Air compressors

In short

An air compressor raises the pressure of atmospheric air by doing mechanical work on it, so that the stored energy can later drive tools, actuators, valves, process nozzles or a gas-turbine combustor. Compressed air is the costliest common utility in an Indian factory — roughly 5.5 to 7 kW of shaft power for every m³/min of free air at 7 bar — so compressor selection, staging and leak control are directly money questions. In university exams and GATE the topic is examined as work-done integrals on the p–V diagram, volumetric efficiency with clearance, optimum multistage pressure and intercooling, and the surging/choking behaviour of dynamic machines. Interviews at Elgi, Kirloskar Pneumatic, Atlas Copco, Ingersoll Rand, Siemens Energy, Thermax, cement and steel plants, and railway workshops lean on the same material plus compressed-air system practice.

Explained

Classification and basic terms

• Positive displacement: a fixed volume of air is trapped and squeezed. Sub-types: reciprocating (piston, single- or double-acting, single- or multi-stage) and rotary (screw, vane, Roots/lobe, scroll, liquid ring).
• Dynamic (roto-dynamic): air is accelerated by an impeller and the velocity is converted to pressure in a diffuser — centrifugal and axial machines. Continuous flow, no trapped volume.
• Fan / blower / compressor by pressure rise: fan up to about 1.1 pressure ratio, blower about 1.1–2, compressor above that (loose convention, not a standard).
• Single-acting: compression on one side of the piston, one cycle per revolution. Double-acting: both sides work, two cycles per revolution.
• Swept (displacement) volume Vs: volume generated by the piston in one stroke. Clearance volume Vc: gas space left at TDC (valve pockets, gasket, piston-to-head gap). Clearance ratio C = Vc/Vs, typically 4–10% for air compressors.
• Free Air Delivered (FAD): the delivered mass expressed as a volume at agreed reference conditions — ISO 1217 uses 1 bar absolute, 20 °C, 0% relative humidity. Always say at what conditions a "m³/min" figure is quoted; a compressor at Leh delivers far less FAD than the same machine at Chennai.
• Compression index n: the actual process is polytropic, n ≈ 1.25–1.35 for air. Perfect cooling → n = 1 (isothermal); no cooling → n = γ = 1.4 (isentropic).
• Receiver: the pressure vessel that damps pulsations, stores air for peak demand, lets the compressor unload, and drops out condensate.

Reciprocating compressor: indicator diagram and work

• The ideal cycle on p–V: induction at p₁ (line a–b), polytropic compression b–c, delivery at p₂ (c–d), then re-expansion of the clearance gas d–a. Work per cycle = enclosed area.
• Without clearance, integrating ∫V dp over the three lines gives the standard result W = [n/(n−1)] p₁V₁ [(p₂/p₁)^(n−1)/n − 1]. The same area for n = 1 reduces to p₁V₁ ln(p₂/p₁).
• Isothermal work is the minimum, isentropic the maximum, polytropic in between — because cooling during compression keeps the volume (and so ∫V dp) small. Hence water jackets, fins, oil injection and multistaging.
• With clearance, the re-expansion line d–a gives back some work, so work per cycle falls — but the induced volume falls in exactly the same proportion. Ideally, work per unit mass (or per m³ of FAD) is unchanged by clearance; clearance hurts capacity, not specific work. In practice it does hurt both, through extra re-expansion losses and heating.
• Real indicator diagram differs: the suction valve needs under-pressure to lift against its spring and the delivery valve needs over-pressure (wire drawing), the walls heat the incoming charge, and valves flutter. These losses are lumped into the measured indicated power.
• Valves are spring-loaded plate, reed or poppet types — automatic, opened only by pressure difference. A leaking or broken valve shows up as high delivery temperature and lost FAD.

Volumetric efficiency, other efficiencies and capacity control

• Theoretical (clearance) volumetric efficiency: η_vol = 1 + C − C(p₂/p₁)^1/n. It falls as the pressure ratio rises; at a high enough ratio it reaches zero — the machine churns the clearance gas and delivers nothing. This is the hard limit that forces multistaging.
• Actual volumetric efficiency = FAD / displacement, typically 70–85%; lower than theoretical because of intake pressure drop, charge heating, leakage past rings and valves, and residual moisture.
• Isothermal efficiency = isothermal power / indicated power — the normal figure of merit for a *cooled, reciprocating* machine, usually 70–80%.
• Isentropic (adiabatic) efficiency = isentropic power / actual power — used for rotary and dynamic machines, 70–85% typical.
• Mechanical efficiency = indicated power / shaft power, about 85–93%. Overall (motor) efficiency adds drive and motor losses.
• Specific power (kW per m³/min FAD at a stated pressure) is what plants actually buy and audit.
• Capacity control: reciprocating — suction-valve lifting (unloaders), clearance pockets, speed control, start/stop; screw — load/unload (on-line/off-line), inlet modulation, variable speed (VSD, the most efficient at part load); centrifugal — inlet guide vanes, suction throttling, blow-off to atmosphere (wasteful, used only to avoid surge).

