Question

In a single stage reciprocating air compressor, the work done on air to compress it from suction pressure to delivery pressure will be minimum, when the compression is:

• A. Isothermal process
• B. Adiabatic process
• C. Polytropic process
• D. Constant pressure process

Hint:

Think of heat as "resistance" to being squeezed. If you keep the air Cool (Isothermal), it's easier to compress. If it gets Hot (Adiabatic), the pressure rises faster, making you work harder to push the piston.

Answer 1

The Correct Option is A

1. Comparing Processes on a P-V Diagram: When air is compressed from a low pressure ($P_1$) to a higher delivery pressure ($P_2$):

• Isothermal Compression ($PV = C$): Temperature remains constant. This follows the lowest path on the P-V diagram, resulting in the smallest area and thus the

minimum work done.

• Adiabatic Compression ($PV^\gamma = C$): No heat is removed during the process. The temperature rises sharply, following the steepest path and requiring the maximum work.

• Polytropic Compression ($PV^n = C$): Represents real-world scenarios where some heat is removed ($1 \lt n \lt \gamma$). The work required is between isothermal and adiabatic.

2. The Concept of Isothermal Efficiency: Since isothermal compression is the ideal goal for energy saving, we use "Isothermal Efficiency" to evaluate real compressors by comparing their actual work to the theoretical isothermal work.

3. Practical Implementation: In practice, achieving perfect isothermal compression is impossible because it would require an infinitely slow stroke. However, engineers use

intercooling in multi-stage compressors to bring the process as close to isothermal as possible to save energy.