TYPES OF TUBE IN WATER TUBE BOILER


Generating Tubes
These consist of numerous small diameter tubes placed in the main flow of hot gases, so forming a large heat exchange surface; the generation of steam takes place mainly by convection. In general the number of generating tubes tends to be reduced in modern boilers, until indeed in radiant heat boilers no generating tubes as such are fitted. Water walls
receiving radiant heat are used instead.

Screen Tubes
These are placed adjacent to the furnace, so receiving heat from the flame together with heat from the hot gases leaving the furnace; therefore they need a relatively large diameter to keep the ratio of steam to water low enough to prevent overheating. The duty of the screen tubes is to protect the superheater tubes from the direct radiant heat of the furnace flame.

Water Wall Tubes
These are used basically to contain the heat of the furnace, thus reducing the amount of refractory material required. In some types of boilers, water cooled refractory walls are used. These consist of tubes with studs welded onto them, covered with refractory material, which can now withstand the high temperatures without damage. In other designs part of the tube surface is exposed to radiant heat which helps to generate steam. In some radiant heat boilers the tubes are welded together along their length by fins or strips, and no refractory is required.

Downcomer Tubes
These consist of large diameter, unheated tubes placed outside the gas stream which act as feeders to the water drum and headers.

Riser or Return Tubes
These return steam and water from the top water wall headers to the steam drum.

Superheater Tubes
These consist of small diameter tubes placed in the main gas stream, after the screen tubes. Their duty is to superheat the saturated steam leaving the drum to a temperature suitable for use in the main turbines. They must be protected from direct radiant heat as they are liable to overheating due to the much smaller specific heat of steam compared to that of water.

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