Thermal - Ies Vol.1 (CRPQ)
Thermal - Ies Vol.1 (CRPQ)
Thermal - Ies Vol.1 (CRPQ)
Engineering Academy
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08. An ideal–gas mixture consists of 2 kmol of N2 13. An ideal-gas mixture consists of 30 percent
and 6 kmol of CO2. The mass fraction of CO2 Helium and 70 percent Argon gases by mass.
in the mixture is The mixture is now expanded reversible
(a) 0.175 (b) 0.250 adiabatically in a turbine from 400°C and
(c) 0.500 (d) 0.825 1.2 MPa to a pressure of 200 kPa. The mixture
temperature at turbine exit is
09. An ideal – gas mixture consists of 2kmol of N2 (a) 195oC (b) 56oC
and 4kmol of CO2. The apparent gas constant (c) 112oC (d) 130oC
of the mixture is
(a) 0.215kJ/kg.K 14. A piston-cylinder device contains an ideal-
(b) 0.225 kJ/kg.K gas mixture of 3 kmol of He gas and 7 kmol
(c) 0.243 kJ/kg.K of Ar gas at 50°C and 400 kPa. Now the gas
(d) 0.875 kJ/kg.K expands at constant pressure until its volume
double. The amount of heat transfer to the
10. A rigid tank is divided into two compartments gas mixture is
by a partition. One compartment contains (a) 6.2 MJ
3 kmol of N2 at 600 kPa and the other (b) 4.2 MJ
compartment contains 7 kmol of CO2 at 200 (c) 27 MJ
kPa. Now the partition is removed, and the (d) 67 MJ
two gases form a homogenous mixture at 300
kPa. The partial pressure of N2 in the mixture is 15. A spherical balloon rises up and the radius
(a) 75 kPa (b) 90 kPa becomes thrice that on the ground. What
(c) 150 kPa (d) 175 kPa is the pressure at that altitude? Assume the
11. An 80 L rigid tank contains an ideal-gas temperature to be constant and pressure on
mixture of 5g of N2 and 5g of CO2 at a ground level is 72cm of Hg.
Specified pressure and temperature. If N2
were separated from the mixture and stored 16. The volume vs. temperature ‘T’ graphs for
at mixture temperature and pressure, its a certain amount of a perfect gas at two
volume would be pressures ‘P1’ and ‘P2’ are shown in the fig.
(a) 32 L (b) 36 L Which is greater, ‘P1’ or ‘P2’?
(c) 40 L (d) 49 L
V P2
12. An ideal-gas mixture consists of 3 kg of Ar and
P1
6 kg of CO2 gases. The mixture is now heated
at constant volume from 250 K to 350K. The
amount of heat transfer is T
(a) 374 kJ (b) 436 kJ
(c) 488 kJ (d) 525 kJ
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3 Objective Practice Questions
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4 Thermal Engineering
friction free.
Which of the above conditions are correct? Win
(a) 1, 2 and 3 (b) 1 and 2 only (a) boiler and condenser are open system
(c) 1 and 3 only (d) 2 and 3 only and turbine, pump and overall plant are
closed systems
26. Experimental data obtained from a constant (b) turbine and pump are open systems and
volume gas thermometer is shown is the figure boiler, condenser and overall plant are
below. The value of l in °C is closed system
P (c) boiler, turbine, condenser and pump
are open systems and overall plant is a
closed system
(d) overall plant is an open system and
boiler, turbine, condenser and pump are
closed systems
T(°C)
l
KEY for Practice Questions
(a) 373.15 (b) 1.0
(c) –100 (d) –273.15 01. (d) 02. (a) 03. (b) 04. (b) 05. (d)
06. (b) 07. (b) 08. (d) 09. (a) 10. (b)
27. In the figure showing a schematic of a thermal 11. (d) 12. (b) 13. (b) 14. (d)
power plant, the closed system (S) and open 15. (2.67 cm of Hg) 16. (P1 > P2) 17. (d)
system (S) are correctly identified as
18. (26.77°C) 19. (21.6°C)
20. (550°N, -50°C) 21. (c) 22. (c) 23. (a)
24. (a) 25. (a) 26. (d) 27. (c)
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5 Objective Practice Questions
S
B. Work done in steady flow process.
C. Heat transfer in a reversible adiabatic
Q process
D. Work done in an isentropic process
U P R T List - II
Volume 1. –∫vdP
(a) 12 Nm (b) 18 Nm 2. Zero
(c) 24 Nm (d) 36 Nm 3. (P1V1 – P2V2) / (γ-1)
4. (P1V1 – P2V2) / (n-1)
02. A system at a given state undergoes change
through the following expansion processes to (a) A-4, B-1, C-3, D-2
reach the same final volume: (b) A-1, B-4, C-2, D-3
1. Isothermal (c) A-4, B-1, C-2, D-3
2. Isobaric (d) A-1, B-2, C-3, D-4
3. Adiabatic (γ=1.4)
05. A gas expands from pressure P1 to pressure
4. Polytropic (n=1.3)
P2 (P2 = P1/10). If the process of expansion
The correct ascending order of the work
is isothermal, the volume at the end of
output in these four processes is
expansion is 0.55 m3. If the process of
(a) 1,2,4,3 (b) 1,4,3,2
expansion is adiabatic, the volume at the
(c) 4,1,3,2 (d) 3,4,1,2
end of expansion will be closer to
03. The values of heat transfer and work transfer
(a) 0.45 m3 (b) 0.55 m3
for the four processes of a thermodynamic
(c) 0.65 m3 (d) 0.28 m3
cycle are given below:
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14. The amount of heat lost by the air in the working fluid can be represented as a circle
football and the gauge pressure of air in the of 10 cm diameter on Pv diagram on which
football the stadium respectively are 1 cm = 300 kPa, 1 cm = 0.1 m3/kg.
(a) 30.6 J, 1.94 bar (b) 21.8 J, 0.93 bar Find the net work during cycle in kJ/kg.
(c) 61.1 J, 1.94 bar (d) 43.7 J, 0.93 bar
20. An engine cylinder has a piston of area 0.12
15. Gauge pressure of air to which the ball must m2 and contains gas at a pressure of 1.5 MPa.
have been originally inflated so that it would The gas expands according to a process
be equal to 1 bar gauge at the stadium is which is represented by a straight line on a
(a) 2.23 bar (b) 1.94 bar pressure-volume diagram. The final pressure
(c) 1.07 bar (d) 1.00 bar is 0.15 MPa. Calculate the work done by the
gas on the piston if the stroke is 0.30 m.
16. A 0.8 m3 rigid tank contains nitrogen gas at
500 kPa and 300 K. Now the gas is compressed 21. A system of volume ‘V’ contains a mass ‘m’ of
isothermally to a volume of 0.1 m3. The work gas at pressure ‘P’ and temperature ‘T’. The
done on the gas during this compression macroscopic properties of the system obeys
process is the following relationship.
(a) 831 kJ (b) 0 kJ (c) 420 kJ (d) 2440
;cP + m^V − bhE = mRT ,
a
kJ
V2
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8 Thermal Engineering
23. The gas space above water in a closed continuously agitated by means of a stirrer
storage tank contains ‘N2’ at 25°C and 100 passing through the cylinder cover. The
kPa. Total tank volume is 4m and there is 3
cylinder diameter is 0.40 m. During the stirring
500 kg water at 25°C. An additional 500 kg process lasting 10 minutes, the piston slowly
is now forced into tank. Assuming constant moves out a distance of 0.485 m against the
temperature throughout find the final pressure atmosphere. The net work done by the fluid
of ‘N2’ and work done on ‘N2’ in the process ? during the process is 2 kJ. The speed of the
electric motor driving the stirrer is 840 rpm.
