Thermal power plant report pdf

Remember me on this computer. Enter the email address you signed up with and we'll email you a reset link. Need an account? Click here to sign up. Download Free PDF. A short summary of this paper. Download Download PDF. Translate PDF. The expansion of the steam turbine produces mechanical power which drives the Excitation Generator Alternator coupled to the turbine. In the first stage, the heat of the hot gases flowing through the heat exchanger is transferred to the packing of the heater and it is accumulated in the packing and the hot gases are cooled to sufficiently low temperature before exhaust to atmosphere.

This stage is referred to as Heating period. In the second stage, the cold air is passed through the hot packing where the heat is accumulated and the heat from the packing is transferred to the cold air. This stage is known as Cooling period. The superheating raises overall cycle efficiency as well as avoids too much condensation in the last stages of the turbine which avoids the blade erosion.

The heat of combustion gases from furnace is utilised for the removal of moisture from steam and to superheat the steam. Super heaters usually have several tube circuits in parallel with one or more return bends, connected between headers.

Superheated steam has the following advantages: I. Steam consumption of the turbine is reduced. Losses due to condensation in the cylinder and the steam pipe are reduced.

Erosion of turbine blade is reduced. Efficiency of the steam plant is increased. Types of Superheater There are two types of super heaters: 1.

Convective superheater 2. Radiant superheater Convective superheater makes use of heat in flue gases whereas a radiant superheater is placed in the furnace and a wall tube receives heat from the burning fuels through radiant process. The radiant type of superheater is generally used where a high amount of superheat temperature is required. Heat from the hot gases to the vapour in the superheater is transferred at high temperatures. Therefore primary section of superheater is arranged in counter flow and secondary section in parallel flow to reduce the temperature stressing of the tube wall.

The metal used for superheat tubes must have high temperature strength, high creep strength and high resistance to oxidation as superheater tubes get rougher service than water wall of the modern boilers. Carbon steels C and chromium-molybdenum alloys C are commonly used for superheater tubes.

The superheater tubes are subjected to corrosion when they are exposed to oxidising and reducing conditions alternately. This destroys the protective oxide film and exposes the metal surface open to further corrosion. The alkali deposits formed also have corrosion effect Low chromium ferritic steels confer some corrosion resistance but marked resistance is obtained by the use of austenitic alloys.

The use of wet steam in an engine or turbine is uneconomical besides involving some risk; hence it is usual to need to separate any water that may be present from the steam before the latter enters the engine. This is accomplished by the use of a steam separator. Thus the function of a steam separator is to remove the entrained water particles from the steam conveyed to the steam turbine.

The turbine shaft, directly or with the help of a reduction gearing, is connected with the driven mechanism. Depending on the type of the driven mechanism a steam turbine may be utilised in most diverse fields of industry, for power generation. The steam turbines are mainly divided into two groups as: a Impulse turbine b Reaction turbine In both types of turbine, first the heat energy of the steam at high pressure is converted into kinetic energy passing through the nozzles.

The turbines are classified as impulse or reaction according to the action of high velocity steam used to develop the power. In impulse turbine, the steam coming out at a very high velocity through the fixed nozzles impinges on the blades fixed on the periphery of a rotor. The blades change the direction of the steam flow without changing its pressure. The resulting motive force due to the change in momentum gives the rotation to the turbine shaft. Fig 5. In the reaction turbine, the high pressure steam from the boiler is passed through the nozzles.

When the steam comes out through this nozzles the velocity of the steam increases relative to the rotating disc. The resulting reaction force of the steam on nozzle gives the The shaft rotates in the opposite direction to the direction of the steam jet.

In an impulse reaction turbine, the steam expands both in fixed and moving blades continuously as the steam passes over them. Therefore, the pressure drop occurs gradually and continuously over both moving and fixed blades. The turbine rotor velocity will be very high, of the order of 30, r. Such high R. There is also danger of structural failure of the blade due to excessive centrifugal stresses. The velocity of the steam at the exit of the turbine is sufficiently high when single stage blades are used.