Multistage compression and intercooling

• Why stage: (1) less work, because intercooling pulls the compression line towards isothermal; (2) lower delivery temperature, protecting valves and lubricating oil; (3) better volumetric efficiency per stage; (4) smaller pressure difference per cylinder, so lighter parts and better balance; (5) condensate can be drained between stages.
• Perfect intercooling: air leaves the intercooler at the original suction temperature T₁; on p–V the second-stage line starts on the T₁ isothermal. Imperfect intercooling leaves it hotter and the saving is smaller.
• Minimum total work occurs when each stage has the same pressure ratio and intercooling is perfect; for z stages the ratio per stage is (p_final/p₁)^1/z. For two stages the ideal intermediate pressure is the geometric mean, p₂ = √(p₁p₃).
• At minimum work, the work is shared equally between stages and the heat rejected in each intercooler is equal.
• Typical practice for air: pressure ratio 3–5 per stage; two stages to about 15–25 bar, three or more for PET blowing (40 bar), breathing air (200–300 bar) and starting-air bottles.
• Aftercooler: cools the final delivery, condensing out most of the water before the receiver — not a work-saving device but essential for air quality.

Rotary positive-displacement compressors

• Screw (helical lobe): male and female rotors mesh; the trapped volume shrinks axially. Oil-flooded (oil seals, cools and lubricates) or oil-free/dry (timing gears, two-stage, needed for food, pharma, electronics — ISO 8573-1 Class 0). Up to about 13 bar single package, continuous duty, compact, low vibration, no valves. Now the default industrial machine in India.
• Vane (sliding-vane): slots in an eccentric rotor hold sliding vanes; simple, moderate pressures, used in small packages and vacuum duty.
• Roots blower (lobe): two meshing lobes displace air without internal compression — pressure builds only against the downstream resistance, so it is a blower (up to roughly 1 bar gauge). Used for aeration in ETPs, pneumatic conveying and some engine superchargers.
• Scroll: two interleaved spirals, oil-free, quiet, small capacities — dental, laboratory and medical air.
• Liquid ring: water forms the sealing ring; chiefly a vacuum pump.

Dynamic compressors and their instabilities

• Centrifugal: radial impeller plus diffuser and volute; work input per kg = ψ·σ·u₂² for radial-vaned impellers (σ = slip factor, ψ = power input factor). High flow, oil-free air, 2–8 bar per stage; multistage integrally geared units serve large plants.
• Axial: many rows of aerofoil blades, each stage giving a small pressure ratio (1.1–1.4) at very high flow — the compressor of a gas turbine or jet engine.
• Surging: at low flow the diffuser or blade passages stall, flow momentarily reverses and the whole machine oscillates violently — damaging and noisy. The surge line bounds the operating map on the left; avoided by blow-off, variable guide vanes or recirculation.
• Choking (stonewalling): at high flow the Mach number somewhere reaches unity, flow cannot increase and the characteristic drops vertically — the right-hand bound of the map.
• Rotating stall: stalled cells travel around the annulus at part of rotor speed; precedes surge and excites blade vibration.
• Dynamic machines have no fixed displacement: delivery pressure depends on speed and on the system resistance, unlike positive-displacement machines where flow is nearly fixed and pressure is set by the system.

