1.4 Steam Boilers:
Boiler is defined as a closed metallic vessel in which the water is heated beyond the boiling state by the application of heat liberated by the combustion of fuels to convert it into steam. The steam thus generated may be used for the following purposes.
oo To generate power as in the case of steam engines and steam turbines.
oo For process work in cotton mills, sugar mills, breweries, etc.
oo For home heating in cold countries.
1.4.1 Function of a Boiler:
oo To maintain equality in the steam generation rate and the steam flow rate.
oo To supply the steam at the required constant pressure with its quality either dry or superheated.
1.4.2 Classification of Boiler:
Define boiler. Differentiate between Water tube boiler & fire tube boiler with the help of neat sketch.
Boilers are classified based on the principle of working as:
o Fire tube boilers and
o Water tube boilers.
Fire Tube Boiler:
oo The hot flue gases produced by the combustion of fuels are passed through a tube as shown in Fig.
oo The water surrounding the tube gets heated and is converted into steam
oo These boilers are suitable for pressures up to 20 bars. o Eg: Cochran, Cornish boiler, Lancashire boiler, Locomotive boiler and Scotch Marine boiler.
Figure 2.2 Fire Tube Boiler Water tube boiler:
Figure 2.3 Water Tube Boiler o The water circulates inside the tubes while the hot gases produced by the combustion of the fuels pass around them externally as shown in Fig.
o These boilers are suitable for generation of steam at very high pressures Eg: Babcock & Wilcox boiler, Stirling boiler, yarrow boiler
1.4.3 Babcock and Wilcox boiler:
Sketch and explain the working of a Babcock and Wilcox boiler. Show the path of the flue gases and water.
o Babcock and Wilcox is a horizontal, externally fired, natural circulation, water tube Boiler
o A high capacity boiler of this type can raise steam pressure up to 40 bars and steaming rate as high as 4000 kg per hour.
Construction:
o It consists of a steam and water drum, water tubes, super heater tubes.
o The steam and water drum is suspended from iron girders resting on iron columns. o A number of horizontally inclined straight water tubes are connected to the front headers forming uptakes at the front end and to the rear headers forming down takes at the rear end.
o The headers are in turn connected to the boiler drum by means of steel tubes to complete the water circuit.
o The headers are fitted with covered inspection holes for cleaning of water tubes periodically.
o A mud box is attached to the bottom of the down take header in which sediment collects, and from which it may be blown off by means of a blow-off valve.
o The super heater consists of number of U-tubes placed between the drum and water tubes for the purpose of superheating the steam.
o The super heater tubes are connected to a T-tube placed inside boiler drum.
o The furnace is placed below the uptake header
o Baffles are introduced across the water tubes to act as deflectors, which cause the furnace gases to cross the tubes three times before leaving the boiler.
o Doors are provided for a man to enter for repairing and cleaning.
Working:
o Water enters the boiler drum through the feed valve. The water level in the boiler is kept up to the middle of the drum.
o The coal is charged through the fire hole. The coal burns in the grate.
o The hot gases of combustion first rise up, then move down and once again rise up due to the presence of baffles. It finally escapes to the chimney through the smoke chamber.
o As the hot gases pass around the tubes, the water in tubes gets evaporated
o The mixture of water and steam from the water tubes passes through the uptake headers and enters the boiler drum.
o The steam being lighter gets separated from the surface of the water inside the boiler drum and collects in the steam space.
o The steam from the steam space passes into the T-tube, and then enters U-tubes of the super heater. o Since the super heater tubes are fitted in the combustion chamber and directly exposed to the hot gases, the steam passing in it will be superheated.
o The superheated steam can be tapped off through the steam stop valve.
o The Steam from the steam stop valve is passed directly to the turbines. Thus, the steam generated can be used for various applications.
Figure 2.4 Babcock-Wilcox Boiler Advantages and Disadvantages of Water Tube Boilers over Fire Tube Boilers:
Advantages:
o In water tube boilers, the ratio of water content to the steam capacity is less; hence water tube boilers can quickly generate steam at the required pressure than the fire tube boiler. o Water tube boilers can develop higher pressures than the fire tube boilers because they do not contain any tubes inside the boiler drum thereby can withstand high pressures for the same wall thickness
o Water is contained in a large number of small diameter tubes; therefore the heating surface of a water tube boiler is more than that of the fire tube boiler. Large heating surface increases the evaporation rate.
o As the circulation of water is more positive than that of the fire tube boilers, hence there is a less tendency of the deposits to settle on the heating surfaces.
o Since the water tube boilers are externally fired, the size and proportions of the furnace can be altered to suit any type of fuel.
o The heat transfer is more effective since the hot gases flow at right angles to the water tubes.
o Failure or Bursting of any of the water tubes does not pose any serious problems, whereas the bursting any of the flue tubes cause serious problems in fire tube boilers.
o Occupies less Space.
o Easy maintenance.
o Water tube boilers can be easily dismantled, conveniently transported and erected quickly at the site.
Disadvantages:
o Water tube boilers require relatively pure feed water because impure feed water forms scale inside the water tubes.
o Water tube boilers are not suited for mobile purposes.
o High initial cost and hence not economical
1.4.4. Lancashire boiler:
Sketch and explain the working of a Lancashire boiler. Show the path of the flue gases and water.
o Lancashire boiler is a horizontal, internally fired, natural circulation fire tube boiler. o This boiler raises steam up to a pressure of /5 bar and maximum evaporative capacity of 8500 kg of steam per hour.
o It is used for supplying steam in sugar mills and chemical industries
Construction:
o It consists of two flues or furnace tubes, which extend over the entire length of the boiler.
o The flue tubes are made large in diameter at the front to accommodate the furnace and are tapered to a smaller diameter at the rear end.
o There are two grates one for each flue tube and two fire holes.
o At the back end of the grate is a low firebrick bridge. The function of the bridge is to prevent the entry of ash particles into the flue tubes and also helps in deflecting the hot gases upwards to provide better heat transfer.
Figure 2.5 Lancashire Boiler o The boiler is set in a brickwork forming one bottom flue and two side flues
o A man hole is provided at the top of the shell for cleaning the boiler and inspection or repairs.
o A mud hole is provided at the bottom of the shell to remove the sediment, which gets accumulated.
o The boiler is filled with water to 3/4th of its volume which will submerge both the flue tubes.
o The remaining 1/4th space above the surface of the water in the boiler shell is the steam space.
Working:
o The fuel is charged through the furnace doors. The fuel burns in the grates.
o The hot gases produced by combustion of fuel are passed first through the flue tubes from the front end to the rear end of the boiler shell. As a result, the water surrounding the flue tubes gets heated.
o The gases then return along a brick built flue under the boiler to the front end, as a result the water at the bottom portion of the boiler shell gets heated up.
o At the front end hot gases divide and flow along the two side flues to the rear end as a result the water in the side portions of the boiler shell gets heated up
o The gases are then passed to the chimney.
o The steam is accumulated in the steam space above the surface of water and can be tapped off through the steam stop valve
Advantages and disadvantages of Lancashire boiler:
Advantages:
o It is simple in design and reliable.
o It is easy to operate.
o It is easy to clean and inspect.
o Less maintenance.