The above-mentioned difficulties associated with the single stage turbine can be solved by compound. The combinations of stages are known as compounding. The different methods of compounding are: 1.

Velocity Compounding 2. Pressure Compounding 3. Pressure And Velocity Compounding 1. Velocity Compounding. There is only one set of nozzles and two or more rows of moving blades. There is also a row of fixed blades in between the moving blades. The function of fixed blade is only to direct the steam coming out from first moving row to next moving row.

The heat energy drop takes place only in the nozzle at the first stage and it converts into kinetic energy. The kinetic energy of the steam gained in the nozzles is successively used by the rows of moving blades and finally exhausted from the last row of the blades on the turbine rotor. The function of the fixed blades is merely to turn the steam into the direction required for entry into the next row of rotor blades without altering pressure and velocity of the steam.

A turbine working on this principle is known as velocity compounded impulse turbine. Pressure Compounding. A number of simple impulse turbine sets arranged in series is known as pressure compounding.

In this arrangement, the turbine is provided with one row of fixed blades at the entry of each row of moving blades. The total pressure drop of the steam does not take place in a single stage nozzle but is divided equally in all the rows of fixed blades which work as nozzles. Pressure and Velocity Compounding. This compounding is a combination of pressure and velocity compounding.

The total pressure drop of the steam from boiler to condenser pressure is divided into a number of stages as done in pressure compounding and velocity obtained in each stage is also compounded.

This arrangement requires less stages and compact turbine can be designed for a given pressure drop. This compounding has an advantage of pressure compounding to provide higher pressure drop in each stage and hence less number of stages and an advantage of velocity compounding to reduce the velocity of each blade row. Advantages and Disadvantages of Velocity Compounding Advantages: 1. It requires less number 2 to 3 only of stages, therefore initial cost is less.

The space required is less. The system is easy to operate and more reliable. The turbine housing need not be made strong as pressure in the housing is considerably less because the total pressure falls in the nozzle only. Disadvantages: 1.

The friction losses are too larger due to the high velocity of steam. The maximum blade efficiency and efficiency range decreases with an increase in number of stages.

The power developed in each successive blade row decreases with an increase in number of rows, even though all the rows require same space, material and initial cost.

Therefore all the stages are not economically used. Velocity compounded steam turbines are generally used as drives for centrifugal compressors, centrifugal pumps, and small generators and feed pumps of high capacity power plants.

Residual Velocity Loss. The steam leaves the turbine with some absolute velocity. This loss is reduced by using the multistage. Loss Due To Friction and Turbulence. Friction loss occurs in nozzles, turbine blades and between the steam and rotating discs. The friction loss in the nozzle is taken into account with introducing factor nozzle efficiency.

Leakage Loss. The leakage of steam occurs at the points mentioned below: a Between the turbine shaft and bearing. Loss Due To Mechanical Friction. The loss due to friction between the shaft and bearing comes under this category. Some loss also occurs in regulating the valves. This friction loss can be reduced with the help of an efficient lubricating system.

Radiation Loss. The heat is lost from the turbine to the surroundings as its temperature is higher than atmospheric temperature. Usually the turbines are highly insulated to reduce this loss.

The loss due to radiation is always negligible. Loss Due To Moisture. The steam contains water particles passing through the lower stages of the turbine as it becomes wet. The velocity of the water particles is less than the steam and therefore the water particles have to be dragged along with the steam and consequently part of the K.

The different methods which are commonly used for governing the steam turbines are listed below: 1. Throttle Governing. Nozzle Control Governing. By-Pass Governing. Combination of Throttle and Nozzle Governing. Combination of Throttle and By-Pass Governing. The quantity of steam entering into the turbine is reduced by the throttling of the steam.

The throttling is achieved with the help of double heat balanced valve which is operated by a centrifugal governor through the servomechanism.

The effort of the governor may not be sufficient to move the valve against the piston in big units. Therefore an oil operated relay is incorporated in the circuit to magnify the small force produced by the governor to operate the valve. In this method of control, the steam supplied to the different nozzle groups is controlled by uncovering as many steam passages as necessary to meet the load by poppet valves. An arrangement often used for large steam power plants is shown in fig.

