Test: Power Plant Level - 1 - Mechanical Engineering MCQ

• Melting point: At pressures below atmospheric, the melting point of water rises slightly. This is because at lower pressures, it requires more energy to overcome intermolecular forces and change the solid phase to liquid phase.


• Boiling point: At pressures below atmospheric, the boiling point of water drops markedly. This is because at lower pressures, the vapor pressure of the liquid becomes equal to the atmospheric pressure more quickly, leading to a lower boiling point.

• Definition: Carbonisation of coal is the process of strongly heating coal continuously for about 48 hours in the absence of air in a closed vessel.


• Process: The carbonisation process involves subjecting coal to high temperatures in the absence of oxygen, which causes the coal to break down and release volatile components.


• Result: This process results in the formation of coke, which is a porous carbon-rich material used in various industrial processes, such as iron and steel production.


• Importance: Carbonisation of coal is important as it helps in obtaining coke, which has high carbon content and is essential for the production of steel.


• Applications: Coke produced through carbonisation of coal is used in blast furnaces for the production of iron and steel, as well as in other industrial processes.

Coke. It is produced when coal is strongly heated continuously for 42 to 48 hours in the absence of air in a closed vessel. This process is known as carbonisation of coal. Coke is dull black in colour, porous and smokeless. It has a high carbon content (85 to 90%) and has a higher calorific value than coal.

• Definition: Heating of dry steam above its saturation temperature means increasing the temperature of the steam beyond the point where it is saturated and no longer in equilibrium with water.



• Superheating: This process is known as superheating, where the steam is heated to a temperature above its saturation temperature.


• Characteristics of superheated steam:
  
  
  
  • Superheated steam has higher energy content compared to saturated steam at the same pressure and temperature.
  
  
  • It is dry and has no moisture content.
  
  
  • Superheated steam is used in various industrial processes due to its higher energy content.
  
  
  
  


• Importance of superheating:
  
  
  
  • Superheating increases the efficiency of steam turbines and engines by providing more energy for work.
  
  
  • It helps in reducing erosion and corrosion in steam pipes and turbines by ensuring dry steam flow.
  
  
  
  


• Methods of superheating:
  
  
  
  • Steam can be superheated by passing it through a superheater, which is a device that increases the steam temperature.
  
  
  • Superheating can also be achieved by using a reheater in a steam power plant, where steam is heated again after passing through the turbine.
  
  
  
  

• Definition: Superheating of steam refers to the process of heating steam beyond its saturation temperature without changing its pressure.


• Constant pressure: When superheating steam, it is important to maintain a constant pressure to ensure that the steam remains in the gas phase throughout the process.


• Advantages: Superheating steam at constant pressure allows for increased energy efficiency in various industrial applications, as the superheated steam carries more energy per unit mass compared to saturated steam.


• Applications: Superheated steam is commonly used in power generation, heating systems, and industrial processes where high temperatures are required.


• Constant pressure superheating: This process is typically achieved by passing the saturated steam through a superheater, where it absorbs additional heat energy without any change in pressure.

• 1 kg.m: This is a unit of work or energy.


• Work or energy: The unit of work or energy is the Joule (J).


• Conversion: 1 Joule is equal to 1 Newton meter (Nm).


• Given: 1 kg.m


• Conversion factor: 1 kg = 9.81 N (acceleration due to gravity)


• Calculation: 1 kg.m = 1 kg * 1 m = 1 * 9.81 N * 1 m = 9.81 N.m = 9.81 J

• Sublimation region: The sublimation region is the region where solid and vapour phases are in equilibrium.


• Equilibrium: In this region, the rate of sublimation (solid to vapour) is equal to the rate of deposition (vapour to solid), resulting in a stable equilibrium state.


• Triple point: The triple point is the point on a phase diagram where solid, liquid, and vapour phases coexist in equilibrium. This is not the same as the sublimation region.


• Phase transitions: Sublimation is the process where a solid changes directly into a vapour without passing through the liquid phase. This occurs in the sublimation region.

• Definition: Stoichiometric quantity of air refers to the exact amount of air required for complete combustion of a fuel with no excess air present.



• Importance: It is crucial to determine the stoichiometric quantity of air to ensure efficient combustion and minimize emissions.



• Calculation: The stoichiometric air-fuel ratio can be calculated based on the chemical equation of the combustion reaction.



• Achieving stoichiometry: By providing the exact amount of air required, we can achieve complete combustion of the fuel without any leftover fuel or excess air.



• Optimum combustion: By using the stoichiometric quantity of air, we can achieve optimum combustion efficiency and reduce the formation of harmful emissions.

• 1 kg of steam sample contains 0.8 kg dry steam.

• Dryness Fraction = Mass of dry steam / Total mass of steam


• Dryness Fraction = 0.8 kg / 1 kg

• Dryness Fraction = 0.8

• The dryness fraction of the given steam sample is 0.8.

