Steam Boiler on Heat Power Engineering i

Heat Power Engineering

1. STEAM BOILERS A NAGA RAJU

2. OUTLINE Introduction of Steam Boilers Selection of Boilers Classifications of Boilers Boiler Mountings Boiler Accessories Performance and Thermal Efficiency of Boilers Heat Balance Sheet Types of Draughts

3. Introduction of steam boiler A Steam boiler or Steam Generator is a closed vessel made of steel. Its function is to convert water into steam by the application of heat. The source of heat for a boiler is combustion of fuel.

4. Functions of boilers: Boilers are used mainly in power plants for generating high pressure steam to produce electricity. In textile industries, boilers are used for sizing and bleaching. For Generating power in steam engines or steam turbines For heating the buildings in cold weather and for producing hot water for hot water supply. To Drive the locomotives and ships

5. Selection of boilers The following factors are to be considered for selection of Boiler Quality of steam required Steam Generation Rate Accessibility of repair and inspection Area Cost Erection facility Fuel and Water availability

6. REQUIREMENTS OF A GOOD BOILER A Boiler should have the minimum fuel consumption It should occupy less floor area It should start quickly Cheaper in cost Generate maximum steam

7. Classification of boilers According to the contents in the tube: The Steam boilers according to the contents in the tube may be classified as : a) Fire tube boilers b) Water tube boilers According to the location of the furnace: The Steam boilers according to the location of furnace a) Internally fired boilers b) Externally fired boilers According to the axis of the shell: The Steam boilers according to the axis of the shell a) Horizontal Boilers b) Vertical Boilers c) Inclined Boilers

8. According to the method of circulation: According to the circulation of water and steam a) Natural Circulation boilers b) Forced C irculation boilers According to the Pressure of steam produced: According to the pressure of steam produced a) High Pressure boilers b) Low Pressure boilers According to use: According to use of steam boiler a) Locomotive boilers b) Marine boilers c) Stationary boilers d) Portable boilers According to the number of tubes: a) Single tube boilers b) Multi tube boilers

9. Fire tube boilers In fire tube boiler the furnace is generally located inside the boiler shell. The hot gases are passing through a inside in tubes and water is surrounded by the tube. The water surrounding the tube evaporates with the heat of fuel gases. Super heater tubes are generally placed in the path of the fuel gases at the exit side. Fire tube boilers are suitable for capacity upto 10000kg/ hr and low pressure upto about 20 bar

10. Advantages of fire tube boilers Simple design and more accessible for repairs Easy maintenance Low initial cost Large water content provides safety in operation Suitable for fluctuating loads.

11. water tube boilers In water tube boiler the furnace is located outside the furnace. The hot gases pass outside the tubes and water is passing through the tubes. It consists of a large number of parallel tubes containing water and connected to boiler drum through the header Evaporation takes place in the tubes due to heat of hot gases. Water tube boiler is suitable for high pressure steam generation

12. Advantages of water tube boilers Suitable for high pressure and high capacity Quick starting Smaller in size for a given output Higher efficiency Suitable for power plant

13. FIRE TUBE BOILER WATER TUBE BOILER In fire tube boilers, hot gases pass through tubes and water surrounds them In Water tube boilers, water passes through tubes and hot flue gases surround them It can generate steam up to a pressure of 24 bar abs It can generate steam at a high pressure up to 162 bar abs. The rate of steam generation and quality of steam are very low The rate of steam generation and quality of steam are better and suitable for power generation The area required for the type of boiler is more The area required for this type of boiler is less Operating cost is less Operating cost is more Overall efficiency is only 75% Overall efficiency with an economizer is up to 90% Load fluctuations cannot be handled Load fluctuations can be easily handled Simple in design Complex in design Not suitable for power plants Used in large power pants Comparison of fire and water tube boilers

14. Boiler mountings boiler mountings are mounted on the boiler for the safety of the boiler. Safety valve Water level indicator Pressure gauge Fusible plug Steam stop valve Feed check valve Blow off cock Man and mud hole

15. Safety valve The function of the safety valve is to blow-off the steam from the boiler to the atmosphere. When the pressure of the steam in the boiler exceeds the design pressure. Then safety valve is used to maintain a constant safe pressure inside the boiler. Types of safety valves are Dead weight safety valve Spring loaded safety valve Lever safety valve High steam and low water safety valve

16. Dead weight safety valve The principle of dead weight safety valve, the steam pressure in the upward direction is balanced the downward force of the dead weight acting on the valve It consists of a gun metal valve rests on the valve seat and is fixed to the top of the vertical steam pipe are covered case with dead weights. The dead weight must be sufficient to keep the valve on its seat against the normal working pressure. If the steam pressure in the boiler exceeds the normal working pressure, the valve is up from its seat and escapes through discharge pipe to the atmosphere, until the pressure reaches its normal valve.

17. SPRING LOADED SAFETY VALVE It is used to maintain safe pressure inside boiler If the steam pressure in the boiler drum exceeds the working pressure, it permits the steam from the boiler to escape t to the atmosphere until the safe working pressure in the boiler is reached. The steam pressure in the upward direction is balanced by the downward spring force.

18. Lever safety valve The principle of operation of lever safety valve depends on the second system of lever mechanism. When the upward force due to steam pressure exceeds the load on the valve, the valve lifts from its seat and escapes the steam to atmosphere until pressure of steam come to the normal valve.

