How gas compression differs from air
An air compressor handles one medium with well-known properties, takes it from atmosphere and can vent it anywhere. A gas compressor must contain a gas that may be flammable, toxic, asphyxiating or valuable, take it from a pipeline, generator or digester at a pressure that varies, and return every leak and relief to a safe place. Four gas properties change the machine itself.
| Property | What it is | What it changes |
|---|---|---|
| k (ratio of specific heats, cp/cv) | About 1.40 for air and nitrogen, about 1.31 for methane, about 1.29 for carbon dioxide, 1.66 for helium | Discharge temperature at a given pressure ratio, and therefore how many stages are needed |
| Molecular weight | About 29 for air, 16 for methane, 28 for nitrogen, around 27 for a 60/40 methane–CO2 biogas, 2 for hydrogen | Density, mass per stroke, power, and the head a centrifugal machine must generate |
| Compressibility (Z) | Correction to the ideal gas law; natural gas at high pressure typically has Z below 1 | Actual volume at suction and discharge, cylinder sizing and absorbed power; ignoring it misstates capacity |
| Dew points and contaminants | Water, hydrocarbon liquids, H2S, siloxanes, oxygen, dust | Whether liquid forms after intercoolers, corrosion, valve fouling and material choice |
The full treatment, with worked temperature and flow-basis examples, is in gas properties and compression behaviour.
Worked example: stage count for a CNG booster from 2 to 250 bar(g)
A CNG station on a low-pressure city gas line must lift natural gas to about 250 bar(g) (3,626 psig) for storage cascades. How many stages does that take?
Assumptions (illustrative): gas treated as methane, k = 1.31, ideal gas (Z = 1, which slightly overstates temperature at the top stages but is fine for staging); suction 2 bar(g) at 30 °C; atmosphere 0.950 bar(a) (Hyderabad); intercooling to 45 °C between stages on a hot day; isentropic efficiency 80% per stage; equal pressure ratio per stage.
- Absolute pressures: suction 2 + 0.95 = 2.95 bar(a); discharge 250 + 0.95 = 250.95 bar(a).
- Overall pressure ratio: 250.95 ÷ 2.95 = 85.1.
- Temperature exponent: (k − 1) ÷ k = 0.31 ÷ 1.31 = 0.237.
- Single stage (impossible, but instructive): T2 = 303.15 K × 85.10.237 = 867.6 K, about 594 °C ideal; at 80% efficiency about 736 °C. No cylinder, valve or lubricant survives that.
- Per-stage ratio for n stages is 85.11/n. The actual discharge temperature for a stage starting at 45 °C is 318.15 + (318.15 × r0.237 − 318.15) ÷ 0.80.
| Stages | Ratio per stage | Interstage pressures, bar(g) | Stage 1 discharge (30 °C suction) | Later stages (45 °C suction) |
|---|---|---|---|---|
| 2 | 9.22 | 26.3 | 292 °C | 320 °C |
| 3 | 4.40 | 12.0, 56.1 | 189 °C | 212 °C |
| 4 | 3.04 | 8.0, 26.3, 81.7 | 144 °C | 165 °C |
| 5 | 2.43 | 6.2, 16.5, 41.5, 102.2 | 119 °C | 138 °C |
Result: three stages run too hot for reliable valves and lubrication on continuous duty; four stages bring discharge into the 140–165 °C region that makers commonly design around; five stages give margin for 45 °C summers and low suction pressure. Two practical points follow. A drop in suction pressure raises the overall ratio and every stage temperature, so the lowest credible line pressure, not the normal one, sets the design. And intercooler performance in May decides whether the machine trips on high temperature. The same calculation for air (k = 1.40) at four stages gives about 194 °C per later stage, which is why an air compressor’s staging cannot be copied for gas.
Which gas compressor for which duty
The table is a starting point for discussion, not a selection. Each row links to the specialist guide.
