The Fire-Tube Boiler: Surface, Not Fire, Makes Steam

Cut two test sections, each 400 mm long, to sit across an insulated box of water: - **A:** one copper tube of 22 mm bore. - **B:** a bundle of small copper tubes whose bores add up to the same area — four tubes of about 11 mm bore, or nine of about 7 mm — soldered through two end plates of copper sheet, so the hot air must go through the small tubes. Pass each section through holes in opposite walls of the insulated box, sealed with silicone, so the tubes run underwater and both ends stay dry and open. The box is open at the top. **Nothing here is ever closed or pressurised.**

Fill the box with 5 litres of cold water and put a thermometer in it. Clamp the heat gun at a fixed distance from the inlet of section A, on its fixed high setting, and run it for ten minutes, stirring and reading the water every minute. Hold the thermometer in the air stream at the tube's outlet and note its temperature too. Refill with water at the same starting temperature and repeat with section B. Section B heats the water faster and its outlet air comes out cooler: the same air, the same flow area, more wall. Work out the heat each delivered: 5 kg × 4.19 kJ/kg·K × the temperature rise, divided by 600 seconds, in kilowatts.

A locomotive fire-tube boiler has four parts, and each exists for surface or for safety: - the **firebox**, a box of water-backed plates around the fire, taking radiant heat; - the **tubes**, carrying the hot gas the length of the barrel through the water — the surface this rung is about; - the **smokebox**, where the gas collects and the exhaust steam blast draws it up the chimney; - the **stays**, hundreds of rods tying the flat firebox plates to the outer shell, because a flat plate under pressure bulges and a round shell does not. The weakness is scale: every mineral in the water is left behind on the tubes as it boils, and scale is an insulator. Tubes are cleaned and replaced as a routine.

The multi-tubular boiler is usually credited to **Marc Seguin**, who patented a boiler with many small tubes in France in the late 1820s, and to **Henry Booth**, who proposed the same arrangement for Robert Stephenson's *Rocket*, winner of the Rainhill trials of 1829; the *Rocket*'s boiler had 25 copper tubes. No patent number is asserted here — the principle is the payload, and it is fully documented. It followed the single-flue Cornish boiler and the twin-flue Lancashire boiler, and it was followed by the water-tube boiler (rung 3 of this batch), which turned the idea inside out. **Honest limits.** A fire-tube boiler holds a large mass of water in a big shell, so the stored energy if the shell fails is enormous, and the shell's size limits the pressure it can hold. It is slow to raise steam. It scales. **Boilers are pressure vessels and are built, tested and inspected under regulation; do not build one.**