The Cochran boiler was produced by Cochran & Co. of Annan, Scotland. 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. Where such a boiler may be heated either by the exhaust gases of the main propulsion plant, or else separately fired when in port (usually by oil rather than coal) it is referred to as a composite boiler.
The boiler is a cylindrical vertical water drum with a hemispherical domed top. This domed shape is strong enough not to require staying. 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 first of these plates forms a shallow combustion chamber and is connected to the firebox by a short diagonal neck. The combustion chamber is of the "dry back" form and is closed by a steel and firebrick plate, rather than a water jacket. The exhaust from the fire-tubes is into an external smokebox and a vertical flue. For maintenance access to the tubes, a manhole is provided in the hemispherical dome.
A typical Cochran boiler, as illustrated, might be 15 feet (4.6 m) high and 7 feet (2.1 m) in diameter, giving a heating surface of 500 square feet (46 m2) and a grate area of 24 square feet (2.2 m2). Working pressure is between 100 and 125 psi.
Where composite firing is used, there are several possible arrangements for the heating gases. Most use a double-pass tube arrangement where another dry back combustion chamber routes the gases from one tube bank to return through the other. Some arrangements use a separate tube bank for the heat recovery exhaust gases or the direct firing gases, others pass the exhaust gases into the top of the (unlit) firebox. A pure heat-recovery boiler may have no firebox at all, other than a shallow domed plate for strength.
Cochran also offer a modified "Sinuflo" fire-tube, which is claimed to offer better heat transfer from the gases. This is bent into a number of horizontal sinusoidal waves, rather than being straight.