• Specific heat: Specific heat is the amount of heat required to raise the temperature of a unit mass of a substance by one degree Celsius.


• Superheated steam: Superheated steam is steam heated to a temperature higher than its boiling point at the absolute pressure where the temperature is measured.


• Specific heat of superheated steam: The specific heat of superheated steam is generally lower than that of water due to the absence of liquid water molecules in its composition.

• 0.1: This value is too low for the specific heat of superheated steam.


• 0.3: This value is also low for the specific heat of superheated steam.


• 0.5: This value is generally of the order of the specific heat of superheated steam, making it a suitable choice.


• 0.8: This value is higher than the usual specific heat of superheated steam.


• 1.0: This value is too high for the specific heat of superheated steam.

Therefore, the specific heat of superheated steam in kcal/kg is generally of the order of 0.5.

• Mollier Chart: A Mollier chart is a graphical representation of the thermodynamic properties of materials. It is commonly used in the field of engineering to analyze processes involving steam or air.


• Flow Through Turbine: When steam flows through a turbine, it undergoes a series of thermodynamic processes which can be represented on a Mollier chart.


• Representation: The flow through a turbine on a Mollier chart is typically represented by a vertical straight line.


• Explanation: As steam passes through a turbine, it typically undergoes a vertical drop in enthalpy due to the expansion and work done by the turbine.


• Vertical Straight Line: This vertical straight line on the Mollier chart indicates the change in enthalpy as the steam passes through the turbine.


• Other Lines: Horizontal lines on the Mollier chart typically represent constant pressure processes, inclined lines represent changes in temperature, and curved lines represent changes in entropy.

Therefore, on a Mollier chart, flow through a turbine is represented by a vertical straight line due to the change in enthalpy as the steam undergoes expansion and work in the turbine.

• Bituminous coal: It is a type of coal that contains a high amount of volatile matter and is commonly used for energy production.


• Non-caking: Non-caking coal does not soften and swell when heated, unlike caking coal which forms a cake-like mass when heated.


• Carbon content: The carbon content of coal is an important factor in determining its properties.


• Criteria for non-caking bituminous coal: The carbon content range for non-caking bituminous coal is typically between 78% and 81%.


• Explanation: When the carbon content of bituminous coal falls within the range of 78% to 81%, it does not exhibit caking properties. This means that it retains its solid form when heated and does not form a sticky mass. Therefore, the correct answer is option A: 78 - 81%.

• Definition: Throttling process is an irreversible adiabatic process in which a fluid undergoes a rapid change in its pressure and temperature without any heat transfer.


• Characteristics:
  
  
  
  • Occurs in a constant enthalpy process.
  
  
  • Results in a decrease in pressure and temperature of the fluid.
  
  
  
  

• During a throttling process, the fluid (in this case, steam) experiences a rapid expansion through a valve, causing a drop in pressure and temperature.


• As the process is adiabatic, there is no heat transfer involved, so the enthalpy of the steam remains constant.


• Enthalpy is a measure of the total energy of a system, including internal energy and pressure-volume work done. In a throttling process, the work done is negligible, so the enthalpy remains constant.


• Therefore, in a throttling process, steam enthalpy remains constant while pressure and temperature change.

• Steam enthalpy remains constant in a throttling process due to the adiabatic and rapid nature of the expansion, which results in no heat transfer and negligible work done.


• Understanding the behavior of steam in throttling processes is crucial for various engineering applications, such as in steam turbines and refrigeration systems.

• Heat transfer takes place: In a throttling process, there is no heat transfer involved. The process is adiabatic, meaning there is no heat exchange with the surroundings.



• Work is done by the expanding steam: Throttling is an irreversible process where there is a drop in pressure of the steam resulting in an increase in its volume. This expansion of steam does not involve any work done.



• Internal energy of steam changes: During throttling, there is no change in the internal energy of the steam. The process is isenthalpic, meaning the enthalpy remains constant.



• All of the above: None of the statements mentioned in the options are true for a throttling process. Therefore, the correct answer is none of the above.

• Definition: An adiabatic process is a process in which there is no heat transfer between the system and its surroundings.


• Non-Heat Transfer Process: Adiabatic processes are characterized by the absence of heat transfer, meaning that no heat is added to or removed from the system during the process.


• Isentropic Process: Adiabatic processes are essentially isentropic, meaning that they occur without any change in entropy within the system.


• Reversible Process: Adiabatic processes can be reversible, meaning that they can be undone without any net change in the system or its surroundings.


• Constant Temperature Process: While adiabatic processes do not involve heat transfer, they do not necessarily occur at constant temperature. The temperature of the system can change during an adiabatic process due to work being done on or by the system.

Therefore, the correct answer is B: Non-Heat Transfer Process.

