Firetube (fire tube) boilers

In a fire tube boiler, combustion takes place within a cylindrical furnace located within the boiler and combustion gases pass through tubes surrounded by water.

The combustion gases leave the furnace through the rear of the boiler, then reverse direction and pass through the boiler tubes several times, increasing the amount of heat transfer.

To maintain high gas velocity throughout the tubes, the number and diameters of tubes in each succeeding pass are reduced.

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