How do a kitchen chimney and gas hob work?
A spark of 14,000 volts, a jet of gas that drags in its own air, and a hood that catches smoke it can barely suck. Take an Indian kitchen's gas stove and chimney apart in 3D and see why a good flame is blue.
▶ Play with it · Read the 60-second explainer · Watch the 40-second video
- Gas from a cylinder, smoke out through a duct. LPG waits as a liquid in a 14.2 kg cylinder at about 5 bar. A regulator lets it out at a gentle 29 millibar, a hose carries it to the stove, and a valve behind each knob feeds a brass burner. Above, a chimney's blower pulls smoky air through grease filters and out through a duct.
- Why a good flame is blue. The gas shoots from a tiny injector into a mixing tube and drags air in with it, the Bunsen principle. With about half the air it needs premixed, the flame burns clean and blue at up to about 1,900 °C. Starved of air, unburnt carbon glows yellow as soot and the flame makes carbon monoxide.
- Thousands of volts to light it, millivolts to keep it safe. The igniter needs about 14,000 volts to make a spark jump a 4 mm gap. On stoves with a flame-failure device, a thermocouple in the flame makes about 28 millivolts to power a magnet that holds the gas valve open. If the flame dies, the voltage falls and a spring shuts the gas off.
- The chimney doesn't suck the smoke up. At the pan, 70 cm below the hood, a chimney's suction is only about 0.05 m/s. The smoke rises by itself in a hot plume at about 1 m/s. The hood has to be low and wide enough to swallow that plume before a draught pushes it out from under the edge.
- Oil can't turn corners. A baffle filter forces the air to zig-zag. Air follows easily, but oil droplets are about 750 times denser, so they keep going straight and hit the steel, then drain into the oil collector. Droplets smaller than a few microns slip through.
- The number on the box is not the air you get. Rated suction is measured with nothing in the way. The filter, a narrow duct, bends and a shut window all push back, so a 1,200 m³/h chimney often moves half to two thirds of that. Where the fan curve meets the duct's system curve is the airflow you really get.
| Term | Meaning |
|---|---|
| LPG | Liquefied petroleum gas: mostly propane and butane, stored as a liquid under pressure in a cylinder. |
| Regulator | The valve on the cylinder that lowers the gas pressure to a steady 29 millibar for the stove. |
| Primary air | Air mixed with the gas inside the burner before it reaches the flame. |
| Bunsen principle | A fast jet of gas drags the surrounding air into a tube with it, premixing fuel and air. |
| Carbon monoxide | A poisonous, invisible gas made when fuel burns without enough oxygen. |
| Thermocouple | Two different metals joined at a tip that make a small voltage when the tip is heated. |
| Flame-failure device | A thermocouple and magnet valve that shut off the gas if the flame goes out. |
| Inertial impaction | Catching particles by turning the air so sharply that they cannot follow and hit a surface. |
| System curve | How much pressure a duct needs for each airflow. Where it meets the fan curve is the real airflow. |
9,000 years from a hole in the roof above an open fire to a blue gas flame under a humming hood in hundreds of millions of kitchens.
- 7000 BCE · A way out through the roof (People of Çatalhöyük, Çatalhöyük, Anatolia (today Turkey))
- 1185 · Chimneys arrive in Europe (Castle builders, Conisbrough Castle, Yorkshire, England)
- 1796 · Count Rumford fixes the smoky fireplace (Benjamin Thompson, Count Rumford, London, England)
- 1826 · A gas stove you could buy (James Sharp, Northampton, England)
- 1855 · The Bunsen burner (Robert Bunsen and Peter Desaga, University of Heidelberg, Germany)
- 1912 · Gas in a bottle (Walter O. Snelling, USA)
- 1933 · The range hood becomes a product (Vent-A-Hood, Dallas, Texas, USA)
- 1965 · Indane lights up Kolkata (Indian Oil Corporation, Kolkata, India)
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