DME-II_Thermal Engineering-II_Boiler-IV

Babcock and Wilcox boiler:

      The Babcock and Wilcox boiler are also is known as Longitudinal Drum Boiler or Horizontal Tubes Boiler. It is probably the first water tube boiler designed and widely used across the globe.

A schematic of the Babcock and Wilcox boiler is shown in figure.  

Construction:

•A horizontal steam and water drum: This is the main part of the boiler. It is supported by a steel structure at a certain height and is independent of brickworks. The size of the boiler drum is small as compared with the boiler drum of a fire-tube boiler of the same capacity. It has water and steam. All safety and control devices are mounted on the boiler drum.

•A bundle of steel tubes: The front end of the boiler drum is connected to the uptake header (water box) by a short tube and the rear end is connected to the down take header (water box) by a long tube. In between the headers, a number of small-diameter steel tubes are fitted at an angle of 5° to 15° with the horizontal to promote the water circulation. These steel tubes are arranged in the combustion chamber in a zigzag way so that more surface area of the tube is exposed to hot gases.

•Combustion chamber: It is the space above the grate, below the front end of the drum where combustion of fuel takes place. This chamber enclosed by brickwork and is lined from inside by fire bricks. Doors are provided to give access for cleaning, inspection, and repairing. The combustion chamber is divided into the separate compartments above the furnace is the hottest and the last chamber is of lowest temperature. This makes the path of hot gases longer before leaving the boiler through the chimney.The superheater is placed between the drum and water tubes. Dampers are provided at the rear end of the chamber to regulate the fresh air supply for maintaining proper combustion of fuel.

•Safety and control devices: They are called mountings, as basically these devices mount over boiler drum. These are the safety valve, pressure gauge, water-level indicator, feed check valve, steam-stop valve, blow of cock, fusible plug, and manhole.

Operation: 

•    The water in Babcock and Wilcox boilers pumped by a feed pump and it enters the drum through the feed check valve up to the prespecified level so that the headers and tubes are always flooded. When the combustion takes place above the grate, the products of hot gases come out and rush through each compartment of the combustion chamber. Hence, the rear part of the tubes has the lowest temperature and the front part of the tubes as highest temperature. When water is heated inside the tube. Due to continuous heat supply, some of the water gets vaporized into steam inside the tubes and a mixture of water and steam enters the boiler drum through the uptake header. The cold water from the boiler drum comes down through the uptake header and enters the lower end of the water tubes for getting heated further. This natural circulation is called thermosiphon system.

•   The steam generated gets collected in the steam space above water space in the boiler drum. In order to remove all water particles from the steam, it is finally through the superheating. The superheated steam is then available for use.

Special features of water tube boiler:

   a) Water tube boilers have the high evaporating capacity (20,000 to 40,000 kg/h) compared with other boilers. The operating pressure lies between 11.5 to 17.5 bar.b) The draft losses are minimum.c) The defective tubes in water tube boilers can be identified and replaced easily.d) The entire boiler rests on iron structures, independent of brick structure.