JNTU
JNTU B.Tech III Year THERMAL ENGINEERING - II Previous Question Paper (February - 2022 - R18)
Answer any Five Questions. All Questions Carry Equal Marks.
Main
With the help of neat sketch explain the working of Rankine Cycle and write the methods to improve the cycle efficiency.
Answer
The Rankine cycle is the theoretical cycle for steam power plants. It consists of four processes: 1-2: Isentropic compression of water in a pump, 2-3: Constant pressure heat addition in a boiler to produce steam, 3-4: Isentropic expansion of steam in a turbine to generate work, and 4-1: Constant pressure heat rejection in a condenser. The cycle is represented on a T-s diagram where the area under the curve 2-3 represents heat added and the area under 4-1 represents heat rejected.
Methods to improve Rankine cycle efficiency include:
- Increasing the boiler pressure, which raises the average temperature of heat addition.
- Superheating the steam to higher temperatures, which increases the mean temperature of heat addition and reduces moisture content at the turbine exhaust.
- Reducing the condenser pressure (vacuum), which increases the expansion range and work output.
- Implementing regeneration (feedwater heating) to increase the temperature of water entering the boiler.
- Employing reheat cycles to allow for higher pressures without excessive moisture in the later turbine stages.
A boiler generates 13000 kg of steam at 7 bar during a period of 24 hrs and consume 1250 kg of coal whose CV = 30000 kJ/kg. Taking the enthalpy of steam coming out of boiler = 2507.7 kJ/kg and water is supplied to the boiler at 40°C. Find (i) the efficiency of the boiler (ii) equivalent evaporation per kg of coal.
Answer
Given: Mass of steam (m_s) = 13000 kg/24 hrs = 541.67 kg/hr; Mass of coal (m_c) = 1250 kg/24 hrs = 52.08 kg/hr; CV = 30000 kJ/kg; h_s = 2507.7 kJ/kg; h_w = c_p * T = 4.187 * 40 = 167.48 kJ/kg.
Heat supplied by coal per hour = m_c * CV = 52.08 * 30000 = 1562400 kJ/hr.
Heat absorbed by steam per hour = m_s * (h_s - h_w) = 541.67 * (2507.7 - 167.48) = 1267858.5 kJ/hr.
(i) Boiler Efficiency = (Heat absorbed / Heat supplied) * 100 = (1267858.5 / 1562400) * 100 = 81.15%.
(ii) Equivalent Evaporation (E_e) = m_s * (h_s - h_w) / (m_c * 2257) = 1267858.5 / (52.08 * 2257) = 10.78 kg of steam per kg of coal.
What are the functions of boiler mountings and accessories? Explain any one mountings and accessories with neat line sketch.
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Derive the expression for critical pressure ratio in terms of index of expansion.
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Steam having a pressure of 10.5 bar 0.95 dryness is expanded through a convergent-divergent nozzle and the pressure of steam leaving the nozzle is 0.85 bar. Find the velocity at throat for maximum discharge conditions. Index of expansion may be assumed as 1.135. Calculate the mass flow rate of steam through the nozzle?
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What is meant by over expansion and under expansion?
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In a steam nozzle, the steam expands from 4 bar to 1 bar. The initial velocity is 60 m/s and the initial temperature is 200°C. Determine the exit velocity if the nozzle efficiency is 92%?
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Steam enters the blade row of an impulse turbine with a velocity of 600 m/s at an angle of 25° to the plane of rotation of blades. The mean blade speed is 250 m/s. The blade angle on the exit side is 30°. The blade friction coefficient is 10%. Determine the (a) Blade angle at inlet (b) work done per kg of steam (c) Diagram efficiency (d) axial thrust per kg of steam per second?
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250 kg/min of steam (3 bar, 0.97 dry) flows through a given stage of reaction turbine. The exit angle of fixed blades as well as moving blades is 20° and 37.6 kW of power is developed. If the rotor speed is 380 rpm and the tip leakage is 5 percent, calculate the mean drum diameter and the blade height. The axial flow velocity is 0.8 times the blade velocity.
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Define the term Degree of Reaction used in Reaction Turbines. Prove that moving and fixed blades should have the same shape for 50% reaction.
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Discuss the merits and demerits of surface condensers over jet condensers.
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In gas turbine plant, the compressor takes air at 15°C and compresses with pressure ratio of 4 with isentropic efficiency 82%. Then the air is heated in the heat exchanger using 75% of the available heat with exhaust gases and then heated in combustion chamber to 600°C. Isentropic efficiency of turbine is 70%. Taking the properties of air and gases same, find work developed per kg of air flow and thermal efficiency of the cycle; Take effectiveness of heat exchanger as 0.75?
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Discuss in detail the needs and demands met by Turbo jet.
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How are rockets classified? Explain in detail.
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