24. A piston-cylinder device contains a 0.05 m of 3
Determine the torque in the shaft and the
a gas initially at 200 kPa. At this state a linear power output of the motor.
spring which have spring constant of 150
kN/m is just touching the piston but exerting 27. An electric heater is put inside an insulated
no force on it. Now heat is transfer to the gas chamber containing a gas. Considering the
causing the piston to raise and compress the system boundaries A and B as shown in figure,
spring until the volume inside the cylinder we have
double. If the cross-section area of the piston
A
is 0.25 m2. Find (i) final pressure inside the
cylinder, (ii) workdone by the gas.
B
25. A cylinder is fitted with a piston as shown
contains 0.01 m3 of gas at a pressure of 0.1
Insulated
MPa. The C.S.A. of piston is 0.05 m2. Initially chamber
the spring does not touch the piston but
atmospheric pressure of 0.1 MPa acts on Electric
the piston. The gas is heated till the volume heater
is three times the original value. If the spring (a) Heat transfer across A and B
constant is 25 kN/m. Calculate work done by (b) Heat transfer across A, work transfer across
gas. B
(c) Work transfer across A, work transfer across
Pa = 0.1 MPa B
(d) Work transfer across A, heat transfer across
0.2m B
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9 Objective Practice Questions
Pressure
19. (2356.2 kJ/kg) 20. (29.7 kJ)
21. (1742 kJ) 22. (686.62 kJ) A D
23. (116.67 kPa, 54.42 kJ)
Volume
24. (320 kPa, 13 kJ) 25. (2.5 kJ)
How much heat will flow into the system along
26. (0.079 N-m, 6.95 W) 27. (d) 28. (a)
the path A-D-B if the work done by it along
the path is 40 kJ?
(a) 40 kJ (b) 60 kJ
(c) 90 kJ (d) 135 kJ
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(c) A rigid vessel containing ammonia resistance heater are running continuously
gas connected through a valve to an but the air temperature in the room remains
evacuated rigid vessel, the valve and the constant. The rate of heat loss from the room
connecting pipes being well insulated that day is,
and the valve being opened and after (a) 3312 kJ/h (b) 4752 kJ/h
a time, condition through the two vessels (c) 5112 kJ/h (d) 2952 kJ/h
becoming uniform.
(d) 1 kg of air flowing adiabatically from the 07. A rigid insulated tank of 3 m3 volume is divided
atmosphere into a previously evacuated into two compartments one compartment of
bottle. volume 1 m3 contains an ideal gas at 0.1 MPa
and 300 K while the second compartment of
04. A well-sealed room contains 60 kg of air at volume 2m3 contains the same gas at 1 MPa,
200 kPa and 25°C. Now solar energy enters and 1000 K. If the partition between the two is
the room at an average rate of 0.8 kJ/s while ruptured determine,
a 120W fan is turned on to circulate the air in
the room. If heat transfer through the walls is (i) Number of moles in chamber A and
negligible, the air temperature in the room in chamber B are
30 min will be (a) 400 & 2400 moles
(a) 25.6°C (b) 49.8°C (b) 40.093 & 240.558 moles
(c) 53.4°C (d) 54.4°C (c) 0.040 & 0.24 moles
(e) 63.4°C (d) 0.045093 & 0.240558 moles
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There are no significant changes in KE and PE. required to move it. Heat is now transferred to
(i) Heat transfer and change in internal energy the air until volume doubles. Determine total
for the process1- 2 are work done and the total heat transferred
(a) 36.9 kJ & 26.4 kJ to the air during the process. Also show the
(b) –15.9 kJ & – 26.4 kJ process on a P-V diagram.
(c) 36.9 kJ & - 26.4 kJ
(d) –15.9 kJ & 26.4 kJ 12. A piston-cylinder device contains an ideal
gas. The gas undergoes two successive
(ii) The heat transfer in process 2-3 is cooling processes by rejecting heat to
(a) 18.78 kJ (b) –18.78 kJ the surroundings. First the gas is cooled at
(c) zero (d) 10.5 kJ 3
constant pressure until T2 = T.
4 1
(iii) Heat transfer in process 3-1 is Then the piston is held stationary while the
1
(a) 26. 4 kJ (b)–26.4 kJ gas is further cooled to T3 = T , where all
2 1
(c) –18. 78 kJ (d) 18.78 kJ temperatures are in K.
(i) The ratio of the final volume to the initial
09. A gas undergoes a thermodynamic cycle volume of the gas is
consisting of three processes beginning at an (a) 0.25 (b) 0.50
initial state where P1 = 1 bar, V1 = 1.5 m and 3
(c) 0.67 (d) 0.75 (e) 1.0
U1 = 512 kJ. The processes are as follows:
(a) Process 1-2: Compression with (ii) The work done on the gas by the piston is
PV = constant to P2 = 2 bar, U2 = 690 kJ (a) RT1/4 (b) CvT1/2
(b) Process 2-3: W23 = 0, Q23 = –150 kJ and (c) CpT1/2 (d) (Cv+Cp)T1/4
(c) Process 3-1: W31 = +50 kJ, Neglecting KE (e) Cv(T1 +T2)/2
and PE changes, determine the heat
interactions Q12 and Q31. (iii) The total heat transferred from the gas is
(a) RT1/4 (b) CvT1/2
10. Air in a rigid tank is at 100 kPa, 300 K with a (c) CpT1/2 (d) (Cv +Cp)T1/4
volume of 0.75 m . The tank is heated to 400 K,
3
(e) Cv(T1+T3)/2
state2. Now one side of the tank acts as piston,
letting the air expand slowly at constant 13. A gas of mass 1.5 kg, undergoes a quasi-static
temperature to state 3 with a volume of 1.5 expansion which follows a relationship P = a +
m . Find the pressure at states 2 and 3. Find
3
bV, where a and b are constants. The initial
the total work and total heat transfer. and final pressures are 1000 kPa and 200 kPa
respectively and the corresponding volumes
11. A piston-cylinder device, whose piston is are 0.20 m3 and 1.20m3. The specific internal
resting on a set of stops, initially contains 3 energy of the gas is given by the relation.
kg of air at 200 kPa and 27°C. The mass of u = 1.5 Pv – 85 kJ/kg, where P is in kPa and v is
the piston is such that a pressure of 400 kPa is in m3/kg.
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(i) The values of a and b are For the fluid system, calculate E2 and E3, if
(a) –1160 kPa & – 800 kPa/m 3
E1 = 0.
