1. An economizer in a steam generator performs the function of:
(a) Preheating the combustion air
(b) Preheating the feed water
(c) Preheating the input fuel
(d) Raising the temperature of steam
(2 Mark, 1989)

Ans: b
The function of economizer is to preheat the feed water.
2. The current level of the maximum temperature at steam turbine inlet is much lower than that at Gas turbine inlet because:
(a) The fuel combustion temperature in a steam generator is lower than that in a gas turbine engine.
(b) Of the corrosive nature of high temperature steam on super heater tubes.
(c) The materials used for the gas turbine blades are not suitable for the steam turbine blades.
(d) Unlike the gas turbine blades the steam turbine blades cannot be cooled.
(2 Mark, 1989)

Ans: d
Gas turbine blades have cooling arrangement.
3. In a gas turbine power plant intercoolers are used to cool the ____(hot gases/ compressed air) in order to decrease the ____ (expansion work/ compression work).
(2 Mark, 1989)

Ans: Compressed air, Compression work
4. In an Rankine cycle when superheated steam is used
(a) Thermal efficiency increases
(b) Steam consumption decreases
(c) Steam dryness after expansion increases
(d) All of the above
(2 Mark, 1990)

Ans: d
In a Rankine cycle when super heated steam is used:
i) Thermal efficiency increases because the mean temperature of heat addition increases.
ii) For same work output steam consumption decreases because specific workdone increases.
iii) The above figure shows that the steam dryness after expansion increases.
5. In an Rankine cycle heat is added
(a) Reversibly at constant volume
(b) Reversibly at constant temperature
(c) Reversibly at constant pressure and temperature
(d) Reversibly at constant pressure
(2 Mark, 1991)

Ans: d
In Rankine cycle heat is added reversibly at constant pressure.
6. A gas turbine cycle with heat exchange and reheating improves
(a) Only the thermal efficiency
(b) Only the specific power output
(c) Both thermal efficiency and specific power output
(d) Neither thermal efficiency nor specific power output
(2 Mark, 1993)

Ans: c
Both thermal efficiency and specific power out put increases.
7. For a given set of operating pressure limits of a Rankine cycle, the highest efficiency occurs for
(a) Saturated cycle
(b) Superheated cycle
(c) Reheat cycle
(d) Regenerative cycle
(1 Mark, 1994)

Ans: d
The highest efficiency occurs for regenerative cycle.
8. A gas turbine cycle with infinitely large number of stages during compression and expansion leads to
(a) Stirling cycle
(b) Atkinson cycle
(c) Ericsson cycle
(d) Brayton cycle
(1 Mark, 1994)

Ans: c
With infinitely large number of stages during compression and expansion, the adiabatic processes leads to isothermal processes.
So, the gas turbine cycle (Brayton cycle) leads to Ericssion cycle.
9. For a single stage impulse turbine with a rotor diameter of 2 m and a speed of 3000 rpm when the nozzle angle is 20°, the optimum velocity of steam is m/s is?
(a) 334
(b) 356
(c) 668
(d) 711
(1 Mark, 1994)

Ans: c
Mean blade velocity (u) = \frac { \pi DN }{ 60 }
=> u = \frac { \pi \times 2\times 3000 }{ 60 } = 314. 16 m/s
We know, \frac { u }{ v } =\frac { cos\alpha }{ 2 }
Optimum steam velocity (v) = \frac { 2u }{ cos\alpha }
=> v = \frac { 2\times 314.16 }{ cos20 } = 668.64 m/s
10. In adiabatic flow with friction, the stagnation temperature along a streamline ____ (increases/ decreases/ remains constant).
(1 Mark, 1995)

Ans: remain constant
In adiabatic flow with friction, the stagnation temperature along a streamline remains constant but the pressure may decrease.
11. Consider a Rankine cycle with superheat. If the maximum pressure in the cycle is increased without changing the maximum temperature and the minimum pressure, the dryness fraction of stream after the isentropic expansion will increase/ decrease.
(1 Mark, 1995)
12. Consider a two stage reciprocating air compressor with a perfect intercooler operating at the best intermediate pressure. Air enters the lowpressure cylinder at 1 bar, 27°C and leaves the highpressure cylinder at 9 bar. Assume the index of compression and expansion in each stage is 1.4, and that for air R = 286.7 J/kg K, the work done per kg air in the high pressure cylinder is:
(a) 111 kJ
(b) 222 kJ
(c) 37 kJ
(d) 74 kJ
(2 Mark, 1997)

Ans: a
Explanation:
In perfect intercooling: each stage of compressor requires equal amount of work.
Work done in the high pressure cylinder (W) = \frac { n }{ n1 } R{ T }_{ 1 }\left[ { \left( \frac { { P }_{ h } }{ { P }_{ L } } \right) }^{ \frac { n1 }{ 2n } }1 \right]
=> W = \frac { 1.4 }{ 1.41 } \times 0.2867\times 300\left[ { \left( \frac { 9 }{ 1 } \right) }^{ \frac { 1.41 }{ 2\times 1.4 } }1 \right]
=> W = 111 kJ
13. A steam plant has the boiler efficiency of 92%, turbine efficiency (mechanical) of 94%, generator efficiency of 95% and cycle efficiency of 44%. If 6% of the generated power is used to run the auxiliaries, the overall plant efficiency is:
(a) 34%
(b) 39%
(c) 45%
(d) 30%
(2 Mark, 1996)

Ans: a
Power used in auxiliaries = 6%
Overall efficiency { (\eta }_{ o }) = { \eta }_{ b }\times { \eta }_{ t }\times { \eta }_{ g }\times { \eta }_{ c }\times (1 6%) = { 0.92\times 0.94\times 0.95\times 0.44\times 0.94 } = 34%
14. Which among the following is the boiler mounting?
(a) Blow off cock
(b) Feed pump
(c) Economiser
(d) Superheater
(1 Mark, 1997)

