Rankine Cycle MCQ Quiz in తెలుగు - Objective Question with Answer for Rankine Cycle - ముఫ్త్ [PDF] డౌన్‌లోడ్ కరెన్

Explanation:

Concept of Regeneration in the Rankine cycle:

• Regeneration means to take some part of the heat from the expanding steam in the turbine to decrease the overall heat supplied in the process.
• The thermal efficiency of the Rankine cycle can be increased by the use of a regenerative heat exchanger.
• In the regenerative cycle, a portion of the partially expanded steam is drawn off between the high and low-pressure turbines.
• The steam is used to preheat the condensed liquid before it returned to the boiler.
• In this way, the amount of heat added at the low temperatures is reduced and the mean effective temperature of heat addition is increased, thus cycle efficiency is increased.
• At point 2 some amount of steam is taken out for the heating purpose of water.

Effect of Regeneration:

• Decrease in turbine work due to a decrease in the mass flow rate of steam.
• Decrease in heat rejection in condenser due to a reduction in mass flow rate.
• Increased of efficiency and mean temperature of heat addition (Tm).

Concept:

Rankine Cycle:

Rankine cycle is a reversible cycle that has two constant pressure and two isentropic processes.

There are four processes in the Rankine cycle:

Process 1 – 2: Isentropic compression

Working fluid is pumped from low to high pressure.

W_pump = h₂ - h₁

Process 2 – 3: Isobaric heat addition

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.

Q_input = h₃ - h₂

Process 3 – 4: Isentropic expansion

The dry saturated vapour expands through a turbine, generating power.

W_tutbine = h₃ - h₄

Process 4 – 1: Isobaric heat rejection

The wet vapour then enters a condenser where it is condensed at constant pressure and temperature to become a saturated liquid.

Q_out = h₄ - h₁

Efficiency of Rankine cycle:

\(\eta_{Rankine}=\frac{W_{net}}{Q_{input}}=\frac{W_{turbine}-W_{Pump}}{Q_{input}}\)

\(\eta_{Rankine}=\frac{W_{net}}{Q_{input}}=\frac{(h_3-h_4)-(h_2-h_1)}{(h_3-h_2)}\)

Concept:

\(Specific\;Steam\;Consumption = \frac{{3600}}{{{W_{net}}}}\)

\(Specific\;Steam\;Consumption = \frac{{3600}}{{({W_T} - {W_{pump}}})}\)

Calculation:

Given:

h1 – h2 = 840 kJ/kg

h1 - h₄­ = 294 kJ/kg

pump work is negligible! hf4 ≈ hf3 = 191.81 kJ/kg

W_net = W_T - W_P = (h1 – h2) - (h₄ - hf3 ) = 840 - 0 = 840 kJ/kg

\(Specific\;Steam\;Consumption = \frac{{3600}}{{{W_{net}}}}= \frac{3600}{840}= 4.28\) kg/kWh

Concept:

mh₁ + (1 - m) h₂ = h₃

m h₁ + h₂ – m h₂  = h₃

m (h₁ – h₂) = h₃ – h₂

\(m = \frac{{{h_3} - {h_2}}}{{{h_1} - {h_2}}}\)

\(m = \frac{{810 - 200}}{{2850 - 200}}\)

∴ m = 0.230

Concept:

Stirling and Ericsson Cycles:

• The ideal Otto and Diesel cycles are internally reversible, but not totally reversible.
• Hence their efficiencies will always be less than that of Carnot efficiency.
• For a cycle to approach a Carnot cycle, heat addition and heat rejection must take place isothermally.
• Stirling and Ericsson cycles comprise of isothermal heat addition and heat rejection.
• Both these cycles also have a regeneration process.
• Regeneration, a process during which heat is transferred to a thermal energy storage device (called a regenerator) during one part of the cycle and is transferred back to the working fluid during another part of the cycle.

Explanation:

Bleeding:

• Bleeding is the process of draining steam from the turbine, at certain points during its expansion, and using this steam for heating the feed-water supplied to the boiler.
• The Ideal Rankine cycle, modified to take into account the effect of bleeding is known as the Regenerative cycle. 

