The Cochran boiler is a cylindrical vertical water drum boiler commonly used in marine applications. It has a hemispherical domed top and bottom firebox riveted to a foundation ring, creating a narrow water space. Fire-tubes arranged above the firebox are mounted between two vertical plates that form a shallow combustion chamber connected to the firebox. Exhaust exits through an external smokebox and vertical flue. Coal or oil fuels are burned on a grate to produce hot gases that pass through the fire-tubes and heat water into steam, collected in the domed top of the boiler.
2. • 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 water space.
• 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 smoke box and a
vertical flue. For maintenance access to the tubes, a manhole is provided in the hemispherical
dome.
3. • 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 "Sinful" 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.
5. Working of Cochran Boiler:
• The unit consists of a cylindrical shell with a dome shaped top where the space is provided for
steam. The shell is formed of steel plates joined together with rivets. The fuel is burnt on grate in
the furnace provided at the bottom most part of the boiler. The grate consists of iron bars which are
arranged with space between them. The spacing allows the air to pass onto the fuel for combustion.
The fire box is hemispherical so that the unburnt fuel, if any, is deflected back to the grate and
complete combustion is achieved.
• An ash pit is attached beneath the furnace for collecting ash after regular intervals. The coal, on
burning, produces hot flue gases and these hot products of combustion from the fire box enter
through the small flue pipe into the combustion chamber which is lined with fire bricks on the
outer walls of the boiler. The lining prevents the shell from being damaged due to the overheating.
• The un-burnt fuel is deflected back to the grate and complete combustion is achieved in
combustion chamber where the high temperatures are maintained.
6. • The hot gases passing through the horizontal smoke tubes give their heat to the water and in doing
so convert water into steam which gets accumulated in the upper portion of the shell from where it
can be supplied to the user.
• Finally the flue gases are discharged to the atmosphere through the smoke box and the chimney.
The smoke box door enables the cleaning and inspection of the smoke box and fire tubes. Through
a manhole provided at the crown of the shell, a man can enter the boiler for periodic cleaning and
maintenance of the boiler. The Cochran boiler is compact in design and there is good external and
internal accessibility. Its efficiency is up to 70 to 75%.