Ans: a
Blow off cock is a boiler mounting.
16. The following data pertain to a single stage impulse steam turbine:
Nozzle angle = 20°,
Blade velocity = 200 m/s
Relative steam velocity at entry = 350 m/s,
Blade inlet = 30°
Blade exit angle = 25°
If blade friction is neglected, the work done per kg steam is:
(a) 124 kJ
(b) 164 kJ
(c) 169 kJ
(d) 174 kJ
(2 Mark, 1997)

Ans: a
Work done (W) = \dot { m } \left( { v }_{ w1 }+{ v }_{ w2 } \right) u
From the above diagram, putting the values of v_{w1 }and v_{w2}
As blade friction is neglected, So v_{r1 }= v_{r2}
=> W = \dot { m } { v }_{ r1 }\left( cos\theta +cos\phi \right) u
=> W = 350 (cos 30^{0} + cos 25^{0}) * 200 = 124 kJ
17. Consider an actual regenerative Rankine cycle with one open feed water heater. For each kg steam entering the turbine, if m kg steam with a specific enthalpy of h_{1} is bled from the turbine, and the specific enthalpy of liquid water entering the heater is h_{2} , then h_{3} specific enthalpy of saturated liquid leaving the heater is equal to
(a) mh_{1} − (h_{2} − h_{1})
(b) h_{1} − m(h_{2} − h_{1})
(c) h_{2} − m(h_{2} − h_{1})
(d) mh_{2} − (h_{2} − h_{1})
(2 Mark, 1997)
18. If V_{N} and α are the nozzle exit velocity and nozzle angle in an impulse turbine, the optimum blade velocity is given by
(a) V_{N }cos 2α
(b) V_{N }sin 2α
(c) \frac { { V }_{ N }cos\alpha }{ 2 }
(d) \frac { { V }_{ N }sin\alpha }{ 2 }
(1 Mark, 1998)

Ans: c
let blade velocity = u
Nozzle exit velocity = V_{N}
We know, \frac { u }{ { V }_{ N } } =\frac { cos\alpha }{ 2 }
=> u = \frac { { V }_{ N }cos\alpha }{ 2 }
19. A Curtis stage, Rateau stage and a 50% reaction stage in a steam turbine are examples of
(a) different types of impulse stages
(b) different types of reaction stages
(c) a simple impulse stage, a velocity compounded impulse stage and reaction stage
(d) a velocity compounded impulse stage, a simple impulse stage and a reaction stage
(1 Mark, 1998)

Ans: d
Option ‘d’ is correct.
20. The isentropic heat drop in the nozzle of an impulse steam turbine with a nozzle efficiency 0.9, blade velocity ratio 0.5, and mean blade velocity 150 m/s in kJ/kg is
(a) 50
(b) 40
(c) 60
(d) 75
(2 Mark, 1998)

Ans: a
let mean blade velocity (u) = 150 m/s
Absolute velocity of steam = v
Blade velocity ratio \left( \frac { u }{ v } \right) = 0.5
=> v = \frac { 150 }{ 0.5 } = 300 m/s
Kinetic energy at inlet per kg = \frac { 1 }{ 2 } { v }^{ 2 }=\frac { 1 }{ 2 } \times { 300 }^{ 2 } = 45 kJ/kg
Isentropic heat drop in nozzle = \frac { KE }{ \eta } =\frac { 45 }{ 0.9 } = 50 kJ/kg
22. Which of the following is a pressure compounded turbine?
(a) Parsons
(b) Curtis
(c) Rateau
(d) all the three
(1 Mark, 2000)

Ans: c
Rateau is a pressure compounded turbine.
23. The Rateau turbine belongs to the category of
(a) pressure compounded turbine
(b) reaction turbine
(c) velocity compounded turbine
(d) radial flow turbine
(1 Mark, 2001)

Ans: a
24. A singleacting twostage compressor with complete intercooling delivers air at 16 bar. Assuming an intake state of 1 bar at 15°C, the pressure ratio per stage is
(a) 16
(b) 8
(c) 4
(d) 2
(2 Mark, 2001)

Ans: c
Explanation:
For perfect inter cooling, the pressure ratio is equal in each stage.
In this question, the compressor has 2 stages.
So, { r }_{ p }^{ 2 }=\frac { 16 }{ 1 }
=> { r }_{ p } = 4
24. The efficiency of superheat Rankine cycle is higher than that of simple Rankine cycle because
(a) the enthalpy of main steam is higher for superheat cycle
(b) the mean temperature of heat addition is higher for superheat cycle
(c) the temperature of steam in the condenser is high
(d) the quality of steam in the condenser is low
(2 Mark, 2002)

Ans: b
Super heating increases the mean temperature of heat addition. So, by super heating the efficiency increases.
25. In a Rankine cycle, regeneration results in higher efficiency because
(a) pressure inside the boiler increases
(b) heat is added before steam enters the low pressure turbine
(c) average temperature of heat addition in the boiler increases
(d) total work delivered by the turbine increases
(1 Mark, 2003)

Ans: c
Regeneration increases the mean temperature of heat addition.
26. Considering the variation of static pressure and absolute velocity in an impulse stream turbine, across one row of moving blades
(a) both pressure and velocity decrease
(b) pressure decreases but velocity increases
(c) pressure remains constant, while velocity increases
(d) pressure remains constant, while velocity decreases
(1 Mark, 2003)