(b) –1160 kPa & 800 kPa/ m 3
(a) 29.7 kJ & 110.7 kJ
(c) 1160 kPa & – 800 kPa/m 3
(b) –29.7 kJ & –110.7 kJ
(d) 1160 kPa & 800 kPa/m 3
(c) 29.7 kJ & zero
(d) zero & 110.7 kJ
(ii) The heat transferred is
(a) 640 kJ (b) 600 kJ 16. The heat capacity at constant pressure of a
(c) 660 kJ (d) 620 kJ certain system is a function of temperature
only and may be expressed as
(iii) The maximum Internal energy attained
41.87
during the expansion is C P = 2.093 + + J/cC
t 100
(a) 335. 53 kJ (b) 503.3 kJ
(c) –335.3 kJ (d) –503.3 kJ where t is the temp of the system in °C. The
system is heated while it is maintained at a
14. One mole of ideal gas Cv = 20.785 J/mol-K at pressure of 1atm until its volume increases
300 K and 0.1 MPa is contained in an insulated from 2000 cm3 to 2400 cm3 and its temp
rigid cylinder. A paddle wheel is inserted into increases from 0°C to 100°C.
gas space and it was rotated by an electric
motor of 50 W capacity for 2 minutes. Estimate (i) Find the magnitude of heat interaction.
the final temperature and pressure of gas. (a) 238.32 J (b) 219.24 J
(a) 588.67 K; 0.1962 MPa (c) 238.32 kJ (d) 219.22 kJ
(b) 58.88 K & 1.962 MPa (ii) How much does the Internal energy
(c) 578.67 K; 19.62 MPa increase ?
(d) None (a) 197.79 kJ (b) 197.79 J
(c) 179.24 kJ (d) 179.24 J
The fluid is surrounded by a perfect heat of a steadily flowing stream. At the inlet to a
insulator during the reaction which begins at certain nozzle, the enthalpy of the fluid passing
state 1 and ends at state 2. The insulation is is 3000 kJ/kg and the velocity is 60 m/s. At the
then removed and105 kJ of heat flows to the discharge end, the enthalpy is 2762 kJ/kg.
surroundings as the fluid goes to state 3. The The nozzle is horizontal and there is negligible
following data are observed for the fluid at heat loss from it. (A1 = 0.1 m2, v1 = 0.187 m3/kg,
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13 Objective Practice Questions
24. An ideal-gas mixture of helium and argon 28. Consider an ideal gas in a frictionless piston
gases with identical mass fractions enters a cylinder assembly. The weightless piston is
turbine at 1200 K and 1 MPa at a rate 0.3 kg/s, initially loaded with a large number of small
and expands isentropically to 100 kPa. The weights. How would you carry out a reversible
power output of the turbine is isothermal expansion process ? Assume that
(a) 478 kW (b) 619 kW a large number of very small weights, and an
(c) 926 kW (d) 729 kW arrangement for reversible heat transfer are
available
25. Helium at 20 atm and 40°C is contained (a) without adding or removing weights on
in a small steel cylinder having a volume the piston, transfer heat to the system
of 15 cm . The cylinder is placed in a large
3
(b) without adding or removing weighs on
container having a volume of 1500 cm . The 3
the piston, transfer heat from the system
large cylinder is perfectly evacuated and (c) adding weights on the piston, transfer
insulated. By an appropriate means Helium is heat to the system
discharged to fill the container. Calculate the (d) removing weight from the piston, transfer
final pressure after equilibrium is reached. heat to the system
(a) 0.2 atm (b) 0.202 atm
(c) 0.198 atm (d) none 29. Consider steady flow of air (Cp = 1.005 kJ/
kgK) in an adiabatic passage. Air enters the
26. An insulated storage tank that is initially passage at 100 kPa, 500 K at a velocity of 150
evacuated is connected to a supply pipeline m/s and exits the passage at 510 K. Assume air
carrying a fluid specific internal energy ui and to be an ideal gas and neglect gravitational
specific enthalpy hi. The valve is opened and effects. The passage is a
fluid flows into the tank through the supply (a) diffuser and the velocity at the exit is
line and reaches the pressure same as that of approximately 49 m/s
supply pipeline. (b) diffuser and the velocity at the exit is
(i) Show that final specific internal energy of approximately 79 m/s
fluid in tank is equal to hi. (c) nozzle and the velocity at the exit is
(ii) Find the final temperature of fluid inside approximately 179 m/s
the tank if the fluid flowing is an ideal gas (d) nozzle and the velocity at the exit is
and supply line temperature is Ti. approximately 249 m/s
27. Statement (I): If the internal energy of a closed 30. If δQ is the heat transferred to the system and
system decreases by 25 kJ while the system δω is the work done by the system, then which
receives 30 kJ of energy by heat transfer, the of the following is an exact differential
work done by the system is 55 kJ. (a) δQ (b) δω
Statement (II): The first law energy balance for (c) δQ +δω (d) δQ – δω
a closed system is (notations have their usual
meaning) ∆E = Q – W.
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15 Objective Practice Questions
P
J
M
K L
List – I
A. J → K B. K → L
C. L → M D. M → J
List – II
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16 Thermal Engineering
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17 Objective Practice Questions
stipulates the law of conservation of 11. A heat pump with a COP of 3.2 is used to heat
energy and entropy. a perfectly sealed house (no leaks). The entire
Which of these statements are correct? mass with in the house (air, furniture, etc.) is
(a) 1 alone (b) 2 and 4 equivalent to 1200 kg of air. When running
(c) 2, 3 and 4 (d) 1, 2 and 3 the heat pump consumes electric power at
a rate of 5 kW. The temperature of the house
08. The heat absorbed and rejected during a was 7°C when the heat pump was turned on.
polytropic process is equal to If heat transfer through the envelop of the
(a) {(γ-n)/(γ-1)} 1/2
x work done house (walls roof, etc) is negligible, the length
(b) (γ-n)/(n-1) x work done of time the heat pump must run to raise the
(c) (γ-n)/(γ-1) x work done temperature of the entire contents of the
(d) {(γ-n)/(γ-1)} x work done
2
house to 22°C is,
(a) 13.5 min (b) 43.1 min
09. The data given in the table refers to an engine (c) 138 min (d) 18.8 min (e) 808 min
based on Carnot cycle, where
Q1 = Heat received (kJ/min), 12. A typical new household refrigerator
Q2 = heat rejected (kJ/s), consumes about 680 kWh of electricity per
W = work output (kW) year and has a coefficient of performance
of 1.4. The amount of heat removed by this
S.No Q1 Q2 W refrigerator from the refrigerated space per
1. 1500 16.80 8.20 year is
2. 1600 17.92 8.75 (a) 952 MJ/yr (b) 1749 MJ/yr
3. 1700 19.03 9.30 (c) 2448 MJ/yr (d) 3427 MJ/yr
4. 1800 20.15 9.85 (e) 4048 MJ/yr
If heat received by the engine is 2000 kJ/ 13. A window air conditioner that consumes
minute the work output will be, nearly, 1kW of electricity when running and has a
(a) 9.98 (b) 10.39 coefficient of performance of 4 is placed in
(c) 11.54 (d) 10.95 the middle of a room, and is plugged in. The
rate of cooling or heating this air conditioner
10. The drinking water needs are met by cooling will provide to the air in the room when
tap water in a refrigerated water fountain running is
from 23 to 6°C at an average rate of 10 (a) 4 kJ/s, cooling (b) 1 kJ/s, cooling
kg/h. if the COP of this refrigerator is 3.1, the (c) 0.25 kJ/s, heating (d) 1 kJ/s, heating
required power input to this refrigerator is (e) 4 kJ/s, heating
(a) 197 W (b) 612 W
(c) 64W (d) 109 W 14. In the figure, E is a heat engine with efficiency
(e) 403 W of 0.4 and R is a refrigerator. Given that Q2+Q4
= 3Q1 the COP of the refrigerator is
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19 Objective Practice Questions
(b) It is proposed to use the same heat 26. Statement (I): Negative temperatures are
pump to cool the house in summer. For impossible on the Kelvin scale.
the same room temperature, the same Statement (II): The Kelvin scale is
heat loss rate, and the same power thermodynamic temperature scale.
input to the pump, what is the maximum (ESE – 14)
permissible atmospheric temperature?s
27. Statement (I): Though heat is added during
23. A reversible engine, as shown in below figure a polytropic expansion process for which g >
during a cycle of operation draws 5 MJ n > 1, the temperature of the gas decreases
from the 400 K reservoir and does 840 kJ of during the process.
work. Find the amount and direction of heat Statement (II): The work done by the system
interaction with other reservoirs. exceeds the heat added to the system.
