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Fire tube boilers working principle and operational procedure

A boiler is used to heat feed water in order to produce steam. The energy released by the burning fuel in the boiler furnace is stored (as temperature and pressure) in the steam produced.

The firetube boiler is usually chosen for low-pressure steam production on vessels requiring steam for auxiliary purposes. Operation is simple and feedwater of medium quality may be employed. The name 'tank boiler is sometimes used for firetube boilers because of their large water capacity. The terms 'smoke tube' and 'donkey boiler are also in use.


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Most firetube boilers are now supplied as a completely packaged unit. This will include the oil burner, fuel pump, forced-draught fan, feed pumps and automatic controls for the system. The boiler will be fitted with all the appropriate boiler mountings.


Fire Tube Boiler arrangement

Fig: Fire Tube Boiler arrangement

A single-furnace three-pass design is shown in Figure above. The first pass is through the partly corrugated furnace and into the cylindrical wetback combustion chamber. The second pass is back over the furnace through small-bore smoke tubes and then the flow divides at the front central smoke box. The third pass is through outer smoke tubes to the gas exit at the back of the boiler.

There is no combustion chamber refractory lining other than a lining to the combustion chamber access door and the primary and secondary quart. Fully automatic controls are provided and located in a control panel at the side of the boiler.


Cochran spheroid boiler

Fig: Cochran spheroid boiler

Cochran boilers

The modern vertical Cochran boiler has a fully spherical furnace and is known as the 'spheroid' . The furnace is surrounded by water and therefore requires no refractory lining. The hot gases make a single pass through the horizontal tube bank before passing away to exhaust. The use of small-bore tubes fitted with retarders ensures better heat transfer and cleaner tubes as a result of the turbulent gas flow.


Composite boilers

A composite boiler arrangement permits steam generation either by oil firing when necessary or by using the engine exhaust gases when the ship is at sea. Composite boilers are based on firetube boiler designs. The Cochran boiler, for example, would have a section of the tube bank separately arranged for the engine exhaust gases to pass through and exit via their own exhaust duct.



Related Info:

Requirement for various boiler types on board cargo ships

Safety precautions for working with marine boiler

Raising liquids from a low point - Use of Pumps

Domestic water systems for general cargo ship

General arrangement for marine boiler

Water tube boilers

Fire tube boilers

The use of boiler mountings

Combustion process - supply of air

Various designs burners

Purity of boiler feedwater

Boiler feedwater treatment

The steam to steam generator

Combustion process - supply of fuel oil

Safety valves

Water level gauges

Double evaporation boilers

Exhaust gas heat exchangers

Safety precautions

Bilge and ballast systems on board


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