Ans: d
In impulse turbine the Kinetic energy decreases across the moving blade. So the velocity decreases.
But no change in static pressure.
27. In a gas turbine, hot combustion products with the specific heats C_{p} = 0.98 kJ/kgK, C_{v }= 0.7538 kJ/kgK enter the turbine at 20 bar, 1500 K and exit at 1 bar. The isoentropic efficiency of the turbine is 0.94. The work developed by the turbine per kg of gas flow is
(a) 689.64 kJ/kg
(b) 794.66 kJ/kg
(c) 1009.72 kJ/kg
(d) 1312.00 kJ/kg
(2 Mark, 2003)

Ans: a
\gamma =\frac { { c }_{ p } }{ { c }_{ v } } =\frac { 0.98 }{ 0.7538 } = 1.3
\frac { { T }_{ 2 } }{ { T }_{ 1 } } ={ \left( \frac { { p }_{ 2 } }{ { p }_{ 1 } } \right) }^{ \frac { \gamma 1 }{ \gamma } }=> \frac { { T }_{ 2 } }{ 1500 } ={ \left( \frac { 1 }{ 20 } \right) }^{ \frac { 0.3 }{ 1.3 } }
=>{ T }_{ 2 } = 751.36 K
Isentropic efficiency (\eta )=\frac { { T }_{ 1 }{ T }_{ 2' } }{ { T }_{ 1 }{ T }_{ 2 } }
=> \eta =\frac { 1500{ T }_{ 2' } }{ 1500751.37 }
=> { T }_{ 2' } = 796.3 K
Turbine work { (W }_{ T })={ c }_{ p }\left( { T }_{ 1 }{ T }_{ 2' } \right)
=> { W }_{ T }=0.98\times \left( 1500796.3 \right) = 689.64 kJ/kg
28. The compression ratio of a gas power plant cycle corresponding to maximum work output for the given temperature limits of T_{min} and T_{max} will be
(a) { \left( \frac { { T }_{ max } }{ { T }_{ min } } \right) }^{ \frac { \gamma }{ 2\left( \gamma 1 \right) } }
(b) { \left( \frac { { T }_{ min } }{ { T }_{ max } } \right) }^{ \frac { \gamma }{ 2\left( \gamma 1 \right) } }
(c) { \left( \frac { { T }_{ max } }{ { T }_{ min } } \right) }^{ \frac { \gamma 1 }{ \gamma } }
(d) { \left( \frac { { T }_{ min } }{ { T }_{ max } } \right) }^{ \frac { \gamma 1 }{ \gamma } }
(2 Mark, 2004)

Ans: a
For maximum work output:
Pressure ratio = { \left( \frac { { T }_{ max } }{ { T }_{ min } } \right) }^{ \frac { \gamma }{ 2\left( \gamma 1 \right) } }
Data for Questions 29 and 30 are given below.
Consider a steam power plant using a reheat cycle as shown. Steam leaves the boiler and enters the turbine at 4 MPa, 350^{0}C (h_{3} = 3095 kJ/kg). After expansion in the turbine to 400 kPa (h_{4} = 2609 kJ/kg), the steam is reheated to 350^{0}C (h_{5} = 3170 kJ/kg), and then expanded in a low pressure turbine to 10 kPa (h_{6} = 2165 kJ/kg).
29. The thermal efficiency of the plant neglecting pump work is
(a) 15.8%
(b) 41.1%
(c) 48.5%
(d) 58.6%
(2 Mark, 2004)

Ans: b
Thermal efficiency of the plant (\eta )=\frac { { W }_{ net } }{ Q }
=> \eta =\frac { \left( { h }_{ 3 }{ h }_{ 4 } \right) +\left( { h }_{ 5 }{ h }_{ 6 } \right) }{ { (h }_{ 3 }{ h }_{ 1 })+\left( { h }_{ 5 }{ h }_{ 4 } \right) }
=> \eta =\frac { \left( 30952609 \right) +\left( 31702165 \right) }{ (309529.3)+(31702609) }
=> \eta = 41.1%
30. The enthalpy at the pump discharge (h_{2}) is
(a) 0.33 kJ/kg
(b) 3.33 kJ/kg
(c) 4.0 kJ/kg
(d) 33.3 kJ/kg
(2 Mark, 2004)

Ans: d
The enthalpy at the pump discharge must be greater than 29.3 kJ/kg.
So the answer is 33.3 kJ/kg.
30. A pv diagram has been obtained from a test on a reciprocating compressor. Which of the following represents that diagram?
(1 Mark, 2005)

Ans: d
Explanation:
At intake: When the clearance air has reduced to the atmospheric pressure, the inlet valve do not openly immediately. The pressure drops lower than the atmospheric pressure and the inertia of the valves are overcome by the pressure difference. Thus the valve is forced open by the atmospheric air and rushes into the cylinder chamber. This negative pressure difference is called as the Intake depression.
At exit: The delivery pipe delays to open. The compressed air pressure inside the cylinder of compressor reaches a pressure slightly more than than the air receiver pressure.
31. Assertion [a]: In a power plant working on a Ranking cycle, the regenerative feed water heating improves the efficiency of the steam turbine.
Reason [r]: The regenerative feed water heating raises the average temperature of heat addition in the Rankine cycle.
Determine the correctness or otherwise of the following Assertion [a] and the Reason [r].
(a) Both [a] and [r] are true and [r] is the correct reason for [a].
(b) Both [a] and [r] are true but [r] is NOT the correct reason for [a].
(c) Both [a] and [r] are false.
(d) [a] is false and [r] is true.
(2 Mark, 2006)

Ans: a
The regeneration in Rankine cycle increases the mean absolute temperature of heat addition and the cycle efficiency.
32. Assertion [a]: Condenser is an essential equipment in a steam power plant.
Reason [r]: For the same mass flow rate and the same pressure rise, a water pump requires substantially less power than a steam compressor.
Determine the correctness or otherwise of the following Assertion [a] and the Reason [r].
(a) Both [a] and [r] are true and [r] is the correct reason for [a].
(b) Both [a] and [r] are true but [r] is NOT the correct reason for [a].
(c) Both [a] and [r] are false.
(d) [a] is false and [r] is true.
(2 Mark, 2006)

