Introduction to Steam Generating Unit (Boiler)

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1 Introduction to Steam Generating Unit (Boiler) A boiler is a closed vessel in which steam is produced from water by combustion of fuel. How a Boiler works: Applications of steam generators (Boilers) Components of a steam generator (Boiler) Water is pumped into the boiler at operating pressure Heat of flue gases vaporizes water to form steam Steam formed is passed into steam space above the water space The steam generators or boilers are integral components of steam turbines which are used as prime movers to drive generators to produce electricity in all thermal and nuclear power plants. These are used in the industry for example in heating systems or for cement production, textiles. These are used to produce distilled water for medicines, pharmaceuticals and other usage. The boilers are used in cold countries for heating large buildings, in agriculture as well for soil steaming. Boiler drum, tubes, furnace Boiler mountings Boiler Accessories Boiler mountings are devices which are required for proper operation, safety and control of the boiler. Examples are water level indicator (WLI), pressure gauges (PGs), steam stop valve, safety valves, fusible plug, feed-check valve, blow-off cock, manhole and mudhole, etc. Boiler accessories are devices which are used to increase the efficiency of a boiler. Examples are air preheater, economizer, superheater, feedpump or injector, baffles, etc. Boiler classification: 1. Tube content: (i) Fire tube boiler and (ii) water tube boiler 2. Axis of shell: (i) Horizontal, (ii) vertical, (iii) inclined 3. Location of furnace: (i) Externally fired, (ii) internally fired 4. Method of circulation: (i) Natural, (ii) forced 5. Mobility: (i) Stationary, (ii) portable 6. Usage: (i) Packaged, (ii) unpackaged 7. Pressure: (i) High, (ii) low 8. Tubes: (i) Single-tube, (ii) Multi-tube

2 How a Fire Tube Boiler works: Hot flue gases are fed through the tubes. Tubes are surrounded by water. Cochran Boiler (A Fire Tube Boiler): Fig: Cochran Boiler Vertical mulititubular Efficiency 70-75% 20 kg/hr to 3000 kg/hr Low initial cost Suitable for fluctuating loads

3 How a Water Tube Boiler works: Small parallel tubes contain water connected to drum with the header Gases pass outside tubes containing water Babcock and Wilcox Boiler ( A Water Tube Boiler: High pressures and capacity Higher Efficiency Suitable for power plant Fig: Babcock and Wilcox Boiler

4 Comparision of Fire tube and water tube boilers Criterion Fire Tube Water tube 1. Flow of hot gases and Gases in tubes surrounded by water water outside Water in tubes and gases flow outside 2. Location of furnace Internal External 3. Floor area for given output Large Small 4. Capacity 10000kg/hr 50000kg/hr 5. Evaporation Slow Fast 6. Pressure range 15 to 20 bar 170 to 200 bar 7. Efficiency 80% 92% 8. Safety Large water content and low steam so better safety Small water content and large steam generation. Needs expert attention. 9. Explosion Lesser risk due to lower P Higher risk due to higher P 10. Application Not suitable for large power Suitable plant 11. Skill Less More 12. Water Treatment No Yes 13. Construction Difficult Simple 14. Shell Diameter More Less 15. Transportation Difficult Simple Boiler Efficiency : Boiler efficiency is the ratio of the quantity of heat utilized actually by the water and steam to the quantity of heat supplied, i.e., Boiler efficiency = Heat actually absorbed by the water and steam / quantity of heat supplied Boiler Capacity: Boiler capacity is defined as the amount of steam a particular boiler can supply per hour usually expressed in kg (of steam)/hr or tons/day. Boiler Pressure: The designated operating pressure of a boiler at which it can safely deliver the steam at its rated capacity.

5 Boiler mountings and accessories: Water Level Indicator Bourdon Tube Pressure Gauge Fusible Plug Lever Safety Valve Flue gas before entering chimney Economizer and Superheater Functions of Boiler mountings: Water level indicator: This is required to indicate the safety level of water inside the boiler shell. Pressure gauge: This is required to indicate the steam pressure inside the boiler shell. Steam Stop Valve: The valve used to regulate the supply of steam to the turbine. Fusible plug: This is required to stop the boiler if the boiler gets dry to prevent damage to the boiler. It is a metallic plug installed in the water pathway. If the boiler temperature exceeds a safe value, it melts and allows shell water to pass through extinguish the furnace fire. Safety valves: These are required to release the internal boiler pressure if the pressure exceeds a predetermined safety level. Functions of Boiler accessories: Air preheater: This is usually used to utilize the waste heat of the chimney to preheat the furnace air thus increasing the boiler efficiency. Economizer: This is usually used to utilize the waste heat of the chimney to preheat the boiler feed water thus increasing the boiler efficiency. Superheater: This is usually used to utilize the waste heat to dry and/or superheat the steam leaving the boiler on the delivery side, thus increasing the steam quality and boiler efficiency. Baffles: These are built inside the boilers. These baffles significantly increase the efficiency by helping feed water in extracting heat from the flue gas.