Rankine Cycle MCQ Quiz in தமிழ் - Objective Question with Answer for Rankine Cycle - இலவச PDF ஐப் பதிவிறக்கவும்

Explanation:

Let us consider the Rankine cycle.

1-2-3-4-1 is a normal Rankine cycle process

Process 1 - 2 → Isentropic expansion (Turbine)

Process 2 – 3 → Constant pressure heat rejection (Condenser)

Process 3 - 4 → Pump work

Process 4 – 1 → Constant pressure heat addition (Boiler)

1´- 2´- 3 - 4´- 1 is a Rankine cycle when the maximum steam temperature is fixed.

As the maximum steam temperature is fixed because of the metallurgical condition and the boiler pressure is increased.

The process 1 – 2 shifted to a process 1´ - 2´ and the boiler pressure changed from PB to P'B.

∴ we can conclude from the Rankine cycle that the quality of turbine exhaust will decrease i.e., from 2 to 2´.

Explanation:

Carnot cycle

• This cycle was developed by Nicolas Leonard Sadi Carnot, to analyse the problem of the efficiency of the heat engine. 
• In the Carnot cycle, the working substance is subjected to cyclic operations consisting of two isothermals and two reversible adiabatic operations.

Otto cycle

• Petrol engine works on this cycle.
• It is known as the constant volume cycle, as the addition and heat and heat rejection in this cycle take place at constant volume.
• This cycle is taken as a standard of comparison for internal combustion engines. For the purpose of comparison with other cycles, the air is assumed to be the working substance.

Joule cycle

• A gas turbine is a rotary engine. In a gas turbine plant, the air is compressed in a rotary compressor and passed into a combustion chamber where fuel is burnt. The product of combustion then made to impinge over rings of turbine blades with high velocity and work is produced.
• Closed cycle gas turbine works on the Joule cycle.

Rankine cycle

• A steam turbine is a prime mover in which the rotary motion is obtained by the gradual change of momentum of the steam.
• The steam turbine works on the Rankine cycle.
• Rankine cycle is an ideal cycle which uses water and steam as a working fluid to generate power with the help of a steam turbine.

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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:

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:

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:

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

Concept: 

Rankine cycle with superheating is shown in the figure.

• Increased heat addition and rejection
• Due to increase heat addition, mean temperature of heat addition increases
• The increase in mean temperature of heat addition increases the efficiency

​∴ In the superheat Rankine cycle the mean temperature of heat addition increases and hence the efficiency increases.