Firetube boilers wherein the combustion gas passes inside the boiler tubes and heat is transferred to water between the tubes and the outer shell,Watertube boiler wherein the boiler water passes through the tubes, while the exhaust gases remain in the shell sides, passing over the tube surfaces. Usually, watertube boilers are used where high steam pressures are required.

Firetube boilers wherein the combustion gas passes inside the boiler tubes and heat is transferred to water between the tubes and the outer shell,Watertube boiler wherein the boiler water passes through the tubes, while the exhaust gases remain in the shell sides, passing over the tube surfaces. Usually, watertube boilers are used where high steam pressures are required.