The numbers of nozzles supplying the steam to the turbine are divided into three groups and the supply to these nozzles is controlled by the three valves. More than one stage is used for high pressure impulse turbine to reduce the diameter of the wheel. The nozzle control governing cannot be used for multi stage impulse turbine due to small heat drop in first stage.

It is also desirable in multistage impulse turbine to have full admission into high pressure stages to reduce the partial admission losses. In such cases by-pass governing is generally employed.

Sudden increase in the vibration of the turbine is the most usual indication of any trouble caused during running of the turbine. Austenite alloys are preferred for still higher temperature.

Blades of L. It converts mechanical energy into electrical energy, by electro-magnetic induction. In a simple version, a bar magnet rotates in an iron yoke which concentrates the magnetic field.

A coil of wire is wound around the stem of the yoke. As the magnet turns, voltage is induced in the coil, producing a current flow. When the North Pole is up, and South is down, voltage is induced in the coil, producing current flow in one direction. As the magnet rotates, and the position of the poles reverses, the polarity of the voltage reverses too, and as a result, so does the direction of current flow. Current that The change in direction occurs once for every complete revolution of the magnet.

When magnetic field lines cut across a conductor, a current is induced in the conductor. In general, an alternator has a stationary part stator and a rotating part rotor. The stator contains windings of conductors and the rotor contains a moving magnetic field. The field cuts across the conductors, generating an electrical current, as the mechanical input causes the rotor to turn.

The rotor magnetic field may be produced by induction in a "brushless" generator , by permanent magnets, or by a rotor winding energized with direct current through slip rings and brushes. Automotive alternators invariably use brushes and slip rings, which allows control of the alternator generated voltage by varying the current in the rotor field winding.

Permanent magnet machines avoid the loss due to magnetizing current in the rotor but are restricted in size owing to the cost of the magnet material. Since the permanent magnet field is constant, the terminal voltage varies directly with the speed of the generator.

Brushless AC generators are usually larger machines than those used in automotive applications. Any kind of obstacle in its performance can mar the working of the power plant's overall electrical system. It is for this reason that it requires adequate protection systems to prevent any kind of hindrance to the power plant's functionality. The main types of protection system are: 1. Over Current Protection 2.

Every alternator has an over current protection. With the help of this trip, the alternator and distribution system can be protected from various faults. For this reason, the protection device has been designed in such a way that in case the over current is not high enough, a time delay provided by an inverse definite minimum time IDMT relay occurs, which prevents the alternator from tripping in case the over current values reduces back to normal within the IDMT characteristics.

But in case of a major fault such as short circuit, the alternator will trip instantaneously without any delay, protecting all devices on the distribution system. Overload of alternator is caused either due to increased switchboard load or serious fault causing very high current flow. If sudden over load occurs then, the load is reduced with the help of preferential trips which removes non essential load such as of air conditioning, ventilation fans etc.

Reverse Power Protection. There is not much difference between an alternator and electric motors from the engineer's perspective. They are both based on similar principles. So just imagine what would happen if an alternator suddenly would act as a motor. This is only possible in systems where two or more generators are running in parallel, Hence this type of protection system is used only if there is more than one alternator on board a ship.

The system is designed in such a way that it will release the breaker and prevent motoring of alternator if a reversal of power occurs.

This protection device is also used to prevent damage to the prime mover, which might be stopped due to some fault. Though it is extremely difficult to detect reverse current with an alternating current system, reverse power can be detected and protection can be provided by reverse power relay.

The use of condenser in the power plant improves the efficiency of the power plant by decreasing the exhaust pressure of the steam below atmosphere. Another advantage of the condenser is that the steam condensed may be recovered to provide a source of good pure feed water to the boiler and reduces the water softening plant capacity to a considerable extent. This expansion of efficiency shows that the efficiency increases with an increase in temperature T1 and with the decrease in temperature T2.

