Category: Thermodynamics Objective Questions

tough Thermodynamics questions and answers on “Actual Vapor Cycle Processes and Comparison of Rankine and Carnot Cycles”.

1. The thermal efficiency of the cycle is
a) Q2 / Wnet
b) Wnet / Q2
c) Wnet / Q1
d) Q1 / Wnet
Answer: c
Clarification: These work and heat quantities are the measured values for actual cycle.

2. Which of the following losses occur in a cycle?
a) piping losses
b) pump losses
c) turbine losses
d) all of the mentioned
Answer: d
Clarification: These are the major losses that happen in a cycle including condenser losses.

3. The important piping losses include
a) pressure drop due to friction
b) heat loss to surroundings
c) both of the mentioned
d) none of the mentioned
Answer: c
Clarification: These two factors contribute to the piping losses.

4. The heat transfer and the pressure drop reduces the availability of steam.
a) true
b) false
Answer: a
Clarification: These two factors reduces the steam availability entering the turbine.

5. The losses in boiler include
a) pressure drop from pump to boiler
b) pressure drop in boiler
c) both of the mentioned
d) none of the mentioned
Answer: c
Clarification: Due to this, water entering the boiler is pumped at a very high pressure hence requiring additional pump work.

6. The main losses in turbine include
a) heat loss to surroundings
b) friction losses
c) both of the mentioned
d) none of the mentioned
Answer: c
Clarification: These are the two major losses in turbine.

7. Heat loss from turbine is generally neglected.
a) true
b) false
Answer: a
Clarification: This is true except for small turbines where we have to consider turbine losses.

8. The losses in pump is due to
a) heat loss to surroundings
b) irreversibilities associated with fluid friction
c) both of the mentioned
d) none of the mentioned
Answer: b
Clarification: The losses in pump are similar to those of turbine.

9. The losses in condenser are ____
a) small
b) large
c) always constant
d) none of the mentioned
Answer: a
Clarification: Hence condenser losses are mostly neglected.

10. The condenser losses include
a) loss of pressure
b) cooling of condensate below saturation temperature
c) both of the mentioned
d) none of the mentioned
Answer: c
Clarification: These are main losses in condenser which are very small.

11. The efficiency of Carnot cycle is ____ the efficiency of Rankine cycle.
a) less than
b) greater than
c) equal to
d) none of the mentioned
Answer: b
Clarification: The Carnot cycle has the maximum possible efficiency.

12. The only process which is different in Carnot and Rankine cycle is
a) compression in pump
b) expansion in turbine
c) heat rejection process
d) heat addition process
Answer: d
Clarification: In Rankine cycle, the heat addition process is reversible and at constant pressure and in Carnot cycle it is reversible and isothermal.

13. The Carnot cycle cannot be realized in practice.
a) true
b) false
Answer: a
Clarification: The reason being that the pump work is very large.

Thermodynamics Multiple Choice Questions on “Vapor Compression Refrigeration Cycle-1”.

1. In vapour refrigeration cycle, which of the following is used for expansion?
a) expansion engine
b) throttling valve or capillary tube
c) both of the mentioned
d) none of the mentioned
Answer: b
Clarification: This is because in expansion engine, power recovery is small and hence its cost is not justified.

2. Which of the following operations occur in a vapour refrigeration cycle?
a) compression
b) cooling and condensing
c) expansion and evaporation
d) all of the mentioned
Answer: d
Clarification: These are the processes which constitute the vapour refrigeration cycle.

3. Compression can be
a) dry compression
b) wet compression
c) both of the mentioned
d) none of the mentioned
Answer: c
Clarification: Dry compression starts with saturated vapour and wet compression starts with wet vapour.

4. Wet compression is preferred over dry compression.
a) true
b) false
Answer: b
Clarification: Dry compression is always preferred.