Ans: b
Condenser is an essential equipment in steam power plant because it condenses the vapours in to liquid. This liquid can be easily sent to the boiler by using a pump.
For the same mass flow rate and same pressure rise pump requires less power compare to steam compressor because water has lesser specific volume compare to steam. High specific volume requires more power.
33. Which combination of the following statements is correct ?
The incorporation of reheater in a steam power plant:
P: always increases the thermal efficiency of the plant.
Q: always increases the dryness fraction of steam at condenser inlet
R: always increases the mean temperature of heat addition.
S: always increases the specific work output.
(a) P and S
(b) Q and S
(c) P, R and S
(d) P, Q, R and S
(2 Mark, 2007)
34. A thermal power plant operates on a regenerative cycle with a single open feedwater heater, as shown in the figure. For the state points shown, the specific enthalpies are: h_{1} = 2800 kJ/kg and h_{2} = 200 kJ/kg. The bleed to the feedwater heater is 20% of the boiler steam generation rate. The specific enthaply at state 3 is
(a) 720 kJ/kg
(b) 2280 kJ/kg
(c) 1500 kJ/kg
(d) 3000 kJ/kg
(2 Mark, 2008)

Ans: a
Using energy balance:
mh_{3} = 0.2m h_{1} + 0.8m h_{2}
=> h_{3} = 0.2h_{1} + 0.8h_{2}
=> h_{3} = 0.2(2800) + 0.8(200) = 720 kJ/kg
Common Data Questions: 35 & 36.
The inlet and the outlet conditions of stream for an adiabatic steam turbine are as indicated in the notations are as usually followed.
35. If mass flow rate of steam through the turbine is 20 kg/s, the power output of the turbine (in MW) is
(a) 12.157
(b) 12.941
(c) 168.001
(d) 168.785
(2 Mark, 2009)

Ans: a
Using Steady flow energy equation:
\dot { m } \left( { h }_{ 1 }+\frac { { v }_{ 1 }^{ 2 } }{ 2000 } +\frac { g{ z }_{ 1 } }{ 1000 } \right) +Q=\dot { m } \left( { h }_{ 2 }+\frac { { v }_{ 2 }^{ 2 } }{ 2000 } +\frac { g{ z }_{ 2 } }{ 1000 } \right) +W=> 20\left( 3200+\frac { 160^{ 2 } }{ 2000 } +\frac { 9.81\times 10 }{ 1000 } \right) =20\left( 2600+\frac { 100^{ 2 } }{ 2000 } +\frac { 9.81\times 6 }{ 1000 } \right) +W
=> W = 12156.79 kW = 12.157 MW
36. Assume the above turbine to be part of a simple Rankine cycle. The density of water at the inlet to the pump is 1000 kg/m^{3}. Ignoring kinetic and potential energy effects, the specific work (in kJ/kg) supplied to the pump is
(a) 0.293
(b) 0.351
(c) 2.930
(d) 3.510
(2 Mark, 2009)

Ans: c
Specific pump work { (W }_{ p })=\nu \left( { p }_{ 1 }{ p }_{ 2 } \right)
=> { W }_{ p }=\frac { 1 }{ \rho } \left( { p }_{ 1 }{ p }_{ 2 } \right)
=> { W }_{ p }=\frac { 300070 }{ 1000 } = 2.93 kJ/kg
Common Data Questions: 37 & 38.
In a steam power plant operating on the Rankine cycle, steam enters the turbine at 4 MPa, 350ºC and exits at a pressure of 15 kPa. Then it enters the condenser and exits as saturated water. Next, a pump feeds back the water to the boiler. The adiabatic efficiency of the turbine is 90%. The thermodynamic states of water and steam are given in the table.
h is specific enthalpy, s is specific entropy and v the specific volume; subscripts f and g denote saturated liquid state and saturated vapour state.
37. The net work output (kJ/kg) of the cycle is
(a) 498
(b) 775
(c) 860
(d) 957
(2 Mark, 2010)

Ans: c
Process 1 – 2 is isentropic.
So, s_{1} = s_{2}
=> s_{1} = s_{f2} + x * s_{fg2}
=> 6.5281 = 0.7549 + x (8.0085 – 0.7549)
=> x = 0.8033
Enthalpy at point 2,
h_{2} = h_{f2} + x * h_{fg2}
=> h_{2 }= 225.94 + 0.8033(2599.1 – 225.94) = 2132.3 kJ/kg
Actual turbine work { (W }_{ T })=\eta \times \left( { h }_{ 1 }{ h }_{ 2 } \right)
=> W_{T }= 0.9(3092.5 – 2132.3) = 864.18 kJ/kg
Pump work { (W }_{ p })={ \nu }_{ f }\left( { p }_{ 1 }{ p }_{ 2 } \right)
=> W_{P }= 0.001014(4000 – 15) = 4.04 kJ/kg
Net work = W_{T }– W_{P}
= 864.18 – 4.04 = 860.14 kJ/kg
38. Heat supplied (kJ/kg) to the cycle is
(a) 2372
(b) 2576
(c) 2863
(d) 3092
(2 Mark, 2010)
39. The values of enthalpy of steam at the inlet and outlet of a steam turbine in a Rankine cycle are 2800 kJ/kg and 1800 kJ/kg respectively. Neglecting pump work, the specific steam consumption in kg/kWhour is
(a) 3.60
(b) 0.36
(c) 0.06
(d) 0.01
(2 Mark, 2011)

