Herein a boiler tube (2) having a longitudinal extension (L) and comprising radially inner and outer tubular portions (4, 6) extending along at least a first part (5) of the longitudinal extension (L). The radially outer tubular portion (6) is metallurgically bonded to the radially inner tubular portion (4). A sensor space (8) is arranged between the radially inner tubular portion (4) and the radially outer tubular portion (6), wherein the sensor space (8) is configured to accommodate a sensor arranged to detect a physical property of the radially outer tubular portion (6). A duct (10) is connected to the sensor space (8) and extends through the radially outer tubular portion (6) to an exit portion (12) of a surface of the radially outer tubular portion (6). The radially inner and outer tubular portions comprise materials of different chemical composition. Also, a boiler tube unit and a furnace are disclosed herein.

A segmented watertube boiler system allows for rapidly expanding the capacity of a boiler. The system joins boiler segments to form a boiler. The segments have portions of the boiler so that the storage capacity for water and heat are increased with the number of segments uses. Any openings in the segments on the ends are capped off and heat and water sources are attached. The boiler can be cleaned and maintained by simply disassembling the segments to provide access to the whole boiler.

A segmented watertube boiler system allows for rapidly expanding the capacity of a boiler. The system joins boiler segments to form a boiler. The segments have portions of the boiler so that the storage capacity for water and heat are increased with the number of segments uses. Any openings in the segments on the ends are capped off and heat and water sources are attached. The boiler can be cleaned and maintained by simply disassembling the segments to provide access to the whole boiler.