Darshan Institute of Engineering & Technology. Certificate

Transcription

1 Darshan Institute of Engineering & Technology Certificate This is to certify that Mr./Ms. Enrollment No. Branch: - Mechanical Engineering Semester: VII has satisfactory completed the course in the subject Power Plant Engineering ( ) in this institute. Date of Submission: - Staff in Charge Head of Department

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3 DARSHAN INSTITUTE OF ENGG. & TECH. B.E. Semester VII List of Experiments Sr. No. Title Date of Performance Date of Submission Sign Remark 1. To study of modern steam power plant 2. To study of steam turbines (impulse, reaction and governing) 3. To study of gas and steam turbine combined cycles 4. To study of nuclear power plant 5. To study of various draught system To study about feed water treatment plant and processes To study of different types of steam nozzle and design a nozzle To comparative study of different types of high pressure boilers 9. To study of different pulverizing mill 10. To study of condenser and cooling tower

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5 STEAM POWER PLANT 1. Objective: EXPERIMENT - 1 To study of modern steam power plant. 2. Introduction: The availability of electrical energy and its per capita consumption is regarded as index of national standard of living in the present day civilization and for development of any country, energy is the basic input. Next to the food, fuel and power are the most important items on which national standard life depends. That s why once the food requirement is fulfilled; next task for any country is to increase the power generation. The production of food also increases with increases of power generation. Therefore, the increase in power potential of a nation is considered most important among all. The energy in the form of electricity is most desired as it is easy to transport, easy to control, clean in its surroundings and can be easily converted in heat or work as per requirements. If we see the data of last ten years, we can realize that consumption of power increases continuously, not only in India but worldwide and it will increase with development of industries and the improvement of living standard. As the coal is easily available in India, large percentage of electricity is developed by thermal power plant in which coal is used as fuel. 3. Components of Thermal Power Plant: Steam power plant basically works on the Rankine cycle in which steam and water is used as working fluid. In Rankine cycle high pressure and temperature steam is generated in the boiler by burning of fuel. That high pressure and temperature steam is then expanded in the turbine to produce power which in turn used to drive the generator to produce electricity. After the expansion of steam low pressure and temperature steam is condensed in the condenser and the condensate is fed back to the boiler with the help of the pump and cycle is repeated. The main components of cycle are boiler, turbine, condenser and feed pump. In modern thermal power plant Rankine cycle is used. Process 1-2: Isentropic expansion: Steam at high pressure and temperature is expanded in turbine isentropically. Pressure of the steam is decreases from P1 to P2. During the process work is done by the turbine. Process 2-3: Isobaric heat rejection: Exhaust steam from the turbine is condensed at constant pressure in the condenser so steam is condensed into water. During the process latent heat of steam is rejected to cooling water. Process 3-4: Isentropic compression: Condensate from the condenser is pumped back to the boiler. During with work is done on the water by the pump. 1-1

6 STEAM POWER PLANT Process 4-1: Isobaric heat supply: In the boiler heat is supplied at constant pressure by burning of the fuel. Fig. 1.1 Schematic diagram of Rankine cycle Fig. 1.2 T-S diagram of Rankine cycle 4. General Layout of Thermal Power Plant: The general layout of the thermal power plant consists of mainly 4 circuits as shown in figure 1.3. The four main circuits are: a) Coal and ash circuit b) Air and gas circuit c) Feed water and steam flow circuit d) Cooling water circuit a) Coal and Ash Circuit Coal from the coal storage is fed to the boiler through coal handling system to generate the steam. Ash produced due to combustion of coal is removed and dump into ash sump through ash handling system. b) Air and Gas Circuit Air from the atmosphere is supplied to the boiler either through F.D. or I.D. fan or by using both. The dust from the air is removed with the help of air filter. Air preheater is used to utilize the heat of exhaust gases which increase the efficiency of the plant. To remove fly ash from the gases electrostatic precipitator is used. Exhaust gases from the boiler is discharge to the atmosphere through chimney. 1-2

7 STEAM POWER PLANT c) Feed Water and Steam Circuit Fig. 1.2 T-S diagram of Rankine cycle High pressure and high temperature steam is generated in the boiler by burning of the fuel. That high pressure and temperature steam is then expanded in the turbine to produce mechanical work which in turn is used to drive generator to produce electrical power. The steam coming out from the turbine is condensed in the condenser and then fed back to the boiler with the help of feed pump. The condensate may be heated in feed water heater using the steam tapped from the different points of the turbine. Some of the steam and water may be lost due to leakage through different components. To compensate the loss, make up water is supplied to the boiler through feed water treatment plant to remove the impurities. d) Cooling Water Circuit The considerable amount of cooling water is required to condense the steam in condenser. It is taken either from lake, river or sea. If adequate water is available throughout the year, then cooling water is taken from the upstream of the river, passed through the condenser and discharge to downstream of the river. If adequate water is not available, then hot water from the condenser is cooled in the cooling tower and cold water from the cooling tower is passed through the condenser. 1-3

8 STEAM POWER PLANT Exercise 1. Draw general layout of modern thermal power plant. 1-4

9 STEAM TURBINE 1. Objective: EXPERIMENT - 2 To study of steam turbine (impulse, reaction and governing) 2. Introduction: A steam turbine converts the energy of high-pressure, high temperature steam produced by a steam generator into shaft work. The energy conversion is brought about in the following ways: The high-pressure, high-temperature steam first expands in the nozzles emanates as a high velocity fluid stream. The high velocity steam coming out of the nozzles impinges on the blades mounted on a wheel. The fluid stream suffers a loss of momentum while flowing past the blades that is absorbed by the rotating wheel entailing production of torque. The moving blades move as a result of the impulse of steam (caused by the change of momentum) and also as a result of expansion and acceleration of the steam relative to them. In other words they also act as the nozzles. 3. Types of Steam Turbine: Impulse steam turbine: Impulse turbines (single-rotor or multirotor) are simple stages of the turbines. Here the impulse blades are attached to the shaft. Impulse blades can be recognized by their shape. They are usually symmetrical and have entrance and exit angles respectively, around 20 C. Because they are usually used in the entrance high-pressure stages of a steam turbine, when the specific volume of steam is low and requires much smaller flow than at lower pressures, the impulse blades are short and have constant cross sections. The single-stage impulse turbine is also called the de Laval turbine after its inventor. The turbine consists of a single rotor to which impulse blades are attached. The steam is fed through one or several convergent-divergent nozzles which do not extend completely around the circumference of the rotor, so that only part of the blades is impinged upon by the steam at any one time. The nozzles also allow governing of the turbine by shutting off one or more them. Reaction steam turbine A reaction turbine, therefore, is one that is constructed of rows of fixed and rows of moving blades. The fixed blades act as nozzles. The moving blades move as a result of the impulse of steam received (caused by change in momentum) and also as a result of expansion and acceleration of the steam relative to them. In other words, they also act as nozzles. The enthalpy drop per stage of one row fixed and one row moving blades is divided among them, often equally. Thus a blade with a 50 percent degree of reaction, or a 50 percent reaction stage, is one in which half the enthalpy drop of the stage occurs in the fixed blades 2-1

10 STEAM TURBINE and half in the moving blades. The pressure drops will not be equal, however. They are greater for the fixed blades and greater for the high-pressure than the low-pressure stages. The moving blades of a reaction turbine are easily distinguishable from those of an impulse turbine in that they are not symmetrical and, because they act partly as nozzles, have a shape similar to that of the fixed blades, although curved in the opposite direction. 4. Compounding of Steam Turbine Fig. 2.1 Impulse and Reaction Turbine Compounding of steam turbines is the method in which energy from the steam is extracted in a number of stages rather than a single stage in a turbine. A compounded steam turbine has multiple stages i.e. it has more than one set of nozzles and rotors, in series, keyed to the shaft or fixed to the casing, so that either the steam pressure or the jet velocity is absorbed by the turbine in number of stages. Necessity The steam produced in the boiler has sufficiently high enthalpy when superheated. In all turbines the blade velocity is directly proportional to the velocity of the steam passing over the blade. Now, if the entire energy of the steam is extracted in one stage, i.e. if the steam is expanded from the boiler pressure to the condenser pressure in a single stage, then its velocity will be very high. Hence the velocity of the rotor (to which the blades are keyed) can reach to about 30,000 rpm, which is pretty high for practical uses because of very high vibration. Moreover at such high speeds the centrifugal forces are immense, which can damage the structure. Hence, compounding is needed. The high velocity which is used for 2-2

11 STEAM TURBINE impulse turbine just strikes on single ring of rotor that cause wastage of steam ranges 10% to 12%. To overcome the wastage of steam compounding of steam turbine is used. Types of compounding a) Velocity compounding b) Pressure compounding c) Velocity pressure compounding Velocity compounding of Impulse Turbine: The velocity compounded Impulse turbine was first proposed by C G Curtis to solve the problem of single stage Impulse turbine for use of high pressure and temperature steam. The rings of moving blades are separated by rings of fixed blades. The moving blades are keyed to the turbine shaft and the fixed blades are fixed to the casing. The high pressure steam coming from the boiler is expanded in the nozzle first. The Nozzle converts the pressure energy of the steam into kinetic energy. It is interesting to note that the total enthalpy drop and hence the pressure drop occurs in the nozzle. Hence, the pressure thereafter remains constant. This high velocity steam is directed on to the first set (ring) of moving blades. As the steam flows over the blades, due the shape of the blades, it imparts some of its momentum to the blades and loses some velocity. Only a part of the high kinetic energy is absorbed by these blades. The remainder is exhausted on to the next ring of fixed blade. The function of the fixed blades is to redirect the steam leaving from the first ring of moving blades to the second ring of moving blades. There is no change in the velocity of the steam as it passes through the fixed blades. The steam then enters the next ring of moving blades; this process is repeated until practically all the energy of the steam has been absorbed. Fig. 2.2 Velocity Compounding for Impulse Turbine Where,p i = Pressure of steam at inlet 2-3

12 STEAM TURBINE V i = Velocity of steam at inlet p o = Pressure of steam at outlet V o = Velocity of steam at outlet A schematic diagram of the Curtis stage impulse turbine, with two rings of moving blades one ring of fixed blades is shown in figure 2.2. The figure also shows the changes in the pressure and the absolute steam velocity as it passes through the stages. In the fig. 2.2 figure there are two rings of moving blades separated by a single of ring of fixed blades. As discussed earlier the entire pressure drop occurs in the nozzle, and there are no subsequent pressure losses in any of the following stages. Velocity drop occurs in the moving blades and not in fixed blades. Pressure compounding of Impulse Turbine: The pressure compounded Impulse turbine is also called as Rateau turbine, after its inventor. This is used to solve the problem of high blade velocity in the single-stage impulse turbine. It consists of alternate rings of nozzles and turbine blades. The nozzles are fitted to the casing and the blades are keyed to the turbine shaft. In this type of compounding the steam is expanded in a number of stages, instead of just one (nozzle) in the velocity compounding. It is done by the fixed blades which act as nozzles. The steam expands equally in all rows of fixed blade. The steam coming from the boiler is fed to the first set of fixed blades i.e. the nozzle ring. The steam is partially expanded in the nozzle ring. Hence, there is a partial decrease in pressure of the incoming steam. This leads to an increase in the velocity of the steam. Therefore the pressure decreases and velocity increases partially in the nozzle. Fig. 2.3 Pressure Compounding for Impulse Turbine Where, the symbols have the same meaning as given above. 2-4