Ans: a
Specific steam consumption = \frac { 3600 }{ { W }_{ net } } =\frac { 3600 }{ { W }_{ T }{ W }_{ P } }
=> sfc = \frac { 3600 }{ (28001800)0 } = 3.6 kg/kWh
40. An ideal Brayton cycle, operating between the pressure limits of 1 bar and 6 bar, has minimum and maximum temperatures of 300K and 1500K. The ratio of specific heats of the working fluid is 1.4. The approximate final temperatures in Kelvin at the end of the compression and expansion processes are respectively
(a) 500 and 900
(b) 900 and 500
(c) 500 and 500
(d) 900 and 900
(2 Mark, 2011)

Ans: a
For process 12:
\frac { { T }_{ 2 } }{ { T }_{ 1 } } ={ \left( \frac { { p }_{ 2 } }{ { p }_{ 1 } } \right) }^{ \frac { \gamma 1 }{ \gamma } }=> \frac { { T }_{ 2 } }{ 300 } ={ \left( \frac { 6 }{ 1 } \right) }^{ \frac { 0.4 }{ 1.4 } }
=> T_{2} = 500 K
For process 3 – 4:
\frac { { T }_{ 3 } }{ { T }_{ 4 } } ={ \left( \frac { { p }_{ 3 } }{ { p }_{ 4 } } \right) }^{ \frac { \gamma 1 }{ \gamma } }
=> \frac { 1500 }{ { T }_{ 4 } } ={ \left( \frac { 6 }{ 1 } \right) }^{ \frac { 0.4 }{ 1.4 } }=> T_{2} = 900 K
40. Steam enters an adiabatic turbine operating at steady state with an enthalpy of 3251.0 kJ/kg and leaves as a saturated mixture at 15 kPa with quality (dryness fraction) 0.9. The enthalpies of the saturated liquid and vapor at 15 kPa are h_{f} = 225.94 kJ/kg and h_{g} = 2598.3 kJ/kg respectively. The mass flow rate of steam is 10 kg/s. Kinetic and potential energy changes are negligible. The power output of the turbine in MW is
(a) 6.5
(b) 8.9
(c) 9.1
(d) 27.0
(1 Mark, 2012)

Ans: b
Enthalpy at the exit of turbine:
{ h }_{ 2 }={ h }_{ f }+x\left( { h }_{ g }{ h }_{ f } \right)=> { h }_{ 2 } = 225.94 + 0.9(2598.3 – 225.9)
=> { h }_{ 2 } = 2361.1 kJ/kg
Turbine power output:
{ W }_{ T }=\dot { m } \times \left( { h }_{ 1 }{ h }_{ 2 } \right)=> { W }_{ T }=10\times \left( 32512361.1 \right)
=> { W }_{ T } = 8900 kW = 8.9 MW
Statement for Linked Answer Questions 41 and 42:
In a simple Brayton cycle, the pressure ratio is 8 and temperatures at the entrance of compressor and turbine are 300 K and 1400 K, respectively. Both compressor and gas turbine have isentropic efficiencies equal to 0.8. For the gas, assume a constant value of c_{p} (specific heat at constant pressure) equal to 1 kJ/kgK and ratio of specific heats as 1.4. Neglect changes in kinetic and potential energies.
41. The power required by the compressor in kW/kg of gas flow rate is
(a) 194.7
(b) 243.4
(c) 304.3
(d) 378.5
(2 Mark, 2013)

Ans: c
For isentropic process (1 – 2):
\frac { { T }_{ 2 } }{ { T }_{ 1 } } ={ \left( { r }_{ p } \right) }^{ \frac { \gamma 1 }{ \gamma } }=> { T }_{ 2 }={ 300\times 8 }^{ \frac { 1.41 }{ 1.4 } }=543.3 K
Isentropic efficiency \left( { \eta }_{ c } \right) =\frac { { T }_{ 2 }{ T }_{ 1 } }{ { { T }_{ 2' }{ T }_{ 1 } } }
=> 0.8=\frac { 543.3300 }{ { T }_{ 2' }300 }
=> { T }_{ 2' } = 604.12 K
Compressor power (W_{c}) = { c }_{ p }\left( { T }_{ 2' }{ T }_{ 1 } \right)
=> W_{c} = 1(604.12 – 300) = 304.12 kJ/kg
42. The thermal efficiency of the cycle in percentage (%) is
(a) 24.8
(b) 38.6
(c) 44.8
(d) 53.1
(2 Mark, 2013)

Ans: a
For isentropic process (3 – 4):
\frac { { T }_{ 3 } }{ { T }_{ 4 } } ={ \left( { r }_{ p } \right) }^{ \frac { \gamma 1 }{ \gamma } }=> { T }_{ 4 }={ \frac { 1400 }{ { 8 }^{ \frac { 1.41 }{ 1.4 } } } }=773.05 K
Isentropic efficiency for turbine:
{ \eta }_{ T }=\frac { { T }_{ 3 }{ T }_{ 4' } }{ { { T }_{ 3 }{ T }_{ 4 } } }=> 0.8=\frac { 1400{ T }_{ 4' } }{ 1400773.05 }
=> { T }_{ 4' } = 898.44 K
Turbine work (W_{T}) = { c }_{ p }\left( { T }_{ 3 }{ T }_{ 4' } \right)
=> W_{T} = 1(1400 – 898.44) = 501.56 kJ/kg
Net work done (W_{net}) = W_{T} – W_{C}
=> W_{net }= 501.56 – 304.12 = 197.44 kJ/kg
Heat supplied (Q) = { c }_{ p }\left( { T }_{ 3 }{ T }_{ 2' } \right)
=> Q = 1(1400 – 604.12) = 795.88 kJ/kg
Thermal efficiency { \left( \eta \right) }=\frac { { W }_{ net } }{ Q } =\frac { 197.44 }{ 795.88 }
=> \eta = 0.248 = 24.8%
43. An ideal reheat Rankine cycle operates between the pressure limits of 10 kPa and 8 MPa, with reheat being done at 4 MPa. The temperature of steam at the inlets of both turbines is 500°C and the enthalpy of steam is 3185 kJ/kg at the exit of the high pressure turbine and 2247 kJ/kg at the exit of low pressure turbine. The enthalpy of water at the exit from the pump is 191 kJ/kg. Use the following table for relevant data.
Disregarding the pump work, the cycle efficiency (in percentage) is _______.
(2 Mark, 2014[1])