5. Why is wet compression not preferred?
a) the liquid refrigerant can be trapped in the head of cylinder
b) this may damage the valves or cylinder head
c) liquid refrigerant can wash away the lubricating oil thus accelerating wear
d) all of the mentioned
Answer: d
Clarification: These are the reasons why dry compression is preferred over wet compression.

6. In the cooling and condensing, correct sequence of processes is
a) desuperheated->condensed->saturated liquid
b) desuperheated->saturated liquid->condensed
c) condensed->desuperheated->saturated liquid
d) saturated liquid->condensed->desuperheated
Answer: a
Clarification: This is the correct sequence of processes and heat is transferred out.

7. The expansion process is
a) isentropic
b) reversible
c) adiabatic
d) all of the mentioned
Answer: c
Clarification: The expansion process is adiabatic but not isentropic and is irreversible.

8. The evaporation process is a
a) constant volume reversible process
b) constant pressure reversible process
c) adiabatic throttling process
d) reversible adiabatic process
Answer: b
Clarification: This is the last process and it completes the cycle.

9. The evaporator produces the cooling or refrigerating effect.
a) true
b) false
Answer: a
Clarification: It absorbs heat from the surroundings by evaporation.

10. In the expansion process, which of the following remains constant?
a) work done
b) heat supplied
c) internal energy
d) enthalpy
Answer: d
Clarification: The expansion is an adiabatic throttling process in which enthalpy remains unchanged.

11. The COP of cycle is given by(Q2=heat absorbed by evaporator and Wc=work done by compressor)
a) 1- (Q2/Wc)
b) 1- (Wc/Q2)
c) Q2/Wc
d) Wc/Q2
Answer: c
Clarification: This is the COP of vapour refrigeration cycle.

12. One tonne of refrigeration is given as the rate of heat removal from surroundings equivalent to heat required for melting one tonne of ice in a day.
a) true
b) false
Answer: a
Clarification: This is the definition of “one tonne of refrigeration”.

13. Which of the following is recommended in a refrigeration cycle?
a) superheating of vapour
b) subcooling of liquid
c) both of the mentioned
d) none of the mentioned
Answer: c
Clarification: The superheating of vapour is done at evaporator outlet and subcooling of liquid occurs at condenser outlet.

14. Superheating of vapour and subcooling of liquid ____ the refrigerating effect.
a) decreases
b) increases
c) no change
d) none of the mentioned
Answer: b
Clarification: The refrigerating effect is increased by using these techniques.

15. A condenser must ____ and then ____ the compressed refrigerant.
a) superheat, evaporate
b) desuperheat, evaporate
c) superheat, condense
d) desuperheat, condense
Answer: d
Clarification: This is the function of a condenser.

Thermodynamics Questions and Answers for freshers on “Specific Heat at Constant Volume and Pressure and Control Volume”.

1. The specific heat of a substance at constant volume is defined as the rate of change of ___ with respect to ___
a) specific internal energy, temperature
b) work, pressure
c) specific internal energy, pressure
d) heat, temperature
Answer: a
Clarification: cv=∂u/∂T at constant volume.

2. Heat transferred at constant _____ increases the _____ of a system.
a) pressure, increases
b) volume, increases
c) both of the mentioned
d) none of the mentioned
Answer: c
Clarification: At constant pressure, (dQ)=dh and at constant volume, Q=Δu.

3. Specific heat of a substance at constant volume is a property of the system.
a) true
b) false
Answer: a
Clarification: Since T,v and u are the properties of the system, specific heat at a constant volume is a property of the system.

4. The specific heat of a substance at constant pressure is defined as the rate of change of ___ with respect to ___
a) work, pressure
b) enthalpy, temperature
c) enthalpy, pressure
d) heat, temperature
Answer: b
Clarification: cp=∂h/∂T at constant pressure.

5. The heat capacity at constant pressure Cp
a) m/cp
b) cp/m
c) mcp
d) none of the mentioned
Answer: c
Clarification: Cp=(mass*specific heat at constant pressure).

6. Specific heat of a substance at constant pressure is a property of the system.
a) true
b) false
Answer: a
Clarification: cp is a property of a substance just like cv.