13 STEAM TURBINE This is then passed over the set of moving blades. As the steam flows over the moving blades nearly all its velocity is absorbed. However, the pressure remains constant during this process. After this it is passed into the nozzle ring and is again partially expanded. Then it is fed into the next set of moving blades, and this process is repeated until the condenser pressure is reached. This process has been illustrated in figure 2.3. It is a three stage pressure compounded impulse turbine. Each stage consists of one ring of fixed blades, which act as nozzles, and one ring of moving blades. As shown in the figure pressure drop takes place in the nozzles and is distributed in many stages. An important point to note here is that the inlet steam velocities to each stage of moving blades are essentially equal. It is because the velocity corresponds to the lowering of the pressure. Since, in a pressure compounded steam turbine only a part of the steam is expanded in each nozzle, the steam velocity is lower than of the previous case. It can be explained mathematically from the following formula i.e. V h 1 = V h 2 Where,h 1 = Enthalpy of fluid at exit h 2 = Enthalpy of fluid at inlet Pressure-Velocity compounded Impulse Turbine: It is a combination of the above two types of compounding. The total pressure drop of the steam is divided into a number of stages. Each stage consists of rings of fixed and moving blades. Each set of rings of moving blades is separated by a single ring of fixed blades. In each stage there is one ring of fixed blades and 3-4 rings of moving blades. Each stage acts as a velocity compounded impulse turbine. Fig. 2.4 Pressure-velocity Compounding for Impulse Turbine 2-5

14 STEAM TURBINE Where, symbols have their usual meaning. The fixed blades act as nozzles. The steam coming from the boiler is passed to the first ring of fixed blades, where it gets partially expanded. The pressure partially decreases and the velocity rises correspondingly. The velocity is absorbed by the following rings of moving blades until it reaches the next ring of fixed blades and the whole process is repeated once again. This process is shown diagrammatically in figure 2.4. Pressure compounding of Reaction Turbine: As explained earlier a reaction turbine is one which there is pressure and velocity loss in the moving blades. The moving blades have a converging steam nozzle. Hence when the steam passes over the fixed blades, it expands with decrease in steam pressure and increase in kinetic energy. Fig. 2.5 Pressure Compounding for Reaction Turbine This type of turbine has a number of rings of moving blades attached to the rotor and an equal number of fixed blades attached to the casing. In this type of turbine the pressure drops take place in a number of stages. The steam passes over a series of alternate fixed and moving blades. The fixed blades act as nozzles i.e. they change the direction of the steam and also expand it. Then steam is passed on the moving blades, which further expand the steam and also absorb its velocity. This is explained in figure 2.5. Where, symbols have the same meaning as above. 5. Governing of Steam Turbine Steam turbine governing is the procedure of controlling the flow rate of steam to a steam turbine so as to maintain its speed of rotation as constant. The variation in load during the operation of a steam turbine can have a significant impact on its performance. In a practical 2-6

15 STEAM TURBINE situation the load frequently varies from the designed or economic load and thus there always exists a considerable deviation from the desired performance of the turbine. The primary objective in the steam turbine operation is to maintain a constant speed of rotation irrespective of the varying load. This can be achieved by means of governing in a steam turbine. Depending upon the particular method adopted for control of steam flow rate, different types of governing methods are being practiced. The principal methods used for governing are described below. Throttle governing: In throttle governing the pressure of steam is reduced at the turbine entry thereby decreasing the availability of energy. In this method steam is passed through a restricted passage thereby reducing its pressure across the governing valve. The flow rate is controlled using a partially opened steam control valve. The reduction in pressure leads to a throttling process in which the enthalpy of steam remains constant. Throttle governing small turbines Low initial cost and simple mechanism makes throttle governing the most apt method for small steam turbines. The mechanism is illustrated in figure 2.6. The valve is actuated by using a centrifugal governor which consists of flying balls attached to the arm of the sleeve. A geared mechanism connects the turbine shaft to the rotating shaft on which the sleeve reciprocates axially. With a reduction in the load the turbine shaft speed increases and brings about the movement of the flying balls away from the sleeve axis. This result in an axial movement of the sleeve followed by the activation of a lever, which in turn actuates the main stop valve to a partially opened position to control the flow rate. Throttle governing big turbines In larger steam turbines an oil operated servo mechanism is used in order to enhance the lever sensitivity. Fig. 2.6 Throttle Governor 2-7

16 STEAM TURBINE The use of a relay system magnifies the small deflections of the lever connected to the governor sleeve. The differential lever is connected at both the ends to the governor sleeve and the throttle valve spindle respectively. The pilot valves spindle is also connected to the same lever at some intermediate position. Both the pilot valves cover one port each in the oil chamber. The outlets of the oil chamber are connected to an oil drain tank through pipes. The decrease in load during operation of the turbine will bring about increase in the shaft speed thereby lifting the governor sleeve. Deflection occurs in the lever and due to this the pilot valve spindle raises up opening the upper port for oil entry and lower port for oil exit. Pressurized oil from the oil tank enters the cylinder and pushes the relay piston downwards. As the relay piston moves the throttle valve spindle attached to it also descends and partially closes the valve. Thus the steam flow rates can be controlled. When the load on the turbine increases the deflections in the lever are such that the lower port is opened for oil entry and upper port for oil exit. The relay piston moves upwards and the throttle valve spindle ascend upwards opening the valve. The variation of the steam consumption rate ṁ (kg/h) with the turbine load during throttle governing is linear and is given by the willan s line. The equation for the willan s line is given by: m = al + C Where a steam rate in kg/kwh is is, L is the load on turbine in KW and C is no load steam consumption. Nozzle governing: In nozzle governing the flow rate of steam is regulated by opening and shutting of sets of nozzles rather than regulating its pressure. In this method groups of two, three or more nozzles form a set and each set is controlled by a separate valve. The actuation of individual valve closes the corresponding set of nozzle thereby controlling the flow rate. Fig. 2.7 Nozzle Governor 2-8

17 STEAM TURBINE In actual turbine, nozzle governing is applied only to the first stage whereas the subsequent stages remain unaffected. Since no regulation to the pressure is applied, the advantage of this method lies in the exploitation of full boiler pressure and temperature. Figure 2.7 shows the mechanism of nozzle governing applied to steam turbines. As shown in the figure the three sets of nozzles are controlled by means of three separate valves. 2-9

18 STEAM TURBINE Exercise 1. Draw the diagram of pressure-velocity compounding of impulse turbine. 2-10

19 STEAM TURBINES 2. Draw neat sketch of Nozzle governing system. 2-11

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21 COMBINED CYCLE POWER PLANT 1. Objective: EXPERIMENT - 3 To study of gas and steam turbine combined cycle. 2. Introduction: Inner Workings of a Combined-Cycle Power Plant A combined-cycle power plant uses both a gas and a steam turbine together to produce up to 50 percent more electricity from the same fuel than a traditional simple-cycle plant. The waste heat from the gas turbine is routed to the nearby steam turbine, which generates extra power. How a Combined-Cycle Power Plant Produces Electricity? This is how a combined-cycle plant works to produce electricity and captures waste heat from the gas turbine to increase efficiency and electrical output. a) Gas turbine burns fuel. The gas turbine compresses air and mixes it with fuel that is heated to a very high temperature. The hot air-fuel mixture moves through the gas turbine blades, making them spin. The fast-spinning turbine drives a generator that converts a portion of the spinning energy into electricity. b) Heat recovery system captures exhaust. A Heat Recovery Steam Generator (HRSG) captures exhaust heat from the gas turbine that would otherwise escape through the exhaust stack. The HRSG creates steam from the gas turbine exhaust heat and delivers it to the steam turbine. c) Steam turbine delivers additional electricity. The steam turbine sends its energy to the generator drive shaft, where it is converted into additional electricity. 3. Combine Cycle (Gas - Vapour Power Cycle) The continued quest for higher thermal efficiencies has resulted in rather innovative modifications to conventional power plants. The binary vapor cycle discussed later is one such modification. A more popular modification involves a gas power cycle topping a vapor power cycle, which is called the combined gas vapor cycle, or just the combined cycle. The combined cycle of greatest interest is the gas-turbine (Brayton) cycle topping a steam turbine (Rankine) cycle, which has a higher thermal efficiency than either of the cycles executed individually. Gas-turbine cycles typically operate at considerably higher temperatures than steam cycles. The maximum fluid temperature at the turbine inlet is about 620 C (1150 F) for modern steam power plants, but over 1425 C (2600 F) for gas-turbine power plants. It is over 3-1

22 COMBINED CYCLE POWER PLANT 1500 C at the burner exit of turbojet engines. The use of higher temperatures in gas turbines is made possible by recent developments in cooling the turbine blades and coating the blades with high-temperature-resistant materials such as ceramics. Because of the higher average temperature at which heat is supplied, gas-turbine cycles have a greater potential for higher thermal efficiencies. However, the gas-turbine cycles have one inherent disadvantage: The gas leaves the gas turbine at very high temperatures (usually above 500 C), which erases any potential gains in the thermal efficiency. The situation can be improved somewhat by using regeneration, but the improvement is limited. It makes engineering sense to take advantage of the very desirable characteristics of the gas-turbine cycle at high temperatures and to use the high temperature exhaust gases as the energy source for the bottoming cycle such as a steam power cycle. The result is a combined gas steam cycle, as shown in fig. 3.1 & 3.2. In this cycle, energy is recovered from the exhaust gases by transferring it to the steam in a heat exchanger that serves as the boiler. In general, more than one gas turbine is needed to supply sufficient heat to the steam. Also, the steam cycle may involve regeneration as well as reheating. Energy for the reheating process can be supplied by burning some additional fuel in the oxygen-rich exhaust gases. Fig. 3.1 Schematic diagram combined gas steam power plant 3-2

23 COMBINED CYCLE POWER PLANT Fig. 3.2 T-s diagram combined gas steam power plant Recent developments in gas-turbine technology have made the combined gas steam cycle economically very attractive. The combined cycle increases the efficiency without increasing the initial cost greatly. Consequently, many new power plants operate on combined cycles, and many more existing steam- or gas-turbine plants are being converted to combined-cycle power plants. Thermal efficiencies well over 40 percent are reported as a result of conversion. A 1090-MW Tohoku combined plant that was put in commercial operation in 1985 in Niigata, Japan, is reported to operate at a thermal efficiency of 44 percent. This plant has two 191-MW steam turbines and six 118-MW gas turbines. Hot combustion gases enter the gas turbines at 1154 C, and steam enters the steam turbines at 500 C. Steam is cooled in the condenser by cooling water at an average temperature of 15 C. The compressors have a pressure ratio of 14, and the mass flow rate of air through the compressors is 443 kg/s. A 1350-MW combined-cycle power plant built in Ambarli, Turkey, in 1988 by Siemens of Germany is the first commercially operating thermal plant in the world to attain an efficiency level as high as 52.5 percent at design operating conditions. This plant has six 150- MW gas turbines and three 173-MW steam turbines. Some recent combined-cycle power plants have achieved efficiencies above 60 percent. 3-3

