250+ TOP MCQs on Work of Compression and Answers

Thermodynamics Multiple Choice Questions on “Work of Compression”.

1. A gas compression process is
a) adiabatic
b) involves heat transfer
c) both of the mentioned
d) none of the mentioned

Answer: c
Clarification: A gas compression process can be either adiabatic or can involve heat transfer.

2. If the gas is cooled during compression, work required will be ____ the adiabatic compression work.
a) more than
b) less than
c) equal to
d) none of the mentioned

Answer: b
Clarification: Here the work required will be less than that required for adiabatic compression.

3. Which of the following is an advantage of cooling?
a) less pipe friction losses
b) reduction in volume of gas
c) both of the mentioned
d) none of the mentioned

Answer: c
Clarification: These are the two advantages of cooling.

4. We use after-coolers to cool the gas which leaves the compressor.
a) true
b) false

Answer: a
Clarification: This is done because compression process is somewhat ineffective.

5. The work of compression is ____ the shaft work.
a) positive of
b) negative of
c) equal to
d) less than

Answer: b
Clarification: This is true for reversible adiabatic compression.

6. For ɣ>n>1 and for 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.

7. In isothermal compression, all work done on gas is transformed into
a) heat added into system
b) heat going out of system
c) internal energy increase
d) none of the mentioned

Answer: c
Clarification: This is the case of isothermal compression.

8. When isothermal compression is taken as ideal process, the energy imparted
a) raises the temperature of gas
b) raises the pressure of gas
c) both of the mentioned
d) none of the mentioned

Answer: b
Clarification: In isothermal compression considered as ideal process, no energy is imparted to the gas.

9. The adiabatic efficiency is given by
a) Ws/Wc
b) Ws/Wt
c) Wt/Wc
d) Wt/Ws

Answer: a
Clarification: This is the efficiency of compressor working in a steady flow process.

10. The isothermal efficiency is given by
a) Ws/Wc
b) Ws/Wt
c) Wt/Wc
d) Wt/Ws

Answer: c
Clarification: This is the efficiency of compressor working in a steady flow process and Wt=work in reversible isothermal compression.

11. The adiabatic efficiency of real compressor can be ____
a) less than unity
b) greater than unity
c) equal to unity
d) none of the mentioned

Answer: b
Clarification: This is due to the effects of cooling.

12. For an adiabatic machine, work of compression is greater than enthalpy rise of gas.
a) true
b) false

Answer: b
Clarification: For an adiabatic machine, work of compression is equal to the enthalpy rise of gas.

13. Argon is kept in a 5 m³ tank at −30°C and 3 MPa. Determine the mass using compressibility factor.
a) 208.75 kg
b) 308.75 kg
c) 303.75 kg
d) 203.75 kg

Answer: b
Clarification: Tr = 243.15/150.8 = 1.612 and Pr = 3000/4870 = 0.616 hence Z = 0.96
m = PV/ZRT = (3000 × 5)/(0.96 × 0.2081 × 243.2)
= 308.75 kg.

14. Find the error in specific volume if ideal gas model is used to represent the behaviour of superheated ammonia at 40°C and 500 kPa?
a) 1.5%
b) 3.5%
c) 4.5%
d) 2.5%

Answer: c
Clarification: NH3, T = 40°C = 313.15 K, Tc = 405.5 K, Pc = 11.35 MPa
v = 0.2923 m³/kg
Ideal gas: v = RT/P = (0.48819 × 313)/(500) = 0.3056 m³/kg
thus error = 4.5%.

15. Find the volume of ethylene having mass of 125 kg at 7.5 MPa and 296.5 K.
a) 0.369 m³
b) 0.669 m³
c) 0.569 m³
d) 0.469 m³

Answer: d
Clarification: For ethylene, Tc = 282.4 K and Pc = 5.04 MPa
Tr = T/Tc = 296.5 / 282.4 = 1.05 and Pr = P/Pc = 7.5 / 5.04 = 1.49
thus Z = 0.32
hence V = mZRT / P = 125 × 0.32 × 0.2964 × 296.5 / 7500 = 0.469 m³.