Power Engineering MCQ Quiz - Objective Question with Answer for Power Engineering - Download Free PDF

Explanation:

Rankine Cycle: Specific Volume of Water in Pump vs. Steam Expanding in Turbine

• The Rankine cycle is a fundamental thermodynamic cycle commonly used in power generation systems, particularly in steam power plants. It consists of four main processes: isentropic compression in a pump, heat addition in a boiler, isentropic expansion in a turbine, and heat rejection in a condenser. One of the critical aspects of the Rankine cycle is the specific volume of the working fluid at different stages of the cycle.

In the Rankine cycle, the working fluid undergoes a phase change during the cycle. Here’s why the specific volume of water in the pump is much less than that of the steam expanding in the turbine:

1. Specific Volume of Water in the Pump:

• The pump handles liquid water (subcooled or saturated) at a low temperature and pressure. Liquid water has a very low specific volume compared to steam.
• The specific volume of liquid water is nearly constant and extremely small, typically in the range of 0.001 m³/kg (depending on the pressure and temperature).
• Since the pump compresses liquid water to a higher pressure, the increase in specific volume during this process is negligible.

2. Specific Volume of Steam Expanding in the Turbine:

• The turbine handles steam, which is either dry saturated or superheated, depending on the operating conditions of the cycle.
• Steam has a significantly larger specific volume than liquid water because it exists in the gaseous phase. For instance, the specific volume of steam can be several hundred times that of liquid water, depending on the pressure and temperature.
• As the steam expands in the turbine, its specific volume increases further due to the drop in pressure. This is a characteristic of the expansion process in the Rankine cycle.

3. Comparison:

• The specific volume of water in the pump is orders of magnitude smaller than the specific volume of steam in the turbine. For example, at a pressure of 1 bar, the specific volume of saturated liquid water is approximately 0.001 m³/kg, while the specific volume of saturated steam is around 1.67 m³/kg—a difference of more than 1000 times.
• This significant difference arises because of the phase change from liquid to vapor and the associated increase in volume during boiling and expansion.

Key Points

• Hydrogen fuel cells produce electricity by combining hydrogen and oxygen through an electrochemical process.
• Water is produced as a byproduct of the reaction, making it an environmentally friendly technology.
• The chemical reaction in a hydrogen fuel cell is 2H₂ + O₂ → 2H₂O + Energy.
• The only byproduct of this reaction is water, which is clean and does not contribute to pollution.

Important Points

• Hydrogen fuel cells are considered a sustainable energy source as they generate electricity with zero emissions apart from water vapor.
• They are used in a variety of applications, including powering electric vehicles, stationary power generation, and backup power systems.
• Hydrogen is stored in pressurized tanks and is supplied to the fuel cell, where it undergoes oxidation to produce energy.

Explanation:

Steam rate:

• The capacity of a steam plant is often expressed in terms of steam rate or specific steam consumption.
• It is defined as the rate of steam flow (kg/s) required to produce unit shaft output (1 kW).

\(Steam\;Rate = \frac{{3600}}{{{W_{net}}}}\;kg/kW - hr\)

Explanation:

Work Input in a Reciprocating Air Compressor

• The work input in a reciprocating air compressor refers to the amount of energy required to compress a given mass of air from an initial pressure and temperature to a higher pressure. This energy depends on the process path the compression follows (e.g., isothermal, isentropic, or polytropic).

Compression follows PV = Constant (isothermal process).

Thermodynamic Analysis:

• In a reciprocating air compressor, the compression process can follow different thermodynamic paths, such as isothermal, adiabatic (isentropic), or polytropic processes.
• The work input in a compression process is given by the area under the pressure-volume (P-V) curve. Hence, the work done depends on the process path.
• For an isothermal process (PV = Constant), the temperature of the air remains constant during compression. Achieving this requires perfect heat transfer with the surroundings, ensuring that the heat generated during compression is removed continuously.

Work Done in an Isothermal Process:

The work done during isothermal compression is given by:

W_isothermal = mRT ln (P₂/P₁)

• m: Mass of air being compressed
• R: Specific gas constant
• T: Absolute temperature of the air (constant in an isothermal process)
• P₁: Initial pressure of the air
• P₂: Final pressure of the air

In an isothermal process, the work input is proportional to the natural logarithm of the pressure ratio (P₂/P₁), and the temperature remains constant. This results in the least amount of work compared to other processes because the heat generated during compression is removed, preventing a rise in temperature and pressure beyond what is necessary for the given pressure ratio.

Why Isothermal Compression Requires Minimum Work:

• During isothermal compression, the temperature of the air remains constant, which helps reduce the pressure rise for a given volume reduction.
• The reduction in pressure rise results in a lower area under the P-V curve, minimizing the work input.
• In contrast, in adiabatic or polytropic processes, the temperature increases during compression, leading to a higher pressure rise and, consequently, more work input.

Practical Challenges:

• While isothermal compression is theoretically the most efficient, achieving perfect isothermal conditions in practice is challenging due to the difficulty in maintaining continuous heat transfer during rapid compression.
• To approximate isothermal compression, intercoolers are often used in multi-stage compressors to cool the air between stages, reducing the overall work input.