The maximum value of temperature T1 of the steam supplied to a steam prime-mover is limited by the material consideration. The temperature T2 can be reduced if the exhaust of the steam prime mover takes place below the atmospheric pressure. This is because; there is definite relation between the steam temperature and pressure. Low exhaust pressure means low exhaust temperature. The steam cannot be exhausted to atmosphere if it is expanded in the turbine below atmospheric pressure.

Under this condition, the steam is made to exhaust in a vessel known as condenser where the pressure inside is maintained below the atmospheric pressure by condensing the steam with the circulation of the cold water.

A closed vessel in which steam is condensed by abstracting heat from steam and the pressure is maintained below atmospheric pressure is known as condenser. The efficiency of the steam plant is considerably increased by the use of condenser. The condensed steam from the condenser is used as feed water for boiler.

Using the condensate as feed for boiler reduces the cost of power generation as the condensate is supplied at higher temperature to the boiler and it reduces the capacity of the feed water cleaning system. The efficiency of the plant increases as the enthalpy drop increases by increasing the vacuum in the condenser.

The specific steam consumption of the plant also decreases as the available enthalpy drop or work developed per kg of steam increases with decrease in back pressure by using condenser.

The deposition of salt in the boiler is prevented with the use of condensate instead of using the feed water from outer source with contained salt. The deposition of salt in boiler shell also reduces the boiler efficiency. This is particularly important in marine steam power plant.

The use of condenser in steam power plant reduces the overall cost of generation by increasing the thermal efficiency of the power plant. The efficient condenser plant must be capable of producing and maintaining a high vacuum with the quality of cooling water available and should be designed to operate for the prolonged periods without trouble. The desirable features of good condensing plant are: i. Minimum quantity of circulating water. Minimum cooling surface area per KW capacity. Minimum auxiliary power.

Maximum area of condensed per m2 of surface area. The effect of low vacuum is very pronounced. The efficiency of the power plant depends to a greater extent on the pressure at the exhaust than the high pressure condition of the steam at inlet.

In mixing type condensers, the exhaust steam form prime mover and cooling water come in direct contact with each other and steam condenses in water directly. The condensate coming out from the mixing type condenser cannot be used as feed to the boiler as it is not free form salt and pollutant.

These type of condenser are generally preferred where the good quality water are feed to the boiler are easily available in ample quantity. Mixing condenser is seldom used in modern power plants. In non-mixing type of condenser, steam and cooling water do not come in direct contact with each other. The cooling water passes through the number of tubes attached to condenser shell and steam surrounds the tubes.

These type of condensers are universally used in all high capacity modern steam power plants as the condensate coming out from the condenser is used as feed for the boiler. Mixing or Jet Type of Condenser. The jet condensers are mainly divided as parallel flow and counter flow jet condenser. In parallel flow condensers, the steam and cooling water flow in same direction where as the flow in opposite direction in counter flow condenser.

Mixing type condenser is mainly classified into three categories depending upon the arrangement used for the removal of condensate as low level, high level and ejector condenser.

Non-Mixing Type or Surface Condenser. In this type of condenser, the cooling tower and exhaust steam do not come in direct contact with each other as in case of jet condenser. This is generally used where large quantity of inferior water is available and better quality of feed water to the boiler must be used most economically. Surface condenser consists of a cast iron air-tight cylindrical shell closed at each end.

A number of water tubes are fixed in the tube plates which are located between each cover head and shell. The exhaust steam from the prime mover enters at the top of the condenser and surrounds the condenser tubes through which cooling water is circulated under force. The steam gets condensed as it comes in contact with cold surface of the tubes.

The cooling water flows in one direction through the first set of the tubes situated in the lower half of condenser and returns in the opposite direction through the second set of the tubes situated in the upper half of the condenser. The cooling water comes out from the condenser is discharged into the river or pond. The condensed steam is taken out form the condenser by a separate extraction pump and air is removed by an air pump.

The requirements of an ideal surface condenser used for power plants are listed below. The steam should be evenly distributed over the whole cooling surface of the condenser vessel with minimum pressure loss. There should not be under cooling of condensate. To achieve this, the quantity of cooling water circulated should be so regulated that the temperature of the steam corresponding to the steam pressure in the condenser. The water should be passed through the tubes and steam must surround the tubes from outside.