(ESE– 07)
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20 Thermal Engineering
1 1
50 kJ
(c) η max (d) 1 − η max
100 kJ
60 kJ
31. According to clausius statement of second
Cyclic
device law of thermodynamics, the COP of a
refrigerator is never
(a) ∞ (b) > 1 (c) 1 (d) < 1
The cyclic device can be
(a) reversible heat engine
(b) a reversible heat pump or a refrigerator KEY for Practice Questions
(c) an irreversible heat engine
01. (b) 02. (c) 03. (c) 04. (b) 05. (b)
(d) an irreversible heat pump or an
06. (a) 07. (a) 08. (c) 09. (d) 10. (c)
irreversible refrigerator
11. (a) 12. (d) 13. (d) 14. (d)
29. Match list – I and list – II and select the correct 15. (0.667 MJ) 16.(0.68) 17. (86 kW)
answer using the codes given below the lists: 18. a-(23891 kJ, 5531 kJ) 19. (15.168)
List – I b-(256.3 kJ, 1896.3 kJ)
A. Cycle is reversible 20. (6326.5 kJ/hr) 21. a(12.55 kW), b(0.708),
c(223.37 kW
B. Cycle is irreversible and possible
22. (2 kW, 50oC) 23. Q2 = 820 kJ,
C. Cycle is impossible Q3 = -4980 kkJ
List – II 24. (d) 25. (B) 26. (a) 27. (a) 28. (a)
# dQ
3. T <0
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21 Objective Practice Questions
5. Entropy
T 1,1′ 2 2′
C
B V
A
(d)
P
S 1,1′
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22 Thermal Engineering
05. Match List-I with List-II and select the correct 07. Match the curves in Diagram I with the curves
answer: from the codes given below the lists in Diagram II and select the correct answer.
List – I Diagram I (Process on p-v plane).
A. The entropy of a pure crystalline
P A
substance is zero at absolute zero
temperature
O B
B. Spontaneous processes occur in a
C
certain direction
C. If two bodies are in thermal equilibrium D
with a third body, then they are also in V
thermal equilibrium with each other. Diagram II (Process on T-s plane)
D. The law of conservation of energy.
2
T 1
3
List – II
1. First law of thermodynamics 0 4
2. Second law of thermodynamics
5
3. Third law of thermodynamics
4. Zeroth law of thermodynamics s
Codes:
(a) A-2, B-3, C-4, D-1
(a) A-3, B-2, C-4, D-5
(b) A-3, B-2, C-1, D-4
(b) A-2, B-3, C-4, D-5
(c) A-3, B-2, C-4, D-1
(c) A-2, B-3, C-4, D-1
(d) A-2, B-3, C-1, D-4
(d) A-1, B-4, C-2, D-3
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23 Objective Practice Questions
09. Which one of the following statements 13. A heat engine receives 1000 kW of heat at
applicable to a perfect gas will also be true a constant temperature of 285°C and rejects
for an irreversible process? (Symbols have the 492 kW of heat at 5°C. Consider the following
usual meanings). thermodynamic cycles in this regard:
(a) δQ = du + p dv (b) δQ = T ds 1. Carnot cycle
(c) T ds = du + p dv (d) None of the above. 2. Reversible cycle
3. Irreversible cycle.
10. Assertion (A): If a graph is plotted for absolute Which of these cycles could possibly be
temperature as a function of entropy, the executed by the engine?
area under the curve would give the amount (a) 1 alone (b) 3 alone
of heat supplied. (c) 1 and 2 (d) None of 1, 2 and 3
Reason (R): Entropy represents the maximum
fraction of work obtainable from heat per 14. An electric motor of 5 kW is subjected to a
degree drop in temperature. braking test for 1 hour. The heat generated
by the frictional forces in the process is
11. Four processes of a thermodynamic cycle transferred to the surroundings at 20°C. The
are shown in fig. 1 on the T-s plane in the resulting entropy change will be
sequences 1-2-3-4. The corresponding correct (a) 22.1 kJ/K (b) 30.2 kJ/K
sequence of these processes in P-v plane as (c) 61.4 kJ/K (d) 82.1 kJ/K
shown in fig. 2 will be
T 3 15. 1600 kJ of energy is transferred from a heat
P
C reservoir at 800 K to another heat reservoir
D
4 2 B at 400 K. The amount of entropy generated
1 A during the process would be
(a) 6 kJ/K (b) 4 kJ/K
Fig 1 s v
Fig 2 (c) 2 kJ/K (d) zero
(a) (C-D-A-B) (b) (D-A-B-C) 16. A system of 100 kg mass undergoes a process
(c) (A-B-C-D) (d) (B-C-D-A) in which its specific entropy increases from 0.3
kJ/kg-K to 0.4 kJ/kg-K. At the same time, the
12. A cyclic heat engine receives 600 kJ of heat entropy of the surroundings decreases from
from a 1000 K source and rejects 450 kJ to a 80 kJ/K to 75 kJ/K. The process is:
dQ (a) Reversible and isothermal
300K sink. The quantity # T and efficiency (b) Irreversible
of the engine are respectively. (c) Reversible
(d) Impossible
(a) 2.1 kJ/K and 70 %
(b) – 0.9 kJ/K and 25 %
(c) +0.9 kJ/K and 70 %
(d) –2.1 kJ/K and 25 %
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24 Thermal Engineering
17. Entropy of a saturated liquid at 227°C is by two compressors: one isothermal and
2.6 kJ/kg K. and Its latent heat of vaporization the other adiabatic. If the entropy change
is 1800 kJ/kg; then the entropy of saturated of air is Δsisot during the reversible isothermal
vapour at 227°C would be compression, and Δsadia during the reversible
(a) 2.88 kJ/kg K (b) 6.2 kJ/kg K adiabatic compression, the correct
(c) 7.93 kJ/kg K (d) 10.53 kJ/kg K statement regarding entropy change of air
per unit mass is
18. Steam is condensed at a constant (a) Δsisot = Δsadia = 0 (b) Δsisot = Δsadia > 0
temperature of 30°C as it flows through the (c) Δsadia > 0 (d) Δsisot < 0
condenser of a power plant by rejecting
heat at a rate of 55 MW. The rate of entropy 22. The heat added to a closed system during
change of steam as it flows through the
a reversible process is given by Q = αT + βT2,
condenser is
where α and β are constants. The entropy
(a) – 1.83 MW/K (b) – 0.18 MW/K
change of the system as its temperature
(c) 0.56 MW/K (d) 0 MW/K
changes from T1 to T2 is equal to
(a) α + β(T2 – T1)
19. A unit mass of a substance undergoes an
(b) [α(T2-T1) + (β/2) (T22 – T12)]/T1
irreversible process from state 1 to state 2
(c) [(α/2)(T22-T12)+(β/3)(T23–T13)]/T12
while gaining heat from the surroundings
(d) αln(T2/T1) + 2β(T2 – T1)
at temperature T in the amount of q. If the
entropy of the substance is s1 at state 1,
23. Heat is lost through a plane wall steadily at a
and s2 at state 2, the entropy change of the
rate of 600W. If the inner and outer surface
substance Δs during this process is
(a) Δs < s2 – s1 (b) Δs > s2 – s1 temperatures of the wall are 20oC and 5oC,
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25 Objective Practice Questions
heat of evaporation is 2066.3 kJ/kg. Find the 29. Calculate the entropy change of the universe
increase of entropy of water, if the final state as a result of the following processes:
