Foster Wheeler ESRD Boiler: Design, Construction, Operation

Foster Wheeler ESRD Boiler is a water-tube boiler designed by Amec Foster Wheeler.

The acronym ESRD stands for External Superheat Reheat D-type boiler, which means that it is a single furnace boiler with the placement of superheaters external to the main tube bank that resembles D-shape geometry.

Design and Construction

It is a boiler that is versatile in design and operation. Its design and construction are described and illustrated below:

design and construction of foster wheeler esrd boiler

  1. It is a roof-fired boiler, which means the burners are placed on the roof, as shown in the figure.
  2. The convective column retains superheaters (both primary and secondary) and reheaters. The primary superheater contains two separate superheating passes, while the secondary keeps only one pass.
  3. The ESRD unit is provided with added features, such as supported by reheaters with controlled dampers and regenerative gas air-heaters.
  4. Steam drum feeds the screen headers via downcomers with large diameters.
  5. Superheat control of the final steam is provided with the attemperator placed between the primary and the secondary superheaters.
  6. The furnace is encased with a monowall, as in the case of ESD IV. Likewise, it is also provided as a screen between the furnace and the convective column. It is constructed in such a way that it opens out to form two rows of open-pitched tubes for the passing out of furnace gases on their way to the superheating convective column. Moreover, the latter monowall tubes directly connect the screen header to the steam drum and thus act as a support for superheaters, reheaters, and by-pass economizers.
  7. The large superheating convective column is divided into two halves: One compartment is known as a reheat section, and the other is a by-pass section. The by-pass section retains the first pass of the primary superheater and the bare tube by-pass economizer. Meanwhile, the reheat section retains the second pass of the primary superheater, the secondary superheater, and the reheater.
  8. In the reheat section, the reheater is placed above both the second pass of the primary superheater and the secondary superheater. Above the reheaters and the bare tube by-pass economizer are provided dampers for the reheat steam temperature control. (see figure below)
  9. In the by-pass section, the bare tube economizer consists of athwartship elements and is placed above the first pass of the primary superheater. (see figure below)
  10. A second economizer is the extended surface economizer. It is a steel fin-tube economizer and is fitted above the reheat control dampers. (see figure below)
  11. The primary and secondary superheaters are arranged crosswise on a shared horizontal plane. Such an arrangement is called athwartships.
  12. The steam-flow is contra-flow to the furnace gases in the first pass of the primary superheater for the maximized exchange of heat, and it is parallel-flow in case of the second pass of the primary superheater to keep the metal tube temperature within the safe limits. Likewise, the steam-flow is a parallel flow to the furnace gases in the secondary superheater for the sake of maintaining tube-temperature to a designed minimum value.
  13. There is a by-pass opening in the monowall based division wall of the superheating convective column. The top four rows of superheater tubes in each bank are positioned above the by-pass opening in order to protect the reheater.
  14. An extended surface steel-gilled economizer is placed above the control dampers.

Operation

Since there are two movements of a ship, ahead and astern (reverse), re-heating of the intermittent steam from the H.P. turbine is required only during the forward motion.

It is for this reason that the reheating function of the boiler stands stalled during the astern motion of the ship, and thereupon, the reheater is cooled by a cooling air stream caused by the forced-draft fan.

During astern movement, the reheat shut-off dampers automatically closed with the simultaneous opening of the cooling dampers.

Ahead Motion

During the ship ahead motion, the flow of furnace gases is schematically shown in the figure below.

ahead motion of foster wheeler esrd boiler

Furnace gases rise and pass through the cavities left in the monowall. Now, a major proportion of gases passes through the second pass of the primary superheater, the secondary superheater, and the reheater. A minor gas volume rises above yet in a parallel fashion and passes over the first pass of the primary superheater and the bare tube by-pass economizer.

The reheat control damper is modulated to control the reheat temperature of the steam. The reheat shut-off damper is signaled to remain open up to the required extent in the ahead condition of motion. 

Thus a well-modulated reheat control damper determines the flow of gas across the bare-tube economizer and permits the proportioned volumes of two parallel gas streams to mix through an opening in the division wall.

Astern Motion

During the astern (reverse) condition, the flow of furnace gases is schematically shown in the figure below.

astern motion of foster wheeler esrd boiler

During astern motion, the steam-flow through the reheater is kept virtually zero. Reheat shut-off dampers operate to become fully closed: the flow of gases is such that by passing over the second pass of the secondary superheater and secondary superheater, the gases are by-passed via an opening in the division wall in order to get mixed with the gas coming from the first pass of the primary superheater. On meeting, the joint gas-stream now passes across the bare-tube economizer.

In the astern condition, the reheaters are cooled off with the cooling air that is let in via the burner windbox to keep the reheater from thermal shocks once the reheating of steam starts again during the ahead motion. 

In both the ahead and astern motions of the ship, the joint gas-stream is allowed to pass through the steel gilled economizer en-route to the regenerative air heater.

Advantages

  1. Inspection and maintenance are caesar as all the superheating elements are in line. They connect to the header studs through butt welds.

Disadvantages

  1. Intricate boiler design
  2. Critical start-up and shut-down
  3. Higher capital investment
  4. Skilled labour