Ans: 40.7%
Turbine work (W_{T}) = (h_{4} – h_{5}) + (h_{6} – h_{1})=> W_{T } = (3399 – 3185) + (3446 – 2247) = 1413 kJ/kg
Heat addition (Q) = (h_{4} – h_{3}) + (h_{6} – h_{5})
=> Q = (3399 – 191) + (3446 – 3185) = 3469 kJ/kg
Efficiency \left( \eta \right) =\frac { { W }_{ T } }{ Q } =\frac { 1413 }{ 3469 } = 0.407 = 40.7%
44. In a power plant, water (density = 1000 kg/m^{3}) is pumped from 80 kPa to 3 MPa. The pump has an isentropic efficiency of 0.85. Assuming that the temperature of the water remains the same, the specific work (in kJ/kg) supplied to the pump is
(a) 0.34
(b) 2.48
(c) 2.92
(d) 3.43
(1 Mark, 2014[1])

Ans: d
Isentropic work = \frac { 1 }{ \rho } \left( { p }_{ 2 }{ p }_{ 1 } \right) = \frac { 1 }{ 1000 } \left( 300080 \right) = 2.92 kJ/kg
Actual work = \frac { Isentropic\quad work }{ \eta } =\frac { 2.92 }{ 0.85 } = 3.43 kJ/kg
45. The thermal efficiency of an airstandard Brayton cycle in terms of pressure ratio (r_{p}) and γ = (c_{p}/c_{v}) is given by
(a) 1 – \frac { 1 }{ { r }_{ p }^{ \gamma 1 } }
(b) 1 – \frac { 1 }{ { r }_{ p }^{ \gamma } }
(c) 1 – \frac { 1 }{ { r }_{ p }^{ 1/\gamma } }
(d) 1 – \frac { 1 }{ { r }_{ p }^{ \frac { (\gamma 1) }{ \gamma } } }
(1 Mark, 2014[2])

Ans: d
Thermal efficiency of Brayton cycle is given by = 1 – \frac { 1 }{ { r }_{ p }^{ \frac { (\gamma 1) }{ \gamma } } }
46. In an ideal Brayton cycle, atmospheric air (ratio of specific heats, c_{p}/c_{v} = 1.4, specific heat at constant pressure = 1.005 kJ/kg.K) at 1 bar and 300 K is compressed to 8 bar. The maximum temperature in the cycle is limited to 1280 K. If the heat is supplied at the rate of 80 MW, the mass flow rate (in kg/s) of air required in the cycle is _____.
(2 Mark, 2014[2])

Ans: 108.07
In the process (1 – 2):
\frac { { T }_{ 2 } }{ { T }_{ 1 } } ={ \left( \frac { { p }_{ 2 } }{ { p }_{ 1 } } \right) }^{ \frac { \gamma 1 }{ \gamma } }=> \frac { { T }_{ 2 } }{ 300 } ={ 8 }^{ \frac { 0.4 }{ 1.4 } }
=> T_{2 }= 543.43 K
In process 2 – 3:
Heat supply (Q) = \dot { m } { c }_{ p }\left( { T }_{ 3 }{ T }_{ 2 } \right)
=> 80\times { 10 }^{ 3 }=\dot { m } \times 1.005\left( 1280543.43 \right)
=> \dot { m } = 108.07 kg/s
47. Steam at a velocity of 10 m/s enters the impulse turbine stage with symmetrical blading having blade angle 30°. The enthalpy drop in the stage is 100 kJ. The nozzle angle is 20°. The maximum blade efficiency (in percent) is ______.
(2 Mark, 2014[2])

Ans: 88.3%
Maximum blade efficiency ({ \eta }_{ max })={ cos }^{ 2 }\alpha
Where, \alpha = nozzle angle
=> { \eta }_{ max }={ cos }^{ 2 }{ 20 }^{ o }
=> { \eta }_{ max } = 0.833 = 88.3%
48. For a gas turbine power plant, identify the correct pair of statements.
P. Smaller in size compared to steam power plant for same power output
Q. Starts quickly compared to steam power plant
R. Works on the principle of Rankine cycle
S. Good compatibility with solid fuel
(a) P, Q
(b) R, S
(c) Q, R
(d) P, S
(1 Mark, 2014[3])

Ans: a
Option P and Q are correct.
Gas power plant works on the principle of Brayton cycle.
In gas power plant liquid fuel, natural gas are mostly used.
49. Steam with specific enthalpy (h) 3214 kJ/kg enters an adiabatic turbine operating at steady state with a flow rate 10 kg/s. As it expands, at a point where h is 2920 kJ/kg, 1.5 kg/s is extracted for heating purposes. The remaining 8.5 kg/s further expands to the turbine exit, where h = 2374 kJ/kg. Neglecting changes in kinetic and potential energies, the net power output (in kW) of the turbine is ____.
(2 Mark, 2014[4])