is steam. (a) A copper block of 600 g mass and with
Cp of 150 J/K at 100oC is placed in lake at
26. In a Carnot cycle, heat is supplied at 350oC 8oC.
and rejected at 27oC. The working fluid is water (b) The same block at 8oC, is dropped from
which, while receiving heat, evaporates a height of 100m into the lake.
from liquid at 350oC to steam at 350oC. The (c) Two such blocks, at 100oC and 0oC, are
associated entropy change is 1.44 kJ/kg K. jointed together.
(a) If the cycle operates on a stationary
mass of 1 kg of water, how much is the 30.
work done per cycle, and how much is (a) One kg of water at 273 K is brought into
the heat supplied? contact with a heat reservoir at 373 K. When
(b) If the cycle operates in steady flow with a the water has reached 373 K, find the entropy
power output of 20 kW, what is the steam change of the water, of the heat reservoir,
flow rate? and of the universe.
(b) If water is heated from 273K to 373 K by first
27. Ten grams of water at 20oC is converted bringing it in contact with a reservoir at 323K
in to ice at –10oC at constant atmospheric and then with a reservoir at 373K, what will
pressure. Assuming the specific heat of liquid the entropy change of the Universe is?
water to remain constant at 4.2 J/gmK and (c) Explain how water might be heated from
that of ice to be half of this value, and taking 273 K to 373 K with almost no change in the
the latent heat of fusion of ice at 0o to be 335J/ entropy of the universe.
gm, calculate the total entropy changes of
the system. 31. The value of Cp for a certain substance can
be represented by Cp = a + bT.
28. A resister of 30 Ω is maintained at a constant (a) Determine the heat absorbed and
temperature of 27oC while a current of 10 the increase in entropy of a mass m of
A is allowed to flow for 1 sec. Determine the substance when its temperature is
the entropy change of the resistor and the increased at a constant pressure from T1
universe. If the resistor initially at 27oC is now to T2.
insulated and the same current is passed (b) Find the increase in the molar
for the same time, determine the entropy specific entropy of copper, when the
change of the resistor and the universe. The temperature is increased at constant
specific heat of the resistor is 0.9 kJ/kg K and pressure from 500 to 1200 K.
the mass of the resistor is 10 gms. Given for copper:
when T = 500 K, Cp = 25.2x103 and
when T =1200 K, Cp = 30.1x103 J/k.mol.K
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27 Objective Practice Questions
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28 Thermal Engineering
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29 Objective Practice Questions
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30 Thermal Engineering
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31 Objective Practice Questions
06. Steam expands in an adiabatic turbine from Mass of water drained from the separator
8 MPa and 500°C to 0.1 MPa at a rate of 3 kg/ ....... 0.55 kg
sec. If steam leaves the turbine as saturated Mass of steam condensed after passing
vapor, the power output of the turbine is through the throttle valve ...…. 4.20 kg
(a) 2174 kW (b) 698 kW Pressure and temperature after throttling
(c) 288 kW (d) 1674 kW ...…... 1 bar, 120°C
(e) 3240 kW Evaluate the dryness fraction of the steam in
the main, and state with reasons, whether the
07. A 300 m rigid tank is filled with saturated
3
throttling calorimeter alone could have been
liquid vapor mixture of water at 200 kPa. If 25 used for this test.
percent of the mass is liquid and 75 percent
of the mass is vapor, the total mass in the tank 12. Steam from an engine exhaust at 1.25 bar
is flows steadily through an electric calorimeter
(a) 451 kg (b) 556 kg and comes out at 1 bar, 130°C. The
(c) 300 kg (d) 331 kg (e) 195 kg calorimeter has two 1 kW heaters and the
flow is measured to be 3.4 kg in 5 min. Find the
08. 3 m rigid vessel contains steam at 10 MPa
3
quality of steam in the engine exhaust. For
and 500 C. The mass of the steam is
o
the same mass flow and pressure, what is the
(a) 3.0 kg (b) 19 kg maximum moisture that can be determined if
(c) 84 kg (d) 91 kg (e) 130 Kg the outlet temperature is at least 105°C?
09. Steam expands in a turbine from 4 MPa and 13. Water has a critical specific volume of
500°C to 0.5 MPa and 250°C at a rate of 1350 0.003155 m3/kg. A closed and rigid steel tank
kg/h. Heat is lost from the turbine at a rate of of volume 0.025 m3 contains a mixture of
25 kJ/s during the process. The power output water and steam at 0.1 MPa. The mass of the
of the turbine is mixture is 10 kg. The tank is now slowly heated.
(a) 157 kW (b) 207 kW The liquid level inside the tank
(c) 182 kW (d) 287 kW (e) 246 kW (a) will rise
(b) will fall
10. A sample of steam from a boiler drum at 3 (c) will remain same.
MPa is put through a throttling calorimeter (d) may rise or fall depending on the amount
in which the pressure and temperature are of heat transferred
found to be 0.1 MPa, 120°C. Find the quality
of the sample taken from the boiler. 14. Statement (I): At a given temperature, the
enthalpy of super-heated steam is the same
11. The following observations were recorded in as that of saturated steam.
an experiment with a combined separating Statement (II): The enthalpy of vapour at
and throttling calorimeter: lower pressures dependent on temperature
Pressure in the steam main ...….15 bar alone. (ESE– 98)
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32 Thermal Engineering
15. Statement (I): For a mixture of solid, liquid 20. Which one of the following statements is
and vapour phases of a pure substance correct? (ESE – 94)
in equilibrium, the number of independent (a) Pressure and temperature are
intrinsic properties needed is equal to one. independent during phase change
Statement (II): The three phases can coexist (b) An isothermal line is also a constant
only at one particular pressure. (ESE – 05) pressure
(c) Entropy decreases during evaporation
16. State to a wet vapor cannot be specified by (d) The term dryness fraction is used to
only (ESE – 11) specify the fraction by mass of liquid in
(a) Pressure and temperature mixture of liquid and vapor
(b) Pressure and dryness fraction
(c) Temperature and dryness fraction
(d) Pressure and volume 21. If a pure substance is below the triple point
temperature, the solid on being heated will
17. Statement (I): One the enthalpy – entropy only (ESE – 13)
diagram of a pure substance the constant (a) solidify or freeze
dryness fraction lines start from the critical (b) liquefy
point. (c) vaporize or sublimate
Statement (II): All the three phases co-exist at (d) have its temperature increased
the critical point.