24 COMBINED CYCLE POWER PLANT Exercise 1. Draw the schematic diagram of combine cycle power plant. 3-4

25 COMBINED CYCLE POWER PLANT 2. Draw schematic, P-V and T-s diagram of gas turbine with inter cooler. 3-5

26 COMBINED CYCLE POWER PLANT 3. Draw schematic, P-V and T-s diagram of gas turbine with regenerator. 3-6

27 COMBINED CYCLE POWER PLANT 4. Draw schematic, P-V and T-s diagram of gas turbine with reheater. 3-7

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29 NUCLEAR POWER PLANT 1. Objective: To study of nuclear power plant. 2. Introduction: EXPERIMENT - 4 Steam, diesel and gas turbine power plants are based on conventional sources of energy. Availability of this fuels are fast depleting while power demand is ever increasing. Thermal power plants also based on coal. We have to seek for large alternative source of energy like solar energy, geothermal energy, nuclear energy, tidal energy. Nuclear has bright future due availability of uranium and thorium in earth crest. Uranium and Thorium in earth crest are estimated to be 1011 tones at a depth 5km. 20 x 106 tons of Uranium, 1x 106 tons of Thorium can economically extracted. Fission of 1 kg of uranium can produce the energy equivalent to burning about 4x 106 of high grade coal. Initial cost is high but operating cost is low as compare to thermal power plant. 3. Nuclear Fission and Fusion Reaction: Nuclear Fission Reaction A heavy nucleus splits in to two or more lighter nuclei. This process is possible at room temperature. Fission can be caused by bombarding with high energy alpha particles, protons, electrons, deuterons, x-rays as well as neutrons. However neutrons are most suitable for fission because of electrically neutral and thus no require high kinetic energy to overcome electrical repulsion for positively charged nuclei. Fig. 5.1 Pictorial View of Nuclear Fission Reaction Nucleus of heavy electrons like U 233, U 235, Pu 239 has ability to capture and absorbs neutrons where upon its get converted to a compound nucleus sometimes this compound nucleus is highly unstable undergoing spontaneous fragmentation in several equal lighter nuclei or two to three neutrons this whole phenomenon called nuclear fission reaction. 4-1

30 NUCLEAR POWER PLANT Fig. 4.2 Nuclear Fission Reaction Nuclear Fusion Reaction It is process in which two light nucleuses makes heavy nucleus called fusion reaction. Its required high temperature (1.2 X 10 9 K). High particle density. High particle confinement. It is not possible in earth. Sun generates heat and energy is a best example of fusion process. There is a 1.6 X 10 7 K temp. Middle of the sun and pressure is more 105 times then earth. Nuclear Chain Reaction 4.3 Nuclear Chain Reaction Essential condition for the practical condition of nuclear energy is that self-sustaining chain reaction should be maintained. The neutrons released have very high velocity of the order of 1.5 X 10 7 m/s. The energy liberated in chain reaction is according to Einstein law E=MC 2. To maintain continues chain reaction it should be leakage of neutrons is less. 4. Nuclear Fuels Nuclear fuels which are generally used in reactor are like U 233, U 235, Pu 239. Natural uranium are consists of three isotopes of uranium U 238, U 235, U

31 NUCLEAR POWER PLANT In natural uranium availability U 238 is largest up to the extent of 99.28%. U 235 is only 0.715% which is most unstable and fissionable and the remainder 0.006% is U 234. The other two fuels 94Pu 239 and 92U 233 are formed in nuclear reactor during fission process from 94Pu 239 and 90Th 232. Respectively due to absorption of neutron without fission. Fissionable Materials: This kind of materials is those which are capable of sustaining a fission chain reaction. U 235 is the only fissionable isotope found in nature. Fertile Materials: The non-fissionable materials and they can be converted in to fissionable materials. U 238 and Th 232 can be converted in to fissionable materials. The chain reaction cannot be maintained in some reactors with natural uranium having only 0.7% fissionable U 235 therefore it is necessary to increase the percentage of u-235 in the fuel if it is used in reactor. The process used to increase the percentage of U 235 is known as enrichment of fuel. Enrichment also reduces size of the reactor. Enrichment method: a) Gaseous diffusion method. b) Thermal diffusion method. c) Centrifugal method. d) Electromagnetic method. Estimation of all resources of uranium in USA is 33%, South Africa 20%, Canada 20% largest reserve of most economical and low of cost uranium lies in Australia. 5. Components of Nuclear Reactors Components of nuclear power plant categorize as under, 1.FUELS 2.MODERATOR 3. CONTROL RODS 4.COOLENT 5.REFLECTOR 6.SHIELDING 7. REACTOR VESSEL. Fig. 4.4 components of Nuclear Power Plants Fuel Nuclear reactors use fissionable materials like U233, U 235, Pu 239. Natural uranium found in earth crest has 0.714% which unstable and capable of sustainable chain reactions. 4-3

32 NUCLEAR POWER PLANT The fuel is shaped in various shapes like rods, plates, pallets, tins etc. and located in the reactor in such a manner that the heat production within the reactor is uniform. The fuel elements are designed taking account the heat transfer, corrosion and structural strength. Homogeneous reactors and heterogeneous reactor. Moderator The function of the moderator is to slow down the neutrons from high kinetic energy to low kinetic energy in a fraction of the second. The main function of the moderator is to increase the probability of the reaction. The fission chain reaction in the nuclear reactor is maintained due to slow neutrons when ordinary uranium used as a fuel. Moderators are lighter then fuel. Water, heavy water. Graphite and beryllium. Graphite and heavy water is used as a moderator with natural uranium. For enriched uranium ordinary water is used. Characteristic of moderator a. It must be as light as possible. b. It must not absorb the neutrons. c. It must be work under high temperature and pressure with good corrosion resistance. d. It must have high chemical stability. e. It must have high heat conductivity. f. If it is in form of solid then it must have high melting point and good machinability. Control Rods The control road controls the rate of energy which is generated. Function of control rod is.., a. When we need to start the reactor. b. It is also used to increase, decrease and stop the reaction. c. The rod may be shaped like fuel rod themselves and are interspersed throughout the reactor core. d. The control is necessary to prevent the melting of fuel rods, disintegration of the coolant and destruction of reactor as the amount of energy released is enormous. e. Control rod materials like cadmium, boron etc.., Coolant The main purpose of the coolant in the reactor is to transfer the produced heat in to the reactor and keep fuel assembly at a safe temperature to avoid their melting and destruction. The heat carried away of coolant by using heat exchanger and use to generation of steam or hot gas. As a coolant generally water, heavy water carbon dioxide helium gas, liquid metal like sodium (Na) potassium (k) or organic liquid. 4-4

33 NUCLEAR POWER PLANT CHARECTERISTICS OF COOLANT a. It must have high chemical and nuclear stability. b. It must have high corrosion resistance. c. It must have high boiling point and low melting point. d. It should be nontoxic. e. It must have high sp. heat and high thermal heat transfer co efficient. f. It should have high density and low viscosity. Reflector It is important to conserve the neutrons as much as possible for reducing consumption of fissile material and keep the size of reactor small. This is possible by surrounding the reactor core with a material which reflects escaping neutrons back in to the core. This material is called reflectors. The require properties of good reflectors should have low absorption and high reflection for neutrons. It should have high resistance to oxidation and high radiation stability. Materials are used for moderators are also used for reflectors. Shielding Shielding is the radioactive zones in the reactor from possible radiation hazard are essential to protect the human life from harmful effects. To prevent effects those radiation on the human life it is necessary to absorb them before emitting to atmosphere. Shielding consists of inner lining of 50 to 60 cm thick steel plate on the reactor core called thermal shield. With a few meters of thick concrete wall surrounding the inner shield called biological shield. Thermal shield cooled by circulation of water. Reactor Vessel It is enclose the reactor core, reflector and shield. It also provides coolant inlet and outlet passages. It has to with stand the pressure at 200 bar or above. The reactor core, fuel and assembly are generally placed at the bottom of the vessel. 6. Classification of Reactors: On the basis of neutron energy a. Fast reactors: In these reactors, the fission is affected by fast neutrons with out any use of moderator. b. Thermal reactors: In these reactors the fast moving neutrons are slowed down with help of moderator. c. Intermediate reactors: In this reactor velocity of the neutrons is kept between fast reactors and thermal reactors. On the basis of fuel used a. Natural uranium fuel reactors. b. Enriched uranium fuel reactor 4-5

34 NUCLEAR POWER PLANT On the basis of coolant used a. Water /heavy water cooled reactors. b. Gas cooled reactors c. Liquid metal /organic liquid cooled reactors. On the basis of moderator a. Water moderated. b. Heavy water moderated. c. Graphite moderated. d. Beryllium moderated. On the basis of reactor core used a. Homogeneous reactor. b. Heterogeneous reactor. Pressurized Water Reactor In a PWR the primary coolant (water) is pumped under high pressure to the reactor core where it is heated by the energy generated by the fission of atoms. The heated water then flows to a steam generator where it transfers its thermal energy to a secondary system where steam is generated and flows to turbines which, in turn, spins an electric generator. Fig. 4.5 Pressurized Water Reactor. Nuclear fuel in the reactor vessel is engaged in a fission chain reaction, which produces heat, heating the water in the primary coolant loop by thermal conduction through the fuel cladding. The hot primary coolant is pumped into a heat exchanger called the steam generator, where it flows through hundreds or thousands of tubes (usually 3/4 inch in diameter). 4-6

35 NUCLEAR POWER PLANT Heat is transferred through the walls of these tubes to the lower pressure secondary coolant located on the sheet side of the exchanger where it evaporates to pressurized steam. The transfer of heat is accomplished without mixing the two fluids, which is desirable since the primary coolant might become radioactive. Some common steam generator arrangements are u-tubes or single pass heat exchangers. In a nuclear power station, the pressurized steam is fed through a steam turbine which drives an electrical generator connected to the electric grid for distribution. After passing through the turbine the secondary coolant (water-steam mixture) is cooled down and condensed in a condenser. The condenser converts the steam to a liquid so that it can be pumped back into the steam generator, and maintains a vacuum at the turbine outlet so that the pressure drop across the turbine, and hence the energy extracted from the steam, is maximized. Before being fed into the steam generator, the condensed steam (referred to as feed water) is sometimes preheated in order to minimize thermal shock. BOILINIG WATER REACTOR The BWR uses dematerialized water as a coolant and neutron moderator. Heat is produced by nuclear fission in the reactor core, and this causes the cooling water to boil, producing steam. The steam is directly used to drive a turbine, after which it is cooled in a condenser and converted back to liquid water. This water is then returned to the reactor core, completing the loop. Fig.4.6 Boiling Water Reactor CANDU REACTOR The CANDU, short for Canada Deuterium-Uranium reactor is a Canadian-invented, pressurized heavy water reactor. The acronym refers to its deuterium-oxide (heavy water) moderator and its use of (originally, natural) uranium fuel. These reactors are more economical to those nations which do not produce enriched uranium as the enrichment of uranium very costly. In this reactor the fuel is normal uranium oxide as small cylinder pallets the pallets are packed in corrosion resistance zirconium alloy tube. The coolant heavy water is passed through the fuel pressure tubes and heat exchanger. The heavy water is circulated in the primary circuit and the same way the steam 4-7