Ans: 7581
Power output = 10(3214 – 2920) + 8.5(2920 – 2374) = 7581 kW
50. Steam enters a well insulated turbine and expands isentropically throughout. At an intermediate pressure, 20 percent of the mass is extracted for process heating and the remaining steam expands isentropically to 9 kPa.
Inlet to turbine: P = 14 MPa, T = 560ºC, h = 3486 kJ/kg, s = 6.6 kJ/(kg.K)
Intermediate stage: h = 2776 kJ/kg
Exit of turbine: P = 9 kPa, h_{f} = 174 kJ/kg, h_{g} = 2574 kJ/kg, s_{f} = 0.6 kJ/(kg.K), s_{g}= 8.1 kJ/(kg.K).
If the flow rate of steam entering the turbine is 100 kg/s, then the work output (in MW) is ____.
(2 Mark, 2015[1])

Ans: 125.56
Here h_{1 }= 3486 kJ/kg, h_{2 }= 2776 kJ/kg
Again, s_{1} = s_{3}
=> s_{1} = s_{f3} + x * s_{fg3}
=> 6.6 = 0.6 + x * (8.1 – 0.6)
=> x = 0.8
Enthalpy at point 3: h_{3} = h_{f3} + x * h_{fg3}
=> h_{3 }= 174 + 0.8 * (2574 – 174) = 2094 kJ/kg
Work output (W) = 100 * (h_{1} – h_{2}) + 80 * (h_{2} – h_{3})
=> W = 100 * (3486 – 2776) + 80 * (2776 – 2094)
=> W = 125.56 MW
51. In a Rankine cycle, the enthalpies at turbine entry and outlet are 3159 kJ/kg and 2187 kJ/kg,respectively. If the specific pump work is 2 kJ/kg, the specific steam consumption (in kg/kWh) of the cycle based on net output is _____.
(2 Mark, 2015[2])

Ans: 3.711
Specific steam consumption = \frac { 3600 }{ { W }_{ net } } =\frac { 3600 }{ { W }_{ T }{ W }_{ P } }
=> sfc = \frac { 3600 }{ (31592187)(2) } = 3.711 kg/kWh
52. Which of the following statements regarding a Rankine cycle with reheating are TRUE?
(i) increase in average temperature of heat addition
(ii) reduction in thermal efficiency
(iii) drier steam at the turbine exit
(a) only (i) and (ii) are correct
(b) only (ii) and (iii) are correct
(c) only (i) and (iii) are correct
(d) (i), (ii) and (iii) are correct
(1 Mark, 2015[2])

Ans: c
In reheating, the mean temperature of heat addition increases. So the thermal efficiency increases.
53. The INCORRECT statement about regeneration in vapor power cycle is that
(a) it increases the irreversibility by adding the liquid with higher energy content to the steam generator
(b) heat is exchanged between the expanding fluid in the turbine and the compressed fluid before heat addition
(c) the principle is similar to the principle of Stirling gas cycle
(d) it is practically implemented by providing feed water heaters
(1 Mark, 2016[1])

Ans: a
In a vapour power cycle,
The major exergy destruction due to irreversibilities takes place in the steam generation. To improve the performance of the steam plant the finite source temperatures must be closer to the working fluid temperatures to reduce thermal irreversibility.
So, adding the liquid with higher energy content to the steam generator, decreases the irreversibility.
54. In a steam power plant operating on an ideal Rankine cycle, superheated steam enters the turbine at 3 MPa and 350^{0}C. The condenser pressure is 75 kPa. The thermal efficiency of the cycle is ____ percent.
Given data:
For saturated liquid, at P = 75 kPa, h_{f} = 384.39 kJ/kg, v_{f} = 0.001037 m^{3}/kg, s_{f} = 1.213 kJ/kgK
At 75 kPa, h_{fg} = 2278.6 kJ/kg, s_{fg} = 6.2434 kJ/kgK
At P = 3 MPa and T = 350^{0}C (superheated steam), h = 3115.3kJ/kg, s = 6.7428 kJ/kgK
(2 Mark, 2016[1])

Ans: 26
Process 1 – 2 is isentropic.
So, s_{1} = s_{2}
=> s_{1} = s_{f2} + x * s_{fg2}
=> 6.7428 = 1.213 + x *6.2434
=> x = 0.8857
Enthalpy at point 2,
h_{2} = h_{f2} + x * h_{fg2}
=> h_{2 }= 384.39 + 0.8857 * 2278.6 = 2402.546 kJ/kg
Pump work ( W_{pump}) = v_{f3}(p_{4} – p_{3})
=> W_{pump }= 0.001037 * (3000 – 75) = 3.03 kJ/kg
Enthalpy at point 4:
h_{4} = h_{3} + W_{pump}
=> h_{4 }= 384.39 + 3.03 = 387.42 kJ/kg
Efficiency \left( \eta \right) =\frac { { W }_{ net } }{ Heat\quad supply } =\frac { { W }_{ T }{ W }_{ p } }{ { Q }_{ s } }
=> \eta =\frac { \left( { h }_{ 1 }{ h }_{ 2 } \right) { W }_{ p } }{ { h }_{ 1 }{ h }_{ 4 } }
=> \eta =\frac { 3115.32403.233.03 }{ 3115.3387.42 }
=> \eta = 0.26 = 26%
55. Consider a simple gas turbine (Brayton) cycle and a gas turbine cycle with perfect regeneration. In both the cycles, the pressure ratio is 6 and the ratio of the specific heats of the working medium is 1.4. The ratio of minimum to maximum temperatures is 0.3 (with temperatures expressed in K) in the regenerative cycle. The ratio of the thermal efficiency of the simple cycle to that of the regenerative cycle is ______.
(2 Mark, 2016[2])

