PESIT-Bangalore South Campus

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1 USN PESIT Bangalore South Campus Hosur road, 1km before Electronic City, Bengaluru Department of Mechanical Engineering INTERNAL ASSESSMENT TEST 2 Date : 03/04/2018 Max Marks : 50 Subject & Code : Power Plant Engineering & 10ME833 Name of faculty : SK/SS/TA Sec : A, B,C Time : 90min Note: Answer any FIVE full questions selecting at least TWO questions from each part PART A Marks 1. With the help of schematic diagrams, explain Natural draught and mechanical draught systems of chimney? Why a balanced draught is preferred over other draught systems? Draught produced by a chimney is 2 cm of water column. Temperature of flue gas is C and ambient temperature is 33 0 C. The flue gases formed per kg of fuel burned is 24kg. Neglect the losses and take the diameter of the chimney as 1.75m. Calculate height of the chimney and mass of flue gases flowing through the chimney. 3. Explain the classification of diesel power plants Explain the working of a gas turbine with intercooler, reheat and regenerator with a T-S diagram 10 PART B 5 With neat sketches explain radiant type super heater and convection type super heater. What is the advantage of using a radiant-convection (combined) type super heater over these two? 10 6 List different system in a diesel power plant. Explain air intake system and starting system in detail 10 7 What is a cooling tower? Explain the working of an induced draft cooling tower with a neat diagram a What are the advantages of a forced draft cooling tower over induced draft cooling tower? 5 b What is cooling pond? Explain the working any one type of cooling pond with neat sketch 5 * * * * * * If you have finished answering the questions and there is still time left, you need not leave the hall in a hurry! Double check that your answers are correct and the representation is professionally outstanding. 10 B.E/8 th Semester PESIT-Bangalore South Campus

2 Q.1 Natural draught

3 Forced Draught

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8 Q.2

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10 Q.3 The diesel electric power-plants are chiefly used in the fields mentioned below. Peak load plant: The diesel plants are used in combination with thermal or hydro-plants as peak load plants. This plant is particularly preferable as peak load plant as it can be started quickly and it has no standby losses as in the case of thermal plants where boilers always must be kept hot. Mobile plants: Mobile diesel plants mounted on skids or trailers can be used for temporary or emergency purposes such as for supplying power to large civil engineering works for supplementing electricity supply systems that are temporarily short of power. Stand-by Units: This can be used as a standby unit to supply part load when required. For example, this can be used with hydro-plant as standby unit. If the water available is not sufficient due to reduced rainfall, a diesel station supply power in parallel with hydro-station. The use is made temporarily till the water is available to take the full load.

11 Emergency plant: The plants used for emergency purposes are at to standby units, normally idle but are used where power interruption would mean financial loss or danger in key industrial processes, tunnel lighting and operating rooms of hospitals. They are also used for telecommunication and water supply under emergency conditions. Nursery station: When the diesel plant is used to supply the power to a small town in the absence of main grid and which can be moved to another area which needs power on a small scale when the main grid is available is known as "Nursery Station". The main grid cannot extend to every corner of the country till there is enough load. Many times the extension of grid is not possible due to the constructional difficulties as in Assam. Diesel unit of small capacity can be installed to supply the load to a small town during the process of development and it can be removed to another required place till the main grid for tapping the power is available.

12 Starting stations: The diesel units are used to run the auxiliaries for starting the large steam plants. Central stations: This can be used as central station where the capacity required is small (5 to 10 MW). The limit is generally decided by the cost of the plant and local conditions regarding the availability of fuel and water, space requirements and non-availability of the grid. Small supply units for commercial purposes and public utilities e.g.,, cinema hall, hospital and municipalities are commonly used in practice.

13 Q.4 Physical arrangement of an ideal two-stage gas-turbine cycle with intercooling, reheating, and regeneration.

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15 Q.5 Radiant Type Super heater The radiant type of superheater receives its heat by radiation in the furnace area of the boiler. An increase in load on a boiler increases the rate of steam flow through the superheater tubes. To maintain a constant superheater temperature the heat input to the superheater must also increase. Since radiant heat is proportional to the furnace temperature, and the furnace temperature remains fairly constant with an increase in the number of fires or firing rate the amount of heat entering the Superheater per kg of steam flow will decrease. Therefore, with an increase in load with a radiant type Superheater, the outlet steam temperature decreases

16 Radiant Type Superheater Underdeck Superheater

17 Convection Type Superheater The convection type superheater is located in the path of the combustion gas flow and receives its heat from the convective flow of these hot combustion gases past the tubes. With an increase in the load the rate of steam flow through the superheater increases. To support the load increase more fuel is burned and more air is used, increasing the amount of combustion gases, and increasing the convective flow of heat to the superheater. This increase in the convection air flow is greater than the increase in steam flow, hence the amount of heat entering the superheater per pound of steam increases. Therefore, with the convection type superheater, an increase in load causes the outlet temperature of the superheater to increase.

18 Convection Type Superheater Overdeck Superheater

19 Radiant-Convection Type Superheater The radiant-convection type superheater receives its heat both by radiation and convection. For this reason it is located in flow path of the combustion gases near enough to the furnace so that it may also receive heat by radiation. By proper positioning of the superheater the best characteristics of the other types of superheaters may be blended. This results in a constant superheat temperature throughout the load range of the boiler. On certain type of multi-drum boilers a separately fired radiant-convection superheater may be employed.