This helps to prevent the deposition of dirt on the outer surface of the tubes. There should not be air-leakage at all in the condenser as it destroys the vacuum in the condenser and reduces the work done per kg of steam. The presence of air also reduces the heat transfer rates in the condenser very rapidly. A high vacuum can be attained in the surface condenser providing a high thermal efficiency. The condensate can be directly used as boiler feed water.

This is very important in any large power plant. Any kind of cooling water can be used in the condenser as it does not directly contact with steam. The limitations of this type are: 1. The surface condenser is bulky and therefore requires more space. Its capital, running and maintenance costs are considerably greater than that of jet condenser.

Evaporative Condenser. These condensers are more preferable acute shortage of cooling water exits. The arrangement of the condenser is shown in fig. Water is sprayed through the nozzles over the pipe carrying exhaust steam and forms a thin film over it. The air is drawn over the surface of the coil with the help of induced fan as shown in fig. The air passing over the coil carries the water from the surface of condenser coil in the form of vapour. The latent heat required from the evaporation of water vapour is taken from the water film formed on the condenser coil enter the temperature of the water field and this helps for heat transfer from steam to the water.

The water particle carried with air due to high velocity of air is removed with the help of eliminator. The makeup water is supplied from outside source. The quantity of water sprayed over the condenser coil should be just sufficient to keep the condenser coil thoroughly wetted.

The water flow rate higher than this will only increase the power requirement of water pump without increasing the condenser capacity. This type of condenser works better in dry weather compared with wet weather as the water vapour carrying capacity of dry air is higher than wet air at the same temperature.

The arrangement of this type of condenser is simple and cheap in first cost. It does not require large quantity of water therefore needs a small capacity cooling water pump. The vacuum maintained in this condenser is not as high as in surface condensers therefore the work done per kg of steam is less with this condenser compared with surface condenser.

These condensers are generally preferred for small power plants and where there is acute shortage of cooling water.

The importance of water flow and prevention of air leakage are already discussed. The success of heat transfer with minimum power consumption for a long time mostly depends upon the clean lines of the condenser tubes. The corrosion and scale formation are the common phenomenon in condenser tube during operation due to the action of chemical compounds and deposited collected on the tube surface carried with the water.

The life of the tubes is also reduced due to erosion which ids the effect of abrasive materials like sand carried with cooling water. In the average condenser installation on a river or lake, provision must be made for cleaning the condenser tubes.

The fouling of tubes occurs because of algae, organic matter, leaves or other floating debris. Grills and screens removes most of the floating debris, even the small particle will eventually accumulate on the tubes and reduces the heat transfer. It is also desirable to clean the condenser while it is under load. A single pass condenser during working condition can be cleaned by using back-washing.

A valve arrangement is generally provided for back-washing purpose. With the most waters, there is general tendency for algae growth to build up on the tube surface. Algae growth is considerably more rapid under warm water conditions therefore summer periods are of the greatest trouble from this source in North American power plants.

The algae often serve as a binder for mud or scale and if algae deposits are removed or controlled, other deposits are also minimised as well. In closed type cooling system, where the cooling water is concentrated by evaporation, the possibility of scale formation is more if the water is not chemically treated Two general methods of treatment are used for condenser tubes cleaning.

First is the sterilization of the heat exchange surface of the condenser. This sterilization can be. The major growth of this application has occurred in past two decade only. Originally these materials were only considered for highly corrosion environments or areas exposed to severe erosion. The cost of stainless steel tubes and available heat transfer data, a decade ago, restricted there used to the really difficult problems areas.

Since that time, a number of important advances have been achieved which have permitted a more use of these materials for condenser application. The determination of the overall heat transfer properties of stainless steel condenser tubes in the early led to more extensive use of these materials. In case of stainless steel tubes, the fouling is due to the formation of deposits from the cooling water only but the fouling of the brass is caused by deposits and corrosion of the inside tube surface also.

