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. Firetube boilers are used for auxiliary purposes to provide smaller quantities of low-pressure steam on diesel engine powered ships. So we will discuss about most commonly used water tube boilers
Below picture shows a typical schematic diagram of a water tube boiler.
Water tube boilers mainly consists of 3 important sections
> Water Drum
> Steam Drum
> Tubes
Water Drum
It is located at the bottom of the boiler and carries the hot water. The same water is supplied to the steam drum where the steam is generated.
Steam drum:
It is located at the top part of the boiler and is fitted with external and internal mountings, which include safety valves and steam stop valves (main and auxiliary), to supply steam to the ship’s systems.
Tubes:
Modern water tube boiler consists of various types of water tubes with different functions. The main types are:
Superheater tubes: They are placed behind the screen tubes to carry the superheated steam from the boiler
Generating tubes (Riser): These tubes connect the water drum and the steam drum. As the name suggests, hot water is carried upward from water drum to steam drum where the steam is produced
Down comers: Downcomer tubes transfer the not-so-hot water to the water drum, where the water is heated up in the generating tube before going back to the steam drum.
Screen wall or Screen Tubes: Water circulates rapidly through these tubes and their purpose is to block or screen the boiler fly ash and prevent its carryover into the superheater elements and generators ( see below pic)
Working
Water in the lower drum and in the riser connected to it, is heated and steam is produced in them which comes to the upper drums naturally. In the upper drum the steam is separated from water naturally and stored above the water surface. The colder water is fed from feed water inlet at upper drum and as this water is heavier than the hotter water of lower drum and that in the riser, the colder water pushes the hotter water upwards through the riser. So there is one convectional flow of water in the boiler system.
Below picture shows an actual marine boiler with its main parts
