Compressors, Steam & Gas Turbines Notes

Compressors, Steam & Gas Turbines

Air compressors — uses, classification (reciprocating and rotary; positive displacement and dynamic), reciprocating compressor work (isothermal, polytropic, isentropic), isothermal efficiency, clearance and volumetric efficiency, multistage compression with intercooling and optimum intermediate pressure; rotary compressors — vane, screw, centrifugal and axial; compressed air on construction sites; steam turbines — principle, impulse (De Laval, Curtis) and reaction (Parsons) turbines, compounding, governing, comparison; gas turbines — open and closed cycles, Brayton cycle efficiency and work ratio, improvements (regeneration, intercooling, reheating), combined cycles, jet propulsion basics — with fully worked numericals.

Last reviewed 16 Sept 2026 · 8 min read

Air compressors

A compressor raises the pressure of air or gas using mechanical work.

Uses

Construction sites: pneumatic rock drills, jackhammers/breakers, rivet guns, sand blasting, shotcrete/guniting, grouting, dewatering (air lift), spray painting, cleaning.
Industry: pneumatic tools and controls, conveying, refrigeration and air conditioning, gas turbines, supercharging engines.

Classification

Basis	Types
Principle	Positive displacement (reciprocating, rotary vane, screw, lobe) — trapped volume reduced; dynamic (rotodynamic) (centrifugal, axial) — velocity imparted then converted to pressure
Stages	Single-stage, multistage
Action	Single-acting, double-acting
Delivery pressure	Low, medium, high
Cooling	Air-cooled, water-cooled
Mobility	Stationary, portable (diesel-driven trailer compressors on sites)

Reciprocating air compressor

Work of compression (without clearance, per cycle)

FormulaWork of compression

Polytropic ( $p V^{n}$ = const):

W = \frac{n}{n - 1} p_{1} V_{1} [(\frac{p _{2}}{p _{1}})^{(n - 1) / n} - 1]

Isentropic: replace $n$ by γ. Isothermal (minimum work):

W_{i so} = p_{1} V_{1} ln \frac{p _{2}}{p _{1}}

Isothermal efficiency $= \frac{W _{i so}}{W _{a c t u a l}}$

Work required: isothermal < polytropic < isentropic — cooling during compression saves work.
Delivery temperature: $T_{2} = T_{1} (\frac{p _{2}}{p _{1}})^{(n - 1) / n}$ .

Clearance and volumetric efficiency

Clearance volume is necessary (valves, thermal expansion) — trapped air re-expands, reducing intake.
Volumetric efficiency $η_{v} = 1 + C - C (\frac{p _{2}}{p _{1}})^{1/ n}$ ( $C$ = clearance ratio $V_{c} / V_{s}$ ) — decreases as pressure ratio increases.
Free air delivery (FAD) — delivered volume reduced to atmospheric conditions.

Multistage compression

High pressure ratios in a single stage cause very high delivery temperature, low volumetric efficiency and more work.
Multistage compression with intercooling (cooling air between stages back to initial temperature) — reduces work, improves volumetric efficiency, lower temperatures (better lubrication, safety).

FormulaOptimum intermediate pressure (two-stage, perfect intercooling)

p_{2} = p_{1} p_{3}

Equal pressure ratio and equal work in each stage. For $k$ stages, stage pressure ratio $= (p_{f ina l} / p_{1})^{1/ k}$ .

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Air compressors

Uses

Classification

Reciprocating air compressor

Work of compression (without clearance, per cycle)

Clearance and volumetric efficiency

Multistage compression

This chapter is in the syllabus of