19. A substance expands on freezing only if 09. (a) 10. (0.95) 11. (0.8488)
12. (0.94, 0.92) 13. (a) 14. (d) 15. (d)
(a) the slope of the sublimation line on P-T
16. (a) 17. (c) 18. (d) 19. (c)
chart is –ve.
20. (b) 21. (c)
(b) the slope of the sublimation line of P-T
chart is +ve
(c) the slope of the fusion line on P-T chart is
–ve
(d) the slope of the fusion line on P-T chart is
+ve
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33 Objective Practice Questions
1
1 (a) A-4, B-3, C-1, D-2 (b) A-4, B-3, C-2, D-1
V V
(c) A-3, B-4, C-1, D-2 (d) A-3, B-4, C-2, D-1
(c)
P (d)
2 P 2
3 04. A system comprising of a pure substance
3
executes reversibly a cycle 1-2-3-4-1 consisting
1 1 of two isentropic and two isochoric processes
V
V as shown in the fig.
2
P
3
02. For maximum specific output of a constant 1
volume cycle (Otto cycle) 4
(a) The working fluid should be air V
(b) The speed should be high Which one of the following is the correct
(c) Suction temperature should be high representation of this cycle on the
(d) Temperature of the working fluid at the temperature-entropy coordinates?
end
of compression and expansion should be
equal
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34 Thermal Engineering
(a) (b) 07. Consider air standard Otto and Diesel cycles,
both having the same state of air at the start
T 4 T 1
of compression. If the maximum pressure in
3 1 both the cycles is the same, the compression
4 2
ratio ‘r’ and the efficiency ‘η’ are related by
2 3
(a) r Diesel > r Otto (b) r Diesel < r Otto
S S
(c) η Otto > η Diesel (d) η Otto < η Diesel
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35 Objective Practice Questions
200
12. Helium gas in an ideal Otto cycle is
compressed from 20°C and 2·5 L to 0.25 L, and
its temperature increases by an additional 0.01 0.03 V(m )
3
700°C during the heat addition process. The 17. A four-stroke Diesel engine has swept volume
temperature of helium before the expansion of 0.03 m3. The engine has an output of 1000
process is kW at 2000 rpm. The mean effective pressure
(a) 1790oC (b) 2060oC (in MPa) is
(c) 1240oC (d) 620oC (a) 2 (b) 1 (c) 0.2 ( d )
0.1
13. The compression ratio used in a Diesel engine
is 15 and cut-off occurs at 6.5% of stroke. 18. In an ideal Otto cycle, air is compressed
Determine the air standard efficiency. from 1.20kg/m3 and 2.2 L to 0.26 L, and the
net work output of the cycle is 440 kJ/kg. The
14. Air in an ideal Diesel cycle is compressed mean effective pressure (MEP) for this cycle is
from 3L to 0.15L, and then it expands during (a) 612 kPa (b) 599 kPa
the constant pressure heat addition process (c) 528 kPa (d) 416 kPa
to 0.30L. Under cold air standard conditions,
the thermal efficiency of this cycle is 19. The efficiency of an Otto cycle is 54%. The
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36 Thermal Engineering
(c) Otto and Joule 10. (c) 11.(47.4) 12. (a) 13. (60.8%)
(d) Joule and Diesel 14. (c) 15. (63.44 %) 16. (235.62 kPa)
17. (a) 18. (b) 19. (1775.93 kJ, 959 kJ,
25. Consider the following statements 816.9, 13.55 bar)
20. (54%) 21. (66.5%) 22. (503 mm2)
1. Carnot, Ercisson and stirling are ideal
power cycles that are completely 23. (c) 24. (c) 25. (a) 26. (d)
reversible
2. Ercisson cycle is not a practical engine
cycle
3. Stirling cycle is the practical power cycle
among the above
4. All these cycles have the same thermal
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37 Objective Practice Questions
04. In case A, moist air is adiabatically saturated 07. With respect to the following figure which
and in case B, moist air is isobarically saturated. shows four processes on the Psychrometric
The saturation temperatures in cases A and B chart, match List-I with List-II and select the
are respectively correct answer using the codes given below
the lists:
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38 Thermal Engineering
List – I List – II
A. Process RS T 1. Sensible cooling
S W
B. Process RT 2. Cooling and dehumidification
U R
C. Process RU 3. Heating and humidification
D. Process RW 4. Sensible heating
List – II
Codes:
1. Cooling and humidifying
(a) A-2, B-1, C-3, D-4
2. Sensible heating
(b) A-3, B-1, C-2, D-4
3. Cooling and dehumidifying
(c) A-3, B-4, C-2, D-1
4. Humidifying
(d) A-4, B-3, C-2, D-1
Codes:
10. If air dry-bulb temperature of 350C and dew
(a) A-2, B-3, C-1, D-4
point temperature of 200C passes through a
(b) A-1, B-4, C-3, D-2
cooling coil which is maintained at 250C, then
(c) A-3, B-1, C-4, D-2
the process would be
(d) A-3, B-2, C-1, D-4
(a) sensible cooling
(b) cooling and dehumidification
08. During chemical dehumidification
(c) cooling and humidification
(a) wet bulb temperature remains constant
(d) cooling with constant wet bulb
but enthalpy changes.
temperature
(b) dry bulb temperature remains constant.
(c) both dew point and wet bulb
11. The process in a hot water spray washer
temperature remain constant .
maintained at a temperature of 40°C,
(d) enthalpy and wet bulb temperature
through which unsaturated air at 10°C dry
remain constant.
bulb temperature and 50% relative humidity
passes, is
09. Match List – I with List – II and select the
(a) sensible heating
correct answer using the codes given below
(b) humidification
the lists:
(c) heating and humidification
List – I
(d) heating and dehumidification
A. Steam spray into air
B. Air passing over a coil carrying steam
12. A sample of moist air is at a temperature
C. Air passing over coil having temperature
‘T’ and relative humidity 50%. A part of the
less than dew point
moisture is removed adiabatically by using
D. Air passing over a coil having temperature
an adsorbent. If the heat of absorption is
above the dew point but below the wet
negligible, the resulting air will have the same
(a) dry bulb temperature but a lower wet
bulb temperature.
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39 Objective Practice Questions
(b) wet bulb temperature but a higher dry 17. A room contains 50 kg of dry air and 0.6 kg of
bulb temperature. water vapor at 25oC and 95 kPa total pressure.
(c) dry bulb temperature but a higher wet The relative humidity of air in the room is
bulb temperature. (a) 1.2% (b) 18.4%
(d) wet bulb temperature but a lower dry (c) 56.7% (d) 65.2%
bulb temperature.