7. When there is mass transfer across the system boundary, the system is called
a) isolated system
b) closed system
c) open system
d) none of the mentioned
Answer: c
Clarification: Basic definition of an open system.

8. If a certain mass of steam is considered as the thermodynamic system, then the energy equation becomes
a) Q=ΔKE + ΔPE – ΔU + W
b) Q=ΔKE + ΔPE – ΔU – W
c) Q=-ΔKE – ΔPE + ΔU + W
d) Q=ΔKE + ΔPE + ΔU + W
Answer: d
Clarification: Q=ΔE + W and E=KE + PE + U.

9. The surface of the control volume is known as the control surface.
a) true
b) false
Answer: a
Clarification: This is same as the system boundary of the open system.

10. Steady flow means that the rates of flow of mass and energy across the control surface
a) varies
b) remains constant
c) depends on the control surface
d) none of the mentioned
Answer: b
Clarification: In a steady flow rate of flow remains constant.

Thermodynamics for Freshers,

Thermodynamics Multiple Choice Questions on “Entropy Principle and its Applications”.

1. Which of the following is true?
a) for an isolated system, dS>=0
b) for a reversible process, dS=0
c) for an irreversible process, dS>0
d) all of the mentioned
Answer: d
Clarification: For an isolated system which does not undergo any energy interaction with the surroundings, dQ=0 and also dS>=dQ/T.

2. The entropy of an isolated system can never ____
a) increase
b) decrease
c) be zero
d) none of the mentioned
Answer: b
Clarification: The entropy of an isolated system always increases and remains constant only when the process is reversible.

3. According to entropy principle, the entropy of an isolated system can never decrease and remains constant only when the process is reversible.
a) true
b) false
Answer: a
Clarification: This is the statement for the principle of increase of entropy.

4. Entropy may decrease locally at some region within the isolated system. How can this statement be justified?
a) this cannot be possible
b) this is possible because entropy of an isolated system can decrease.
c) it must be compensated by a greater increase of entropy somewhere within the system.
d) none of the mentioned
Answer: c
Clarification: The net effect of an irreversible process is an entropy increase of the whole system.

5. Clausius summarized the first and second laws of thermodynamics as
a) the energy of the world is constant
b) the entropy of the world tends towards a maximum
c) both of the mentioned
d) none of the mentioned
Answer: c
Clarification: These two statements were given by Clausius.

6. The entropy of an isolated system always ____ and becomes a ____ at the state of equilibrium.
a) decreases, minimum
b) increases, maximum
c) increases, minimum
d) decreases, maximum
Answer: b
Clarification: If entropy of an isolated system varies with some parameter, then there is a certain value of that parameter which maximizes the entropy.

7. Entropy principle is the quantitative statement of the second law of thermodynamics.
a) true
b) false
Answer: a
Clarification: This is a general fact about entropy principle.

8. Which of the following can be considered as an application of entropy principle?
a) transfer of heat through a finite temperature difference
b) mixing of two fluids
c) maximum temperature obtainable from two finite bodies
d) all of the mentioned
Answer: d
Clarification: These are some basic applications of entropy principle.

9. The final temperatures of two bodies, initially at T1 and T2 can range from
a) (T1-T2)/2 to sqrt(T1*T2)
b) (T1+T2)/2 to sqrt(T1*T2)
c) (T1+T2)/2 to (T1*T2)
d) (T1-T2)/2 to (T1*T2)
Answer: b
Clarification: (T1+T2)/2 is the temperature when there is no delivery of work and sqrt(T1*T2) is the temperature with maximum delivery of work.

10. Which of the following processes exhibit external mechanical irreversibility?
a) isothermal dissipation of work
b) adiabatic dissipation of work
c) both of the mentioned
d) none of the mentioned
Answer: c
Clarification: These processes exhibit external mechanical irreversibility.

Thermodynamics online quiz on “Availability-2”.

