A Foster Wheeler D-type boiler is named 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.
Typically, Foster Wheeler D-type boilers operate at an evaporative capacity ranging from 4536 kg/hr. to as high as 52000 kg/hr. at 14 bar saturated steam pressure. The highest temperature of the steam reaches 510 °C at 60 bar. It is shown below in Fig. 1.
A D-type boiler consists of two drums, a water drum, and a steam drum, along with upper and lower headers.
The steam-drum lies above the water-drum. Both drums are connected with banks (groups) of water tubes: one group consists of three rows of fire tubes known as screen tubes (50mm), and the other one consists of a number of main boiler tubes called generating tubes (32mm).
Between the two groups lies the superheater, along with immense space for access, as shown in Fig. 2.
The furnace is constructed in such a unique way that it is surrounded by water-wall tubes (50mm).
Such a furnace is called a water-cooled furnace and is designed exclusively for extreme steaming conditions.
But what are these water-wall tubes for? Their sole purpose is to initiate the evaporation of the water running in them, which is fed by headers.
These water-wall tubes cover the side walls, rear wall, and roof of the furnace. All the water-wall tubes thus make a circulation loop between the two drums and are fed by upper and lower headers.
The two drums are also connected through external downcomers to sustain natural circulation in the generating tubes and water walls.
The superheater consists of u-bend elements and is placed between the screen tubes and generating tubes.
The superheater is arranged at the right angle to the generating tubes (32mm diameter).
The superheating elements are expanded into the headers, which are provided by gas baffles in order to give the number of steam passes required under all steaming conditions.
As one can see from Figure 2, the location of the superheater is between the water and steam drum in order to protect it from the furnace’s hot gases with the help of rows of screen tubes.
The water drum supplies water to the lower header through under-floor water tubes. The lower header provides water further to the side-wall water tubes, which run across the roof to the steam drum.
The upper header is connected to the steam drum through risers. Between the upper and lower headers are fitted rear wall tubes.
Furnace walls are provided with refractory material to prevent heat loss due to convection and radiation, as shown in Fig. 2.
In the D-type design, the furnace floor and front wall are lined with refractory material. The burners are provided on the front wall for good flame quality.
In practice, water is supplied from the water drum to the water walls through underground smaller diameter (82mm) water tubes via. lower headers.
Combustion gases raise the temperature of the water in the water-walls and thereby evaporate it, thus producing a low-quality, highly saturated steam.
The steam-water mixture collects in the steam drum through water walls as well as screen tubes.
From the steam drum, water in the water-steam mixture comes back to the water drum due to its being denser than the steam through external unheated downcomers (100mm).
Why are the downcomers kept external to the boiler and unheated and preserved in the double casing?
The single answer might be to direct the flow of water from the steam drum to the water drum and avoid steaming during its downward travel.
Generally speaking, through the generating tubes, the phase change process occurs: the heat from the combustion gases is absorbed by the hot water, which converts into steam.
This heated water (now called good-quality saturated steam) rises and returns to the steam drum through three rows of screen tubes via. top headers where the separation of steam and water takes place: the steam is collected and further passes to the superheater, and water is returned to the water drum through the external downcomers.
In this way, a thermal syphon sustains the circulation of water in a closed loop. It is noteworthy that with the arrangement of water-tube boiler tubes, there is no rigid fixation of water circulation through downcomers and risers or screen tubes.
The downcomers become risers and vice versa depending upon the temperature of the water circulating in them.
That’s why special insulation or casing is provided to protect the heat loss or heat gain.
The furnace is placed on one side, as illustrated in Fig. 2. Hot flue gases thus produced pass over the three rows of screen tubes to the superheater, which is placed before the generating tubes. There, they pass the generating tube bank and then to the vertical convective section.
Here, the burned gases now enter the economizer section below the uptake. Gas baffles control the direction and quantity of these gases as per the required steaming conditions.
A refractory material is used on the furnace floor and front burner wall as well as on the backside of the water wall, etc., to prevent the loss of heat through radiation.
The combustion air passes through ducting and travels in the air registers to reach the furnace where combustion actually takes place.
All tubes, including downcomers, are expanded and bell-mouthed in the drums and all headers.
The burned gases, before leaving the boiler through uptake, are made to pass through the economizer to give a fair boiler economy by transferring heat of the exiting hot gases to the feed-water passing through the economizer before entering the water drum.
They may, in some cases, pass through the air heater as well. Soot blowers, as shown in the figure, are used to sweep the superheater, generating tubes and other convective surfaces.
One can find internal access to the tubes via. Manholes in the water and drum as well as through handholes in the headers.
Manufacturing Processes and Materials
It is to be noted that the water and steam drums are all-welded construction. All the headers (both upper and lower) are solid-drawn with welded ends. All the boiler tubes are solid-drawn mild steel.
Some of the merits of the D-type foster wheeler boiler are given below:
- The steam generation process is quicker
- Design arrangements are flexible.
- Thermal efficiency is higher.
- Natural feed-water circulation.
Some of the demerits of the D-type foster wheeler boiler are given below:
- The limited superheated temperature of the steam.
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).