The tandem compoundafter Hornblower, Woolf & McNaught · mill practice by 1876
Expand steam a long way in one cylinder and the metal must swing between boiler-hot and condenser-cold, condensing fresh steam on every stroke. The compound divides the fall: a small high-pressure cylinder exhausts into a receiver, and a great low-pressure cylinder finishes the work, each keeping to its own modest range of temperature. Jonathan Hornblower patented the idea in 1781 and was crushed by Watt's lawyers; Woolf revived it; McNaught made a business of compounding older engines. Strengths: the best economy of its century, gentler torque, and — at sea, as the triple-expansion — engines frugal enough to make ocean steamers pay. Weaknesses: two of everything, and a receiver that leaks pressure between the stages. Move the cylinder-ratio lever and watch the receiver float to balance what one cylinder gives and the other swallows.
Key to the plate
a. — from the boiler house. A hundred pounds of steam from Lancashire boilers next door. Pressure this high spends itself badly in one cylinder — the temperature swing wastes charge on the walls — which is precisely the problem this engine exists to solve.
b. — governor. Trip gear on the high-pressure cylinder, after Corliss: the governor trims the first admission, and everything downstream — receiver, low-pressure cylinder — follows its lead.
c. — high-pressure cylinder. Small and hot: it takes the boiler’s full steam, uses the top of its expansion, and exhausts — still at useful pressure — into the receiver. By splitting the temperature range over two cylinders, each cylinder’s walls swing through less, and less charge dies on the iron.
d. — receiver. The vessel between the stages, where the high cylinder’s exhaust waits to become the low cylinder’s supply. Its pressure is not set by any valve — it floats to wherever the two cylinders’ appetites balance, and it moves when you move the cutoff or the ratio. Watch it on the combined indicator diagram.
e. — low-pressure cylinder. The giant of the pair, sized by the ratio lever: it takes the receiver’s milder steam and expands it down to the condenser’s vacuum. Same push on the same rod — a tandem — but each cylinder works only half the temperature hill.
f. — crosshead & guides. Both pistons ride one rod to a single crosshead: a tandem compound. The crank cannot tell there are two cylinders.
g. — fly-wheel. Steadier work than any single cylinder can offer — two admissions per stroke, staggered down the expansion — so the wheel has less ironing to do, and waterworks duty men prized the smoothness almost as much as the coal bill.