The overall corrosion resistance of stainless steel, type is excellent for condenser tube service both the interior and exterior surface resist the formation of corrosion product which has an important influence on the heat transfer characteristics of the tubes. It offers excellent erosion and corrosion resistance in fresh water, immunity to NH3 and sulphide attack and the elimination of potentially troublesome copper ions in feed water.

The high cost of water makes it necessary to use cooling towers for water cooled condenser. The main steam condenser performs the dual function of removing this rejected energy from the plant cycle and keeping the turbine back pressure at the lowest possible level. The rejected energy must be returned to the atmosphere. The condenser does this by transferring the latent heat of the exhaust steam to water exposed to the atmosphere. This water is called circulating or cooling water. The cooling water requirement in an open system is about 50times the flow of the steam to the condenser.

In power plants the hot water from condenser is cooled in cooling tower, so it can be reused in condenser for condensation of steam. In a cooling tower water is made to trickle down drop by drop so that it comes in contact with the air moving in the opposite direction. As a result of this some water is evaporated and is taken away with air. In evaporation the heat is taken away from the bulk of water, which is thus cooled.

Factors affecting cooling of water in a cooling tower are: 1. Temperature of air. Humidity of air. Temperature of hot air. Size and height of tower. Velocity of air entering tower. Accessibility of air to all parts of tower. Degree of uniformly in descending water. Arrangement of plates in tower.

Natural Draught Cooling Tower 2. Natural Draught Cooling Tower. In this type of tower, the hot water from the condenser is pumped to the troughs and nozzles situated near the bottom. Troughs spray the water falls in the form of droplets into a pond situated at the bottom of the tower. The air enters the cooling tower from air openings provided near the base, rises upward and takes up the heat of falling water.

Natural draught cooling tower has the following advantages: I. Low operating and maintenance cost. It gives more or less trouble free operation. Considerable less ground area required. The enlarged top of the tower allows water to fall out of suspension. High initial cost. Its performance varies with the seasonal changes in dry bulb temperature and relative humidity of air. Mechanical Draught Cooling Tower. In these towers the draught of air for cooling the tower is produced mechanically by means of propeller fans.

These towers are usually built in cells or units, the capacity depending upon the number of cells used. It is similar to natural draught tower as far as interior construction is concerned, but the sides of the tower are closed from an air and water tight structure, except for fan opening at the base for the inlet of fresh air, and the outlet at the top for the exit of air and vapour.

There are hoods at the base projecting from the main portion of the tower where the fans are placed for forcing the air, into the tower. Forced Draught Cooling Tower Advantages: 1.

More efficient than induced draught. No problem of fan blade erosion as it handles dry air only. More safe. The vibration and noise are minimum. The fan size is limited to 4 meters. Power requirement high approximately double that of induced draught system for the same capacity. In the cold weather, ice is formed on nearly equipments and buildings or in the fan housing itself. The frost in the fan outlet can break the fan blades. The greatest variation in the design of steam power stations is due to the different fuel sources.

Some prefer to use the term energy center because such facilities convert forms of heat energy into electrical energy. Power Plants is an establishment for power generation. In Bangladesh, the consumption of per capita generation is very low only kWh. The power division of the Ministry of Energy and Mineral Resources is the umbrella organization that controls power generation, transmission and distribution.

An Independent Power Project IPP of the ministry is under implementation for improvement in generation and distribution of electricity by government and private agencies. It began with Thomas Newcomen Dartmouth in the early 's. Early developments were very slow and Newcomen's design was used in England for nearly years.

Newcomen's engine could be better described as a 'vacuum' engine. The vacuum was created by condensing steam. The engine however, was extremely inefficient, and where coal had to be brought from a distance it was expensive to run. James Watt brought about a major increase in power and efficiency with his developments. Watt re-designed the engine so that condensation occurred outside of the cylinder. This meant that the cylinder did not lose heat during each stroke. It also allowed the use of pressurized boilers thus obtaining power on the up-stroke as well as the down-stroke.

