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Water Tube Boilers
Boilers are used on board ships for the production of steam. Steam is utilized for heating, running turbo generators, running cargo oil pump turbines, air conditioning, water sterilization, fresh water production, etc. Ships running on steam engines require large boiler plants called main boilers. For any purposes other than main propulsion, auxiliary boilers fulfill the requirements. Auxiliary boilers are normally oil fires, where either diesel oil or heavy oil can be used for producing hot flue gas which circulate inside the boiler to convert water into steam. When ship is sailing, heat from the main engine exhaust gas is utilized for the production of steam. Such boilers are termed as Exhaust Gas Boilers (EGB). There are boilers which can be used in two modes; oil fired mode and exhaust gas mode. They are known as composite boilers.
Water Tube and Smoke Tube Boilers
There are two types of boilers, considering the path of flue gases; smoke tube or fire tube boilers and water tube boilers. In smoke tube boilers flue gases pass through tubes and water is filled outside the tubes. On the other hand in water tube boilers, water flows through the tubes while flue gases pass outside the tubes. In both cases steam is produced by transferring heat from flue gases to the water.Water tube boilers are commonly used on board most of the ships because of their smaller size and higher capacity compared to smoke tube boilers.
Referring to the figure above, in smoke tube boiler, hot flue gases produced by burning fuel pass through the tubes while water is present at the space around the tubes and steam is produced. In the case of water tube boiler, hot gases pass around the tubes while water is inside the tubes. Here steam is collected at the top of the boiler.
Advantages of Water Tube Boilers
- Generally higher efficiency of 85% and above and thereby reduced fuel consumption.
- Flexible design. Comparatively lesser space required than fire tube boiler of same capacity.
- Very high rate of steam production.
- Possible to achieve higher pressures and temperatures, which is helpful for turbo generators and cargo oil pump turbines.
- Flexibility in operation for fluctuating demands of the system; plant readily responses to sudden changes in steam demands.
- Some of the water tubes are constructed as stay tubes for support. Hence no additional support is required.
- Because of natural or forced water currents inside, boiler can be easily put into service from cold in 3 to 4 hours. It may take up to 24 hours for smoke tube boilers since there is no circulation of water.
- Water tube boilers contain water in tubes and water drum, whereas in smoke tube boilers entire pressure vessel except tubes is filled with water. It makes the overall weight of the boiler much higher when compared to water tube boilers. Also presence of large quantity of water inside causes a lag in the supply-demand of the plant.
- Radiation loss is less in water tube boilers.