Why Water Tube Boilers ?
In contrast to its internal construction, which includes a small steam drum and small diameter tubes, the water tube boiler is used to generate steam having high pressure and temperature. These small internal parts produce high volume steam for high capacity applications.
Water tube boilers are the most widely used boilers. These type has replaced many boilers, including the fire tube type, mainly because of the following reasons :
- The weight of the water tube boiler is much less than any type of boiler of equivalent size.
- Steam raising and steam generation process is much faster
- The design is custom made and flexible to include any kind of modifications
- It has very high efficiency than the rest of the boilers
- The design facilitates good natural circulation of the feed water
The earlier versions of water tube boiler consisted of a single drum arrangement with headers connected using short, bent pipes with straight tubes. The hot gases used to pass over the tubes in only one go or a single pass.
But with the advent of bent tube design, the boilers were fitted with two drums with a an integral furnace.The boilers are known as D shaped design or D shaped Boilers.
Construction of a D type Watertube Boiler
The D type boilers has an arrangement of two drums at the side of the furnace and is surrounded by water tube walls . The steam drum is at the top and the water drum is located at the bottom of the whole system. The water wall tubes are connected either to the top and bottom headers or to the bottom header and the steam drum. The return tubes going to the steam drum are connected to the upper header.
Both the drums are connected with large number of small diameter tubes which carry feed water. These small diameter tubes are known as generating tubes because they provide the main heat transfer surfaces for the production of steam. Water is circulated between both the drums with the help of externally fitted Large bore downcomers. Refractory material is used in the making of the furnace floor, burner wall and behind the water walls. The refractory material acts as an insulation, preventing heat from escaping. The superheater is located away from the furnace and in between the two boilers. The boiler is also provided with a double casing which allows the passage of combustion air to the air control register surrounding the burner.
The D type boiler also has a two stage superheater – primary and secondary, located below the generating tubes. It also has an attemperator to control the temperature of steam. This basic D type boiler is also known as ESD (Emergency Shutdown Boiler).
ESD II Boilers
Later came the ESD II type boilers. This were to accommodate a higher superheated steam temperature range. The whole arrangement of ESD II is similar to ESD I with the only difference of an additional controller fitted in between the two superheater stages. The controller is to maintain the superheat temperature of the steam.
The design of the controller consists of a bypass which diverts the steam when the temperature of the superheated steam required is less.
Introduction of Marine Engineering 2nd Edition by D.A Taylor