water tube boilers

In water tube boilers, water is converted to steam inside the tubes, while hot gases pass over and around the outside of the tubes. Water tube boilers can operate at higher pressures than fire tube boilers.

The flow of steam and water within a water tube boiler is called circulation. This circulation is critical in preventing tubes from overheating. When tubes overheat, metal softens, weakens and may eventually rupture.

In a simple water tube circuit, bubbles of steam form in the heated tubes or "risers".

The resulting steam and water mixture is lighter than cooler water on the unheated side of the boiler, and rises to a steam drum at the top of the boiler. Here the bubbles rise to the surface and steam is released.

The water then flows from the drum down through the cooler rubes, or "downcomers", completing and repeating the cycle.
Because the steamdrum is so important in the effici-ent operation of the boiler, we'll go a bit more into detail about this subject.

The main purpose of the steam drum is the separation of steam from water. This is accomplished by providing sufficient volume and low enough velocity to allow the steam to escape.

This separation of steam and water is assisted by steam separators within the drum. These are mechanical devices, such as baffles installed in the space above the water level to rapidly change the direction of steam flow.

Some steam drums contain more intricate devices called cyclone separators, which swirl the steam in a circular motion. Water droplets being carried by the steam are trapped in steam separators and drain back into the water. This prevents water from leaving the steam. The term "carry-over" refers to any contaminant that leaves the steam drum along with the steam.

In addition water tube boilers usually include one or more "bottom drums" or "mud-drums" where suspended impurities in the water can settle out.

The continuous or intermittent removal of small amounts of water and impurities from these drums is called blow-down.
Water tube boilers are classified according to their design. In a "D" type boiler, the steam drum is placed directly above the mud drum. The furnace and boiler are placed off to one side.

The "O" type boiler also utilizes two drums, with the burner position in the center of the boiler.

The "A" type boiler has two small mud drums with a larger single steam drum. Regardless of design, all water tube boilers rely on circulation to allow steam to rise and pass on to the afterboiler section where it is carried to the plant and used as a source of energy.

Except above mentioned "D", "O", and "A" type, there are also other proprietary more sophisticated designs.

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