1. The compressor in a refrigerator takes R-134a in at 100 kPa, −20°C and then compresses it to 1 MPa, 40°C. With the room temperature at 20°C find the minimum compressor work.
a) -48.19 kJ/kg
b) -58.19 kJ/kg
c) -68.19 kJ/kg
d) -78.19 kJ/kg
Answer: a
Clarification: w(c) = h1 – h2 + q(rev)
w(min) = h1 – h2 + To(s2 – s1) = 387.22 – 420.25 + 293.15 × (1.7148 – 1.7665)
= -48.19 kJ/kg.

2. Find the specific reversible work for a steam turbine with inlet at 4 MPa, 500°C and an actual exit state of 100 kPa, x = 1.0 with a 25°C ambient temperature.
a) 550.0 kJ/kg
b) 650.0 kJ/kg
c) 750.0 kJ/kg
d) 850.0 kJ/kg
Answer: d
Clarification: To = 25°C = 298.15 K, hi = 3445.2 kJ/kg; si = 7.090 kJ/kg K,
he = 2675.5 kJ/kg; se = 7.3593 kJ/kg K
w(rev) = (hi – Tosi) – (he – Tose) = (hi – he) + To(se – si)
= (3445.2 – 2675.5) + 298.2(7.3593 – 7.0900)
= 769.7 + 80.3 = 850.0 kJ/kg.

3. Find the specific reversible work for a compressor using R-134a with inlet state of –20°C, 100 kPa and an exit state of 50°C, 600 kPa. Use 25°C as ambient temperature.
a) -28.878 kJ/kg
b) -38.878 kJ/kg
c) -48.878 kJ/kg
d) -58.878 kJ/kg
Answer: b
Clarification: The compressor is assumed to be adiabatic so q = 0
w(rev) = To(se – si) – (he – hi)
hi = 387.22 kJ/kg; si = 1.7665 kJ/kg K;
he = 438.59 kJ/kg; se = 1.8084 kJ/kg K
w(rev) = 298.15 (1.8084 – 1.7665) – (438.59 – 387.22)
= -38.878 kJ/kg.

4. A steady stream of R-22 at ambient temperature of 10°C, and at 750 kPa enters a solar collector. The stream exits at 80°C, 700 kPa. Calculate the change in availability.
a) 4.237 kJ/kg
b) 5.237 kJ/kg
c) 6.237 kJ/kg
d) 7.237 kJ/kg
Answer: c
Clarification: hi = 56.46 kJ/kg, si = 0.2173 kJ/kg K,
he = 305.91 kJ/kg, se = 1.0761 kJ/kg K
∆ψie = ψe – ψi = (he – hi) – T0(se – si)
= (305.912 – 56.463) – 283.2(1.0761 – 0.2173)
= 6.237 kJ/kg.

5. Cold water is running in a river at 2°C and the air temperature is 20°C. What is the availability of water relative to the ambient temperature?
a) 2.157 kJ/kg
b) 2.857 kJ/kg
c) 3.457 kJ/kg
d) 2.457 kJ/kg
Answer: d
Clarification: ψ = h1 – h0 – T0(s1 – s0)
ψ = 8.392 – 83.96 – 293.15(0.03044 – 0.2966)
= 2.457 kJ/kg.

6. Nitrogen is flowing in a pipe with a of velocity 300 m/s at 500 kPa, 300°C. What is its availability relative to an ambient at 100 kPa, 20°C?
a) 272 kJ/kg
b) 252 kJ/kg
c) 292 kJ/kg
d) 232 kJ/kg
Answer: a
Clarification: ψ = h1 – h0 + (1/2)V^2 – T0(s1 – s0)
= Cp(T1 – T0) + (1/2)V^2 – T0[Cp ln(T1/T0) – R ln(P1/P0)]
= 1.042(300-20)+(300^2)/2000 – 293.15[1.042 ln (573.15/293.15) – 0.2968ln(500/100)]
= 272 kJ/kg.

