Boiler is a pressure vessel in which the water is heated to evaporate and generate the steam and the unit is so arranged that the generated steam accumulated in it.
Boilers are used to produce steam, either for the main propulsion or for auxiliary machinery. A boiler is, in general, any closed liquid-containing vessel to which heat is applied. It is also called steam generator as it transforms water into steam. Boilers generally consist of metal shells (or bodies), headers and tubes that form the container of the steam and water under pressure and, in certain types, of the furnace and passages for the hot gases. Some boilers have additional drums called superheaters.
Boilers fall into two categories: water-tube and fire-tube ones, according to which substance passes through the tube and which flows round the outside. They are used to provide steam for propulsion or for various ship services.
In firetube boilers, the fire or hot gases are directed through the inside of tubes within the boiler shell, which are surrounded by water. The tubes are arranged in banks so that the gases can be passed through the boiler up to 4 times before passing out the stack. This system exposes the maximum heat transfer surface to the water. These can produce up to approximately 750 hp or 25,000 lbs of steam per hour. 80% of boilers in use are of this configuration.
In water-tube boiler, the fire or hot gases are directed to and around the outside of tubes containing water, arranged in a vertical position. Water-tube boilers have two or more drums. The separation of steam and water takes place in the top drum, while the bottom drum serves as a collection point for sludge. This system is usually used when more than 750 hp or several hundred thousand lbs of steam per hour, are needed.
The fire-tube 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.
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. 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.
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.
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. The use of exhaust gases from diesel main propulsion engines to generate steam is a means of heat energy recovery and improved plant efficiency. Individual banks may be arranged to provide feed heating, steam generation and superheating. A boiler drum is required for steam generation and separation to take place and use is usually made of the drum of an auxiliary boiler.
Auxiliary steam plant system
The auxiliary steam installation provided in modern diesel-powered tankers usually uses an exhaust gas heat exchanger at the base of the funnel and one or perhaps two water tube boilers. Saturated or superheated steam may be obtained from the auxiliary boiler. At sea it acts as a steam receiver for the exhaust-gas heat exchanger, which is circulated through it. In port it is oil-fired in the usual way. Auxiliary boilers on diesel main propulsion ships, other than tankers, are usually of composite form, enabling steam generation using oil firing or the exhaust gases from the diesel engine. With this arrangement the boiler acts as the heat exchanger and raises steam in its own drum.