ME6701 – POWER PLANT ENGINEERING INTERNAL ASSESMENT EXAMINATIONS – I PART-A 1. The alternator is used in a power plant which converts....... (a) Electrical (b) Electrical (c) Mechanical (d) Mechanical Energy into energy into Energy into Energy into Mechanical Solar Energy Electrical Nuclear Energy Energy Energy 2. For forced draught system, the function of chimney is mainly........ (a) To produce (b) To discharge (c) To reduce the (d) None of the draught to gases high up temperature of above accelerate the in the the hot gases combustion of atmosphere to discharged fuel avoid hazard 3. Pulverized fuel is used for....... (a) Better (b) Saving fuel (c) Obtaining fuel (d) For economy burning 4. Ideal ‘Rankine Cycle’ is a __________ process. (a) Reversible (b) Irreversible (c) Both of the (d) None of the mentioned mentioned 5. What is the most preferable dryness fraction of the exhaust steam? (a) 0.99 (b) 0.77 (c) 0.66 (d) 0.88 6. The vaccum obtainable in a condenser is dependent upon......... (a) Capacity of (b) Quantity of (c) Any of the two (d) Temperature ejector steam to be is possible of cooling handled water 7. Fluidized bed combustion helps to reduce (a) boiler size (b) pollution (c) both (A) and (d) None of the (B) above 8. The Otto cycle consists of (a) two constant (b) two constant (c) two constant (d) none of the pressure pressure and volume mentioned processes and two constant processes and two constant entropy two constant volume processes entropy processes processes 9 Which of the following is true for the Brayton cycle? (a) first sir is (b) heat is added (c) air expands in (d) all of the compressed reversibly at turbine mentioned reversibly and constant reversibly and adiabatically pressure adiabatically 10. The diesel plants are mainly used ________ (a) As peak load (b) As base load (c) As standby (d) Both peak and plants plants power plants stand by plants PART-B Define steam and heat rate. Steam is water in the gas phase, which is formed when water boils or evaporates. Steam is invisible; however, "steam" often refers to wet steam, the visible mist or aerosol of water droplets formed as this water vapour condenses. If heated further it becomes superheated steam. 11. Heat rate is the common measure of system efficiency in a steam power plant. It is defined as "the energy input to a system, typically in Btu/kWh, divided by the electricity generated, in kW." Mathematically: Efficiency is "a ratio of the useful energy output by the system to the energy input to the system." Why thermal power plants are not suitable for fluctuating loads? All power plants have some measure of response time. Plants with boilers or nuclear 12. reactors might be more like hours, because the boiler has to take more fuel in, then heat more water into steam and that mostly takes time. Give the example for once through boiler Once-through boilers are generally associated with high pressure operation and the feed 13 water enters at high sub-critical >180 bar or supercritical pressure whilst superheated steam leaves at a pressure some 20–30 bar lower. Differentiate stocker firing and pulverized fuel firing. 14 Pulverized Fuel Firing Due to pulverization, the surface much area of coal becomes larger, and in this method air required for combustion is much less. As the quantity of required air and fuel both are less, loss of heat in this method of boiler firing is much less. A mechanical stocker is a mechanical system that feeds solid fuel like coal, coke or anthracite into the furnace of a steam boiler. They are common on steam locomotives after 1900 and are also used on ships and power stations. Characterize compounding of steam turbines. steam's pressure drop and it will increase its velocity. This high-velocity strikes the turbines rotor and the speed of the rotor becomes high. Compounding of steam 15 turbine is used to reduce the rotor speed. It is the process by which rotor speed come to its desired value Outline the quality of steam. A steam quality of 0 indicates 100 % liquid, (condensate) while a steam quality of 100 16 indicates 100 % steam. One (1) lb of steam with 95 % steam and 5 % percent of liquid entrainment has a steam quality of 0.95. Define boiler mountings and accessories. Boiler mounting are the components generally mounted on the surface of he boiler to have safety during operation Safety valve: it is a mechanical device used to safeguard 17 the boiler, in case the pressure inside the boiler rises above its normal working atmosphere List any four applications of diesel power plant. Diesel power plant is used for electrical power generation in capacities ranging from 100 to 5000 H.P. 18 They are commonly used for mobile power generation and are widely used in transportation systems consisting