7. R-12 at 30°C, 0.75 MPa enters a steady flow device and exits at 30°C, 100 kPa. Assuming the process to be isothermal and reversible, find the change in availability of the refrigerant.
a) -26.1 kJ/kg
b) -36.1 kJ/kg
c) -46.1 kJ/kg
d) -56.1 kJ/kg
Answer: b
Clarification: hi = 64.59 kJ/kg, si = 0.2399 kJ/kg K,
and he = 210.02 kJ/kg, se = 0.8488 kJ/kg K
∆ψ = he – hi – T0(se – si) = 210.02 – 64.59 – 298.15(0.8488 – 0.2399)
= -36.1 kJ/kg.

8. A wooden bucket(2 kg) with 10 kg hot liquid water, both at 85°C, is lowered down to 400 m into a mineshaft. What is the availability of water and bucket with respect to the surface with ambient temperature of 20°C?
a) 232.2 kJ
b) 242.2 kJ
c) 212.2 kJ
d) 252.2 kJ
Answer: a
Clarification: φ1 – φ0 = m(wood)[u1 – u0 – T0(s1- s0)]
+ m(H2O)[u1- u0- T0(s1- s0)] + m(tot)g(z1- z0)
= 2[1.26(85 – 20) – 293.15× 1.26 ln{(273.15 + 85)/293.15}
+ 10[ 355.82 – 83.94 – 293(1.1342 – 0.2966)] + 12 × 9.807 × (-400) /1000
= 15.85 + 263.38 – 47.07 = 232.2 kJ.

9. Air in a piston/cylinder arrangement is at 25°C, 110 kPa with a volume of 50 L. It goes through a reversible polytropic process to final state of 500 K, 700 kPa and exchanges heat with the ambient at 25°C. Find the total work from the ambient.
a) -9.28 kJ
b) -9.38 kJ
c) -9.48 kJ
d) -9.58 kJ
Answer: d
Clarification: ma*(u2 – u1) = 1Q2 – 1W2,(tot) ; ma*(s2 – s1) = 1Q2/T0
ma = 110 × 0.05/0.287 × 298.15 = 0.0643 kg
1Q2 = T0*ma*(s2 – s1) = 298.15 × 0.0643[7.3869 – 6.8631 – 0.287 ln (700/110)]
= -0.14 kJ
1W2,(tot) = 1Q2 – ma*(u2 – u1) = -0.14 – 0.0643 × (359.844 – 213.037)
= -9.58 kJ.

10. Find the specific reversible work for a R-134a compressor with inlet state of –20°C, 100 kPa and an exit state of 600 kPa, 50°C. Use a 25°C ambient temperature.
a) 48.878 kJ/kg
b) -38.878 kJ/kg
c) 48.878 kJ/kg
d) -38.878 kJ/kg
Answer: b
Clarification: This is a steady state flow device
and the compressor is assumed to be adiabatic so q = 0,
w(rev) = T0(se – si) – (he – hi)
= 298.15(1.8084 – 1.7665) – (438.59 – 387.22)
= -38.878 kJ/kg.

11. A steady stream of R-22 at ambient temperature, 10°C, and at 750 kPa enters a solar collector. The stream exits at 80°C, 700 kPa. Calculate the change in availability of the R-22 between these two states.
a) 8.762 kJ/kg
b) 8.143 kJ/kg
c) 7.237 kJ/kg
d) 6.237 kJ/kg
Answer: d
Clarification: Change in availability = (he – hi) – T0(se – si)
= (305.912 – 56.463) – 283.2(1.0761 – 0.2173)
= 6.237 kJ/kg.

Thermodynamics for online quizzes,

Thermodynamics Multiple Choice Questions on “Gas Compression”.

1. In a gas compressor,
a) work is done on the gas to raise its pressure
b) there is an appreciable increase in its density
c) both of the mentioned
d) none of the mentioned
Answer: c
Clarification: This is the main function of a gas compressor.

