Introduction to Stirling Boiler
Stirling Boiler is one of the most basic varieties of bent tube boiler. In most of the modern thermal power generating plants we use bent tube boiler. Stirling Boiler is one of the largest capacity boilers. A Stirling Boiler can generate as high as 50,000 kg steam per hour and can produce pressure as high as 60 kgf per cm2. This boiler was first designed in the year of 1888 by Alan Stirling, so we refer the boiler as Stirling Boiler. Because of the huge capacity we can use this boiler in central power stations.
In Stirling Boiler there are three steam drums and two mud drums. Three steam drums are placed on the top portion of the boiler system structure and two mud drums are placed on bottom of the structure. The top steam drums are connected to the bottom mud drums with banks of bent tubes. Since the tubes are bent, the mechanical stresses due to expansion of the pipes during heating can not effect the system much. The steam drums, mud drums and bent tubes are made of steel. Also still structure is used to support the entire system.
The entire system is enclosed by a brickwork. Here the brick enclosure is used to prevent the heat dissipation to the surroundings. The fire door is constructed at the bottom side of the brick enclosure wall. The damper is provided on the other side of the brick enclosure wall to take out the combustion gas as when required.
The fire brick arch is provided above the furnace. Three baffles are provided in the boiler system to allow the combustion gas to flow in zigzag way. There is one water circulating tubes connecting the mud drums. Also, there are steam circulating tubes connecting middle steam drums to outer steam drums. There is also a group of hot water circulating tube from front steam drum to middle steam drum.
A safety value is provided on the back steam drum. Finally, the steam is collected from middle steam drum. The steam compartment is constructed inside the middle steam drum. The super heater is connected to steam compartment through a steel pipe.
The entire system is enclosed by a brickwork. Here the brick enclosure is used to prevent the heat dissipation to the surroundings. The fire door is constructed at the bottom side of the brick enclosure wall. The damper is provided on the other side of the brick enclosure wall to take out the combustion gas as when required.
The fire brick arch is provided above the furnace. Three baffles are provided in the boiler system to allow the combustion gas to flow in zigzag way. There is one water circulating tubes connecting the mud drums. Also, there are steam circulating tubes connecting middle steam drums to outer steam drums. There is also a group of hot water circulating tube from front steam drum to middle steam drum.
A safety value is provided on the back steam drum. Finally, the steam is collected from middle steam drum. The steam compartment is constructed inside the middle steam drum. The super heater is connected to steam compartment through a steel pipe.
Construction of Stirling Boiler
The several parts and their work of the Stirling water tube boiler are listed and described below:- Super Heater: A Superheater is one of the boiler Accessories which is used to convert saturated steam into superheated or dry steam.
- Steam Circulation Tube: As its name indicates, It circulates the steam from one place to another in the boiler and also prevents the steam from being in a single place and thus prevents overheating.
- Water Circulation Tube: This is also one of the boiler accessories which works the same as the steam-circulating tube, the difference is- It circulates water instead of steam.
- Safety Valve: This safety valve prevents the boiler from being burst, releasing the extra steam generated by the boiler over the desired range. It works just like a pressure cooker.
- Flue Gas Outlet: Flue gas outlet is a second name of the chimney where totally used flue gases are released into the atmosphere from the flue gas outlet.
- Brick Work Enclosure: As the name indicates, It is a square-shaped brick structure which contains all parts of the Stirling boiler inside.
- Steam Drum: This boiler contains three steam drums which further collect the generated steam coming from the bent tubes. The generated steam is collected at the upper surface of these drums.
- Mud Drum: The unwanted water particles are settled down into the mud drum and further clean water is supplied.
- Bent Water Tube: The multiple bent tubes are placed into this boiler coming from the mud box and going to all three steam drums. The water inside the bent tubes is heated with the help of flue gas coming from the combustion chamber.
Advantages of Stirling Boiler
- They have a small water content, and therefore respond rapidly to load change and heat input.
- The small diameter tubes and steam drum mean that much higher steam pressures can be tolerated, and up to 160 bar may be used in power stations.
- The design may include many burners in any of the walls, giving horizontal, or vertical firing options, and the facility of control of temperature in various parts of the boiler. This is particularly important if the boiler has an integral superheater, and the temperature of the superheated steam needs to be controlled.
Disadvantages of Stirling Boiler
- They are not as simple to make in the packaged form as shell boilers, which means that more work is required on site.
- The option of multiple burners may give flexibility, but the 30 or more burners used in power stations means that complex control systems are necessary.