manometer connected across the canister shows its loading directly, since each 1 kPa of resistance is 102 mm of water and about 1% of the engine’s intake charge.
Part of Gas Cleaning for Mobile Use
The fabric filter housing is the vehicle’s last gas-cleaning vessel before the engine, holding a replaceable filter medium behind a gas-tight lid.
What The Carrier Specifies
The final stage before the engine is a canister packed with cotton cloth or dry sawdust, with a replaceable element, and the carrier says to carry spare filter material and change it every 200-500 km depending on gas quality.
Size The Cloth, Then Fit It In
The cloth area follows from the gas flow and an assumed filtration speed of 1-3 cm/s (recalled from fabric-filter practice), worked in Particulate Filtration:
- 60 Nm³/h at 40 °C is 68.8 m³/h = 0.0191 m³/s.
- Area: 0.0191 ÷ 0.03 = 0.64 m² to 0.0191 ÷ 0.01 = 1.9 m².
- A cloth tube 0.2 m across and 1 m long gives π × 0.2 × 1 = 0.63 m². A folded or double-wall element in a larger canister gives more cloth than a plain tube; measure the area of the cloth you actually fit.
A canister of 0.3 m by 1 m does not fit every chassis, so the usual compromise is a faster filtration speed and more frequent changes, which matches the carrier’s short interval.
Make The Housing Open And Seal
- Material: steel, thick enough to resist condensate corrosion (the liquid is acidic).
- Lid: a hinged or clamped lid on a heat-resistant gasket, tightened by hand tools and tested with soapy water after each opening.
- Support: a perforated cage so the cloth cannot collapse inward under suction.
- Drain: at the lowest point of the canister, with a valve.
- Position: downstream of the cooler, accessible from outside the vehicle, away from the hot reactor.
A Cheap Loading Gauge
Connect a clear plastic tube, bent into a U and half filled with water, with one leg to the gas line before the filter and one after, or one leg to the line and one open to the air for the total suction. The height difference in millimetres of water is the resistance: 1 kPa is 1,000 ÷ 9.81 = 102 mm. A clean filter might read some tens of millimetres. A steady rise toward several hundred means loading: at 500 mm of water (4.9 kPa) the filter alone costs the engine about 5% of its intake charge. Fit an isolating valve at each gauge connection and close both while the start-up blower is running or the system is pressurised: positive pressure would blow the water out of the tube and release gas. Open them only with the engine pulling suction. Rely on the trend, not on a number I cannot source for your filter.
Method
- Change the element when the manometer reading has climbed or at the carrier’s 200-500 km, whichever comes first.
- Stop and let the system cool, outdoors and upwind, then open the lid.
- Replace the element with dry, clean material; keep a spare in a sealed bag.
- Close, torque the lid, and test with soapy water.
- Put the tarred element in a metal container away from the fuel and the vehicle.
Common Mistakes
- Packing damp sawdust. It blocks and passes tar.
- Opening the lid with the engine running or the unit hot. Tar-loaded media can ignite when air arrives.
- A lid with a gasket that is never tested. It leaks air in, or gas out.
Safety
Opening the filter opens the gas line
The gas in the canister is about 18-22% carbon monoxide by volume, and the media is a tar-loaded fuel. Change it outdoors, upwind, with the engine off and the system cold, never in a garage, shed or enclosed yard. Tar fires in the filter were the most common breakdown in the carrier’s history; keep a bucket of sand to hand, and never use water on a hot gasifier. See Carbon Monoxide Risks.
What’s Next
- Learn next: Operating Procedures — using the whole train day to day
- Combine with: Particulate Filtration — the sizing behind this housing