The beam engine gave way to the reciprocating steam engine which was refined to a high degree. Double and triple expansion steam engines were common and there was scarcely a demand for mechanical energy which steam could not meet. However, reciprocating steam engines were complicated, and hence not always reliable. In Charles Parsons produced the first steam turbine. With Michael Faraday's earlier discovery of electromagnetic induction the widespread use of electricity had begun.

The two technologies came together and with the National grid, progressively eliminated the need for factories to have their own steam plant. Today, mechanical power production using steam is almost wholly confined to electricity generation. Theory: Steam cycles used in electrical power plants and in the production of shaft power in industry are based on the familiar Rankine cycle, studied briefly in most courses in thermodynamics.

Reheating is a practical solution to the excessive moisture problem in turbines, and it is commonly used in modern steam power plants. The schematic and T-s diagram of the ideal reheat Rankine cycle. The ideal reheat Rankine cycle differs from the simple ideal Rankine cycle in that the expansion process take place in two stages.

In first stage the high-pressure turbine , steam is expanded isentropically to an intermediate pressure and sent back to the boiler where it is reheated at constant pressure, usually to the inlet temperature of the first turbine stage. Steam then expands isentropically in the second stage low-pressure turbine to the condenser pressure.

Figure: Ideal Reheat Rankine Cycle Thus the total heat input and the total work output for a reheat cycle become: 2. The heat transfer to the incoming water feed water first increases its temperature until it becomes a saturated liquid, then evaporates it to form saturated vapor, and usually then further raises its temperature to create superheated steam. Steam power plants operate on sophisticated variants of the Rankine cycle. These are considered later. The simple Rankine cycle from which the cycles of large steam power plants are derived.

In the simple Rankine cycle, steam flows to a turbine, where part of its energy is converted to mechanical energy that is transmitted by rotating shaft to drive an electrical generator. The reduced-energy steam flowing out of the turbine condenses to liquid water in the condenser. A feed water pump returns the condensed liquid condensate to the boiler.

The heat rejected from the steam entering the condenser is transferred to a separate cooling water loop that in turn delivers the rejected energy to a neighboring lake or river or to the atmosphere. Fig: Barapukuria steam turbine power plant. As a result of the conversion of much of its thermal energy into mechanical energy, or work, steam leaves the turbine at a pressure and temperature well below the turbine entrance throttle values. At this point the steam could be released into the atmosphere.

But since water resources are seldom adequate to allow the luxury of onetime use, and because water purification of a continuous supply of fresh feed water is costly, steam power plants normally utilize the same pure water over and over again.

We usually say that the working fluid water in the plant operates in a cycle or undergoes of cyclic process. In order to return the steam to the high-pressure of the boiler to continue the cycle, the low- pressure steam leaving the turbine at state 2 is first condensed to a liquid at state 3 and then pressurized in a pump to state 4. The high pressure liquid water is then ready for its next pass through the boiler to state 1 and around the Rankine cycle again. The boiler and condenser both may be thought of as types of heat exchangers, the former with hot combustion gases flowing on the outside of water filled tubes, and the latter with external cooling water passing through tubes on which the low- pressure turbine exhaust steam condenses.

In a well-designed heat exchanger, both fluids pass through with little pressure loss. Figure: Ideal Reheat Rankine Cycle Therefore, as an ideal, it is common to think of boilers and condensers as operating with their fluids at unchanging pressures. It is useful to think of the Rankine cycle as operating between two fixed pressure levels, the pressure in the boiler and pressure in the condenser. A pump provides the pressure increase, and a turbine provides the controlled pressure drop between these levels.

Looking at the overall Rankine cycle as a system, we see that work is delivered to the surroundings the electrical generator and distribution system by the turbine and extracted from the surroundings by a pump driven by an electric motor or a small steam turbine.

Similarly, heat is received from the surroundings combustion gas in the boiler and rejected to cooling water in the condenser. Fig: Barapukuria steam turbine power plant Turbine house. At the start of the twentieth century reciprocating steam engines extracted thermal energy from steam and converted linear reciprocating motion to rotary motion, to provide shaft power for industry. Today, highly efficient steam turbines, convert thermal energy of steam directly to rotary motion.