36 NUCLEAR POWER PLANT is generated as PWR in secondary circuit due to circulate heat in to it from primary circuit. The control of the reactor is achieved by varying the moderator level in the reactor so no control rod is needed. Fig. 4.7 Candu Reactor Gas Cooled Reactor It is also called Gas Cooled Graphite Moderated (GCGM) Reactor. A gas-cooled reactor (GCR) is a nuclear reactor that uses graphite as a neutron moderator and carbon dioxide (helium can also be used) as coolant. Fig.4.8 Gas Cooled Reactor Fuel used is U 233 as fissile material and thorium as fertile material. Because of high melting point of graphite. In the primary circuit, coolant circulated is CO2 for GCGM reactor or (He) for HTGC reactor. The coolant transfers the heat energy to feed water in the heat exchanger and the steam generated is used to generate power in steam turbine. 4-8

37 NUCLEAR POWER PLANT Fast Breeder Reactor THE PROCESS OF CONVERTING MORE FERTILE MATERIAL IN TO FISSILE MATERIAL IN A REACTOR IS CALLED BREEDING. In fast breeder reactor the core containing U 235 is surrounded by a blanket of fertile material U 238. In this reactor no moderator is used the fast moving neutrons liberated due to fission of U235 are absorbed by U 238 which gets converted in to Pu 239 a fissile material. This reactor also uses two liquid metal coolants in which sodium is used as primary coolant and sodium potassium as secondary coolant.(sodium boils at C under atmospheric pressure and freeze at 95 0 C). The reactor also used two liquid metal coolants in which sodium is used as primary coolants and sodium potassium as secondary coolants. Liquid sodium is circulated through the reactor to carry the heat produced. The heat produced by the sodium is transferred to secondary coolant sodium potassium in the primary heat exchanger which in turn transfer the heat in secondary heat exchanger called steam generator. 4-9

38 NUCLEAR POWER PLANT Exercise 1. Draw the neat sketch of Boiling Water Reactor (BWR). 4-10

39 NUCLEAR POWER PLANT 2. Draw the neat sketch of CANDU type reactor. 4-11

40

41 DRAUGHT SYSTEM 1. Objective: EXPERIMENT - 5 To study of various draught system. 2. Aim: To understand the working principle and construction of various draught system. 3. Introduction: The purpose of draught is to supply required quantity of air for combustion and remove the burnt product from the system. To move the air through the fuel bed and to produce a flow of hot gases through the boiler, economizer, preheater and chimney required a difference of pressure equal to that necessary to accelerate the burnt gases to their final velocity and to overcome the pressure losses equivalent to pressure head. This difference of pressure required to maintain the constant flow of air and to discharge the gases through the chimney to atmosphere is known as draught. Because of the emission of large amount of flue gases and other materials environment is polluted, thus to decrease the environmental pollution some techniques and equipment s are used. Generally Electrostatic precipitators and Draughts system is used by coal gas plants to decrease the environment pollution. Here we explain the brief about the Draught system. Draught can be obtained by use of chimney, fan, steam or air jet or combination of these. When the draught is produced with help of chimney only, it is known as Natural Draught and when the draught is produced by any other means except chimney it is known as Artificial Draught. 4. Types of draught system Draught are of two types: a. Natural Draught b. Artificial Draught Natural draught: The natural draught is obtained with the use of tall chimney which may be sufficient or insufficient to overcome the losses in the system. Its usefulness depends upon the capacity of the plant and duct work. This system of producing the draught is useful for small capacity boilers and it does not play much important role in the present high capacity thermal power plants. A chimney is a vertical structure of masonry; brick, steel or reinforced concrete built for the purpose of enclosing a column of hot gases to produce the draught and discharge the gases high enough which will prevent an air pollution the draught produced by the chimney is due to the temperature difference of hot gases in the chimney and cold air outside the chimney. Consider the height of the chimney above the grate level is H. 5-1

42 DRAUGHT SYSTEM Fig. 5.1 Natural draught The pressure acting on the grate from the chimney side, P 1 = P a + W g H And the pressure acting on the grate from the atmospheric side, P 2 = P a + W a H Where, P a = Atmospheric pressure, W a, W g = Weight densities of the atmospheric air and hot gases passing through the chimney respectively. The gas density varies along the height of the chimney as part of the heat is lost by the gas to the chimney. Therefore the average density of the gas should be taken for circulation. The net acting pressure on the grate of the combustion chamber due to the pressure exerted by gas column and air column is given by: P = P 2 P 1 Artificial draught: Artificial draught can be further classified as: Forced draught: In a forced draught system, a lower is installed near the base of the boiler and air is forced to pass through the furnace, flues, economizer, air-preheater and to the stack. This draught system is known as positive draught or forced draught system because the pressure of air throughout the system is above atmospheric draught system or forced draught system because the pressure of air throughout the system is above atmospheric pressure and air is forced to flow through the system. A stack or chimney is also used in this system but its function is to discharge gases high in the atmosphere to prevent the contamination. Fig. 5.2 forced draught 5-2

43 DRAUGHT SYSTEM It is not much significant for producing draught therefore height of the chimney may not be very much. Induced draught: In this system, the blower is located near the base of the chimney instead of near the grate. The air is sucked in the system by reducing the pressure through the system below atmosphere. The induced draught fan sucks the burned gases from the furnace and the pressure inside the furnace is reduced below atmosphere and induces the atmospheric air to flow through the furnace. The action of the induced draught is similar to the action of the chimney. The draught produced is independent of the temperature of the hot gases therefore the gases may be discharged as cold as possible after recovering as much heat as possible in the air-preheater and economizer. This draught is used generally when economizer and air-preheater are incorporated in the system. The fan should be located at such a place that the temperature of the gas handled by the fan is lowest. The chimney is also used in this system and its function is similar as mentioned in forced draught but total draught produced in induced draught is the sum of the draught produced by the chimney and the fan. Balance Draught: Fig. 5.3 Induced draught It is always preferable to use a combination of forces and induces draught instead of forced or induced draught alone. If the forced furnace is used alone, then the furnace cannot be opened either for inspection or for firing because the high pressure air inside the furnace will try to blow out suddenly and there us every chance of blowing out the fire completely and furnace stops. If the induced draught is used alone, then also furnace cannot be opened either for firing or inspection because the cold air will try to rush into the furnace as the pressure inside the furnace is below the atmospheric pressure. This reduces the effective draught and dilutes the combination. To overcome both the difficulties mentioned above either using forced draught or induced draught alone, a balanced draught is always preferred. The balanced draught is a combination of forced and induced draught. The forced draught overcomes the resistance 5-3

44 DRAUGHT SYSTEM of the fuel bed therefore sufficient air is supplied to the fuel bed for proper and complete combustion. The induced draught fan removes the gases from the furnace maintaining the pressure inside the furnace just below atmosphere. This helps to prevent the blow-off of flames when the doors are opened as the leakage of air is inwards. The pressure inside the furnace is near atmospheric so there is no danger of blowout of flames or there is no danger of inrushing the air into the furnace when the doors are opened for inspection. The pressure of air below the grate is above atmosphere and it helps for proper and uniform combustion. The pressure of air above the grate is just below the atmosphere and it helps to remove the exhaust gases as quick as possible from the combustion zone. Fig. 5.2 Balanced draught 5-4

45 DRAUGHT SYSTEM Exercise 1. Distinguish between force draught and induced draught. 5-5

46 DRAUGHT SYSTEM 2. Distinguish between Mechanical draught and Natural draught. 5-6

47 FEED WATER TREATMENT PLANT 1. Objective: To study of feed water treatment plants. EXPERIMENT Introduction: Feed water is the major source of soluble and insoluble impurities entering the boiler and therefore the principal aim of the feed water treatment and monitoring is to minimize the levels of such impurities. The objectives of any chemical treatment for a modern, safe and efficient thermal power plant are: To reduce corrosion of metals and equipment To avoid scale formation 3. Methods of Feed water treatment: Feed water treatment External Treatment Internal Treatment Colloidal conditioning Lime-soda process Zeolite process Ion exchange process Phosphate conditioning Cold lime-soda Process Hot lime-soda Process Carbonate Conditioning Calgon Conditioning Chart 6.1 Methods of Feed water treatment 6-1

48 FEED WATER TREATMENT PLANT External treatment a) Lime-soda process: In this Method, the soluble calcium and Magnesium salts in water are chemically converted into insoluble Compounds by adding calculated amounts of lime Ca (OH) 2 and soda Na2CO3 calcium carbonate CaCO3 and magnesium hydroxide Mg (OH)2. So Precipitated, are filtered off. Cold lime-soda process: In this Method, Calculated quantity of chemical (lime and soda) is mixed with water at room temperature. At room temperature, the precipitates formed are finely divided, so they do not settle down easily and cannot be filtered easily. Consequently, it is essential to add small amounts of coagulants (Like Alum, aluminum sulphate, sodium aluminate, Etc.). Which hydrolyses to flocculent, gelatinous precipitate of aluminum hydroxide, and entraps the fine precipitates. Use of sodium aluminate as coagulant, also helps the removal of silica as well as oil, if present in water. Cold L-S process provides water, containing a residual hardness of 50 to 60 ppm. NaAlO2 + 2H2 O ----> NaOH +Al(OH) 3 Al2 (SO4 ) 3 +3 Ca(HCO3 ) > 2Al(OH) CaSO4 + 6CO2 Method: Raw water and calculated quantities of chemicals (Lime + soda + coagulant) are fed from the top into the inner vertical circular chambers, fitted with a vertical rotating shaft carrying a number of paddles, As the raw water and chemicals flow down there is a vigorous stirring and continuous mixing, whereby softening of water takes place. As the softened ware comes into the outer chamber of the lime the softened water reaches up. The softened water then passes through a filtering media (usually made of wood fibers) to ensure complete removal of sludge. Filtered soft water finally a flow out continuously through the outlet at the top sludge settling at the bottom of the outer chamber is drawn off occasionally. Hot lime-soda process: Figure 6.1 Cold lime soda process Involves in treating water with softening chemicals at a temperature of 80 to C. Since hot process is operated at a temperature close to the boiling point of the solution, so (i) the reaction proceeds faster; (ii) the softening capacity of hot process is increased to may fold; (iii) the precipitate and sludge formed settle down rapidly and hence, no coagulants are needed; (iv) much of the gases (Such as CO2 and air) Driven out of the water; (v) Viscosity of softened water is lower, so filtration of water becomes much easier. This in-turn increases 6-2

49 FEED WATER TREATMENT PLANT the filtering capacity of filters, and (vi) Hot Lime-Soda Produces water of comparatively lower residual hardness of 15 to 30ppm. Hot lime-soda plant consists essentially of three parts (a) reaction tank in which raw water, chemicals and steam are thoroughly mixed; (b) conical sedimentation vessel in which sludge settles down, and (c) Sand filter which ensures complete removal of sludge from the softened water. Advantages of Lime soda process: Figure 6.2 Hot lime soda process It is a very economical If this process is combined with sedimentation with coagulation, lesser amounts of coagulants shall be needed The process increased the ph value of the treated water, thereby corrosion of the distribution pipes is reduced Besides the removal of hardness, the quantity of minerals in the water are reduced To certain extent, iron and manganese are also removed from the water. Due to alkaline nature of treated- water, amount of pathogenic bacteria s in water is considerably reduced Disadvantages of Lime soda process: Disposal of large amounts of sludge (insoluble precipitate) poses a problem. However, the sludge may be disposed off in raising low-lying areas of the city This can remove hardness only up to 15 ppm, which is not good for boilers. b) Zeolite process: Chemical structure of sodium zeolite may be represented as Na2O3, Sio2, YH2O where x=2-10 and y=2-6. Zeolite is hydrated sodium alumino silicate, capable of exchanging reversibly its sodium ions for hardness, producing ions in water Zeolite are two types, 6-3

