Foster Wheeler ESD II Boiler: Construction, Operation, Merits, Demerits

Foster Wheeler ESD II boiler is a water-tube boiler that falls in the category of eternally superheating apparatus. It is a design modification to its predecessor boiler: ESD I.

In the past, Queen Elizabeth 2 (a British liner) was fitted out with three Foster Wheeler ESD II boilers to feed two Brown-Pametrada turbines.

Why ESD II Boiler?

In the ESD I type, there was a problem: with the increase in load, the heat input to the superheater increased proportionally,  thereby removing the excess heat with the help of an air-attemperator. This problem with the ESD I boiler was minimized in its later design which is known as ESD II.

ESD II Boiler Specification

In 1960, there were a number of ships that were commissioned with the ESD II design of the Foster Wheeler Boiler. They were to give an evaporation rate as high as 82000 kg/hr. at 52 bar and 587℃.

Foster Wheeler ESD II Design

In the refined arrangement of ESD I that is the ESD II design, the heat input to the superheater is limited to the required superheat to be added. It is achieved through an outlet damper control (by means of linked dampers) as well as a damper-controlled by-pass route.

With this variation of by-pass, an upward economizer known as a control unit is used to absorb the measured heat of the burned gases which would otherwise go to the superheater under the damper-open conditions thereby unnecessarily raising the steam temperature. It is shown in figure 01.

Foster Wheeler ESD II Design
Fig. 01

Construction

  1. Like the ESD I boiler, the ESD II variation consists of two drums: one steam drum lying above the second water drum.
  2. On one side, a furnace is provided which is water-cooled.
  3. The furnace is designed to have water-walls. These walls consist of close-pitched 50mm thin water-tubes and surround the sides, roof, and top of the furnace.
  4. The burned gases enter a split gas passage: a large proportion of gases pass through the screen tubes connecting the two drums, and only a small amount is allowed to get deflected in the dampers that regulate the gas flow (or its mass) through the superheater thereby controlling the superheat temperature of the steam.
  5. The lower water headers are rectangular in cross-section. They are supplied with feed-water from the water drums through underfloor rubes.
  6. External downcomers (130mm diameter) connect the steam drum to the water drum. 
  7. The furnace is provided with refractory material at the floor, burner front wall as well and behind the water-walls.
  8. Unlike ESD I, the ESD II arrangement is not provided with the attemperator.
  9. The all-welded double casing is used to encase the boiler body.
  10. Soot blowers (fully retractable-type) are used in the furnace as well as the superheater section.
Construction of the Foster Wheeler ESD II
Fig. 02

Operation

The gases produced in the furnace pass over a bank of thin screen tubes (50mm). From there, they split into two streams: either they pass over a multi-loop type superheater or they enter the control unit.

The control unit as shown in Fig. 02, is constructed and supported by the superheater. It is a type of plain tube economizer. It is supplied with the feed-water uninterruptedly on its way from the economizer to the steam drum. In case, steaming is required under low-feed conditions, the excess water is returned to the water drum through a circulating channel fitted on the installation.

The interlinked dampers function in such a way that they regulate the flow of gas through the individual passages thereby controlling the superheat temperature: the higher the gas flow over the control unit, the lower the final steam temperature.

After passing through the control unit, the two gas streams join and finally take a path through the economizer to the atmosphere.

Merits

The merits of the Foster Wheeler II are given below:

  1. It is provided with more economical control of the final superheat temperature of the steam via. control unit and linked dampers.
  2. No extra tubing is required as in the case of the ESD I-type boiler for air-attemperator.

Demerits

The demerits of the Foster Wheeler II are given below:

  1. The evaporation rate is lower than that of an ESD III-type boiler.
  2. The final steam temperature is also lower with an ESD II type boiler when compared with its ESD I counterpart.