Boiler and Desal Tubes
The construction of water-tube boilers, which use small-diameter tubes and have a small steam drum, enables the generation or production of steam at high temperatures and pressures. The weight of the boiler is much less than an equivalent firetube boiler and the steam raising process is much quicker.
Design arrangements are flexible, efficiency is high and the feedwater has a good natural circulation. These are some of the many reasons why the water-tube boiler has replaced the firetube boiler as the major steam producer.
Early water-tube boilers used a single drum. Headers were connected to the drum by short, bent pipes with straight tubes between the headers. The hot gases from the furnace passed over the tubes, often in a single pass,
A later development was the bent tube design. This boiler has two drums, an integral furnace, and is often referred to as the ‘D’ type because of its shape. The furnace is at the side of the two drums and is surrounded on all sides by walls of tubes. These water wall tubes are connected either to upper and lower headers or a lower header and the steam drum. Upper headers are connected by return tubes to the steam drum. Between the steam drum and the smaller water drum below, large numbers of smaller-diameter generating tubes are fitted. These provide the main heat transfer surfaces for steam generation. Large-bore pipes or downcomers are fitted between the steam and water drum to ensure good natural circulation of the water.
In the arrangement shown, the superheater is located between the drums, protected from the very hot furnace gases by several rows of screen tubes. Refractory material or brickwork is used on the furnace floor, the burner wall, and also behind the waterwalls. The double casing of the boiler provides a passage for the combustion air to the air control or register surrounding the burner,
The need for a wider range of superheated steam temperature-controlled to other boiler arrangements being used. The original External Superheater ‘D’ (ESD) type of boiler used a primary and secondary superheater located after the main generating tube bank. An attemperator located in the combustion air path was used to control the steam temperature.
ESD II & ESD III Type Boilers
The later ESD II type boiler was similar in construction to the ESD I but used a control unit (an additional economizer) between the primary and secondary superheaters. Linked dampers directed the hot gases over the control unit or the superheater depending upon the superheat temperature required. The control unit provided a bypass path for the gases when low-temperature superheating was required. In the ESD III boiler, the burners are located in the furnace roof, which provides a long flame path and even heat transfer throughout the furnace.
