A water-tube steam boiler is a type of boiler in which water circulates in tubes, with the heat source (such as hot gases or flue gases) surrounding the tubes. This design is distinct from a fire-tube boiler, where the water surrounds the heat source within the boiler.

In a water-tube boiler, the water is pumped through a system of tubes, which are usually arranged vertically or inclined, with the hot gases passing over the outside of the tubes. The heat from the gases is transferred to the water through the tube walls, causing the water to boil and generate steam.

Water-tube boilers are typically used in large-scale industrial applications, such as power stations and ship propulsion, where high steam pressures and temperatures are required. They are able to handle higher pressures and temperatures than fire-tube boilers due to their more efficient heat transfer design.

The main components of a water-tube boiler include the boiler proper (the main structure containing the water tubes and heat source), the steam drum (where the steam is collected and distributed), the mud drum (where sediment and other impurities settle out of the water), and various pumps, valves, and controls to regulate the water and steam flow.