Air Compressors and Gas Turbines Notes for SSC JE

A compressor raises the pressure of air or gas. A gas turbine uses compressed hot gas to produce power. SSC JE mechanical asks the types of compressors, the best compression process and the Brayton cycle.

Types of Compressors

Work of Compression

• Isothermal compression (pV = constant) needs the least work. Adiabatic compression needs the most. Polytropic compression lies between them.
• Work of a single-stage reciprocating compressor: W = n/(n − 1) × p₁V₁ × [(p₂/p₁)^(n−1)/n − 1].
• Clearance volume reduces the volumetric efficiency, but it does not change the work per unit mass.
• Volumetric efficiency = actual volume of free air delivered / swept volume.
• Multi-stage compression with intercooling reduces the work. The ideal intermediate pressure is p_i = √(p₁p₂) for two stages.

Gas Turbine (Brayton Cycle)

• The Brayton cycle has four processes: isentropic compression, constant pressure heat addition, isentropic expansion and constant pressure heat rejection.
• Efficiency of the ideal Brayton cycle = 1 − 1/(r_p)^(γ−1)/γ, where r_p is the pressure ratio.
• Gas turbine parts: compressor, combustion chamber and turbine. Jet engines use the same cycle.
• Regeneration and reheating improve the performance.

Quick Tips

• Isothermal is the ideal compression, but it is not practical.
• Intermediate pressure for two stages is the geometric mean of the suction and delivery pressures.
• Axial flow compressors are used in jet engines.
• Brayton efficiency increases with the pressure ratio.

Quick Revision Points

• Isothermal compression needs the least work.
• Intercooling reduces the work.
• Intermediate pressure is √(p1p2).
• Brayton cycle: two isentropic and two constant pressure processes.
• Efficiency rises with the pressure ratio.
• Axial compressors give a high flow rate.

Practice Questions With Answers

1. Which compression process requires the minimum work?
   Answer: Isothermal compression
2. What is the ideal intermediate pressure for two-stage compression?
   Answer: √(p1 × p2)
3. The Brayton cycle consists of which processes?
   Answer: Two isentropic and two constant pressure processes
4. Which compressor is used in a jet engine?
   Answer: Axial flow compressor
5. What is the purpose of intercooling?
   Answer: To reduce the work of compression
6. What is volumetric efficiency?
   Answer: Actual free air delivered / swept volume
7. How does the Brayton efficiency change with pressure ratio?
   Answer: It increases
8. Which compressor gives high pressure but low flow?
   Answer: Reciprocating compressor

How to Practise This Chapter

Practise 10 questions every day and write the steps on paper. Then attempt timed sets from our practice papers and the previous year papers page. See all topics on the Mechanical Engineering notes page.

Frequently Asked Questions

What is a compressor?

A machine that raises the pressure of a gas.

What is clearance volume?

The volume left in the cylinder at the end of the delivery stroke.

Why is intercooling used?

To cool the air between stages and save work.

What is the Brayton cycle?

The ideal cycle for gas turbines.

What are the main parts of a gas turbine?

Compressor, combustion chamber and turbine.

What is a polytropic process?

A process following pV^n = constant.

Which cycle is used in jet engines?

Brayton cycle.

What is a centrifugal compressor?

A compressor that uses an impeller to throw air outward.

What is the pressure ratio?

The ratio of the delivery pressure to the suction pressure.

What improves a gas turbine's efficiency?

Regeneration, reheating and intercooling.