20 Radiant-Convection Type Superheater Interdeck Superheater

21 Q.6 Plant Layout of DPP

22 The essential components of a Diesel Electric Plant are: Engine Engine fuel system Engine air intake system Engine exhausts system Engine cooling system Engine lubrication system Engine starting system.

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24 Air intake system The air intake system conveys fresh air through pipes to Air intake manifold of 4 stroke engine The scavenging pump inlet of a two stroke engine The supercharger inlet of a supercharged engine. Because a diesel engine requires close tolerances to achieve its compression ratio, and because most diesel engines are either turbocharged or supercharged, the air entering the engine must be clean, free of debris, and as cool as possible To improve a turbocharged or supercharged engine s efficiency, the compressed air must be cooled after being compressed.

25 Following precautions should be taken while designing air intake systems Air intake should be located outside the engine room. Air intake should not be located in confined places to avoid undesirable acoustic vibrations. Pressure drop in the air intake line should be minimum to avoid engine starvation Air filters should be accessible for periodic cleaning. In some cases a muffler may be introduced to prevent engine noise from reaching outside air.

26 Air intake systems are usually one of two types, wet or dry In wet system, the air is sucked or bubbled through a housing that holds a bath of oil such that the dirt in the air is removed by the oil in the filter. The air then flows through a screen-type material to ensure any entrained oil is removed from the air

27 In a Dry filter system, paper, cloth, or a metal screen material is used to catch and trap dirt before it enters the engine. In addition to cleaning the air, the intake system is usually designed to intake fresh air from as far away from the engine This provides the engine with a supply of air that has not been heated by the engine s own waste heat. The reason for ensuring that an engine's air supply is as cool as possible is that cool air is denser than hot air. This means that, per unit volume, cool air has more oxygen than hot air. Thus, cool air provides more oxygen per cylinder charge than less dense, hot air. More oxygen means a more efficient fuel burn and more power.

28 Starting Systems A functional machine needs a running engine, and if the engine doesn t crank, it doesn t start. A properly operating and reliable starting system is a must for keeping a machine productive. For many years, diesel engines have mostly used electric motors to crank them over to start the combustion process. For some applications, an air or hydraulic motor will create the torque needed to turn the engine over Different systems adopted for starting a diesel engine are Auxiliary Engine starting system Electric Motor Starting System Compressed Air Starting System The function of this system is to start the engine from cold by supplying compressed air at about 17 bar from an air tank It is admitted into few cylinders making them work like reciprocating air motors to run the engine shaft Fuel is then injected into remaining cylinders and ignited in the normal way causing the engine to start

29 Q.7 What is a cooling tower? It is used to cool the water used in the water cooled condenser of the refrigeration/power plant so that the same water can be reused to absorb the heat at the condenser Cooling towers are a special type of heat exchanger that allows water and air to come in contact with each other to lower the temperature of the hot water. Most cooling towers work based on the principle of evaporative cooling That hot water is pumped through pipes directly into the cooling tower. Cooling tower nozzles are used to spray the water onto to the fill media.

30 What is a cooling tower?

31 Fills allows the water flow down and exposes the maximum amount of water surface area possible for the best air-water contact When the air and water come together, a small volume of water evaporates, creating an action of cooling. The colder water gets pumped back to the process/equipment that absorbs heat or the condenser. It repeats the loop over and over again to constantly cool down the heated equipment or condensers.

32 Q.8.a Forced Draft Towers: The advantages of Forced Draft Towers over Induced Draft Towers are: This is more efficient than induced draft as some of the air velocity is converted into static pressure in the tower and recovered in the form of useful work. The vibration and noise are minimum as mechanical equipment are set on a solid foundation. As it handles dry air, problems of fan blade erosion are avoided. It is more safe as it is located on the ground level. Size of the fan will be small as we can accommodate number for fans in the same tower Maintenance is comparatively easy.

33 Q.8.b Cooling ponds Cooling ponds are used where sufficient land is available as an alternative to cooling towers. The pond receives thermal energy in the water from the plant s condensers and the energy is dissipated mainly through evaporation Once the water has cooled in the pond, it is reused by the plant. New water is added to the system ( make-up water) to replace the water lost through evaporation.

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37 . Types of Cooling Ponds: Natural and Directed Flow System. In natural flow system, water coming out from the condenser is just allowed to flow into the pond as shown in Fig.

38 In directed flow system, the hot water coming out of condenser enters the middle channel as shown in Figure On reaching the far end divides into two currents, being directed by the baffle walls so as to traverse the pond several times before uniting at the intake point. The water gets more time and passes over a more surface, so the cooling achieved is very effective.

39 Spray cooling pond is one of the simplest methods of cooling the condenser water although it is not efficient. Thousands of spray ponds were put into operation until cooling towers came into picture in 1920s Single Deck and Double Deck Systems: In a single deck system, spray nozzles are arranged at the same elevation

40 Cooling effect of single deck system is less than double deck system. In double deck system, spray nozzles are arranged at different elevations Its cooling effect is more than single deck system as water comes in contact with air at lower temperature.

41 Open and Louvre Fence: In case of open pond, the drift losses will be more if the wind velocity is high. This can be avoided by providing Louvre fence as shown in Figure.

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43 Disadvantages of cooling ponds are: The considerably large area for cooling is required. Spray losses due to evaporation and wind run high. There is no control over the temperature of cooled water. The cooling effect is reduced with the decrease in wind velocity and if the load on the plant increases, the pond does not respond to the requirement. It provides minimum cooling in absence of wind flow. The cooling efficiency is low compared with cooling tower. Cooling pond system is rarely used nowadays in modern thermal power plants of high capacity as it requires large surface area which is rarely available.