to take taper boilers. Vertical fire-tube boiler A vertical fire-tube boiler (VFT), colloquially known as the "vertical boiler", has a vertical cylindrical shell, containing several vertical flue tubes. Horizontal return tubular boiler Horizontal return tubular boiler (HRT) has a horizontal cylindrical shell, containing several horizontal flue tubes, with the fire located directly below the boiler's shell, usually within a brickwork setting 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 ubiquitous 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. Immersion fired boiler The immersion fired boiler is a single-pass fire-tube boiler that was developed by Sellers Engineering in the 1940s. It has only firetubes, functioning as a furnace and combustion chamber also, with multiple burner nozzles injecting premixed air and natural gas under pressure. It claims reduced thermal stresses, and lacks refractory brickwork completely due to its construction. Variations Water tubes Fire-tube boilers sometimes have water-tubes as well, to increase the heating surface. A Cornish boiler may have several water-tubes across the diameter of the flue (this is common in steam launches). A locomotive boiler with a wide firebox may have arch tubes or thermic syphons. As firebox technology developed, it was found that placing a baffle of firebricks (heat-resistant