19. Water level indicator It indicates the water level inside the boiler shell Normally two water level indicator are fitted into the boiler. These are fitted at the front end side of every boiler. Water level indicator consists of three cocks (steam, water and drain cock) and glass tube. The water of the boiler comes into the glass tube through the lower tube and steam through the upper tube. Two cocks are used to control the passage between the boiler and the glass tube. Third cock is used to discharge some of the water from boiler.

20. FUSIBLE PLUG The function of the fusible plug is put-off the fire in the furnace of the boiler. When the water level falls below an unsafe level in the boiler and thus avoid the explosion due to overheating of the tubes and shell. During normal condition, the fusible plug is covered with water which keeps the temperature of the fusible metal below its melting point When the water level in the boiler falls below the safe limit, the fusible plug is exposed to steam on one side and to flue gases on other side.

21. Steam stop valve It is used to regulate the flow of steam The spindle is rotated by help of a hand wheel. Due to rotation of spindle the valve move up and down. When the valve sits over the valve seat, the passage of steam is completely closed. The steam passage may be partially or fully opened by moving the valve up, by rotating the hand wheel.

22. Feed check valve Feed check valve is a non-return valve and the flow of boiler feed water from the pump to the boiler and to prevent the back flow of water from the boiler. When the pump pressure is less than the boiler pressure, does not allow water to flow back to the pump side and acts as a non-return valve.

23. Blow-off cock It is used to blow off the water from the boiler It is also used to discharge mud and other sediments deposited in the bottom most part of shell. To empty the boiler when necessary for cleaning, repair and inspection.

24. Manhole The function of manhole is provided and opening through which a man can enter a boiler for cleaning and inspection purpose The manhole is provided on the boiler shell at a convenient place.

25. Boiler accessories The devices used in boiler to increase its efficiency and quality of steam are known as accessories

26. Boiler accessories Air preheater Economiser Super heater Feed pump Steam separator Steam strap Steam injector

27. Air preheater Air preheater is placed between economiser and the chimney Air preheaters are generally three types Tubular type Plate type Regenerative type Air preheater is the waste heat recovery device which is placed in the path of the waste flue gases goes from the furnace to the chimney. The hot gases are passed through the tubes and air circulates around them Increases the evaporative capacity per kg of fuel Increase of boiler efficiency by 2% for each 35-40 C rise in air temperature.

28. economiser It is a device, which increases the temperature of feed water using waste heat of flue gases leaving the boiler through chimney. An economiser is placed in the path of the flue gases in between the boiler and the air preheater It consists of a large numbers of vertical water pipes The feed water enters into economiser through the bottom of the pipe and it comes into the top pipe through the vertical pipes Feed water with normal temperature is pumped to the bottom pipe by feed pump. Then water enters into top pipe through vertical pipes, the flue gases pass over external surfaces of vertical pipes. Then heat transfer to the feed water

29. Super heater A super heater is a device used to convert saturated steam or wet steam into superheated steam Super heated steam is used in steam turbines for electricity generation A super heater placed in the path of flue gases It consists of a set of small diameter U tubes in which steam flows This set is connected to the steam headers. The u tubes are placed in the path of flue gases. The hot flues gases passed over the super heater tube, gets super heated. The steam inside the tubes and leaves the outlet header

30. Feed pump It is a device used to pump the feed water into the boiler drum at high pressure Types Reciprocating pump Rotary pump

31. Steam separator It separates water particles from steam before it is supplied to a steam engine or turbine Water droplets in steam reduces the thermal efficiency. A steam separator is placed near the engine/turbine to remove water particles from the steam It consists of a cylindrical vessel. The vessel is fitted with baffle plates Water gauge is fitted to indicate the water collected in the separator. A drain valve is provided to discharge water from vessel The steam strikes the baffle plates and the direction of the flow is changed. As a result heavier water particles in steam falls down to the bottom of the separator The separated steam which is free from water is passed to the turbine

32. Steam trap The function of steam trap is to drain away the condensed steam from steam pipes The bucket type steam trap is most common It consists a outer vessel with sufficient water to float the bucket which carries valve rod The water formed due to condensation of steam in the pipe line flows into the trap due to gravity At normal condition valve is closed. When sufficient water is collected, due to increased weight bucket sinks down the valve is open The door provided at the bottom of the trap is used to remove sediments and dust

33. Babcock and wilcox boilers Babcock and Wilcox boilers is horizontal and External Fired Boiler. Stationary and Water tube boiler. Specifications: Diameter of Drum : 1.2-1.8 m Length of Drum : 6-10 m Diameter of water tubes: 100 mm Working Pressure : 40 bar Capacity of steam : 40000 kg/ hr Efficiency : 60-80%

34. Babcock and wilcox boilers Water from the drum enters the bottom header and flows through the tubes. The hot gases flowing through the tubes heat and evaporate the water flowing through the tubes. Due to low density the mixture of water and steam enters in the drum through the uptake header Steam is separated out in the drum and collected above the water level The steam from the drum is led into super heater which is placed in the path of hot gases

35. Babcock and wilcox boilers Since the super heater is exposes to the hot gases, the steam passed in it will be superheated. Superheated steam is supplied to prime mover through steam stop valve. baffles are provided to deflect then hot gases so as to flow along a longer paths.

36. advantages Generate steam at high pressure and rate of steam generation is high. For given output it occupies less space Parts are accessible for cleaning, inspection and repaired. Parts of a boiler can be separaed , transported easily