Concept:

We use the polytropic process equations and mechanical efficiency to determine the power input required for compressing air in a reciprocating compressor.

Given:

• Mass flow rate of air, \( \dot{m} = 20 \, \text{kg/min} = 0.333 \, \text{kg/s} \)
• Inlet pressure, \( P_1 = 110 \, \text{kPa} \)
• Inlet temperature, \( T_1 = 300 \, \text{K} \)
• Outlet pressure, \( P_2 = 660 \, \text{kPa} \)
• Polytropic index, \( n = 1.25 \)
• Gas constant, \( R = 0.287 \, \text{kJ/kg·K} \)
• Mechanical efficiency, \( \eta_{\text{mech}} = 80\% = 0.8 \)
• Given: \( (6)^{0.2} = 1.43 \)

Step 1: Calculate the Polytropic Work Done

The work done per kg of air for a polytropic process is:

\( W_{\text{polytropic}} = \frac{n}{n-1} \times R \times T_1 \times \left[ \left( \frac{P_2}{P_1} \right)^{\frac{n-1}{n}} - 1 \right] \)

Substitute the values:

\( W_{\text{polytropic}} = \frac{1.25}{0.25} \times 0.287 \times 300 \times \left[ (6)^{0.2} - 1 \right] \)

\( W_{\text{polytropic}} = 5 \times 0.287 \times 300 \times (1.43 - 1) \)

\( W_{\text{polytropic}} = 184.515 \, \text{kJ/kg} \)

Step 2: Calculate the Indicated Power

The indicated power is the work done multiplied by the mass flow rate:

\( P_{\text{indicated}} = \dot{m} \times W_{\text{polytropic}} \)

\( P_{\text{indicated}} = 0.333 \times 184.515 = 61.5 \, \text{kW} \)

Step 3: Calculate the Power Input

The power input accounts for mechanical efficiency:

\( P_{\text{input}} = \frac{P_{\text{indicated}}}{\eta_{\text{mech}}} \)

\( P_{\text{input}} = \frac{61.5}{0.8} = 76.875 \, \text{kW} \)

The correct answer is Mumbai.

• Bhabha Atomic Research Centre (BARC) is located in Mumbai.

Key Points

• Bhabha Atomic Research Centre:
  • It was established in 1954.
  • Its headquarters is in Trombay, Mumbai.
  • Earlier, it was known as Atomic Energy Establishment.
  • At present, BARC house has eight research reactors:
    • Apsara, a one MW Swimming pool-type reactor.
    • Cirus, a 40 MW reactor.
    • Dhruva, a 100 MW high power nuclear research reactor.
    • Zerliana, Purnima I, Purnima II, Purnima III and Kamini
  • It comes under the aegis of the Department of Atomic Energy.

Additional Information

• Other Atomic Research Centres in India: 
  • Atomic Minerals Directorate for Exploration and Research (AMD) is located in Hyderabad.
  • Indira Gandhi Centre for Atomic Research (IGCAR) is located in Kalpakkam, Tamil Nadu.
  • Variable Energy Cyclotron Centre (VECC) is located in Kolkata.

Explanation:

Simple Impulse Turbine (De Laval Turbine):

• It is the first working impulse turbine.
• It consists of a single set of the nozzle and moving blades. Therefore, total pressure drop from boiler pressure to condenser pressure takes place in a single nozzle which gives high rotational speed exceeding pressure limit of 3000 rpm.
• These turbines are mostly used in small power and high-speed purposes.

Additional Information

Concept:

Note that the Rankine cycle has a lower efficiency compared to the corresponding Carnot cycle 2’-3-4-1’ with the same maximum and minimum temperatures. The reason is that the average temperature at which heat is added in the Rankine cycle lies between T₂ and T₂^' and is thus less than the constant temperature T2' at which heat is added to the Carnot cycle.

It is very difficult to build a pump that will handle a mixture of liquid and vapor at state 1' (refer T-s diagram; Carnot Vapor Cycle) and deliver saturated liquid at state 2’.

It is much easier to completely condense the vapor and handle only liquid in the pump (Rankine Cycle)

In the Carnot cycle, the compression is wet compression so the pump work requirement is more compare to the Rankine cycle where the pump compresses only the saturated liquid. Thus the specific work output for the Rankine cycle is more than the Carnot cycle for the same maximum and minimum temperature.

Also, the work ratio is defined as the ratio of net-work to the work done in the turbine.

\(r_w=\frac{W_{net}}{W_T}=\frac{W_T-W_C}{W_T}\)

Thus the work ratio is low for the Carnot Cycle.

Explanation:

Boiler:

• According to A. S. M. E. steam generating unit or boiler is defined as "A combination of apparatus for producing, furnishing or recovering heat together with the apparatus for transferring the heat". So made available to the fluid being heated and vapourised.

Types of boiler:

Water Tube Boiler:

• When water is contained inside the tubes (called water tube) which are surrounded by flames and hot gases from outside, then the boilers are named as water tube boilers.
• They are safe, quick steaming, flexible in construction and operation.
• These boilers are extensively used because they can be built for high pressure and large evaporative capacities.