System accessories, Indian codes and energy practice

• Train: intake filter and silencer → compressor (with intercoolers) → aftercooler → moisture separator and auto drain → air receiver (safety valve, pressure gauge, drain, fusible plug on some designs) → dryer → filters → distribution ring main → point-of-use FRL (filter-regulator-lubricator).
• Drying: refrigerated dryers give about +3 °C pressure dew point (indoor service air); adsorption/desiccant dryers reach −40 °C or lower for instrument air and outdoor lines. ISO 8573-1 classifies air by solid particles, water and total oil, e.g. the common instrument-air specification 1.4.1.
• Testing and design standards: ISO 1217 (acceptance tests, displacement compressors), IS 5456 (code of practice for testing positive-displacement air compressors and exhausters), IS 7938 (air receivers for compressed air installations), IS 2825 (unfired pressure vessels; ASME Section VIII is the common alternative). Editions are revised — check the current version before quoting a clause.
• Statutory: an air receiver is pressure plant — formerly under Section 31 of the Factories Act, 1948, now under the OSH Code, 2020 (in force 21 November 2025, which repealed the Factories Act, 1948) and the rules under it — safe working pressure must be established, a safety valve and gauge fitted, and the vessel examined periodically by a competent person. Examination intervals and test types are set by the State Factories Rules (which continue under the Code's savings clause until State OSH rules replace them) and differ between states, so verify locally. Noise limits follow the Noise Pollution (Regulation and Control) Rules, 2000 and the relevant Environment (Protection) Rules schedules.
• Energy: compressed air is commonly 10–20% of an industrial plant's electricity and is a standing item in BEE energy audits under the Energy Conservation Act, 2001 (PAT scheme for designated consumers). Useful field rules: leakage in an unaudited Indian plant is often 20–30% of generation; 1 bar reduction in discharge pressure saves roughly 6–8% of power; every 4 °C drop in intake air temperature saves about 1%; a clogged intake filter costing 50 mbar of suction loss costs about 3–4% of power. Treat all of these as audit rules of thumb, not exact laws.

Key points

• Air compressor – a machine that does work on air to raise its pressure; positive displacement (fixed volume squeezed) or dynamic (velocity converted to pressure).
• FAD – delivered flow referred to standard inlet conditions (ISO 1217: 1 bar abs, 20 °C, dry); displacement is the geometric volume, FAD is what you get.
• Clearance ratio C = Vc/Vs – 4–10% for air; it cuts volumetric efficiency but, ideally, not the work per kg delivered.
• Volumetric efficiency – η_vol = 1 + C − C·r^1/n; falls steeply with pressure ratio and reaches zero at the limiting ratio.
• Isothermal compression is the minimum-work ideal; isentropic the maximum; real cooled machines sit at n ≈ 1.25–1.35.
• Isothermal efficiency – the figure of merit for cooled reciprocating machines (70–80%); isentropic efficiency is used for rotary and dynamic machines.
• Multistaging – saves work, limits delivery temperature, restores volumetric efficiency, lightens parts and allows condensate drainage.
• Optimum staging – equal pressure ratio per stage with perfect intercooling; two-stage ideal intermediate pressure = √(p₁p₃).
• Perfect intercooling – air re-enters the next stage at the original suction temperature; equal work and equal intercooler heat load per stage follow.
• Double-acting – two compression cycles per revolution, so power uses 2N instead of N.
• Screw compressor – oil-flooded or oil-free, valveless, continuous duty up to about 13 bar; the industrial workhorse.
• Roots blower – displaces air with no internal compression; pressure rise set entirely by downstream resistance.
• Surging – violent flow reversal in a dynamic compressor at low flow; choking is the high-flow limit where a passage reaches Mach 1.
• Positive displacement vs dynamic – the first gives nearly constant flow with pressure set by the system, the second gives a pressure–flow characteristic set by speed.
• Aftercooler and dryer – remove water; pressure dew point must be below the lowest ambient the pipe will see.
• Receiver – damps pulsation, stores for peaks, allows unloading, separates condensate; it is statutory pressure plant.
• Specific power – 5.5–7 kW per m³/min FAD at 7 bar for a good screw package; the number to audit.
• Leakage – usually the single biggest loss; a 3 mm hole at 7 bar gauge wastes about 0.4 m³/min of free air.

Formulas and examples

Example 1 — single-stage reciprocating compressor with clearance. Single-acting, bore 150 mm, stroke 200 mm, N = 300 rpm, p₁ = 1 bar, T₁ = 300 K, p₂ = 7 bar, n = 1.3, C = 5%.

1. Vs = 0.7854 × 0.15² × 0.20 = 0.003534 m³ (3.534 litres).
2. 7^1/1.3 = 7^0.7692 = 4.468 → η_vol = 1 + 0.05 − 0.05 × 4.468 = 0.827 (82.7%).
3. Induced volume V₁ = 0.827 × 0.003534 = 0.002922 m³/cycle → FAD = 0.002922 × 300 = 0.877 m³/min (at 1 bar, 300 K).
4. 7^0.3/1.3 = 7^0.2308 = 1.567 → W = (1.3/0.3) × 10⁵ × 0.002922 × (1.567 − 1) = 4.333 × 10⁵ × 0.002922 × 0.567 = 717 J/cycle.
5. Indicated power = 717 × 300/60 = 3.59 kW. MEP = 717/0.003534 = 2.03 × 10⁵ Pa = 2.03 bar.
6. W_iso = 10⁵ × 0.002922 × ln 7 = 292.2 × 1.946 = 569 J → 2.84 kW → η_iso = 2.84/3.59 = 79.2%.
7. T₂ = 300 × 1.567 = 470 K = 197 °C — already high enough to justify two stages above this ratio.