18. A 40-m3 room contains air at 30oC and a total
13. If Pv is the partial pressure of vapour, Ps is the pressure of 90 kPa with a relative Humidity of
partial pressure of vapour for saturated air and 75 percent. The mass of dry air in the room is
Pb is the barometric pressure, the relationship (a) 24.7 kg (b) 29.9 kg
between relative humidity ‘φ’ and degree of (c) 39.9 kg (d) 41.4 kg
saturation ‘µ’ is given by
19. Degree of saturation of air at 30oC, 100 kPa is
(a) µ = φ [ (Pb-Ps) / ( Pb - Pv)]
24%. The saturation pressure of water vapour
(b) µ = φ [ (Pb-Pv) / ( Pb - Ps)] at 30oC is 4 kPa. Find the relative humidity and
(c) µ = φ Pv / Pb specific humidity.
the moisture in the air will start condensing is At T2 = 20oC Psat = 2.337 kPa
(a) 8.7 C
o
(b) 11.3 C o
T2 = 45 C
o
Psat = 9.557 kPa
(c) 13.8 C o
(d) 9.16 C o
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41 Objective Practice Questions
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42 Thermal Engineering
hf = 191.83 kJ/ kg
10. Consider a steam power plant which
sf = 0.6493 kJ/kgK
operates on a reheat Rankine cycle and
has a net power output of 150 MW. Steam vg = 14.67 m3/kg
enters the high-pressure turbine at 10 MPa hf = 2392.8 kJ/ kg
and 500°C and the low – pressure turbine at 1
sg = 8.1502 kJ/ kgK
MPa and 500°C. Steam leaves the condenser
as a saturated liquid at a pressure of 10 kPa. hg = 2584.7 kJ/ kg
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43 Objective Practice Questions
6 MPa, 450°C : 13. In an ideal steam power cycle with the same
inlet pressure, the low fraction of steam in
v = 0.05214 m3/kg;
the last stage of expansion process can be
h = 3301.8 kJ/ kg; avoided by
(a) Providing regeneration
s = 6.7193 kJ / kg K
(b) Providing reheating
20 kPa, T = 60.06°C :
(c) Reducing the superheat
vf = 1.017 cm3/g (d) Lowering the condenser pressure
hf = 251.40 kJ/ kg
14. Statement (I): The Rankine cycle with
sf = 0.8320 kJ/kg K regenerative feed heating always has a
higher cycle efficiency than the Rankine
vg = 7.649 m3/kg
cycle without regenerative feed heating.
hfg = 2358.3 kJ/ kg Statement (II): The higher efficiency of
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44 Thermal Engineering
01. (d) 02. (d) 03. (d) 04. (b) 05. (c)
03. In an air – standard Brayton cycle the minimum
06. (b) 07. (d) 08. (3.6)
and maximum temperatures are 300 K and
09. a(38.9%), b(2.88 kg/kWhr), c(5.63°C) 1200 K, respectively. The pressure ratio is that
10. a(0.94), b(41.850, c( ) which maximize the net work developed by
11. a(1021.66 kJ/kg), b(37.9 %) 12. (d) the cycle per unit mass of air flow. Calculate
13. (b) 14. (c) 15. (c) 16. (c) 17. (c) the compressor and turbine work, each in
kJ/kg air, and the thermal efficiency of the
cycle.
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45 Objective Practice Questions
Which of these statements are correct? 08. A gas turbine set takes in air at 27°C and 1
(a) 1, 2 and 3 (b) 2 and 3 atm. The pressure ratio is 4 and the maximum
(c) 1 and 2 (d) 1 and 3 temperature is 560°C. The compressor
and turbine efficiencies are 0.83 and 0.85
05. Consider the following statements : respectively. Determine the overall efficiency
(1) Intercooling is effective only at lower if the regenerator effectiveness is 0.75.
pressure ratios and high turbine inlet
temperatures. 09. In an ideal Brayton cycle with regeneration,
(2) There is very little gain in thermal air is compressed from 80kPa and 10oC to
efficiency when intercooling is used 400kPa and 175oC, is heated to 450oC in
without the benefit of regeneration. the regenerator, and then further heated
(3) With higher values of ‘γ’ and cp of the to 1000oC before entering the turbine.
working fluid the net power output of Under cold – air – standard conditions, the
Brayton cycle will increase. effectiveness of the regenerator is
(a) 33 % (b) 44 %
Of these statements (c) 62 % (d) 77 %
(a) 1, 2 and 3 are correct
(b) 1 and 2 are correct 10. Consider a gas turbine that has a pressure
(c) 1 and 3 are correct ratio of 6 and operates on the Brayton cycle
(d) 2 and 3 are correct. with regeneration between the temperature
limits of 20 and 900oC. If the specific heat ratio
06. A gas turbine power plant has a specific of the working fluid is 1.3, the highest thermal
output of 350 kJ/kg and an efficiency of 34%. efficiency this gas turbine can have is
A regenerator is installed and the efficiency (a) 38 % (b) 46%
increases to 51%. The specific output will be (c) 62 % (d) 58 %
closest to
(a) 350 kJ/kg (b) 468 kJ/kg 11. An ideal gas turbine cycle with many stages
(c) 525 kJ/kg (d) 700 kJ/kg of compression and expansion and a
regenerator of 100 percent effectiveness has
07. In an ideal Brayton cycle with regeneration, an overall pressure ratio of 10. Air enters every
Argon gas is compressed from 100 kPa and stage of compressor at 290 K, and every stage
25oC to 400 kPa, and then heated to 1200oC of turbine at 1200 K. The thermal efficiency of
before entering the turbine. The highest this gas turbine cycle is
temperature that argon can be heated in (a) 36 % (b) 40 %
the regenerator is (c) 52 % (d) 76 %
(a) 246oC (b) 846oC
(c) 689oC (d) 573oC 12. Statement (I): In gas turbines, regenerative
heating always improves the efficiency unlike
in that case of reheating.
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46 Thermal Engineering
13. Statement (I): The air-fuel ratio employed in a 01. A Carnot refrigerator requires 1.5 kW per
gas turbine is around 60:1 tonne of refrigeration to maintain a region at
Statement (II): A lean mixture of 60:1 in a –30oC. Then find the COP of refrigerator .
gas turbine is mainly used for complete
combustion. (ESE – 00) 02. The working temperature in evaporator and
condenser coils of refrigerator are –30oC and
14. Statement (I) : With the heat exchanger gas 32oC. If the actual refrigerator has a COP 0.75
turbine cycle, the cycle efficiency reduces as times maximum COP. Find power input for the
the pressure ratio increases. refrigeration capacity of 5 kW.
Statement (II) : As the pressure ratio increases,
the delivery temperature from the compressor 03. A domestic refrigerator set at 2oC handles a
increases and ultimately will exceed that of refrigeration load of 8000 kJ/day. The ambient
the exhaust gas from the turbine. temperature is 30oC. The COP of refrigerator
(ESE – 15) is 0.15 times of maximum COP. Then find the
daily electricity consumption in kW-hr.