Eliminating the intermediate step of conversion of thermal energy into the linear motion of a piston was an important factor in the success of the steam turbine in electric power generation. The resulting high rotational speed, reliability, and power output of the turbine and the development of electrical distribution systems allowed the centralization of power production in a few large plants capable of serving many industrial and residential customers over a wide geographic area.

The final link in the conversion of chemical energy to thermal energy to mechanical energy to electricity is the electrical generator. The rotating shaft of the electrical generator usually is directly coupled to the turbine drive shaft.

Elsewhere, where 50 cycles per second is the standard frequency, the speed of rpm is common. Through transformers at the power plant, the voltage is increased to several hundred thousand volts for transmission to distant distribution centers.

At the distribution centers as well as neighborhood electrical transformers, the electrical potential is reduced, ultimately to the and volt levels used in homes and industry. Since at present there is no economical way to store the large quantities of electricity produced by a power plant, the generating system must adapt, from moment to moment, to the varying demands for electricity from its customers. It is therefore important that a power company have both sufficient generation capacity to reliably satisfy the maximum demand and generation equipment capable of adapting to varying load.

Boiler b. Steam Turbine c. Condenser d. Feed pump e. Economizer f. Pre-heater 3. The steam may emerge wet, dry saturated, or superheated depending on the boiler design. Thermal electrical power generation is one of the major methods.

Heat generated in the furnace is utilized to convert water into steam. In addition to the above equipment the plant requires various auxiliaries and accessories depending upon the availability of water, fuel and the service for which the plant is intended. The flow sheet of a thermal power plant consists of the following four main circuits: a Feed water and steam flow circuit.

A steam power plant using steam as working substance works basically on Rankine cycle. Steam is generated in a boiler, expanded in the prime mover and condensed in the condenser and fed into the boiler again. The different types of systems and components used in steam power plant are as follows: a High pressure boiler b Prime mover c Condensers and cooling towers d Coal handling system e Ash and dust handling system f Draught system g Feed water purification plant h Pumping system i Air preheater, economizer, super heater, feed heaters.

Figure 2. Coal received in coal storage yard of power station is transferred in the furnace by coal handling unit. Heat produced due to burning of coal is utilized in converting water contained in boiler drum into steam at suitable pressure and temperature. The steam generated is passed through the super heater. Superheated steam then flows through the turbine. After doing work in the turbine the pressure of steam is reduced.

Steam pressure in the condenser depends upon flow rate and temperature of cooling water and on effectiveness of air removal equipment. Water circulating through the condenser may be taken from the various sources such as river, lake or sea. If sufficient quantity of water is not available the hot water coming out of the condenser may be cooled in cooling towers and circulated again through the condenser.

Bled steam taken from the turbine at suitable extraction points is sent to low pressure and high pressure water heaters. Air taken from the atmosphere is first passed through the air pre-heater, where it is heated by flue gases. The hot air then passes through the furnace. The flue gases after passing over boiler and super heater tubes, flow through the dust collector and then through economizer, air pre-heater and finally they are exhausted to the atmosphere through the chimney.

Thermal energy released by combustion of fuel is transferred to water, which vaporizes and gets converted into steam at the desired temperature and pressure.

The steam produced is used for: a Producing mechanical work by expanding it in steam engine or steam turbine. Boiler is a closed vessel in which water is converted into steam by the application of heat. Usually boilers are coal or oil fired. According to flow of water and hot gases: a Water tube b Fire tube. In water tube boilers, water circulates through the tubes and hot products of combustion flow over these tubes.

In fire tube boiler the hot products of combustion pass through the tubes, which are surrounded, by water. Fire tube boilers have low initial cost, and are more compacts. But they are more likely to explosion, water volume is large and due to poor circulation they cannot meet quickly the change in steam demand.

For the same output the outer shell of fire tube boilers is much larger than the shell of water-tube boiler. Water tube boilers require less weight of metal for a given size, are less liable to explosion, produce higher pressure, are accessible and can respond quickly to change in steam demand.

Water-tube boilers require lesser floor space. The efficiency of water-tube boilers is more. Therefore steam can be generated easily.