• Definition: Hygrometry is the science of measuring and studying the water vapor content in the air.



• Importance: Understanding the water vapor content in the air is crucial for various applications such as weather forecasting, agriculture, industrial processes, and indoor air quality monitoring.



• Measurement: Hygrometers are devices used to measure humidity levels in the air. They can be based on various principles such as psychrometry, dewpoint measurement, or electrical resistance.



• Effects: The amount of water vapor in the air can affect human comfort, health, and the performance of certain materials. High humidity levels can lead to mold growth, while low humidity levels can cause dry skin and respiratory issues.



• Applications: Hygrometry is used in various fields such as meteorology, agriculture, HVAC systems, pharmaceuticals, and food production to ensure optimal conditions for processes and products.

• Volumetric analysis of the flue gases: The Orsat meter is a device used to analyze the volume percentages of oxygen, carbon dioxide, and carbon monoxide in a sample of flue gas. It works based on the principle of gas absorption and is commonly used in industries to monitor and control emissions.

By using the Orsat meter, technicians can determine the composition of flue gases, which is essential for ensuring compliance with environmental regulations and optimizing combustion processes.

• Usage: Alkaline pyrogallate is used in Orsat's apparatus for the absorption of gases.


• Gas Absorbed: The primary gas absorbed by alkaline pyrogallate in Orsat's apparatus is O₂.


• Function: The alkaline pyrogallate solution helps in the absorption of oxygen from a gas mixture, allowing for the analysis of the remaining gases present.


• Importance: This process is crucial in various industries and laboratories for measuring the composition of gas mixtures accurately.

Therefore, the correct answer is option C: O₂.

• Regenerative Cycle: A regenerative cycle is a thermodynamic cycle that aims to improve the efficiency of a system by utilizing heat transfer between the working fluid and the surroundings.


• Ideal Regenerative Cycle: An ideal regenerative cycle is a theoretical concept where there are no losses in the system, resulting in maximum efficiency.


• Carnot Cycle: The Carnot cycle is a theoretical thermodynamic cycle that operates between two temperature reservoirs and provides the maximum possible efficiency for a heat engine.


• Comparison: In the context of efficiency, an ideal regenerative cycle would aim to be as efficient as possible, but it cannot surpass the efficiency of the Carnot cycle.


• Efficiency Comparison: The efficiency of an ideal regenerative cycle is always less than the efficiency of the Carnot cycle due to the limitations of real-world systems and the presence of losses.

Increasing the operating pressure of the boiler, automatically raises the temperature at which boiling takes place. This raises the average temperature at which heat is added to the steam and thus raises the thermal efficiency of the cycle.

• Type: Stationary fire tube boiler

• Stationary: Lancashire boiler is a stationary boiler, meaning it is fixed at one place and does not move.


• Fire tube boiler: In a fire tube boiler, the hot gases produced by the combustion of fuel pass through the tubes that are surrounded by water. Lancashire boiler follows this principle.


• Construction: The Lancashire boiler has two internal flues, which help in the efficient transfer of heat and provide a large heating surface area.


• Usage: Lancashire boilers were commonly used in industrial settings for steam generation due to their reliability and efficiency.

• Therefore, the Lancashire boiler is classified as a stationary fire tube boiler due to its design and operating principles.

• Regeneration: Regeneration is the process of utilizing the waste heat in the exhaust gases to preheat the feedwater before it enters the boiler. This helps in reducing the amount of fuel needed to raise the temperature of the feedwater, thereby increasing the overall efficiency of the cycle.


• Reheating of steam: Reheating of steam is done in between the stages of a steam turbine. By reheating the steam, its specific volume increases, leading to an increase in the efficiency of the turbine. This process allows more work to be extracted from the steam before it is condensed back into water.


• Both (a) and (b): By incorporating both regeneration and reheating of steam in a thermal cycle, the efficiency of the cycle can be significantly increased. Regeneration reduces the fuel consumption by preheating the feedwater, while reheating of steam allows for more work extraction from the steam in the turbine.

Therefore, the correct answer is option C: both regeneration and reheating of steam.

• 1 kilowatt-hour (kWh) = 1000 watt x 3600 seconds


• 1 watt = 1 joule per second (J/s)


• 1 kWh = 1000 J/s x 3600 s = 3,600,000 J = 3.6 MJ (mega joules)


• 1 MJ = 1000 kJ (kilojoules)


• Therefore, 1 kWh = 3.6 MJ = 3600 kJ

• A: 1000 J - Incorrect, this is the conversion of 1 watt-hour to joules


• B: 360 kJ - Incorrect, this is not the correct conversion


• C: 3600 kJ - Correct, as derived above


• D: 3600 kW/sec - Incorrect, this is not the correct conversion


• E: 1000 kJ - Incorrect, this is not the correct conversion