50 FEED WATER TREATMENT PLANT i. Natural zeolites are non-porous for Ex; Natrolite Na2Al3O3.4sio22h2o ii. Synthetic zeolites possess gel structure. Synthetic Zeolites possess higher exchange capacity than natural Zeolites Process For Softening of water by Zeolite process, hard water is percolated at a specified rate through a bed of zeolite; kept in a cylinder. The Hardness causing ions (ca +2, Mg +2 etc.) are retained by the zeolite as CaZe and MgZe, while the outgoing water contains sodium salts. Reactions taking place during the softening process are, Na2Ze +Ca (HCO3) > CaZe +2NaHCO3 Na2Ze +Mg (HCO3) > MgZe +2NaHCO3 Figure 6.3 Zeolite process Regeneration: After Some time the zeolite is completely converted into calcium and magnesium Zeolites and it ceases to soften water i.e.; it gets exhausted. At this stage the supply of hard water is stopped and the exhausted zeolite is reclaimed by treating the bed with a concentrated NaCl solution CaZe (or MgZe) + 2NaCl ----> Na2Ze + CaCl2 (or MgCl2 ) The washings are led to drain and the regenerated zeolite bed thus obtained is used again for softening process. Limitations: If the supply of water is turbid in will clog the pores of zeolite led Water contains large quantities of colored ions such as Mn +2 and Fe +2 they may be removed first because these ions produce Mn and Fe Zeolites,which can t be easily regenerated 6-4

51 FEED WATER TREATMENT PLANT Mineral acids destiny the zeolite bed Advantages: If removes the hardness almost completely Equipment occupying a small space Requires less time It is quite clean Disadvantages: Treated water contains more sodium salts than in time soda process The method only replaces Ca +2 and Mg +2 ions by Na + ions leaves all the acidic ions. c) Ion-exchange process: Ion exchange resins are insoluble, cross linked long chain organic polymers with micro porous structure, and the functional groups attached to the chains are responsible for the Ion exchanging properties resigns containing acidic functional groups (-cool+,-so3h etc.) are capable of exchanging their H+ ions with other cat ions, which comes in their contact where as those containing basic functional groups(-nh2=nh, hydrochloride) are capable of exchanging their anions with other anions, which comes in their contact Ion exchange resins may be classified as, i. Cat ion exchange resin(rh+) are mainly styrene-divinyl benzene copolymers, which on sulphonation or carboxylation, become capable to exchange their hydrogen ions with the cat ions in water ii. Anions exchange resins (ROH) are styrene-divinyl benzene or amine-formaldehyde, copdymers, which contains amino or quaternary ammonium or quaternary phosphonium or tertiary sulphonium groups as an integral part of the resin matrix these after treatment with dilute. NaOH solutions become capable to exchange their OH-anions with anions. Process: The Hard water is passed first through cat ion exchange column, which removes all the cat ions like Ca+2 etc. from it and equivalent amount of H+ ions released from this column to water, thus 2RH + + Ca 2+ R2Ca H + 2RH+ + Mg 2+ R2Mg H + After Cat ion exchange column, the hard water is passed through anion exchange column which removes all the anions like so4, cl - etc. present in the water and equivalent amount of OH- ions are released from this column to water thus: R OH- + Cl - R Cl - + OH - 2R OH- + SO4 2- R 2SO OH - 2R OH- + CO3 2- R 2CO OH - H + and OH - ions get combined to produce water molecule H + + OH - H2O The water coming out from the exchanger is deionized or demineralized water. Regeneration: 6-5

52 FEED WATER TREATMENT PLANT When capacities of cation and anion exchangers to exchange H+ and OH- ions respectively are lost, they are then said to be exhausted The exhausted cat ion exchange column is regenerated by passing a solution of Diluted HCL or Dilute H2SO4. The regeneration can be represented as, R2Ca H + 2RH + + Ca 2+ The exhausted anions exchange column is regenerated by passing a solution of diluted NaoH. The regeneration can be represented as, R 2SO OH - 2R OH - + SO4 2- Advantages: Figure 6.4 Ion-exchange process Process used to soften highly acidic or alkaline water. It produces water of very low hardness. Disadvantage: The equipment is costly. If water contains turbidity out-out of the process is reduced. Internal treatment In this process, an ion is prohibited to exhibit its original character by converting it into other more soluble salt by adding appropriate reagent. An internal treatment is accomplished by adding a proper chemical to the boiler water either to precipitate the scale forming impurities in the form of sludge, which can be removed by blow down operations, or to convert them into compounds, which will stay in dissolved form in water and they do not cause any harm. Important Internal treatment methods are: 6-6

53 FEED WATER TREATMENT PLANT a) Colloidal conditioning: In low pressure boilers, scale formation can be avoided by adding organic substances like Kerosene, tannin, agar-agar etc. which get coated over the scale firming precipitates, there by yielding coated non sticky and loose deposits b) Phosphate conditioning: In High pressure boilers, scale formation can be avoided by adding sodium phosphate which reacts with hardness of water forming non- adherent and easily removable soft sludge 3CaCl2+2Na3PO4 Ca2(PO4) 2+6NaCl The main phosphates employed are (a) NaH2PO4 (b) Na2HPO4 (c) Na3PO4 c) Carbonate Conditioning: In low pressure boilers, scale formation can be avoided by adding sodium carbonate to boiler water, then caso4 converted into Caco3 in equipment Caco3 forms loose sludge d) Calgon Conditioning: CaSO4+Na2CO3 CaCO3+ Na2SO4 Involves in adding calgon [(NaPO3)6] to boiler water then it forms soluble complex compound with caso4 Na2[Na4(PO3) 6] - 2Na + + [Na4P6O18] 2-2CaSO4 + [Na4P6O18] 2- [CaP6O18] Reverse Osmosis Process Reverse Osmosis is a technology that is used to remove a large majority of contaminants from water by pushing the water under pressure through a semi-- permeable membrane. Osmosis Osmosis is a naturally occurring phenomenon and one of the most important processes in nature. It is a process where a weaker saline solution will tend to migrate to a strong saline solution. A solution that is less concentrated will have a natural tendency to migrate to a solution with a higher concentration. For example, if you had a container full of water with a low salt concentration and another container full of water with a high salt concentration and they were separated by a semi-- permeable membrane, then the water with the lower salt concentration would begin to migrate towards the water container with the higher salt concentration. A semi-permeable membrane is a membrane that will allow some atoms or molecules to pass but not others. A simple example is a screen door which allows air molecules to pass through but not pests or anything larger than the holes in the screen door. 6-7

54 FEED WATER TREATMENT PLANT Figure 6.5 Osmosis Process Reverse Osmosis Reverse Osmosis is the process of Osmosis in reverse. Whereas Osmosis occurs naturally without energy required, to reverse the process of osmosis we need to apply energy to the more saline solution. A reverse osmosis membrane is a semi-- permeable membrane that allows the passage of water molecules but not the majority of dissolved salts, organics, bacteria and pyrogens. However, we need to push the water through the reverse osmosis membrane by applying pressure that is greater than the naturally occurring osmotic pressure in order to desalinate (demineralize or deionize) water in the process, allowing pure water through while holding back a majority of contaminants. Following Figure shows the process of Reverse Osmosis. When pressure is applied to the concentrated solution, the water molecules are forced through the semi permeable membrane and the contaminants are not allowed through. Figure 6.6 Reverse Osmosis Process 6-8

55 FEED WATER TREATMENT PLANT Working Reverse osmosis works by using a high pressure pump to increase the pressure on the salt side of the RO and force the water across the semi-permeable RO membrane, leaving almost all (around 95% to 99%) of dissolved salts behind in the reject stream. The amount of pressure required depends on the salt concentration of the feed water. The more concentrated the feed water, the more pressure is required to overcome the osmotic pressure. In very simple terms, feed water is pumped into a Reverse Osmosis (RO) system and we end up with two types of water coming out of the RO system: good water and bad water. The good water that comes out of an RO system has the majority of contaminants removed and is called permeate. Permeate is the water that was pushed through the RO membrane and contains very little contaminants. Figure 6.7 RO Pumping Process The bad water is the water that contains all of the contaminants that were unable to pass through the RO membrane and is known as the concentrate, reject, or brine. All three terms (concentrate, reject, and brine) are used interchangeably and mean the same thing. Figure 6.7 shows how an RO system works. As the feed water enters the RO membrane under pressure (enough pressure to overcome osmotic pressure) the water molecules pass through the semi-- permeable membrane and the salts and other contaminants are not allowed to pass and are discharged through the concentrate stream, which goes to drain or can be fed back into the feed water supply in some circumstances to be recycled through the RO system to save water. The water that makes it through the RO membrane is called permeate or product water and usually has around 95% to 99% of the Dissolved salts removed from it. 6-9

56 FEED WATER TREATMENT PLANT Exercise 1. Draw the neat sketch of hot water zeolite process. 6-10

57 STEAM NOZZLE 1. Objective: EXPERIMENT - 7 To study about different types of nozzle and calculate dimensions of nozzle for designing. 2. Introduction: A nozzle is often a pipe or tube of varying cross sectional area, and it can be used to direct or modify the flow of a fluid (liquid or gas). Nozzles are frequently used to control the rate of flow, speed, direction, mass, shape, and/or the pressure of the stream that emerges from them. In a nozzle, the velocity of fluid increases or decrease at the expense of its pressure energy. 3. Expansion through nozzle: Characteristics of expansion phenomenon through nozzle is different for compressible fluid and incompressible fluid. For Incompressible fluid: If liquid is passing through nozzle its velocity increase and pressure decrease. If we decrease back pressure (pressure at outlet) higher and higher, we obtain maximum velocity at outlet. As the density remains same for incompressible fluid, there is no change in fluid property and its velocity increasing continuously. Figure 7.1 Convergent nozzle For compressible fluid: Initially if we decreasing the back pressure velocity increases, but at one point we will find that if we decreasing back pressure, there is no further increase in velocity. That means we are having decreasing back pressure but mass flow rate is not increasing, as it the nozzle has got chocked. This is called chocking or critical condition of nozzle. (It happens only in compressible fluids). Initially mass flow rate is zero, so pressure ratio is 1. In left direction we decrease back pressure and mass flow rate increases but at certain point there is no further increase in velocity. So this is called typical characteristics of convergent nozzle. At this condition steam velocity is reached near local sonic velocity. When we are using convergent nozzle, maximum velocity and maximum mass flow rate both are limited. We don t increase velocity beyond local sonic velocity if nozzle geometry is like convergent type and even we don t change velocity by decreasing back pressure also. That s why convergent-divergent nozzle is used. 7-1

58 STEAM NOZZLE 4. Convergent-divergent nozzles Figure 7.2 mass flow vs pressure ratio Generally, in convergent section velocity increase and pressure decrease but it happens up to sonic velocity. In divergent section pressure increase and velocity decrease and maximum velocity obtain at throat. In convergent-divergent nozzle we find some unique change in the divergent section of the nozzle. Once sonic velocity at throat is reached and we are still decreasing back pressure then further expansion will take place and we will have further reduction in pressure and increase in velocity, so supersonic velocity obtained. Graphically, Figure 7.3 Convergent-divergent nozzle Figure 7.4 Convergent divergent nozzle shock waves formation 7-2

