Energy conversion steam boiler

STEAM BOILERS A steam boiler is a closed vessel, strongly constructed of steel, in which steam is generated from water by the application of heat. The steam generated is used for producing power and for industrial work and heating work. The steam boiler is also known as steam generator. The function is to convert chemical energy of fuel by combustion into heat and to transfer this heat to water and thus to produce steam.

Chemical Energy of

fuel

Heat Energy

Flow Energy of steam

COMBUSTION WATER MECHANICAL WORK OR HEATING WORK

WATER TUBE BOILERS Water tube boilers have water inside the tubes and hot gases surrounding the tubes. These boilers are used extensively because they can be built for high pressures and large evaporative capacities.

They have been constructed in which steam is produced at the critical pressure of 220.9 bar , and evaporative capacity of half million kg of steam per hour. 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 bent tube (4) Method of Circulation: Natural Circulation or Forced Circulation

Babcock and Wilcox Water Tube Boiler

THIS IS A HORIZONTAL, EXTERNALLY FIRED, WATER TUBE, NATURAL CIRCULATION TYPE STATIONARY BOILER. THIS BOILER IS THE MOST COMMON TYPE USED IN THERMAL POWER PLANTS FOR GENERATION OF STEAM IN LARGE QUANTITIES. IT CONSISTS OF A HIGH PRESSURE STEEL DRUM MOUNTED AT THE TOP .FROM EACH END OF THE DRUM, CONNECTIONS ARE MADE WITH THE UPTAKE HEADER AND A DOWN TAKE HEADER. THE HEADERS ARE JOINED TO EACH OTHER BY A LARGE NUMBER OF WATER TUBES WHICH ARE KEPT INCLINED AT AN ANGLE OF ABOUT 15DEGREE TO THE HORIZONTAL. THE WATER TUBES ARE STRAIGHT, SOLID DRAWN STEEL TUBES ABOUT 10 CM IN DIAMETER AND ARE EXPANDED INTO THE BORED HOLES OF THE HEADERS.

Flow of flue gases:

THE FURNACE IS LOCATED BELOW THE UPTAKE HEADER. THE COAL IS FED TO THE CHAIN GRATE STOKER THROUGH THE FIRE DOOR BAFFLES ARE PROVIDED ACROSS THE WATER TUBES TO ACT AS DEFLECTORS TO THE FLUE GASES. The hot flue gases rise upward and pass across the left-side portion of the water tubes. The baffles deflect the flue gases and hence the flue gases travel in the zig-zag manner (i.e., the hot gases are deflected by the baffles to move in the upward direction, then downward and again in the upward direction) over the water tubes and along the super heater. The flue gases finally escape to atmosphere through chimney.

Water circulation: • Salient features of Babcock and Wilcox

Boiler: •

Its overall efficiency is higher than a fire tube boiler.

• The defective tubes can be replaced easily. • All the components are accessible for

inspection even during the operation. • The draught loss is minimum compared with

other boiler. • Steam generation capacity and operating

pressure are high compared with other boilers.

• The boiler rests over a steel structure independent of brick work so that the boiler may expand or contract freely.

• The water tubes are kept inclined at an angle of 10 to 15 degree to promote water circulation.

That portion of water tubes which is just above the furnace is heated comparatively at a higher temperature than the rest of it. Water, its density being decreased, rises into the drum through the uptake-header. Here the steam and water are separated in the drum. Steam being lighter is collected in the upper part of the drum. The water from the drum comes down through the down –comer into the water tubes. A continuous circulation of water from the drum to the water tubes and water tubes to the drum is thus maintained. The circulation of water is maintained by convective currents and is known as “natural circulation”. A damper is fitted as shown to regulate the flue gas outlet and hence the draught. The boiler is fitted with necessary mountings. Pressure gauge and water level indicator are mounted on the boiler at its left end. Steam safety valve and stop valve are mounted on the top of the drum. Blow-off cock is provided for the periodical removed of mud and sediments collected in the mud box.

Lets see some basics

• Steam Generation STEAM

GENERATION is the process of converting water into steam, a vapor exhibiting the properties of a gas by application of heat. Heating water at any pressure eventually will cause it to boil and steam will be released.

• SENSIBLE HEAT The heat required

to bring the water from 0 degree celcius to boiling point is the enthalpy or heat content of the liquid measured in Kcal/kg. This is also known as sensible heat. The sensible heat required to bring the water 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.

• Saturation Temperature When required sensible heat is added to water, it starts boiling with the help of continuation of heat addition. But both the water and steam remains at the same temperature though heat is continued to be added till the entire water is converted into steam. This temperature is the saturation temperature. For each boiling pressure there is only one saturation temperature and vice versa. The water and steam at this temperature are termed saturated water and saturated steam respectively.

• .

Lets see some basics

• Latent Heat During the boiling process i.e., after the water attains the saturation temperature, even though heat is being added the temperature remains the constant. This heat is being used to change the water from liquid to vapor state. This heat is the enthalpy of evaporation or latent heat. Thus the latent heat is the heat required to convert the saturated water into saturated steam. Like sensible heat the latent heat required also depends upon the pressure at which boiling occurs. Latent heat decreases with increase in pressure.

• Superheat When the steam is heated out of

contact with water, the steam temperature increases above saturation temperature. Such a heating is known as superheating. On superheating of the steam , the enthalpy ( heat content) of steam will increase by the amount of the heat is added and the temperature of steam also will rise. The rate at which the temperature rise depends to some degree on 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 can't 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.

LETS STUDY THIS BOILER

THANKYOU