In this article, you will learn about Stirling Boiler Definition, Construction, Working, Applications, Advantages and Disadvantages.
Definition of Stirling Boiler
Stirling boiler is a bent tube, externally fired, stationary water tube boiler with natural circulation. It has a pressure range of maximum 60 bar and a steam production of about 50000 kg/hr. Efficiency of Stirling boiler is up to 60% to 80%.
It is one of the most primary type of bent tube boiler. Usage of bent tube boiler is done in the modern thermal power engineering plants. Alan Stirling designed this boiler first in the year 1888, and thus the boiler was named after him. This boiler comes in the list of one of the largest capacity boilers.
Read All About Marine Boilers: Marine Boilers – Working, Types, Parts & Basics of Boilers
Construction of Stirling Boiler
Stirling boiler consists of three straight horizontal steam drums arranged in upper part of boiler settings and a mud drum situated in lower part. The steam drums are radially attached with the mud drum by series of bent water tubes. During heating, the mechanical stress due to expansion of pipes doesn’t affect the system. The mud drum, steam drums and bent tubes are made up of steel. The first steam drum is attached with a safety valve while the second steam drum is connected with a pressure gauge and the third steam drum is attached with a feed check valve. Also, mud drum is connected with a blow off valve.
The Whole system is surrounded by a brickwork. Prevention of heat dissipation to the surrounding is avoided due to brick enclosure. The fire door is built at the base of the brick enclosure wall. To clear out the combustion gas, a damper is provided on the other side of brick enclosure wall. To let the combustion gas flow in meandering way, three baffle plates are situated in the boiler. Also, the fire brick arch is provided over the furnace. A water circulating tube is connected with the mud drums and the steam circulating tubes are connected to the middle steam drums to outer steam drums.
Components of Stirling Boiler
The following are the components of Stirling Boiler:
- Steam drum
- Safety valve
- Steam stop valve
- Pressure gauge
- Feed check valve
- Blow off valve
- Fire door
- Bent water tubes
- Water circulating tube
- Steam circulating tube
- Baffle plates
- Ash pit
- Super heater tubes
- Inspection door
- Steel girder
Working of Stirling Boiler
Feeding of water is done through the feed check valve to the third steam drum which then passes through the bent tubes to the mud drum. In the mud drum, the particles get settled down and are removed via blow off valve at regular interim. Hence, pure water passes through the bank of bent tubes to the other two drums. Steam drums are also connected with each other by equalizing tubes and circulating tubes. The circulating tubes equalize the level of water where as the equalizing tubes equalize the level of steam in the drum.
The firebox is located at the bottom left corner of the boiler where coal is burnt over grate to produce flue gases. These flue gases are deflected by the brick arch to move upwards between the water tubes, where the baffle plates are provided to guide movements of flue gases and ensure proper heating of the water tubes. On absorbing heat, the water flowing in the bent water tubes gets converted into steam. This steam moves upwards and gets collected above the water level in the steam drums. Furthermore, saturated steam is extracted from second steam drum and supplied to the superheated tubes. This converts the saturated steam into superheated steam which can be taken out from the steam stop valve. Ash pit is located at the bottom of the boiler system and the ash generated by fuel burning is collected in the ash pit whereas the smoke is permitted to escape in the surrounding through a chimney. A damper is fixed before the chimney to control the air supply into the firebox. Inspection doors are provided in the boilers to inspect its operations regularly.
Uses of Stirling Boiler
Stirling Boiler finds it’s application in the following areas:
- They are used in sugar mills.
- They are used in paper mills.
- They are used in compound steam engine.
Advantages of Stirling Boiler
The following are the advantages of Stirling Boilers:
- High steam production rate due to number of steam drums.
- They respond quickly to heat input and change of load due to the small water content.
- High pressure operation with less pressure drop.
- It prevents undue thermal stress as bent tubes allow free expansion and contraction.
- It permits great flexibility in design.
Disadvantages of Stirling Boiler
The following are the disadvantages of Stirling Boilers:
- It becomes more difficult to clean and inspect the bent tubes.
- Stirling boilers are larger in size.
- Super heater temperature is meagre.
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