59 STEAM NOZZLE Initially inlet pressure and back pressure are same. So no flow is there. In convergent section velocity increases and in divergent portion velocity decreases, till we have sonic velocity at throat. After that if we further, reduce back pressure to point C, there is no change in convergent section and we will continue about the same line or expansion even in the divergent section. If we proceed along this line we will reach at point f, but we should be at point c. So after certain distance shock waves produced and we will be come back at point C. 5. Application of steam nozzle: To produce high velocity jet to impinge on curved blade of driving turbine shaft Jet engines to produce thrust Rocket motors to produce thrust Artificial Fountains Flow measurements Injectors for pumping feed water Ejectors for removing air from condensers Fire hose to produce water jet 6. Velocity of Steam through nozzle Steam flow through nozzle maybe assumed as adiabatic flow since during expansion of steam there is no any heat transfer. It can be calculated by following formula. C 2 = ((h 1 h 2 ) η n Where, h 1 = Enthalpy at inlet h 2 = Enthalpy at outlet η n = Nozzle efficiency 7. Critical pressure ratio The pressure ratio for the maximum mass flow rate of gas through the nozzle is called the critical pressure ratio. p 2 = ( 2 p 1 n + 1 ) n n 1 Value of critical ratio depends upon index n. for different fluids it is as following. Table: 7.1 Sr. No. Type of Fluid Index Critical pressure ratio 1 Air Wet steam x Depends on value of dryness fraction 3 Dry saturated steam Superheated steam

60 STEAM NOZZLE 8. Nozzle efficiency and Effect of friction Figure 7.5 h-s Diagram Expansion process in nozzle considered as isentropic expansion but in actual practice there is a friction loss in the nozzle, so actual flow in the nozzle is not isentropic flow. Nozzle efficiency is defined as the ratio of actual heat drop to isentropic heat drop or heat drop due to isentropic expansion. η N = h 1 h 2 h 1 h 2Where, h 1 = Initial Enthalpy h 2 = Final Enthalpy h 2 = Actual final Enthalpy Effect of Friction When steam flows through a nozzle the final velocity of steam for given pressure drop is reduced due to following reason. The friction between the nozzle surface and steam. The internal friction of steam itself. The shock losses. The convergent portion of nozzle is smaller than the divergent portion. Thus, the wall friction is small in the convergent portion as compared to divergent portion. The fluid friction is also small in convergent portion than in the divergent portion, since the fluid velocity in the convergent portion is small. Thus, most of the friction occurs in the divergent portion of the nozzle and h-s diagram plot as shown in following figure. 7-4

61 STEAM NOZZLE Figure 7.6 h-s Diagram These frictional losses entail the following effects. The expansion is no more isentropic and the enthalpy and entropy of steam increasing during the process. The final dryness fraction of steam is increased as the kinetic energy gets converted into heat due to friction and is absorbed by steam. The specific volume of steam increased as the steam becomes drier due to this frictional reheating. Exit velocity is reduced as the kinetic energy gets converted into heat due to friction. Mass flow rate is decreased. 7-5

62 STEAM NOZZLE 1. Derive p 2 p 1 = ( 2 n+1 ) n n 1 ; Exercise Where P2 is throat pressure, P1 is inlet pressure and n is the index of isentropic expansion of steam through the nozzle. 7-6

63 HIGH PRESSURE BOILER 1. Objective: EXPERIMENT - 8 Comparative study of different types of high pressure boilers. 2. Introduction: Efficiency of any cycle can be increased by increasing the temperature of source or temperature of heat supplied. So, efficiency of the thermal power plant can be increased by increasing temperature of the steam supplied to the turbine. With development of the material, it is possible to supply high temperature steam to turbine. So to increase the efficiency of the plant it is necessary to high pressure boiler. 3. Unique features of high pressure boiler: Method of water circulation The water may be circulated through the boiler by natural circulation due to density difference or by forced circulation with the help of pump. In all modern thermal power plants forced circulation is used. But with increase of pressure, density difference decreases and at critical pressure it becomes zero. Thus the natural circulation ceases. Therefore, in high pressure boiler, it is necessary to use forced circulation. Further, heat transfer rate can also be increased by increasing the velocity of water with the help of pump. Types of Tubing In most of high pressure boiler, the water is circulated through the tubes and outer surface of tubes are exposed to the gases. If the water is circulated through the one continuous tube, large pressure drop will take place. To minimize the pressure drop, water is circulated through parallel system of tubing. Improved Method of Heating The heat transfer from the hot gases to water can be increased by using following methods: o At critical pressure, water is directly converted into steam. So, by increasing the pressure above the critical pressure latent heat of vaporization can be saved. o If water is supplied to the boiler at high temperature, then efficiency of heat supplied can be increased. So, by using the feed water heater, temperature of feed water can be increased. o The overall heat transfer coefficient can be increased by increasing velocity of water inside the tube or by increasing the velocity of gases. 4. Different high pressure boilers: LaMont Boiler: Construction and Working The arrangement of water circulation and different components is shown in figure

64 HIGH PRESSURE BOILER Fig. 8.1 LaMont Boiler The feed water from the hot well is supplied to a storage and separating drum through economizer. The most of the sensible heat is supplied to feed water through economizer. Water from the storage drum is circulated through the radiant evaporator and convective evaporator with the help of circulating pump. Circulation of water through evaporator is 8 to 10 times the weight of steam evaporated. Such large quantity of water circulation through evaporator tubes prevents the tube from overheating. Part of the water evaporated, as it pass through evaporator, is separated in the separating drum. Dry and saturated steam from the separating drum is passed through the super-heater before supply to prime mover. Distributing headers are used to distribute the water into radiant evaporator through nozzles. Note: - In the radiant evaporator, heat is transferred by radiation so it is called radiant evaporator, whereas in convective evaporator, heat is transferred by convection. In modern high pressure boiler, furnace wall is covered with the water tube, so heat transfer in furnace is by radiation. This water tubes also protect the furnace wall from overheating. Problem with La Mont Boiler and its Solution The main difficulty experience in the La Mont boiler is the formation and attachment of bubbles on the inner surface of the heating tubes. As the attached bubbles offers high thermal resistance to heat transfer compare to water film, it reduce the heat transfer and steam generation. It also increases the thermal stresses. This problem can be reduced by increasing the pressure inside the boiler upto the critical pressure. At the critical pressure, water and steam have same density, so formation of bubble can be eliminated. 8-2

65 HIGH PRESSURE BOILER Benson Boiler: To avoid the formation and attachment of bubbles inside the water tube, Benson boiler is operated at critical pressure. The arrangement of the boiler components is shown in figure 8.2. Construction and Working Fig. 8.2 Benson Boiler Water from the hot well is passed through the economizer where sensible heat is supplied to the water. Part of the water is evaporated when it passes through the radiant evaporator and remaining water is evaporated as it passes through the convective evaporator. Then dry and saturated steam from the convective evaporator is passed through the super-heater before supply to prime mover. Problem with Benson Boiler and its Solution Major problem with the Benson boiler is deposition of salt in the transformation zone when all remaining water is converted into steam. This deposited salt offers the resistance to heat transfer and reduces the steam generation. It also causes the overheating of the tube. To avoid this difficulty, the boiler is normally flashed out after every 4000 working hours to remove the salt. Loeffler Boiler: The major difficulty experienced in Benson boiler is deposition of salt on the inner surface of the tubes. This difficulty was solved in Loeffler boiler by preventing the circulation of water through the tubes. The arrangement of the components of boiler is shown figure

66 HIGH PRESSURE BOILER Construction and Working Fig. 8.3 Loeffler Boiler The water from the hot well is supplied to the evaporating drum through economizer. About 65% of the steam coming from the super-heater is supplied to the evaporating drum for evaporation of the feed water from the economizer. Steam is generated by mixing of the super-heated steam to the feed water in evaporating drum. Dry and saturated steam generated in the evaporating drum is circulated through the radiant super-heater and convective super-heater with the help of steam circulating pump. About 35% of superheated steam generated in the super-heater is supplied to the H.P. turbine and remaining is supplied to evaporating drum. Exhaust steam from the H.P. turbine is reheated in the reheater before supplied to the L.P. turbine. For distribution of super-heated steam throughout the water into evaporator, special design nozzles are used which reduce the priming and noise. Higher salt concentration water can be used in this boiler. Schmidt-Hartmann Boiler: The operation of the boiler is similar to an electric transformer. The arrangement of the boiler components are shown in figure

67 HIGH PRESSURE BOILER Construction and working Fig. 8.4 Schmidt-Hartmann Boiler The boiler consists of two circuits. In the primary circuit, steam is generated from the distilled water at 100 atm. The generated steam is passed through the coil submerged in the evaporating drum. Evaporating drum contains the impure water at 60 atm. so, in the evaporating drum, steam is generated at 60 atm. and high pressure steam is condensed. Steam generated in evaporating drum is passed through the super-heater before supply to the prime mover. The high pressure condensate produced in the submerged coil is passed through the feed pre-heater to raise the temperature of impure water to its saturation temperature. So, only latent heat is supplied in the evaporator drum. Natural circulation is used in the primary circuit and this is sufficient for desired rate of heat transfer and to overcome the thermo-siphon head of about 2 to 10 m. Every care is taken in design and construction to prevent the leakage of distilled water in the primary circuit, so in normal circumstances, make up water is not required. For safety of operation pressure gauge and safety valve are fitted in the primary circuit. Advantages a) There is a rare chance of overheating or burning the highly heated components of primary circuit as there is no chance of obstruction to the circulation by impurities. 8-5

68 HIGH PRESSURE BOILER b) The salt deposited in the evaporator drum due to the circulation of impure water can be easily brushed off just by removing the submerged coil from the drum or by blowing off the water. c) The wide fluctuations of load are easily taken by this boiler without priming problem. d) The absence of water risers in the drum, and moderate temperature difference across the heating coil allows evaporation to proceed without priming. Velox Boiler: High rate of heat transfer can be achieved by increasing the velocity of the flue gases over the velocity of the sound. In the velox boiler, velocity of the gases is more than velocity of the sound. The arrangement of components of the boiler is shown in figure 8.5. Construction and Working Air is compressed to 2.5 bar with the help of the compressor driven by the turbine. That high pressure air is supplied to combustion chamber to get supersonic velocity of the gases. The supersonic gases are passed through the combustion chamber and gas tubes to achieve high heat transfer rate. Fig. 8.5 Velox Boiler The burned gases in the combustion chamber are passed through the annulus of the tubes as shown in figure 2.8. Heat is transfer from the gases to water while passing through the annulus to generate the steam. The mixture of water and steam formed in the water tube is passed to the separator which is design so that the mixture enters with spiral flow. Due to the centrifugal force, heavier water particles are thrown outward on the wall which separates the steam from water. 8-6

