How do a kitchen chimney and gas hob work?

An Indian kitchen stove runs on LPG, liquefied petroleum gas. In the red 14.2 kg cylinder it is a liquid, squeezed by its own vapour to about 5 bar on a warm day. The regulator clipped on top lets it out at a steady, gentle 29 millibar, about 3% more than the air pressure around you.

A rubber hose carries the gas to the stove. Inside, a brass pipe feeds a valve behind each knob. From the valve, gas shoots out of a tiny injector into a mixing tube, drags air in with it, and comes out of the ring of holes in the brass burner as a flame. Beside each burner sit an igniter that sparks and, on newer stoves, a flame-failure sensor.

Above it hangs the chimney. A blower pulls smoky air up through baffle filters that trap oil, which drips into an oil collector. The air leaves through a duct in the wall.

LPG is made of propane and butane, molecules of carbon and hydrogen. When they burn, they join with oxygen from the air and become carbon dioxide and water vapour, giving out heat. Each litre of gas needs about 28 litres of air.

A stove mixes some of that air in before the flame. The gas shoots out of a tiny injector at high speed into a mixing tube, and the fast jet drags air in through the gap beside it. This is the Bunsen principle, the same trick as the burner in a school lab. The rest of the air, the secondary air, reaches the flame from around it.

With about half the air premixed, the flame is blue with a sharp inner cone, and its hottest part reaches about 1,900 °C. Starve it of air (a blocked tube, a dirty burner) and the fuel cannot burn completely: tiny specks of soot glow yellow, blacken your pans, and the flame gives off carbon monoxide, a poisonous gas.

To light the burner, you push and turn the knob. Gas starts to flow, and the igniter fires a spark across a gap of a few millimetres between its metal tip and the burner. Air does not normally carry electricity, so it takes a big push: about 14,000 volts for a 4 mm gap. A piezo lighter makes that by snapping a hammer onto a special crystal; an electronic one uses a battery and a tiny transformer.

Many new stoves also have a flame-failure device. A thin metal probe, a thermocouple, sits in the flame. Two different metals joined at a hot tip make a small voltage, about 28 millivolts. That powers a little electromagnet in the gas valve, which holds the valve open. That's why you must hold the knob in for a few seconds: the probe needs time to warm up.

If milk boils over or a gust blows the flame out, the probe cools, the voltage falls, and below about 4 millivolts the magnet lets go. A spring snaps the valve shut, and the gas stops.

You might think a chimney sucks smoke up from the pan like a vacuum cleaner. It can't. Suction dies away fast: a few centimetres below the hood the air moves quickly, but at the pan, 70 cm down, a typical chimney pulls at only about 0.05 metres per second, slower than you can feel. Factory hoods aim for 0.3 to 0.5 m/s at the source, and a kitchen chimney gets nowhere near that.

What really lifts the smoke is heat. Air over a hot pan is lighter than the air around it, so it rises in a plume at about 1 m/s, dragging oil droplets and steam with it. The plume spreads as it climbs, so the chimney has to be wide enough and low enough to catch all of it, and suck at least as much air as the plume brings. That's why it hangs 65 to 75 cm above the stove.

The enemy is a draught: a fan, an open window, someone walking past. A sideways breeze bends the plume out from under the hood's edge, and the smoke escapes into the room.

Frying and tadka throw tiny droplets of oil into the air, most of them thinner than a hair. If they reach the fan, the duct and your walls, everything turns sticky. The chimney's filter has to catch them, but it can't be a fine sieve, or the air couldn't get through.

A baffle filter uses a trick instead. Its steel blades force the air to zig-zag through narrow channels. Air is so light it follows the turns easily. A droplet of oil is about 750 times denser than air, so it carries on in a straight line, like a cyclist who can't make a hairpin bend, and splats into the metal. This is called inertial impaction. The oil then runs down the blades into the oil collector.

A mesh filter catches droplets on layers of thin aluminium strands. It works well when clean, but the oil stays in the mesh, so it slowly clogs and the airflow drops. Filterless chimneys turn the air round a curved panel and let the spinning fan wheel fling oil into a cup. Some have an auto-clean button that heats the fan chamber so stuck oil melts and drains away.

A chimney sold as "1,200 m³/h" moves that much air only with nothing in its way. Real air has to squeeze through the filter, up a duct, round its bends and out through a flap in the wall. All of that pushes back, and the more air you try to move, the harder it pushes: resistance grows with the square of the flow.

The blower can only push so hard. Its fan curve shows how its airflow drops as the back-pressure rises. The duct's system curve shows how much pressure each airflow needs. The chimney settles where the two curves cross. A narrow 100 mm pipe, a long run of crumpled flexible foil duct or lots of bends can cut the real airflow to half of the number on the box, while the fan gets louder, not better.

And every cubic metre blown out must come back in. With the doors and windows shut, the kitchen's air pressure drops and the chimney chokes. That returning air is called make-up air. A recirculating chimney skips the duct: it blows the air through a carbon filter and back into the kitchen. It traps grease and some smells, but the heat, steam and carbon dioxide stay in the room.