Foster Wheeler DSD Type Boiler: Design Modifications, Merits, Demerits

A DSD type Foster Wheeler boiler is a boiler with a double superheater D-type design. Or, it is a slight modification in the design of the typical D-type boiler arrangement. The DSD-type Foster-Wheeler boiler is a D-type boiler because it is a single furnace boiler and the arrangement of boiler tubes is such that it resembles a D-shape compared with double furnace boilers, which are named M-shaped due to the M-shape geometry of the boiler tubes.

Design Modifications to the D-Type Foster Wheeler Boiler

The principle modifications to the D-type boiler are described and illustrated in Fig. 01 below:

Figure of modifications to d type boiler

  1. The placement of two superheaters (one being primary and the other secondary) immediately behind the three rows of screen tubes (which act as risers in most cases)
  2. The U-bend elements of the superheater are vertically oriented instead of their inclined fitting in the typical D-type.
  3. Where the superheaters extend vertically up, the headers lie below the furnace floor level.
  4. The superheater elements are welded to studs on its headers.
  5. There is an additional screen-header (in some designs more than one) in the DSD-type.
  6. The screen tubes at the lower ends terminate in the screen-header. Whereas, they are fitted in the water drum in the original D-type design.
  7. The downcomers feed screen-header with the water from the steam drum through large bored lines. In addition to this, the water-drum supplies water to the screen-header.
  8. The screen feeder is placed in such a manner so as to leave ample space for the accommodation of two-superheaters as stated in point 1.
  9. Roof-firing is employed as can be seen in the figure the location of the burner on the roof. 
  10. Furnace walls are monowall encased.
  11. Buckstays work as additional wall support and are fitted between the monowall and outer casing.

In its construction, two superheaters (primary and secondary) are fitted in a vertical direction just behind the three rows of screen tubes.

The burners in the marine ships are usually located at the front of the boiler furnace. But in modern boilers, such as Foster Wheeler D and DSD designs and many others, burners are provided on the roof of the boiler and thus commonly known as top-fired boilers.

The furnace of the DSD-type boiler is water-cooled as in the case of its D-type counterpart. The water-wall category is of tangent type. In many cases, they are monowall tubes which are Foster Wheeler’s trade name for membrane tubes.

The water-tubes surrounding the furnace top and side walls are directly fed with water from the screen header (what is a header? Its purpose is the same as that of a drum but both differ in size. The Header serves as a manifold or a collection point to which different tubes and pipes are fitted/expanded)

The water coming from the steam drum through downcomers is supplied to the furnace front and rear water-wall tubes through lower headers  (membrane tubes).

From these walls, the return-supply of water-steam mixture to the steam drum is actualized through upper headers and roof wall tubes.

The connection between the two drums (the steam drum and the water drum) is made through downcomers with the endeavor to naturalize the circulation of water under all steaming conditions.

Steam collected from the steam drum is injected into the superheaters in two sections: primary and secondary.

It is allowed to pass through each section repeatedly unless the required superheat temperature is attained.

In case, the final steam temperature rises well above the required temperature, the desuperheater placed between the two superheating sections is used as a moderator to de-superheat the steam.

The air is drawn to the combustion chamber through natural circulation, which means that no blowers and fans are used to maintain the draft, and due to this, the pressure inside D and DSD-type furnaces is not too high.

Merits

Some of the merits of DSD type boiler are mentioned below:

  1. Increase thermal efficiency of the boiler due to two superheaters.
  2. Better control of the superheat temperature of the final steam through the desuperheater.
  3. Eco-conscious boiler as it causes less emissions due to improved combustion efficiency: less emissions per unit of energy produced.
  4. Better heat recovery performance due to the economizing function of the boiler.
  5. Operational flexibility (varying steam demand adaptability)

Demerits

Some of the demerits of using DSD type boiler are mentioned below:

  1. Somewhat challenging maintenance of the boiler interior due to its complex engineering design.
  2. Higher capital is required.
  3. Reduced thermal efficiency at relatively lower operating loads.
  4. Increased energy input at start-up compared with D-type design.
  5. Higher technical skills to supervise boiler operations and manual controls.

Design Modification

The design modifications to the DSD type boiler are ESD I, ESD II, ESD III, and ESD IV.

Applications

In the 220000 DWT tankers, the DSD-type Foster Wheeler boilers are used in pairs. Together, they run to give maximum evaporative capacity as high as 147000 kgh at 61 bar and 510°C.