69 HIGH PRESSURE BOILER The gases coming out from the annulus at the top is further passed over the super-heater to super heat the steam. The gases coming out from the super-heater is passed through the turbine to utilize the kinetic energy of the gases. The power output of the turbine is used to run the compressor. The exhaust gases coming out from the turbine are passed through the economizer to utilize the heat of exhaust gases. The extra power required to drive the compressor is supplied with the help of electric motor. Advantages a) Very high rate of combustion is possible. b) It is very compact and has greater flexibility. c) Low excess air is required as pressurized air is used and draught problem is simplified. d) It can be quickly started from the cold. Super Critical Boiler: The efficiency of the plant can be increased by increasing the pressure of the steam. So in the modern power plants super critical boilers are used. Boiler which operates above the critical pressure is called super critical boiler. Note:- Critical state: State of a substance beyond which there is no clear distinction between the liquid and gaseous phase A point where saturated liquid and dry saturated vapour lines meet so that latent heat is zero, is called Critical Point (figure 8.6). For water, Critical Point is given by: Pressure: 221 bar Temperature: C Fig. 8.6 T-S Diagram of Rankine Cycle Super critical boiler is also called once through boiler as the water is converted into superheated steam in single continuous pass. It does not required steam separating drum. Design consideration of super critical boiler a) Above super critical pressure, there is no density difference between the steam and water so forced circulation is necessary. 8-7

70 HIGH PRESSURE BOILER b) There is no steam drum so no blow down therefore extremely pure water must be used for which require high quality water treatment plant. c) Boiler tubes must be made of high strength austenitic steels or super alloys to withstand high temperature. d) Normally 3 steps of feed heating are required due to high pressure and to avoid excessive moisture at turbine exhaust. Advantages a) The heat transfer rate is considerable large compare to sub critical boiler. b) The pressure level is more stable due to less heat capacity of the boiler therefore give better response. c) Higher thermal efficiency of power plant can be achieved. d) The problems of erosion and corrosion are minimized in as two phase mixture does not exist. e) The turbo generators connected to super-critical boilers can generate peak loads by changing the pressure of operation. f) It gives better response to load fluctuation. Disadvantages a) Feed pump is necessary. b) More reheats are required, hence increased complexity of the plant and maintenance. c) High capital involved. Super-charged boiler: In the super-charged boiler combustion of the fuel is carried out under the high pressure. Construction and working The arrangement of the different components is shown in figure Air from the atmosphere is supplied to combustion chamber at high pressure with the help of the compressor. In the combustion chamber, combustion is carried out under the high pressure. The exhaust gases from the combustion chamber are used to run the gas turbine as they are exhausted at high pressure and the power produced by the gas turbine is used to run the compressor to compressor the air. The exhaust gases from the turbine are further used to preheat the feed water in the economizer. Advantages a) High heat transfer rate can be achieved as combustion is carried under the high pressure. b) Rapid start of the boiler is possible as the boiler is compact. c) It gives the better response to load fluctuation due to small heat storage capacity. d) The part of the gas turbine output can be used to drive other auxiliaries. Disadvantages a) The tightness of high pressure gas passage is essential b) Capital cost of the boiler is high 8-8

71 HIGH PRESSURE BOILER Fig. 8.7 Super charged Boiler 8-9

72 HIGH PRESSURE BOILER Exercise 1. Draw the neat sketch of Schmidt-Hartmann boiler. 8-10

73 HIGH PRESSURE BOILER 2. Draw the neat sketch of Benson boiler. 8-11

74 HIGH PRESSURE BOILER 3. Draw the neat sketch of La Mont boiler. 8-12

75 PULVERIZING MILL EXPERIMENT Objective: To study of different types of Pulverizing Mill. 2. Introduction: Pulverize: Pulverizes are devices that are used to produce coal in the powder form. They are also called as pulverizing mills. The pulverizing process consists of three stages namely a) Feeding: Feeding system controls automatically air required for drying and transporting pulverized fuel to the burner depending on the boiler demand. For pulverization of coal has to be dry and dusty. b) Drying: Dryer are an integral part of the pulverizing equipment. For drying coal part of primary air passing through the air preheater at 350 0C is utilized. c) Grinding: The third stage of pulverization process is the grinding and equipment used for this action is known as the grinding mill. 3. Advantages and Dis advantages: Advantages of Pulverized Coal Firing: a) The main advantage of pulverized firing system lies in the fact that by breaking a given mass of coal into smaller pieces exposes more surface area for combustion. b) Greater surface area of coal per unit mass of the coal allows faster combustion as more coal surface is exposed to heat and oxygen. This reduces the excess air required to ensure complete combustion and the required fan power. c) Wide variety and low grade coal can be burnt more easily when the coal is pulverized d) Pulverized coal gives faster response to load changes as the rate of combustion can be controlled easily and immediately. Automatic control applied to pulverized coal fired boilers is effective in maintaining an almost constant steam pressure under wide load variations e) This system is free from clinker and slagging troubles. f) This system works successfully with or in combination with the gas and oil. g) It is possible to use highly pre-heated secondary air (350 o C) which helps in rapid flame propagation. h) The pulverized system can be repaired easily without cooling the system as the pulverizing equipment is located outside the furnace. i) Large amount of heat release is possible in this system compared to stoke firing system. j) The banking losses are low compared to stoke firing system k) The boiler can be started from cold very rapidly and efficiently. This is highly important when grid stability is of the important concern 9-1

76 PULVERIZING MILL l) The external heating surface is free from corrosion and fouling as smokeless combustion is possible m) There are no moving parts in the furnace or boiler subjected to high temperature. Therefore the life of the pulverized fuel firing system is more and operation is troubleless n) Practically no ash handling problem in this type of firing system o) The furnace volume required is considerably less as the use of the burners which produce turbulence in the furnace makes it possible to complete combustion with minimum travel Disadvantages of Pulverized Coal firing: a) The capital cost of the pulverized coal firing system is considerably high as it requires many additional auxiliary equipment. Its operation cost is also high compared to stoke firing system. b) The system produces fly ash (fine dust) which requires special and costly fly-ash removal equipment as electrostatic precipitators. c) The flame temperatures are high and the conventional types of refractory lined furnaces are not inadequate. It is always necessary to provide water cooled walls for the safety of the furnace. The maintenance cost is also high as working temperature is high which causes rapid deterioration of the refractory surface of the furnace. d) The possibility of the explosion is more as coal burn like gas. e) The storage of powdered coal requires special attention and high protection from the fire hazards. 4. Types of Pulverizing Mill Four different types of pulverizing mills are used. a) Ball and race mill b) Bowl mill c) Ball mill d) Hammer mill Ball and race mill: This is also known as the contact mill. The coal is crushed between two moving surfaces ball and race. The upper race is stationary and the lower race is driven by worm and gear, holds the steel balls between them. The coal is allowed to fall on the inside of the race from feeder or hopper. Moving balls and race catches coal between them to crush in to a powder. Springs are used to hold down the upper race and adjust the force needed for crushing. Hot air supplied picks up the coal dust as it flows between the ball and races and then enters in to the classifier, moving and fixed vanes make the entering air to form a cyclonic flow which helps to through the oversized particles on to the wall of classifier. The oversized particles slide down for further grinding in the mill. 9-2

77 PULVERIZING MILL The coal particles of required size carried to burners with air from the top of the classifier. Fig. 9.1 Ball and race mill Bowl mill: The bowl mill grinds the coal between a whirling bowl & rollers mounted on pivoted axis. The pulverizer consists of stationary rollers and power driven balls in which pulverization takes place as the coal passes between the bowl and rollers. The hot primary air supplied in to the bowl picks up coal parcels and passes through the classifier. Where oversized coal particles falls back to bowl for further grinding. The required size coal particles along the primary air supplied to the burner. 9-3

78 PULVERIZING MILL Fig. 9.2 Bowl mill Ball Mill with double classifier: The line diagram of the ball mill is as shown in figure 9.3. It consist of a large cylinder partly filled with varying sized steel balls. The coal from coal hopper fed in to the cylinder with the help of crew conveyor. At the same time required quantity of hot air from air preheater is also enters. As the cylinder rotates pulverization takes place between the balls and the coal. The stream of hot air picks up the pulverized coal and pass through the classifier. The oversized coal particles thrown out of the air stream in the classifier and fine coal particles are passed to the burner through exhaust fan. Ball mill capable of pulverizing 10 tons of coal /hr containing 4% moisture requires 28 tons of steel balls and consumes KWh energy per ton of coal pulverized. 9-4

79 PULVERIZING MILL Fig. 9.3 Ball mill Hammer mill: The hammer mills have swinging hammers connected to an inner ring and placed within the rotating drum. The coal to be pulverized is fed in to the path of hammers. Grinding is done by the combination of impact on large particles and attrition on small particles. The hot air is supplied to dry the coal as well as carrying coal particles to burners. It is compact low in cost and simple in operation. However its maintenance is costly and its capacity is limited. The power consumption is high when fine powder is required. Fig. 9.4 Hammer mill 9-5

80 PULVERIZING MILL Exercise 1. Draw the neat sketch of Bowl pulverizing mill. 9-6

81 PULVERIZING MILL 2. Draw the neat sketch of Unit pulverized coal handling system. 9-7

82 PULVERIZING MILL 3. Draw the neat sketch of (i) Long flame burner, (ii) Turbulent burner, and (iii) Tangential burner. 9-8

83 CONDENSERS AND COOLING TOWERS EXPERIMENT Objective: To study of condenser and cooling tower 2. Introduction: In systems involving heat transfer, a condenser is a device or unit used to condense a substance from its gaseous to its liquid state, by cooling it. In so doing, the latent heat is given up by the substance, and will transfer to the condenser coolant. Condensers are typically heat exchangers which have various designs and come in many sizes ranging from rather small (hand-held) to very large industrial-scale units used in plant processes. For example, a refrigerator uses a condenser to get rid of heat extracted from the interior of the unit to the outside air. Condensers are used in air conditioning, industrial chemical processes such as distillation, steam power plants and other heat-exchange systems. Use of cooling water or surrounding air as the coolant is common in many condensers A cooling tower is a heat rejection device which rejects waste heat to the atmosphere through the cooling of a water stream to a lower temperature. Cooling towers may either use the evaporation of water to remove process heat and cool the working fluid to near the wet-bulb air temperature or, in the case of closed circuit dry cooling towers, rely solely on air to cool the working fluid to near the dry-bulb air temperature. Common applications include cooling the circulating water used in oil refineries, petrochemical and other chemical plants, thermal power stations and HVAC systems for cooling buildings. The classification is based on the type of air induction into the tower: the main types of cooling towers are natural draft and induced draft cooling towers. Cooling towers vary in size from small roof-top units to very large hyperboloid structures that can be up to 200 meters (660 ft.) tall and 100 meters (330 ft.) in diameter, or rectangular structures that can be over 40 meters (130 ft.) tall and 80 meters (260 ft.) long. The hyperboloid cooling towers are often associated with nuclear power plants, although they are also used in some coal-fired plants and to some extent in some large chemical and other industrial plants. Although these large towers are very prominent, the vast majority of cooling towers are much smaller, including many units installed on or near buildings to discharge heat from air conditioning. 3. Stages in Condensation & Condensation Process: a) De-superheating of the hot gas b) Condensing of the gas to liquid state and release of the latent heat. c) Sub-cooling of the liquid refrigerant. Sub-cooling only occupies a small portion of condenser s surface area. Therefore, an average heat transfer coefficient is used for the whole condenser s surface area, and the condensation is assumed to occur at the condensing temperature. 10-1