15. For air standard Brayton cycle, increase in the
maximum temperature of the cycle, while 04. Consider a heat pump that operates on the
keeping the pressure ratio the same, would reversed Carnot cycle with R–134a as the
result in working fluid executed under the saturation
(a) Increase in air standard efficiency dome between the pressure limits of 140 and
(b) Decrease in air standard efficiency 800 kPa. R–134a changes from saturated
(c) No change in air standard efficiency vapor to saturated liquid during the heat
(d) Increase in the efficiency but reduction in rejection process. The net work input for this
net work cycle is
(a) 28 kJ/kg (b) 34 kJ/kg
KEY for Practice Questions (c) 49 kJ/kg (d) 144 kJ/kg
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The cooling takes place in 10 hrs. The specific operates between the pressure limits of 120
heat of this substance is 2 kJ/kgK above kPa and 1000 kPa. The mass fraction of the
freezing point and 0.5 kJ/kgK below freezing refrigerant that is in the liquid phase at the
point. The freezing point is –3oC, latent heat of inlet of the evaporator is
freezing is 230 kJ/kg. What is power required (a) 0.65 (b) 0.60 (c) 0.40 (d) 0.55
to drive the plant if the actual COP is half of
ideal COP ? 11. Consider a heat pump that operates on the
ideal vapor compression refrigeration cycle
07. A refrigerator operates on the ideal vapor with R – 134a as the working fluid between
compression refrigeration cycle with R–134a the pressure limits of 0.32 and 1.2 MPa. The
as the working fluid between the pressure coefficient of performance of this heat pump
limits of 120 and 800 kPa. If the rate of heat is
removal from the refrigerated space is 32 (a) 0.17 (b) 1.2
kJ/s, the mass flow rate of the refrigerant is (c) 3.1 (d) 5.9
(a) 0.19 kg/s (b) 0.15 kg/s 12. Consider a refrigerator that operates on the
(c) 0.23 kg/s (d) 0.28 kg/s vapor compression refrigeration cycle with
R–134a as the working fluid. The refrigerant
08. A heat pump operates on the ideal vapor enters the compressor as saturated vapor at
compression refrigeration cycle with R–134a 160 kPa, and exits at 800 kPa and 50°C, and
as the working fluid between the pressure leaves the condenser as saturated liquid at
limits of 0.32 and 1.2 MPa. If the mass flow 800 kPa. The coefficient of performance of
rate of the refrigerant is 0.193 kg/s, the rate of this refrigerator is
heat supply by the heat pump to the heated (a) 2.6 (b) 1.0
space is (c) 4.2 (d) 3.2
(a) 3.3 kW (b) 23 kW 13. R–12 vapour compression refrigeration
(c) 26 kW (d) 31 kW system is operating at a condenser pressure
of 9.6 bar and an evaporator pressure of 2.19
09. A 5 kW cooling capacity refrigeration system bar. Its refrigeration capacity is 15 tons. The
operating on simple VCR cycle, refrigerant values of enthalpy at the inlet and outlet of
enters evaporator with an enthalpy of 75 kJ/ the evaporator are 64.6 and 195.7 kJ/kg. The
kg and leaves with an enthalpy of 183 kJ/kg. specific volume at inlet to the reciprocating
The enthalpy of refrigerant after compression compressor is 0.082m3/kg. The index of
is 210 kJ/kg. compression for the compressor is 1.13.
Calculate : (i) COP Determine:
(ii) Power input to the compressor in kW (a) The power input in kW required for the
(iii) Rate of heat transfer at the condenser compressor,
(b) The COP. Take 1 ton of refrigeration as
10. An ideal vapor compression refrigeration equivalent to heat removal @ of 3.517
cycle with R–134a as the working fluid kW.
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48 Thermal Engineering
14. An ideal gas refrigeration cycle using air 19. Statement (I): Power input per TR of a
as the working fluid operates between the refrigeration system increases with decrease
pressure limits of 80 and 280 kPa. Air is cooled in evaporator temperature
to 35˚C before entering the turbine. The Statement (II): C.O.P of refrigeration system
lowest temperature of this cycle is decreases with decreases in evaporator
(a) – 58°C (b) –26°C temperature.
(c) 5°C (d) 11°C
20. A water chiller with a capacity of 30 tons of
15. Consider an ideal gas refrigeration cycle refrigeration cools 20 m3/hr of water entering
using helium as the working fluid. Helium at 12°C. What is the temperature of water
enters the compressor at 100 kPa and –10°C leaving the chiller?
and compressed to 250 kPa. Helium is then (a) 5°C (b) 6°C
cooled to 20°C before it enters the turbine. (c) 7°C (d) 7.5°C
For a mass flow rate of 0.2 kg/s, the net power
input required is 21. Absorbent in a vapour absorption refrigeration
(a) 9.3 kW (b) 27.6 kW system separates from the refrigerant only
(c) 48.8 kW (d) 93.5 kW when it
(a) Is sufficiently heated
16. A vapour absorption refrigeration system (b) Is sprayed on cooling water
works with three reservoirs. A refrigeration (c) Is cooled
effect of 100 W is required at 250 K. Heat (d) Reacts with refrigerant
source is available at 400 K. Heat rejection
occurs at 300 K. Find the minimum value of 22.
heat required in watt.
Sat.vapour line
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50 Thermal Engineering
2. − T b 2V l b 2T l
2P 2V 2
T P
03. It can be shown that for simple compressible
3. T b 2T l
2s
substance, the relationship V
4. − V b 2P l
1 2V
cp – cv= − T b 2T l b 2V l exists.
2V 2 2P
p T T
Where Cp and Cv are specific heats at
Codes:
constant pressure and constant volume
A B C D
respectively. T is temperature, V is volume (a) 3 4 2 1
and P is pressure. (b) 4 1 3 2
Which one of the following statements is NOT (c) 3 4 1 2
(d) 4 1 2 3
true?
(a) cp is always greater than cv 06. Which one of the following relationships
(b) The right side of the equation reduces to defines the Helmholtz function F?
R for an ideal gas (a) F = H + TS (b) F = H – TS
(c) Since b 2V l can be either positive or
2P
(c) F = U - TS (d) F = U + TS
negative, and b 2T l must be positive, T
T
2V 2
p
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51 Objective Practice Questions
is equal to
B. Joule Thomson experiment
(a) Zero (b) Cp/Cv
C. Joule’s experiment
(c) R (d) RT
D. Reversible engines
List - II
13. Which one of the following is the correct
1. Mechanical equivalent
statement ?
2. Thermodynamic temperature scale
Calpeyron equation is used for
3. Throttling process
(a) Finding specific volume of vapour
4. Loss of availability
(b) Finding specific volume of liquid
(c) Finding latent heat of vaporization
(a) A-1, B-2, C-3, D-4 (b) A-1, B-2, C-4, D-3
(d) Finding sensible heat
(c) A-4, B-3, C-2, D-1 (d) A-4, B-3, C-1, D-2
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52 Thermal Engineering
15. According to the Maxwall relation, which of 02. Consider the following statement:
the following is/are correct? 1. Reciprocating compressors are best
(a) b 2T l = − b 2P l (b) b 2V l = − b 2T l
2v 2s 2s 2P suited for high pressure and low volume
P T T V
capacity.
(c) b 2T l = − b 2V l (d) None of the above
2P 2s 2. The effect of clearance volume on
V T
power consumption is negligible of the
same volume of discharge.
3. While the compressor is idling, the delivery
KEY for Practice Questions
valve is kept open by the control circuit.
01. (b) 02. (a) 03. (c) 04. (a) 05. (c) 4. Intercooling of air between the stages of
06. (c) 07. (b) 08. (b) 09. (d) 10. (b) compression helps to minimize losses.
11. (c) 12. (c) 13. (c) 14. (a) 15. (a) Of these statements
(a) 1 and 2 are correct
(b) 1 and 3 are correct
(c) 2 and 4 are correct
(d) 3 alone is correct
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53 Objective Practice Questions
08. The ratio of the clearance volume to the 12. Following points express the effect of keeping
displacement volume of a R12 reciprocating high clearance volume for the cylinders in
compressor is 0.05. Specific volume at inlet reciprocating compressor. Which one of the
and outlet of compressor are 0.04 and 0.02 following points is disagreeable?
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