of railroads, ships, automobiles, and airplanes. They can be used as standby power plants. Define cut-off ratio and expansion ratio. The cutoff ratio is the ratio of the volume after combustion to the volume before 19 combustion. The compression ratio is the ratio of the maximum volume to the minimum volume. Efficiency goes up with compression ratio and down with cutoff ratio. Mention the major differences between Otto and Diesel cycle. Otto cycle is used for petrol or spark ignition engine while diesel cycle is used for diesel or compression ignition engine. The main difference between Otto 20 cycle and Diesel cycle is that in Otto cycle heat addition takes place at constant volume and in diesel cycle heat addition takes places at constant pressure. PART-C 21. Draw a Rankine cycle for a coal fired and steam thermal power plant. State the various means of a increasing the efficiency of the plant. The Rankine cycle closely describes the process by which steam-operated heat engines commonly found in thermal power generation plants generate power. Power depends on the temperature difference between a heat source and a cold source. The higher the difference, the more mechanical power can be efficiently extracted out of heat energy, as per Carnot's theorem. The heat sources used in these power plants are usually nuclear fission or the combustion of fossil fuels such as coal, natural gas, and oil, or concentrated solar power. The higher the temperature, the better. The efficiency of the Rankine cycle is limited by the high heat of vaporization of the working fluid. Also, unless the pressure and temperature reach super critical levels in the steam boiler, the temperature range the cycle can operate over is quite small: steam turbine entry temperatures are typically around 565 °C and steam condenser temperatures are around 30 °C. This gives a theoretical maximum Carnot efficiency for the steam turbine alone of about 63.8% compared with an actual overall thermal efficiency of up to 42% for a modern coal-fired power station. This low steam turbine entry temperature (compared to a gas turbine) is why the Rankine (steam) cycle is often used as a bottoming cycle to recover otherwise rejected heat in combined-cycle gas turbine power stations. The cold source (the colder the better) used in these power plants are usually cooling towers and a large water body (river or sea). The efficiency of the Rankine cycle is limited on the cold side by the lower practical temperature of the working fluid. The working fluid in a Rankine cycle follows a closed loop and is reused constantly. The water vapor with condensed droplets often seen billowing from power stations is created by the cooling systems (not directly from the closed-loop Rankine power cycle). This 'exhaust' heat is represented by the "Qout" flowing out of the lower side of the cycle shown in the T–s diagram below. Cooling towers operate as large heat exchangers by absorbing the latent heat of vaporization of the working fluid and simultaneously evaporating cooling water to the atmosphere. While many substances could be used as the working fluid in the Rankine cycle, water is usually the fluid of choice due to its favorable properties, such as its non-toxic and unreactive chemistry, abundance, and low cost, as well as its thermodynamic properties. By condensing the working steam vapor to a liquid the pressure at the turbine outlet is lowered and the energy required by the feed pump consumes only 1% to 3% of the turbine output power and these factors contribute to a higher efficiency for the cycle. The benefit of this is offset by the low temperatures of steam admitted to the turbine(s). Gas turbines, for instance, have turbine entry temperatures approaching 1500 °C. However, the thermal efficiency of actual large steam power stations and large modern gas turbine stations are similar. The four processes in the Rankine cycle T–s diagram of a typical Rankine cycle operating between pressures of 0.06 bar and 50 bar. Left from the bell-shaped curve is liquid, right from it is gas, and under it is saturated liquid–vapour equilibrium. There are four processes in the Rankine cycle. The states are identified by numbers (in brown) in the T–s diagram. Process 1–2: The working fluid is pumped from low to high pressure. As the fluid is a liquid at this stage, the pump requires little input energy. In other words Process 1-2 is [Isentropic compression in pump] Process 2–3: The high-pressure liquid enters a boiler, where it is heated at constant pressure by an external heat source to become a dry saturated vapour. The input energy required can be easily calculated graphically, using an enthalpy–entropy chart (h–s chart, or Mollier diagram), or numerically, using steam tables.