Example 2 — two-stage with perfect intercooling versus single-stage. FAD 10 m³/min at 1 bar, 300 K, delivered at 36 bar, n = 1.3.

1. Ideal intermediate pressure = √(1 × 36) = 6 bar, so each stage ratio = 6. 6^0.2308 = 1.512.
2. p₁V̇₁ = 10⁵ × 10 = 10⁶ J/min. W = 2 × 4.333 × 10⁶ × 0.512 = 4.438 × 10⁶ J/min → 74.0 kW.
3. Single-stage: 36^0.2308 = 2.286 → W = 4.333 × 10⁶ × 1.286 = 5.574 × 10⁶ J/min → 92.9 kW.
4. Saving = 18.9 kW, i.e. 20.4% — and the delivery temperature per stage drops from 300 × 2.286 = 686 K (413 °C, which would char the oil) to 300 × 1.512 = 454 K (181 °C).
5. Intercooler duty: ṁ = 10⁵ × 10/(287 × 300) = 11.61 kg/min; Q = 11.61 × 1.005 × (454 − 300) = 1,797 kJ/min = 29.9 kW of heat to be removed.

Example 3 — measuring FAD by a receiver pump-up test. A 1.5 m³ receiver, initially at atmospheric 1 bar abs, is pumped to 7 bar abs in 2 minutes; temperature is allowed to settle back to ambient before reading. FAD = 1.5 × (7 − 1)/(1 × 2) = 4.5 m³/min. If the nameplate says 5.2 m³/min, the machine has lost about 13% of capacity — look for worn valves, rings, a choked intake filter or a leaking intercooler. (The test assumes isothermal conditions, so take the final pressure after the air has cooled, not at cut-off.)

Example 4 — what leaks and over-pressure cost, in rupees. A plant runs a 75 kW screw compressor 6,000 h a year at an average 70% load; electricity ₹8/kWh.

1. Energy = 75 × 0.70 × 6,000 = 315,000 kWh/yr → ₹25.2 lakh/yr.
2. An ultrasonic survey finds leakage equal to 25% of generation. Halving it removes 12.5% of the flow, and with load/unload or VSD control the power follows roughly in proportion: 0.125 × 315,000 = 39,375 kWh ≈ ₹3.15 lakh/yr saved.
3. Reducing the header setting from 7.0 to 6.0 bar (if every user can work on 6) saves about 7%: 22,050 kWh ≈ ₹1.76 lakh/yr.
4. Scale of one defect: a single 3 mm hole at 7 bar gauge passes about 0.4 m³/min FAD; at 6 kW per m³/min that is 2.4 kW, or 19,200 kWh and about ₹1.54 lakh a year at 8,000 h. One unrepaired drain valve pays for a leak-survey contract.

Example 5 — centrifugal stage pressure ratio. Impeller tip diameter 0.50 m, 18,000 rpm, radial vanes, slip factor 0.90, power input factor 1.04, inlet 293 K, isentropic efficiency 0.80.

1. u₂ = π × 0.50 × 18,000/60 = 471.2 m/s.
2. W = 1.04 × 0.90 × 471.2² = 0.936 × 222,070 = 207.9 kJ/kg → ΔT₀ = 207,900/1,005 = 206.9 K.
3. Pressure ratio = (1 + 0.80 × 206.9/293)^3.5 = (1.565)^3.5 = 4.79 — the kind of single-stage ratio an aero-derivative or high-speed process centrifugal reaches; industrial oil-free centrifugals use several lower-ratio stages with intercooling.

Where it is used

• General factory service air: oil-flooded screw packages with refrigerated dryers running pneumatic cylinders, clamps, impact wrenches and blow guns — Elgi, Kirloskar Pneumatic, Atlas Copco India, Ingersoll Rand and Kaeser dominate this market.
• Process industries: instrument air for pneumatic control valves in refineries and fertiliser plants (desiccant-dried, ISO 8573-1 Class 1.4.1 or better); oil-free screw or centrifugal air in pharma, dairy, food and bottling; axial/centrifugal main air compressors in air-separation and blast-furnace blowing.
• High-pressure niches: 40 bar reciprocating or booster machines for PET bottle blowing, 200–300 bar for breathing-air cylinders and starting-air bottles on marine and locomotive diesels, CNG dispensing compressors at retail outlets.
• Transport: air-brake compressors on Indian Railways locomotives, EMUs and coaches, and on trucks and buses; pantograph and horn air; tyre inflation and spray painting in every workshop.
• Mining, construction and infrastructure: portable diesel-driven screw compressors for jackhammers, drifters, DTH drilling and shotcreting; aeration blowers (Roots type) in sewage and effluent treatment plants.
• Job roles: compressed-air system designers and sizing engineers, service and reliability engineers, BEE-accredited energy auditors doing leak surveys and specific-power benchmarking, and test-bed engineers running ISO 1217 / IS 5456 acceptance tests.