Types of Water Tube Boilers

• Babcock and Wilcox boiler
• Stirling boiler
• Lamont boiler
• Benson boiler
• Loeffler boiler, etc.

Fire Tube Boilers:

• When the flames and hot gases, produced by combustion of fuel, pass through the tubes (called multitube) which are surrounded by water, then the boilers are named as fire tube boilers.

Types of Fire Tube Boilers:

• Simple vertical boiler
• Lancashire boiler
• Cochran boiler
• Locomotive boiler

Supercritical boiler:

• A large number of steam generating units are designed between working ranges of 125 atm and 510°C to 300 atm and 660°C. These are basically characterised as subcritical and supercritical. Subcritical consists, preheater, evaporator and superheater while supercritical boiler requires only preheater and superheater.
• Supercritical boilers operate at a pressure greater than 22 MPa and also referred to as Once through boilers since the feed water circulates only once through boiler in each steam cycle.

Concept:

Centrifugal compressor

• Air is drawn into the centre of a rotating impeller with radial blades and is pushed toward the centre by centrifugal force.
• This radial movement of air results in a pressure rise.
• The airflow rate is more than a reciprocating compressor. but the maximum pressure value is less than the reciprocating compressor.
• Centrifugal compressors were used in jet engines and smaller gas turbine engines.
• For larger engines, axial compressors need a lesser frontal area and are more efficient.

Pressure ratio for centrifugal compressor = 4 : 1

Pressure ratio for axial flow compressor = 1.2 : 1

The correct answer is Anthracite.

• Anthracite contains 86%–97% carbon and generally has the highest heating value of all ranks of coal. It has the highest carbon content.
• Anthracite combusts with a hot, clean flame, containing low content of sulfur and volatiles.
• It is used in domestic applications or other specialized industrial uses that require smokeless fuels.
• In India, it is found only in the Union territory of Jammu and Kashmir.

Additional Information

Concept:

Fire Tube Boiler: In this boiler, the hot flue gases are present inside the tubes and water surrounds them. They are low-pressure boilers. The operating pressure is about 25 bar.

Example to fire tube boilers:

• Cornish boiler
• Cochran boiler
• Locomotive boiler
• Lancashire boiler
• Scotish marine boiler

Water Tube Boiler: In this boiler, the water is present inside the tubes and the hot flue gases surround them. They are high-pressure boilers. The operating pressure is about 250 bar.

Examples of water tube boilers:

• Stirling boiler
• Babcock and Wilcox boiler
• Yarrow boiler
• La mont boiler
• Loeffler boiler
• Velox boiler

In a locomotive boiler, the draught is produced by a steam jet.

The draught is one of the most essential systems of the thermal power plant which supplies the required quantity of air for combustion and removes the burnt products from the system.

Natural draught: A draught produced by a chimney due to the difference of densities between the hot gases inside the chimney and cold atmospheric air outside it.

Induced draught: The air pressure at the fuel bed is reduced below that of the atmosphere by means of a fan placed at or near the bottom of the chimney.

Steam jet draught:

a)Induced steam jet: The draught produced by a steam jet issuing from a nozzle placed in the chimney.

b)Forced steam jet: The draught produced by a steam jet issuing from a nozzle placed in the ashpit under the fire grate of the furnace. Example: Locomotive boiler

Balanced draught: It is a combination of induced and forced draught.

The correct answer is Coal.

• Non-Renewable Resources:
  • The sources that cannot be replaced or reused once they are destroyed are called the Non-renewable resources.
  • These are limited resources. so these are used limitedly.
  • These are not environmentally friendly because the amount of carbon emission is high.
  • The cost of these resources is high.
  • They are not pollution-free.
  • They require low maintenance cost.
  • Coal, oil, nuclear energy, petroleum, natural gas, batteries, shale gas, phosphate are some examples.

Additional Information

• Renewable Resources:
  • Resources that are used by humans since human life exists.
  • These resources are used by our ancestors for their daily purposes like lighting, shelter, transportation, cooking, heating, protection from harm.
  • These are also known as non-conventional sources of energy.
  • For example Soil, water bodies, sun, wind, tidal energy, geothermal, forest, mountains, wildlife, atmospheric resources.
  • These can be used unlimited.
  • They are environment friendly.
  • The cost is low.
  • They are pollution-free.
  • They require high maintenance costs.
  • They are sustainable resources.
  • They cause no harm to life to exist on earth.

The correct answer is Russia.

• Russia has developed the world's first floating nuclear plant.

Key Points

• Akademik Lomonosov is the world’s first floating nuclear reactor. 
• Akademik Lomonosov was named after 18th-century Russian scientist Mikhail Lomonosov.
• Recently, It is completed its epic 5,000 km Arctic voyage in 22 days, despite environmentalists warning of serious risks to the region. 
• The plant was launched by Russia on 19 May 2018 at the St Petersburg shipyard.
• Akademik Lomonosov has painted with signature red, white, and blue colors of the nation's flag at its exterior.