The Water-Tube Boiler: Put the Water in the Tubes

US 65,042, 'Steam Generator', G. H. Babcock and S. Wilcox Jr. The drawing shows sets of pipes in two directions — one set *"in a horizontal position, and the other inclined at an angle therewith"* — cast with T-heads so the sections bolt together, over a combustion chamber with an inclined grate. The specification opens by naming the problem it solves: the *"deficient circulating capacity"* of earlier tube generators, and *"the difficulty of removing incrustations from the interior of the generating vessels, which deposits are increased by a sluggish circulation of the water"*. Circulation and scale are the two ideas of this rung.

Make a closed loop of clear vinyl tube about 1.5 m round, with a 30 cm length of copper tube spliced into one side and clamped. Fill it with water, leaving the top open to air through a short vertical tee — **it must stay open; never seal a heated loop.** Tilt the loop so the copper section is the low, inclined leg, and heat it gently with the heat gun from below. Inject a drop of dye near the copper. Within a minute the dye moves: hot water, lighter, rises out of the copper leg and cold water falls down the other side to replace it. Time the dye round the loop. Now lay the loop flat and repeat: the circulation nearly stops. That is Babcock and Wilcox's point about inclined tubes — circulation comes from height and density difference, and a sluggish loop lets steam bubbles and scale settle where they form.

**US 65,042, 'Steam Generator', George H. Babcock and Stephen Wilcox Jr., of Providence, Rhode Island; patented 28 May 1867.** Babcock & Wilcox went on to build water-tube boilers for power stations and ships, and the design line runs to the high-pressure boilers that feed steam turbines today. It inverted the fire-tube boiler of rung 1: small tubes can hold pressures a shell cannot, and the water they hold is small at any one place. **Honest limits.** It needs clean, treated feed water — a scaled water tube overheats and fails where a big shell would only lose efficiency. It holds little water, so it responds fast but needs attentive feed control. **Boilers are pressure vessels; this rung builds only an open loop, and so should you.**