Rankine Cycle:

Process 3-4: Isentropic expansion in Turbine (W_T)

Process 4-1: Constant pressure heat removal in Condenser (Q₂)

Process 1-2: Isentropic compression in Pump (W_P)

Process 2-3: Constant pressure heat addition in Boiler (Q₁)

Important Points

Governing

• Steam turbine governing is the procedure of controlling the flow rate of steam to a steam turbine so as to maintain its speed of rotation is constant.
• The variation in load during the operation of a steam turbine can have a significant impact on its performance.

Reheating

• In the reheat cycle, the total expansion of steam from the boiler to condenser pressure is carried out in more than one stage with reheating of steam in between the stages.
• The main advantage of the reheat cycle is that it increases the dryness fractions of steam at condenser inlet thus making it possible to use higher boiler pressure. It also increases the net-work output thus decreasing the mass flow rate of steam required for the same power output. 
• By reheating it's not sure that efficiency will increase, it may decrease also depending on the mean temperature of heat addition.
• Also, the mean temperature of heat addition doesn't always increase. It depends on the pressure in the reheater and entry pressure. 

Explanation:

Modified Rankine Cycle:

In the Rankine cycle, that the steam is expanded to the extreme toe of the p-v diagram (point 3 mentioned in the diagram), but in actual reciprocating steam engines, it is found to be too uneconomical (due to the larger size of the cylinder) to expand steam to the full time.

It may be noted in the diagram, is very narrow at the toe and the amount of work done (represented by area 5-3-6) during the final portion of the expansion stroke is extremely small. In fact, it is too small to overcome even the friction of the moving parts in the steam engine.

In order to overcome the above-mentioned difficulty, the Rankine cycle is slightly modified where the expansion stroke of the piston is stopped at point 5 by cutting the toe of the Rankine cycle and the steam is expanded at constant volume.

Thus, there is a reduction in the stroke of the cylinder.

Explanation:

The load on a power station varies from time to time due to uncertain demands of the consumers and is known as variable load on the station.

Load Curves: The curve showing the variation of load on the power station with respect to (w.r.t) time is known as a load curve.

Average load: The average of loads occurring on the power station in a given period (day or month or year) is known as average load or average demand.

Load Duration Curve: When the load elements of a load curve are arranged in the order of descending magnitudes, the curve thus obtained is called a load duration curve.

Diversity factor: The ratio of the sum of individual maximum demands to the maximum demand on power station is known as diversity factor.

Load Factor: The ratio of average load to the maximum demand during a given period is known as load factor

Explanation:

Attemperation:

• Attemperation is the primary means for controlling the degree of superheat in a superheated boiler.
• Attemperation is the process of partially desuperheating steam by the controlled injection of water into the superheated steam flow.
• The degree of superheating will depend on the steam load and the heat available, given the design of the superheater.
• The degree of superheat of the final existing steam is generally not subject to wide variation because of the design of the downstream processes. In order to use achieve the proper control of superheat temperature, an attemperator is used.
• A direct contact attemperator injects a stream of high purity water into the superheated steam.
• It is usually located at the exit superheater but may be placed at an intermediate position.
• Usually, boiler feed water is used attemperation and water required for attemperation is taken from Preheater.
• Attemeration is done between the primary and secondary superheater.

Explanation:

Rankine cycle: The Rankine cycle is the fundamental operating cycle of all steam power plants where an operating fluid is continuously evaporated and condensed.

The schematic representation of the Rankine cycle: 

• 1-2 Isentropic compression in a pump
• 2-3 Constant pressure heat addition in a boiler
• 3-4 Isentropic expansion in a turbine
• 4-1 Constant pressure heat rejection in a condenser

∴ The compressor is not a part of the Rankine cycle 

Additional Information

Importance of the parts used in the Rankine cycle:

1. Pump: Water is pumped from low pressure to high pressure, which is the boiler’s operating pressure
2. Boiler: The high-pressure liquid enters a boiler where it is heated at constant pressure by an external heat source to become
3. Turbine: The dry saturated vapor enters a turbine where it expands isentropically, generating power by rotating the shaft connected to an electric generator
4. Condensor: At constant pressure, the steam is condensed in a condenser, to become a saturated liquid