After the cooler, a fabric, dry-sawdust or sand filter removes the fine dust that the cyclone missed, and the main sizing rule is surface area: gas passing the cloth slowly, around 1-3 cm/s, so 60 normal cubic metres an hour at 40 °C needs roughly 0.64-1.9 m² of fabric. A clogged filter starves the engine of gas, so it is cleaned or replaced on schedule, about every 50 hours for the parent’s fabric filter.
Part of Gas Cleaning & Filtering
Particulate filtration is the stage that removes fine dust and residual tar droplets from producer gas after cooling, before the engine.
Three Filter Media In The Parent
- Fabric filter: a bag or tube of closely woven cotton or synthetic fabric. Cotton is the safer choice where any residual heat remains.
- Sawdust filter: a drum packed with dry sawdust or wood shavings; gas percolates through and particles stick.
- Sand filter: a vertical tube of sand. Effective, but it creates more pressure drop.
The parent’s warning is that a clogged filter starves the engine. The reason is the suction budget: the engine pulls the gas through everything, and each 1 kPa of suction lost in the train costs about 1% of the intake charge. A filter that doubles in resistance as it loads can take more of the engine’s power than the gasifier did.
Sizing The Fabric
Filtration speed, the gas flow divided by cloth area, is the controlling number. Fabric filters in dust-collection practice commonly run at about 1-3 cm/s (recalled; the figure comes from general baghouse practice, not from the gasifier carrier).
- Gas at 60 Nm³/h cooled to 40 °C expands by 313 ÷ 273 = 1.146: 68.8 m³/h = 0.0191 m³/s.
- Area at 3 cm/s: 0.0191 ÷ 0.03 = 0.64 m². At 1 cm/s: 0.0191 ÷ 0.01 = 1.9 m².
- For the stationary 5 kW set at about 20 Nm³/h: 22.9 m³/h = 0.0064 m³/s, so 0.21-0.64 m².
- A cloth tube 0.2 m across and 1 m long has π × 0.2 × 1 = 0.63 m² of surface.
A filter at the high-speed end clogs quickly; at the slow end it lasts longer and costs more room, a trade-off the vehicle has to make against weight (Fabric Filter Housing).
Method
- Place the filter after the cooler, so the dust arrives dry and cold.
- Make the housing tight and easy to open, but only when cold and ventilated.
- Clean or replace a fabric filter every 50 hours, as the parent’s maintenance schedule says. Carry spare filter material, as the second carrier’s history advises, and replace tarry or damp media rather than cleaning it back into service.
- Note the suction at the engine over time; a steady rise means the filter is loading.
Common Mistakes
- Packing sawdust damp. Wet sawdust passes tar and water and blocks quickly. The parent specifies dry.
- A filter too small for the flow. It loads in hours.
- Cleaning by shaking in the shed. It puts tarred dust where you breathe it.
Safety
A tar-loaded filter burns, and its housing holds carbon monoxide
The gas inside is about 18-22% carbon monoxide by volume. The second carrier records tar fires in the gasifier or filter as the most common breakdown: a filter loaded with tar can ignite if air enters a hot system. Never open the housing while gas is present or the system is warm, and never anywhere indoors. Treat used media as tarred waste, away from the fuel store. See Carbon Monoxide Risks.
What’s Next
- Learn next: Moisture Separation — the water that the filters also meet
- Combine with: Engine Connection — what the filtered gas feeds