Water-tube boilers are a type of boiler where water circulates inside tubes that are heated externally by combustion gases. This design allows for high-pressure steam generation and efficient heat transfer. They are commonly used in power plants and industrial applications due to their ability to handle large volumes of steam and quick response to load changes. These boilers are typically more efficient than fire-tube boilers, as they can operate at higher pressures and temperatures. Their construction also allows for easier maintenance and cleaning, making them a popular choice for facilities that require reliable and continuous steam supply.