2. For ɣ>n>1 and for the same pressure ratio p2/p1, the maximum work is needed for
a) isothermal compression
b) adiabatic compression
c) polytropic compression
d) all need same work
Answer: b
Clarification: This comes when these three reversible compression processes are plotted on the p-V diagram.

3. Staging of compression process is done with intermediate cooling.
a) true
b) false
Answer: a
Clarification: The work of compression is reduced by staging.

4. A two-stage compression process includes,
a) the gas is first compressed isentropically in the low pressure cylinder
b) it is cooled in the intercooler to its original temperature
c) it is compressed isentropically in the high pressure cylinder
d) all of the mentioned
Answer: d
Clarification: All these processes take place in the order a-b-c in a two-stage compressor.

5. For minimum work the intermediate pressure is the _____ of the suction and discharge pressures.
a) arithmetic mean
b) geometric mean
c) sum
d) difference
Answer: b
Clarification: The intermediate pressure p2=sqrt(p1*p4), where p1 is the suction pressure and p4 is the discharge pressure.

6. The intermediate pressure that produces minimum work will also result in
a) equal pressure ratios in the two stages of compression
b) equal work for the two stages
c) equal discharge temperatures
d) all of the mentioned
Answer: d
Clarification: This comes from the pressure-temperature relations.

7. Heat rejected in the intercooler is given by (here cp is the specific heat at constant pressure)
a) cp*(T3-T2)
b) cp*(T3+T2)
c) cp*(T2-T3)
d) none of the mentioned
Answer: c
Clarification: The process of intercooling is from 2-3.

8. If there are N stages of compression, then the pressure ratio in each stage is
a) p2/p1 = (discharge pressure/suction pressure)^(1/N)
b) p2/p1 = (suction pressure/discharge pressure)^(1/N)
c) p2/p1 = (discharge pressure/suction pressure)^N
d) p2/p1 = (suction pressure/discharge pressure)^N
Answer: a
Clarification: This is true irrespective of the number of stages in compression.

9. The isothermal efficiency of a compressor is given by
a) p1*v1/ total work of compression
b) p1*v1*ln(p2/p1) / total work of compression
c) total work of compression / p1*v1*ln(p2/p1)
d) total work of compression / p1*v1
Answer: b
Clarification: In gas compression, the desirable idealized process is often a reversible isothermal process.

10. The ratio of the actual volume of gas taken into cylinder during suction stroke to the piston displacement volume is called the volumetric efficiency.
a) true
b) false
Answer: a
Clarification: Volumetric efficiency = mass flow rate*specific volume of gas at inlet / piston displacement per cycle.

11. Clearance(C) is defined as
a) (piston displacement per cycle / clearance volume )^n
b) piston displacement per cycle / clearance volume
c) clearance volume / piston displacement per cycle
d) (clearance volume / piston displacement per cycle)^n
Answer: c
Clarification: This is used to make the calculations easier.

12. The volumetric efficiency is given by
a) 1-C+C(p2/p1)^(1/n)
b) 1+C-C(p2/p1)^(1/n)
c) 1+C+C(p2/p1)^(1/n)
d) 1-C-C(p2/p1)^(1/n)
Answer: b
Clarification: This comes from the basic formula of volumetric efficiency.

13. Volumetric efficiency decreases as the clearance _____ and as the pressure _____
a) decreases, increases
b) increases, decreases
c) decreases, decreases
d) increases, increases
Answer: d
Clarification: We know that (p2/p1)^(1/n) is always greater than 1 and volumetric efficiency is given by 1+C-C(p2/p1)^(1/n).

14. Compressors are built with the maximum clearance.
a) true
b) false
Answer: b
Clarification: Compressors are built with minimum clearance because as clearance decreases, volumetric efficiency increases.

15. For a fixed clearance, as the pressure ratio is increased, the volumetric efficiency of a compressor
a) decreases
b) increases
c) remains constant
d) none of the mentioned
Answer: a
Clarification: As the pressure ratio increases, the volumetric efficiency of a compressor decreases.