84 CONDENSERS AND COOLING TOWERS When saturated vapour comes in contact with a surface having a temperature below the saturation temperature, condensation occurs. There are two types of condensation: a) Film-wise condensation: condensed liquid wets the surface and forms a film covering the entire surface. b) Drop-wise condensation: surface is not totally wetted by the saturated vapour, and the condensate forms liquid droplets that fall from the surface. Compared to film-wise condensation, drop-wise condensation has a greater surface heattransfer coefficient as it has a greater area exposed to the saturation vapour. 4. Types of Condensers Condensers may be classified broadly into two major groups according to the manner in which the cooling water cools and condenses the exhaust steam; these are: a) Jet condensers, in which cooling water comes in direct contact with the exhaust steam and the steam as a result is condensed. The condensing or cooling water is usually sprayed into the exhaust steam so that rapid condensation of the steam occurs. b) Surface condensers, in which the cooling water and exhaust steam do not actually mix; the cooling water passes through a number of tubes while the exhaust steam passes over the outer surfaces of the tubes. Condensor Jet Condensor Surface Condensor Parallel Flow Counter Flow Ejector type Down Flow Type Central Flow Type Inverted Flow Type Regener ative Flow Type Evapora tive Flow Type Fig.10.1 Types of Condensers The most common type is a surface condenser which has the great advantage that the condensate (condensed steam) is not thrown to waste but is returned to the boiler through feed water system. A jet condenser is a much simpler and less costly apparatus than a 10-2

85 CONDENSERS AND COOLING TOWERS surface condenser. The jet condenser should be installed where a cheap source of boiler feed water is available. Jet Condenser The exhaust steam and water come in direct contact with each other and temperature of the condensate is the same as that of cooling water leaving the condenser. The cooling water is usually sprayed into the exhaust steam to cause, rapid condensation. Parallel-Flow type of Jet Condenser The exhaust steam and cooling water find their entry at the top of the condenser and then flow downwards and condensate and water are finally collected at the bottom. The baffle plate is provided in it ensures the proper mixing of the steam and cooling water. Fig Low level Parallel-Flow type of Jet Condenser Counter-Flow type Jet Condenser The steam and cooling water enter the condenser from opposite directions. Generally, the exhaust steam travels in upward direction and meets the cooling water which flows downwards. a. Low Level Jet Condenser Figure Shows, L, M and N are the perforated trays which break up water into jets. The steam moving upwards comes in contact with water and gets condensed. The condensate and water mixture is sent to the hot well by means of an extraction pump and the air is removed by an air suction pump provided at the top of the condenser. 10-3

86 CONDENSERS AND COOLING TOWERS b. High Level Jet Condenser Fig Low level Counter-Flow type of Jet Condenser It is also called barometric condenser. In this type the shell is placed at a height about meters above hot well and thus the necessity of providing an extraction pump can be obviated. However provision of own injection pump has to be made if water under pressure is not available. 10-4

87 CONDENSERS AND COOLING TOWERS Fig High level Counter-Flow type of Jet Condenser Ejector Condenser Flow type Jet Condenser Here the exhaust steam and cooling water mix in hollow truncated cones. Due to this decreased pressure exhaust steam along with associated air is drawn through the truncated cones and finally lead to diverging cone. In the diverging cone, a portion of kinetic energy gets converted into pressure energy which is more than the atmospheric so that condensate consisting of condensed steam, cooling water and air is discharged into the hot well. The exhaust steam inlet is provided with a non-return valve which does not allow the water from hot well to rush back to the engine in case a failure of cooling water supply to condenser. Fig Ejector Condenser Surface Condensers Invariably, almost all the steam power plants employ surface condenser. Surface condensers may be sub-divided into: a) Surface condenser in which exhaust steam passes over a series of tubes through which the cooling water is flowing. b) The evaporative surface condenser in which exhaust steam passes through a series of tubes and water is allowed to flow in the form of thin film outside the tubes while air passes upwards outside the tubes. Double Pass Surface Condenser (Two-flow surface condenser) A surface condenser, as illustrated in fig. 10.6, consists of a cast iron shell, cylindrical in shape and closed at each end to form a water box. A tube plate is located between each cover head and the shell. A number of water tubes are fixed to the tube plates. 10-5

88 CONDENSERS AND COOLING TOWERS The exhaust steam from the engine enters at the top of the condenser and is condensed by coming in contact with the cold surface of the tubes through which cooling water is being circulated. The cooling water enters at one end of the tubes situated in the lower half of the condenser and after flowing to the other end returns in the opposite direction through the tubes situated in the upper half of the condenser. The resulting water from the condensation of the exhaust steam and the air associated with the uncondensed water vapour, are extracted from the bottom of the condenser where the temperature is the lowest, so that the work of the wet air pump is reduced. The surface condenser of this type requires two pumps, namely, wet air pump to remove air and condensate, and a water circulating pump to circulate the cooling water under pressure through the tubes of the condenser. Steam driven reciprocating pumps are used, but electric driven centrifugal pumps are used very extensively (commonly) for circulating water and condensate removal. Steam ejectors are also sometimes used for air removal. Fig Double Pass Surface Condenser Surface condensers may be classified as, two-flow or multi-flow condensers. Surface condenser, as illustrated in fig. 10.6, is a two-flow condenser because the circulating water traverses (travels) the whole length of the condenser twice. By introducing more partitions in the water boxes, the condenser may be converted into a three-flow condenser or even four-flow condenser. The velocity of cooling should be increased with the increase of number of flows. The rate of transmission of heat through the tubes to the circulating water, increases with the increase of number of flows, but the power required to circulate the water is increased. Surface condensers: According to the direction of flow of exhaust steam a. Down flow condenser Figure 10.7 shows a sectional view of a down flow condenser. The exhaust team enters at the top and flows downwards over the tubes through which the cooling water is flowing. The exhaust steam as a result is condensed and the condensate is extracted from the bottom by the condensate extraction pump. The cooling water enters at one end of the tubes situated in the lower half of the condenser and after flowing of the other end returns 10-6

89 CONDENSERS AND COOLING TOWERS in the opposite direction through the tubes situated in the upper half of the condenser. The temperature of condensate, therefore, decreases as the exhaust steam passes downwards, and hence partial pressure of steam decreases from top to bottom of the condenser. Fig down flow condenser The air exit is shielded from the downstream of the condensate by means of a baffle plate, and thus air is extracted with only a comparatively small amount of water vapour. As the air passes downwards, it is progressively cooled and becomes denser (partial pressure of air increases) and hence it is extracted from the lowest convenient point. In a condenser of this type, therefore, the partial pressure of steam decreases, the partial pressure of air correspondingly increases, as the mixture passes from top to the bottom of the condenser. The result of all these effects is that the condensate temperature falls below the exhaust steam temperature which enters at the top. Thus, by cooling the air, the capacity of the air pump is considerably reduced. b. Central Flow Surface Condenser In Central Flow Surface Condenser (fig. 10.8), the suction pipe of the air extraction pump is placed in the centre of the tubes nest; this causes the condensate to flow radially towards the centre as shown by the arrows in the figure. The condensate leaves at the bottom where the condensate extraction pump is situated. The air is withdrawn from the centre of the nest of tubes. This method is an improvement on the down flow type as the exhaust steam is directed radially inward by a volute casing around the tubes nest; it has thus access to the whole periphery of the tubes. Fig Central Flow Surface Condenser 10-7

90 CONDENSERS AND COOLING TOWERS c. Evaporative condenser Where the supply of cold water is extremely limited, the evaporative condenser is the only suitable type which can be run on a minimum supply of cooling water, and even without cooling water in cold weather and on light loads. Exhaust steam from the engine is exhausted into a coil of grilled pipes or series of tubes, the outlet of which is connected to the wet air pump (fig. 10.9). Cooling water is allowed to flow in a thin film over the outside of the tubes. A natural or forced air current causes rapid evaporation of this film of water. The effect of this is that not only the steam inside the tubes is condensed but some of the cooling water is also evaporated on the outside of the tubes. The process of evaporation cools the water. The film of water on the outside of the tubes is maintained by allowing water to trickle (fail) over them continuously. The water which is not evaporated falls into an open tank or collecting tank under the condenser, from which it can be drawn by circulating water pump and used over again. The evaporative condenser is placed outside in the open air. On account of nuisance which would result from the production of clouds of steam, this type of condenser is restricted to small power plants. Fig Evaporative condenser 5. Principle of Operation for Cooling Towers The principle of operation of cooling towers is very similar to that of the evaporative type of condensers, in which the warm water gets cooled by means of evaporation. Water evaporates as a result of the hot water droplet coming in contact with the air (which is being pumped out by means of a fan). This evaporating water also absorbs the latent heat from the water surrounding it. By losing latent heat, the water is cooled. 6. Types of Cooling Tower According to the method adopted to circulate the air, cooling towers may be classified as: a) Natural draft cooling towers b) Mechanical draft cooling towers. 10-8

91 CONDENSERS AND COOLING TOWERS Natural Draft Cooling Tower As the name indicates, the air is circulated inside the cooling tower by natural convection. The natural draft cooling towers are further classified as: a. Natural draft cooling towers spray type b. Natural draft cooling towers splash deck type SPRAY TYPE The entire system is housed inside a box-shaped structure which also accommodates spray headers, spray nozzles, and louvers. The louvers (usually made of steel) are placed on the sides to enhance natural circulation of air inside the cooling tower. To prevent the carryover of water droplets to the atmosphere, the louvers are slanted towards the inside. Usually these types of cooling towers are located outside the building, so that the air can pass freely through the tower. The fig shown below explains about the spray type of cooling tower. Fig Natural draft cooling tower spray type Warm water from the condenser is fed to the spray header by means of a pump. The spray header is located on top as shown in the sketch. The spray nozzles spray the warm water inside the tower. Air from the atmosphere comes in contact with the warm water, thereby causing some water droplets to evaporate. The evaporating water also absorbs some amount of latent heat from the surrounding water, which causes the remaining water to cool. The passing air also absorbs some amount of sensible heat from the warm water. A make-up line, which may be controlled by a simple float, may be used to make up the loss of water due to evaporation. The cooled water may be then taken back to the condenser. The size of the spray plays a vital role. If the spray is too fine, a greater amount of water will be taken away by the air. On the other hand, if the size of the spray is too large, the area of contact of water with the air will be reduced. 10-9

92 CONDENSERS AND COOLING TOWERS SPLASH DECK TYPE This type of cooling tower is very similar to that of the spray type. Instead of a spray header, a water box is used. The water box has small holes at the bottom. It also contains decking inside the tower. The hot water from the condenser enters into the water box and splashes via holes in the water box on the decking. The main objective of the decking is to increase the surface area of contact of air with the warm water. This type of cooling tower is 20-30% more effective than the spray type. Mechanical Draft Cooling Towers The mechanical draft cooling towers are very much similar to that of the natural draft cooling towers. As the name indicates, air is circulated inside the tower mechanically instead of natural circulation. Propeller fans or centrifugal fans may be used. According to the location of the fan, they are further classified as: a. Forced draft cooling towers, and b. Induced draft cooling towers. Forced Draft Cooling Towers In this system, fan is located near the bottom and on the side. This fan forces the air from bottom to top as shown in fig The hot water from the condenser is supplied at the top of the cooling tower which is sprayed through series of spray nozzles. Around 3-5 % make up water is added to the pond to compensate the water lost due to evaporation. A drift eliminator is used to prevent loss of water droplets along with the forced air. Fig Forced draft cooling tower 10-10