From the perspective of its engineering design, the Foster Wheeler ESD IV boiler is a duplicate version of its sister boiler ESD III type, except for some structural changes in the construction of water-walls.
It is used extensively with large tankers where steam with evaporative rates higher than those which ESD III is capable of producing is required.
It is a water-tube boiler that was designed by Amec Foster Wheeler and falls within the category of D-type boilers on account of its single furnace arrangement along with the resemblance of an internal water-tube setting with D-shape geometry.
Like ESD III, it is also provided with cutting-edge boiler technology to assist in the optimized control of the superheat temperature of the final steam to be expanded in the H.P. turbine.
Evaporative Capacity of Foster Wheeler ESD IV Boiler
The ESD IV has a qualitative edge over its predecessor designs. With the maximally advantaged use of construction materials in water-tube walls, the single ESD IV boiler produces steam at evaporative rates above 120,000 kg/hr.
Design and Construction
As stated above, the design of the Foster Wheeler ESD IV boiler is the same as that of ESD III. However, there are some unique differences between the two types that are mentioned below:
- The differential characteristic of the ESD IV type boiler is the proficient use of monowalls in its fabrication in comparison to its ESD III counterpart. The complete boiler unit is encased within a single continuous monowall, which is also called a membrane-wall. In addition to the enclosure, a separate monowall is provided between the furnace and the superheater sections that act as a gas-tight screen.
- Provisions are provided for the passing of the furnace gases before admission into the superheating sections over the tubes of the lower section of the mentioned screen that opens out to form two rows of open-pitched tubes. In the ESD III-type, the number of rows of these close-pitched screen tubes varies between four and six. Nevertheless, in the ESD IV, their lesser number (only 2 rows) is advantageous in terms of their comfortable accommodation at their lower ends in the header, eliminating the need for water-drum.
- To prevent heat and mass leakage at the expanded tube ends, all the tubes are welded either to studs or directly to the headers.
- The superheater and economizer are placed in such a fashion that they appear crosswise to each other, as illustrated in Fig. 01. Such a right-angular positioning is called athwartship. It is required to provide needful support for the shorter elements.
In the past, there were a plethora of problems associated with furnace refractory materials. In the ESD IV design, all such problems are mitigated with the exorbitant use of monowalls.
But what are these walls made up of? The monowall is a singular, continuous, and tightly welded wall construction that is used to fabricate an enclosure of the boiler’s body.
It consists of a water-tube arrangement in which close-pitched tubes are welded to a steel-strip (or a stud) placed between them, thus giving a gas-tight enclosure.
Only at the outer side is a layer of insulation or cleading required, as shown below in the schematic diagram of a regular furnace-based monowall construction.
Unlike ESD III and other designs in which the furnace has such a monowall construction at selected places only.
The ESD IV modality of the FW boilers uses monowalls at large (and at multiple places) due to added benefits such as its sturdier and more robust structure, as well as the ability to provide a controlled enclosure for the retention of heat. Its water-cooled design is thermally efficient as heat across it is exchanged optimally.
Control of Steam Superheat Temperature
There are two means by which the superheat temperature of the steam is controlled in the ESD IV boilers.
The two underlying methods are known as attemperation techniques: one is called drum-type desuperheating, and the other is famously known as spray-type thermal (superheat) control. These methods are common with ESD III-type boilers.
In the drum-type control method, the steam after the first superheater pass is diverted to the steam-drum where its excessive heat is transferred to the water in the drum, thereby cooling it off to the required degree.
The desuperheating steam is then mixed with the non-desuperheating steam for its subsequent admission into the superheating secondary pass.
In the spray-type control method, the superheated steam after the first superheater pass is allowed to pass through a special steam pipe where multiple nozzles controlled by a sequential-valve arrangement spray good quality water in a very atomized form directly over the circulating steam flow to absorb its surplus heat.
Both these methods are well explained in the previous blog post: ESD III type boiler.
I am the author of Mechanical Mentor. Graduated in mechanical engineering from University of Engineering and Technology (UET), I currently hold a senior position in one of the largest manufacturers of home appliances in the country: Pak Elektron Limited (PEL).