The Strong Duplex Boiler

Gallery opened: Sept 2016

Updated 16 Nov 2016

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George S Strong was a mechanical engineer of Philadelphia, Pa, who introduced an unusual boiler with twin fireboxes made of corrugated steel plates with welded longitudinal seams, in contrast to the usual flat plates supported by stays. The fire boxes met in a Y-shaped junction that led to single combustion chamber, which in turn led the gases to a conventional fire-tube boiler section fitted with 235 tubes. Most of this construction was basically cylindrical in shape to resist the boiler pressure. The original idea of using corrugated tubes in boilers was due to Samson Fox, an English engineer.

Scientific American for 12 January 1889, took an optimistic view of the project:

"The corrugated furnace chambers have, as our readers know, been extensively introduced in marine boilers. They have effected important economy in this service, as higher pressures can be carried than with the old style flat-sided structures. They have co-operated with the compound engine to bring down the coal consumption to the very low point it has now attained in good practice. Its introduction on a locomotive is a step in the right direction, comparable to compounding the cylinders."

However, it was remarked rather cruelly by Angus Sinclair in 1907 that:

"Although the Fox corrugated flue was found very frequently in marine practice, and had been to a limited extent adapted to locomotives in Germany, it has not been a success on American locomotives. The engine was a failure, and was a good illustration of what an amateur will do when he undertakes to design a locomotive."

Strong's first engine was a 4-4-0 No 383 built at Wilkes-Barre, Pa, in 1885; this had a conventional flat-sided firebox and was used to test Strong's valvegear inventions.

The second was 4-6-2 No 444 "Duplex"

A later engine was the 4-4-2 "A G Darwin", built in 1888

Left: The Duplex locomotive: 1886

The first engine equipped with the Strong twin fire boxes for burning anthracite coal was "Duplex" No. 444, built at Wilkes-Barre in 1886. The boiler was 33 feet long, and was composed of an outer shell in combination with a fire box of two Fox corrugated flues side by side, joining into a combustion chamber. The total length of fire box and combustion chamber was 16 ft 4.5 in; the smallest diameter of flue was 38.25 inches. The fire boxes were 8 ft 9 in long.

The A G Darwin was little different from the Duplex, though it is believed to have had somewhat shorter fixeboxes and boiler barrel. It appears it was converted into an experimental engine called "Balanced Compound No 1"

Left: The A G Darwin: 1888

The Darwin was not short of crew accomodation. The driver occupied a roomy cab on top of the boiler, while the fireman inhabited a strange overlapping double-cab at the rear of the boiler. A speaking tube was provided for communication between the fireman and engineer; one wonders how well it worked in such a noisy situation, and one notes that there is a walkway, with its own little door, joining the two cabs.

It is not obvious in this view, but the rear section of the boiler, behind the driver's cab, was twice the width of the front section to accomodate the two fire boxes side-by-side.

Left: Part of an article on the George Strong locomotives

Mr Strong had a great advantage in his name. Who would not want the safety of a Strong boiler? Who would not want the power of a Strong locomotive? Despite this advantage, the engines were not successful. Details are lacking but the boilers were difficult and expensive to construct, and difficult to keep water-tight. I have found no record of any boiler accidents.

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