The water tube boiler is employed for high-pressure, high-temperature, high-capacity steam applications, e.g. providing steam for main propulsion turbines or cargo pump turbines. Fire tube boilers are used for auxiliary purposes to provide smaller quantities of low-pressure steam on diesel engine powered ships.
The construction of water tube boilers, which use small-diameter tubes and have a small steam drum, enables the generation or production of steam at high temperatures and pressures. The weight of the boiler is much less than an equivalent firetube boiler and the steam raising process is much quicker.
Design arrangements are flexible, efficiency is high and the feedwater has a good natural circulation. These are some of the many reasons why the water tube boiler has replaced the firetube boiler as the major steam producer.
Early water tube boilers used a single drum. Headers were connected to the drum by short, bent pipes with straight tubes between the headers. The hot gases from the furnace passed over the tubes, often in a single pass.