Interview questions

• Q: Why is isothermal compression the ideal for a compressor when isentropic is the ideal for a turbine? A: Compressor work is ∫V dp, so keeping the air cool and dense during compression minimises it; turbine work is maximised by expanding without heat loss, which is isentropic.
• Q: Does clearance volume change the work done per kg of air delivered? A: Ideally no — the clearance gas re-expands and returns its work, so work per unit mass is unchanged; what clearance destroys is volumetric efficiency, hence capacity.
• Q: Why do we stage a compressor instead of using one big cylinder? A: To cut work through intercooling, to keep delivery temperature within the oil's limit, to restore volumetric efficiency, and to reduce the pressure load on each piston and crank.
• Q: For two-stage compression from 1 to 25 bar, what intermediate pressure and why? A: √(1 × 25) = 5 bar, because equal pressure ratios with perfect intercooling give minimum total work and equal work in each stage.
• Q: What is FAD and why insist on it? A: Free Air Delivered is the output referred to standard inlet conditions (ISO 1217: 1 bar, 20 °C, dry); without stated reference conditions a flow figure cannot be compared between machines or sites.
• Q: What is surging and how is it prevented? A: A low-flow instability in a dynamic compressor where blade passages stall and flow reverses cyclically; prevented by blow-off or recirculation, variable inlet guide vanes, and keeping the duty point right of the surge line.
• Q: A Roots blower is rated 0.8 bar gauge. What sets its actual discharge pressure? A: The downstream resistance — it displaces air with no internal compression, so the pressure simply rises until the system accepts the flow, and the rating is a mechanical limit.
• Q: Why does volumetric efficiency fall as pressure ratio rises? A: The clearance gas expands further before suction pressure is reached, so the piston travels more of its stroke before any fresh air enters.
• Q: Your plant air pressure keeps dropping during a shift even though the compressor runs loaded. Where do you look? A: Leaks and open drains first, then undersized or blocked piping and filters, a choked dryer, a stuck unloader or worn valves, and any single large intermittent user that should be fed from a dedicated receiver.
• Q: Why is an air receiver required even with a screw compressor that delivers steady flow? A: To meet peak demand without over-sizing the machine, to let it unload instead of cycling, to separate condensate, and to stabilise header pressure.
• Q: What legal obligation attaches to an air receiver in an Indian factory? A: It is pressure plant under the OSH Code, 2020 (in force 21 November 2025; formerly Section 31 of the Factories Act, 1948) — safe working pressure must be established, a safety valve and gauge fitted, and periodic examination carried out as the State Factories Rules require; intervals vary by state.
• Q: Two quotes offer the same m³/min at 7 bar. How do you choose? A: Compare specific power (kW per m³/min FAD) at the actual duty and at part load, control type (VSD versus load/unload), air quality class, noise, and life-cycle cost — energy dominates the total, not first cost.

Common mistakes

• Using N instead of 2N for a double-acting cylinder, or forgetting the number of cylinders, when converting work per cycle to power.
• Mixing gauge and absolute pressure in the pressure ratio — compression ratios are always absolute.
• Applying η_vol = 1 + C − C·r^1/n with the overall pressure ratio in a multistage machine instead of the individual stage ratio.
• Writing W = [n/(n−1)]p₁V₁[...] with V₁ as the swept volume rather than the volume actually induced (η_vol × Vs).
• Comparing a quoted "displacement" against a competitor's "FAD", or comparing flows measured at different ambient conditions.
• Claiming clearance reduces the work per kg of air delivered, or that an intercooler saves work in a single-stage machine (an aftercooler does not reduce compression work at all).
• Treating a dynamic compressor like a positive-displacement one — throttling it hard towards zero flow drives it into surge instead of simply reducing output.

Related topics

Turbomachinery: turbines, pumps and compressors · Gas turbines and jet propulsion · Thermodynamics: laws and properties · Power cycles: Rankine, Otto, Diesel and Brayton · Refrigeration and air conditioning · Fluid mechanics and pipe flow losses