STEAM BOILERSA steam boiler is a closedvessel, strongly constructed ofsteel, in which steam isgenerated from water by theapplication of heat. The steamgenerated is used for producingpower and for industrial workand heating work. The steamboiler is also known as steamgenerator. The function is toconvert chemical energy of fuelby combustion into heat and totransfer this heat to water andthus to produce steam.
Chemical Heat Flow Energy of MECHANICAL WORK Energy of COMBUSTION WATER OR HEATING WORK Energy steam fuel WATER TUBE BOILERSWater tube boilers have water inside the tubes andhot gases surrounding the tubes. These boilers areused extensively because they can be built for highpressures and large evaporative capacities.They have been constructed in which steam isproduced at the critical pressure of 220.9 bar , andevaporative capacity of half million kg of steam perhour. The water tube boilers may be classified into(1) Service : Stationary or Marine(2)Position of drum: Vertical, cross or longitudinal(3) Types of tubes used : Straight tube or benttube(4) Method of Circulation: Natural Circulation orForced Circulation
Babcock and Wilcox Water Tube BoilerTHIS IS A HORIZONTAL, EXTERNALLYFIRED, WATER TUBE, NATURALCIRCULATION TYPE STATIONARYBOILER. THIS BOILER IS THE MOSTCOMMON TYPE USED IN THERMALPOWER PLANTS FOR GENERATION OFSTEAM IN LARGE QUANTITIES. ITCONSISTS OF A HIGH PRESSURE STEELDRUM MOUNTED AT THE TOP .FROMEACH END OF THE DRUM,CONNECTIONS ARE MADE WITH THEUPTAKE HEADER AND A DOWN TAKEHEADER. THE HEADERS ARE JOINEDTO EACH OTHER BY A LARGE NUMBEROF WATER TUBES WHICH ARE KEPTINCLINED AT AN ANGLE OF ABOUT15DEGREE TO THE HORIZONTAL. THEWATER TUBES ARE STRAIGHT, SOLIDDRAWN STEEL TUBES ABOUT 10 CMIN DIAMETER AND ARE EXPANDEDINTO THE BORED HOLES OF THEHEADERS.
Flow of flue gases:THE FURNACE IS LOCATED BELOWTHE UPTAKE HEADER. THE COAL ISFED TO THE CHAIN GRATE STOKERTHROUGH THE FIRE DOOR BAFFLESARE PROVIDED ACROSS THE WATERTUBES TO ACT AS DEFLECTORS TOTHE FLUE GASES.The hot flue gases rise upward andpass across the left-side portion ofthe water tubes. The baffles deflectthe flue gases and hence the fluegases travel in the zig-zag manner(i.e., the hot gases are deflected bythe baffles to move in the upwarddirection, then downward and againin the upward direction) over thewater tubes and along the superheater. The flue gases finally escapeto atmosphere through chimney.
• Salient features of Babcock and Wilcox Water circulation: Boiler:That portion of water tubes which is •just above the furnace is heated Its overall efficiency is higher than a firecomparatively at a higher temperature tube boiler.than the rest of it. Water, its densitybeing decreased, rises into the drum • The defective tubes can be replaced easily.through the uptake-header. Here thesteam and water are separated in the • All the components are accessible fordrum. Steam being lighter is collected inspection even during the operation.in the upper part of the drum. Thewater from the drum comes down • The draught loss is minimum compared withthrough the down –comer into the other boiler.water tubes. • Steam generation capacity and operatingA continuous circulation of water fromthe drum to the water tubes and water pressure are high compared with othertubes to the drum is thus maintained. boilers.The circulation of water is maintainedby convective currents and is known • The boiler rests over a steel structureas “natural circulation”. independent of brick work so that the boilerA damper is fitted as shown to may expand or contract freely.regulate the flue gas outlet and hencethe draught. • The water tubes are kept inclined at an angle of 10 to 15 degree to promote waterThe boiler is fitted with necessarymountings. Pressure gauge and water circulation.level indicator are mounted on theboiler at its left end. Steam safetyvalve and stop valve are mounted onthe top of the drum. Blow-off cock isprovided for the periodical removed ofmud and sediments collected in themud box.
Lets see some basics• Steam Generation STEAM • Saturation Temperature GENERATION is the process of converting When required sensible heat is added to water into steam, a vapor exhibiting the water, it starts boiling with the help of properties of a gas by application of heat. continuation of heat addition. But both the Heating water at any pressure eventually water and steam remains at the same will cause it to boil and steam will be temperature though heat is continued to released. be added till the entire water is converted into steam. This temperature is the• SENSIBLE HEAT The heat required saturation temperature. For each boiling to bring the water from 0 degree celcius to pressure there is only one saturation boiling point is the enthalpy or heat temperature and vice versa. The water and content of the liquid measured in Kcal/kg. steam at this temperature are termed This is also known as sensible heat. The saturated water and saturated steam sensible heat required to bring the water respectively. to the boiling point depends on the • . pressure at which the water is heated. Higher the pressure, higher will be the requirement of sensible heat.
Lets see some basics• Latent Heat During the • Superheat boiling process i.e., after the When the steam is heated out of water attains the saturation contact with water, the steam temperature, even though heat temperature increases above is being added the temperature saturation temperature. Such a remains the constant. This heat heating is known as superheating. is being used to change the On superheating of the steam , the water from liquid to vapor state. enthalpy ( heat content) of steam This heat is the enthalpy of will increase by the amount of the evaporation or latent heat. Thus heat is added and the temperature the latent heat is the heat of steam also will rise. The rate at required to convert the which the temperature rise saturated water into saturated depends to some degree on steam. Like sensible heat the pressure. latent heat required also depends upon the pressure at which boiling occurs. Latent heat decreases with increase in pressure.
Lets see some basics• Critical Point When water and steam reach the level of absolute pressure 3206.2 psia (221.2 bar) and a corresponding saturation temperature705.40oF (374.15oC), the vapor and liquid are indistinguishable.• This level is called the Critical Point.• At the critical point there is no change of state when pressure is increased or if heat is added. At the critical point the water and steam cant be distinguished, and there is no point referring to water or steam.
Lets see some basics• Critical Point For states above the critical point the steam is supercritical. Supercritical is not the same as superheated - which is saturated steam at lower pressures and temperatures heated above the saturation temperature.
Lets see some basics• Some more discussion on Critical Point It can be seen in figure, with the increase in pressure for steam generation, the sensible heat required increases with decrease in latent heat. At every pressure between saturated water and saturated steam a phase called wet steam exists. How ever at one point the water turns into steam on addition of sensible heat alone without going through the phase of wet steam. This occurs at a temperature of 374 degree celcius and 224.6 kgf/cm2 abs pressure. This point is called critical point and the pressure and saturation temperature corresponding to this point are the critical pressure and critical temperature. At critical point the density of water and steam remains the same.