How does a water tube steam boiler work?

In a water tube steam boiler, unlike a fire tube, water circulates inside the tubes. The heat that is generated and the combustion gases that surround the tubes heat the water that circulates inside them. Many water-tube boilers operate according to the principle of natural water circulation (also known as "thermosiphoning").

The capacity of this type of boiler can be enhanced by increasing the number of tubes in the boiler.