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.