Vertical boiler with horizontal fire-tubes

[2] It is widely used in marine practice, either fired directly by coal or oil fuels, or else used for heat recovery from the exhaust of large diesel engines.

The firebox is another hemispherical dome, riveted to the base foundation ring to give a narrow waterspace.

The fire-tubes are arranged in a single horizontal group above this, mounted between two flat vertical plates that are inset into the boiler barrel.

The combustion chamber is of the "dry back" form and is closed by a steel and firebrick plate, rather than a water jacket.

[6] The Cochran boiler was not applied to locomotives, but was used for a pair of experimental steam railmotors built for the GNSR by Andrew Barclay in 1905.

This dome shape is still sufficiently strong to withstand the pressure, but there is now a sharp corner between the shell and the top plate.

The outer drum is a parallel cylinder but the inner drum is usually stepped in three diameters: wide around the fire grate to provide the most grate area and to promote rapid boiling in its waterjacket, the central section carries the inner ends of the fire-tubes, and the upper section is narrowed to reduce the unjacketed surface area and to provide an open steam space above the waterline.

[9][11] This upper space is deliberately oversized so that when fitted to a road vehicle, any tilting during hill climbing will still leave an adequate depth of water cover above the tubes.

Where the boiler is used for steam wagons it is often coke-fired with a deep firebed, and coke is merely poured down this chute rather than carefully distributed.

The boiler is sometimes superheated by a semi-radiant element arranged as a coil inside the upper part of the central drum.

The combustion chamber is set entirely within the water space so that all of it can provide heating surface, and it is made cylindrical for strength.