Fire-tube Boiler - Types of Fire-tube Boiler - Admiralty-type Direct Tube Boiler

Admiralty-type Direct Tube Boiler

Extensively used by Britain, before and in the early days of ironclads, the only protected place was below the waterline, sometimes under an armoured deck, so to fit below short decks, the tubes were not led back above the furnace but continued straight from it with keeping the combustion chamber in between the two. Hence the name, and considerably reduced diameter, compared to the ubiquituous Scotch or return tube boiler. It was not a great success and its use was being abandoned after the introduction of stronger side armouring – “the furnace crowns, being very near the water-level, are much more liable to over-heating. Further, on account of the length of the boiler, for an equal angle of inclination, the effect on the water-level is much greater. Finally, the unequal expansion of the various parts of the boiler is more pronounced, especially at the top and bottom, due to the increased ratio between the length and the diameter of the boiler; the local strains are also more severe on account of the comparatively feeble circulation in long and low boilers.” All these also resulted in a shorter life. Also, the same length of a combustion chamber was much less effective on a direct tube than on a return tube boiler, at least without baffling.