Ans: 0.8
Efficiency of simple Brayton cycle:
{ \eta }_{ 1 }=1\frac { 1 }{ { r }_{ p }^{ \frac { \gamma 1 }{ \gamma } } } =1\frac { 1 }{ 6^{ \frac { 1.41 }{ 1.4 } } } =0.4006Efficiency of Ideal regenerative Brayton cycle:
{ \eta }_{ 2 }=1\left( \frac { { T }_{ min } }{ { T }_{ max } } \right) { r }_{ p }^{ \frac { \gamma 1 }{ \gamma } }=> { \eta }_{ 2 }=1\left( 0.3 \right) 6^{ \frac { 0.4 }{ 1.4 } } = 0.499
Now, \frac { { \eta }_{ 1 } }{ { \eta }_{ 2 } } =\frac { 0.4006 }{ 0.499 } = 0.8
56. The pressure ratio across a gas turbine (for air, specific heat at constant pressure, c_{p} = 1040 J/kg.K and ratio of specific heats, γ = 1.4) is 10. If the inlet temperature to the turbine is 1200 K and the isentropic efficiency is 0.9, the gas temperature at turbine exit is _____K.
(2 Mark, 2017[1])

Ans: 679.38
For isentropic process (1 – 2):
\frac { { T }_{ 2 } }{ { T }_{ 1 } } ={ \left( \frac { { P }_{ 2 } }{ { P }_{ 1 } } \right) }^{ \frac { \gamma 1 }{ \gamma } }=> { T }_{ 2 }=1200\times { \left( \frac { 1 }{ 10 } \right) }^{ \frac { 0.4 }{ 1.4 } } = 621.54 K
Again, Isentropic efficiency: \left( \eta \right) =\frac { { T }_{ 1 }{ T }'_{ 2 } }{ { T }_{ 1 }{ T }_{ 2 } }
=> 0.9=\frac { 1200{ T }'_{ 2 } }{ 1200621.54 }
=> { T }'_{ 2 } = 679.38 K
57. In the Rankine cycle for steam power plant the turbine entry and exit enthalpies are 2803 kJ/kg and 1800 kJ/kg, respectively. The enthalpies of water at pump entry and exit are 121 kJ/kg and 124 kJ/kg, respectively. The specific steam consumption (in kg/kW.h) of the cycle is ______.
(2 Mark, 2017[2])

Ans: 3.6
Specific steam consumption = \frac { 3600 }{ { W }_{ net } } =\frac { 3600 }{ { W }_{ T }{ W }_{ P } }
=> sfc = \frac { 3600 }{ (28031800)(124121) } =\frac { 3600 }{ 10033 } = 3.6 kg/kWh
58. Select the correct statement for 50% reaction stage in a steam turbine.
(a) The rotor blade is symmetric.
(b) The stator blade is symmetric.
(c) The absolute inlet flow angle is equal to absolute exit flow angle.
(d) The absolute exit flow angle is equal to inlet angle of rotor blade.
(1 Mark, 2018[2])

Ans: d
59. A steam power cycle with regeneration as shown below on the Ts diagram employs a single open feedwater heater for efficiency improvement. The fluids mix with each other in an open feedwater heater. The turbine is isentropic and the input (bleed) to the feedwater heater from the turbine is at state 2 as shown in the figure. Process 34 occurs in the condenser. The pump work is negligible. The input to the boiler is at state 5. The following information is available from the steam tables:
The mass flow rate of steam bled from the turbine as a percentage of the total mass flow rate at the inlet to the turbine at state 1 is ____.
(2 Mark, 2019[1])
60. A gas turbine with air as the working fluid has an isentropic efficiency of 0.70 when operating at a pressure ratio of 3. Now, the pressure ratio of the turbine is increased to 5, while maintaining the same inlet conditions. Assume air as a perfect gas with specific heat ratio γ = 1.4. If the specific work output remains the same for both the cases, the isentropic efficiency of the turbine at the pressure ratio of 5 is _____ (round off to two decimal places).
(2 Mark, 2019[1])

Ans: 0.514
Suppose the inlet temperature to turbine = T_{1}
The outlet temperature to turbine = T_{2}
When pressure ratio is 3 bar:
T_{2 }= \frac { { T }_{ 1 } }{ { r }_{ p }^{ \frac { \gamma 1 }{ \gamma } } } = \frac { { T }_{ 1 } }{ 3^{ \frac { \gamma 1 }{ \gamma } } }
When pressure ratio is 5 bar:
T_{2}‘ = \frac { { T }_{ 1 } }{ 5^{ \frac { \gamma 1 }{ \gamma } } }
Again, W_{actual }= W_{isentropic}\times \eta
Given, Specific work output remains the same for both the cases.
i.e. { c }_{ p }\left( { T }_{ 1 }{ T }_{ 2 } \right) \times { \eta }_{ 1 }={ c }_{ p }\left( { T }_{ 1 }{ T' }_{ 2 } \right) \times { \eta }_{ 2 }
=> { c }_{ p }\left( { T }_{ 1 }\frac { { T }_{ 1 } }{ 3^{ \frac { \gamma 1 }{ \gamma } } } \right) \times 0.7={ c }_{ p }\left( { T }_{ 1 }\frac { { T }_{ 1 } }{ 5^{ \frac { \gamma 1 }{ \gamma } } } \right) \times { \eta }_{ 2 }
=> \left( 1\frac { 1 }{ 3^{ \frac { 0.4 }{ 1.4 } } } \right) \times 0.7=\left( 1\frac { 1 }{ 5^{ \frac { 0.4 }{ 1.4 } } } \right) \times { \eta }_{ 2 }
=> { \eta }_{ 2 } = 0.514
61. Which one of the following modifications of the simple ideal Rankine cycle increases the thermal efficiency and reduces the moisture content of the steam at the turbine outlet?
(a) Decreasing the condenser pressure
(b) Increasing the boiler pressure
(c) Decreasing the boiler pressure
(d) Increasing the turbine inlet temperature
(1 Mark, 2019[2])