Compressors

Compressors raise gas pressure. Learn reciprocating, screw, centrifugal and axial types, isentropic and polytropic work, efficiency, multi-staging with intercooling, free-air delivery and compressed-air system design.

Compressed air is called the 'fourth utility' in factories, gas turbines compress air before combustion, and refrigerators compress refrigerant vapour. Unlike liquids, gases heat up strongly when compressed, which shapes compressor design.

Compressed air is expensive: only ~10–15 % of the electrical input ends up as useful work at the tool.

An analogy

Pumping a bicycle tyre: the pump gets hot. Squeezing gas takes work, and much of it appears as heat.

Types

Positive displacement: reciprocating (piston), rotary screw, scroll, vane — for small–medium flows and high pressures. Dynamic: centrifugal and axial — for large flows (gas turbines, process plants). Screw compressors dominate factory air (5–500 kW).

Compression work and temperature

Adiabatic compression heats the gas: air at 20 °C compressed 8:1 isentropically reaches ≈ 258 °C. Real compressors (η_s ≈ 0.75–0.88) run hotter still. Isothermal compression would need the least work, which is why cooling during compression helps.

Multi-staging and intercooling

Splitting compression into stages with coolers between them brings the process closer to isothermal, saving work and limiting temperatures. With N stages, equal pressure ratios per stage minimise work.

Surge, choke and system efficiency

Centrifugal and axial compressors surge (flow reversal) at low flow and choke at high flow; anti-surge control keeps them on the map. In compressed-air systems, leaks (often 20–30 %), pressure set too high, and poor controls waste energy; heat recovery can reclaim up to 90 % of the input as warm air or water.

Formulas

• Isentropic work (ideal gas): w_s = c_p T₁ [r_p^((γ−1)/γ) − 1]
• Actual work: w = w_s / η_s
• Isothermal (minimum) work: w_T = R T₁ ln r_p
• Optimum stage ratio: r_stage = r_p^(1/N) (N stages, perfect intercooling)

Worked example: Single-stage air compressor

Given: T₁ = 20 °C, r_p = 8; η_s = 0.8, c_p = 1.005

Find: Specific work.

• 8^0.2857 = 1.811
• w_s = 1.005 × 293 × 0.811 = 238.9 kJ/kg
• w = 238.9/0.8 = 298.7 kJ/kg

Answer: ≈ 299 kJ/kg

Worked example: Two stages with intercooling

Given: Same overall ratio 8, each stage √8 = 2.83; Perfect intercooling to 20 °C

Find: Work and saving.

• w = 2 × 1.005 × 293 × (2.83^0.2857 − 1)/0.8 = 254.6 kJ/kg
• Saving = 15 %

Answer: ≈ 255 kJ/kg (15 % less)

Common mistakes

• Wrong: Setting system pressure higher than needed. — Right: Each 1 bar extra costs ≈ 7 % more energy; fix pressure drops instead.
• Wrong: Ignoring leaks. — Right: Leak surveys often find 20–30 % of output lost.

Applications

• Factory air: Screw compressors at 7–8 bar feed pneumatic tools, actuators and blow-off.
• Gas turbines: Multi-stage axial compressors with pressure ratios of 20–45 in jet engines.
• Gas pipelines & refrigeration: Centrifugal compressor stations and refrigeration compressors.

Key takeaways

• w_s = c_pT₁(r_p^((γ−1)/γ) − 1); actual = w_s/η.
• Compression heats gas; intercooling saves work.
• Screw for factories, axial for turbines.
• Leaks and excess pressure waste lots of energy.

Going further

• Polytropic efficiency: Per-stage small-step efficiency, independent of pressure ratio — preferred for multi-stage machines.
• Compressor maps: Pressure ratio vs corrected flow with speed lines and efficiency islands; turbocharger matching uses them.
• Oil-free vs oil-injected: Food, pharma and electronics need oil-free compression (ISO 8573 class 0).