| Duty | Typical pressures | Usual machine | Key selection points | Detailed guide |
|---|---|---|---|---|
| CNG station (mother, online or daughter booster) | Suction from low-pressure line or daughter cascade; discharge about 250 bar(g) | Multi-stage lubricated reciprocating, electric or gas-engine drive | Suction pressure range, stage count, gas drying, hazardous-area package, noise | CNG and process gas compressors; natural gas boosters |
| Nitrogen booster for testing, purging or laser cutting | From 5–8 bar(g) generator or vaporiser to 15–30 bar(g) or much higher for testing | Oil-free or well-separated reciprocating; air-driven gas boosters for small flows | Purity protection, buffer receiver, suction tightness, oxygen monitoring | nitrogen booster compression |
| Biogas collection and upgrading | Near-atmospheric digester gas to a few bar for engines, or 8–15 bar(g) class for upgrading, then to CNG pressure | Blowers and screw machines at low pressure; reciprocating for upgrading and bio-CNG | H2S, moisture, siloxanes, oxygen, corrosion-resistant materials, condensate handling | biogas compression and pretreatment; landfill gas |
| Process gas in pharma and chemicals | Project specific | Oil-free reciprocating, or diaphragm for leak-free, high-purity or toxic gases | Contamination limits, leakage path, materials compatibility, cleaning | oil-free process gas; diaphragm compressors |
| Carbon dioxide recovery or transfer | Watch the critical point near 31 °C and 74 bar | Reciprocating or screw, often with liquefaction | Phase behaviour, dryness, carbonic-acid corrosion | carbon dioxide compression |
| Hydrogen | Project specific | Diaphragm or specially designed reciprocating | Embrittlement, leakage, IIC gas group | hydrogen compression |
Where the gas is already defined but the machine type is not, the step-by-step requirements are in process gas compressor selection requirements.
Safety and hazardous-area essentials
This is a summary of what must be settled before a gas compressor is bought or installed. It does not replace the plant’s own hazard study or the regulations that apply to your gas and site.
- Area classification. The plant owner classifies the area into zones according to how likely a flammable atmosphere is (for gases, Zone 0, 1 or 2). The zone, gas group and temperature class decide the protection required for motors, instruments and panels. Classification of the area is not certification of equipment; both are needed.
- Containment and leakage. Reciprocating gas compressors use rod packing and distance pieces so that leakage is collected and vented rather than reaching the crankcase or the room. Diaphragm machines avoid a dynamic seal on the gas side altogether.
- Venting and relief. Relief valves, packing vents, purge and blowdown lines must discharge to a safe location, sized for the worst case. A relief valve venting natural gas into a compressor shed is a design fault.
- Purging. Before start-up and before maintenance, the machine is purged, usually with nitrogen, so that air and flammable gas never mix inside it.
- Leak and gas detection. Detectors are placed according to gas density: high for methane and hydrogen, low for heavier gases such as propane or CO2. Detection should trip the compressor and isolate the supply through a defined cause-and-effect logic.
- Materials. Wet CO2 and H2S corrode carbon steel; hydrogen embrittles some steels; oxygen service needs clean, compatible materials. Materials are chosen from the gas analysis, not from the air-compressor catalogue.
- Hot surfaces. Discharge piping and cylinders, as in the example above, can approach the ignition temperature limits of some temperature classes. Design temperatures must be checked against the area’s temperature class.
We do not classify areas or issue Ex certificates; we work to the classification and approvals your project specifies. The detail is in hazardous-area compressor package enquiries and seal and leakage management.
What a gas compressor enquiry needs
A gas compressor cannot be quoted from “500 cubic metres, 200 bar”. These items decide the machine, and missing any one of them means the quote is a guess.
| Item | What to give | Why it matters |
|---|---|---|
| Gas analysis | Full composition in mol%, including water, H2S, CO2, oxygen and heavy hydrocarbons; normal, lightest and heaviest cases | Sets k, molecular weight, Z, dew points and materials |
| Suction pressure | Minimum, normal and maximum, stating gauge or absolute | Minimum suction gives the highest ratio and temperature; maximum gives the highest load |
| Discharge pressure | Required pressure at the outlet flange, and any relief setting downstream | Sets stage count and pressure rating |
| Flow | Nm³/h stating reference conditions, or kg/h; normal and peak | Volume flow without a reference basis is ambiguous; mass flow is clearest when composition varies |
| Temperatures | Gas suction temperature range; site ambient range; cooling water if any | Cooler sizing and discharge temperature margin in 45 °C summers |
| Site and area classification | Zone, gas group, temperature class; indoor or outdoor; power supply | Motor, panel and instrument protection; package layout |
| Duty and standby | Running hours, start frequency, N+1 or single | Machine speed, build and maintenance intervals |
As a sense check, 500 Nm³/h of methane (0 °C, 1.01325 bar(a)) is about 358 kg/h. For flows given in other units, convert to mass flow and state the basis.
Gas compressor service and spares
Most gas compressor failures start with valves, packing, cooling or lubrication. Service works from a log of stage pressures and temperatures, packing vent flow and vibration, compared with the maker’s data.
| Area | Common faults | Service and spares |
|---|---|---|
| Suction and discharge valves | Broken plates or springs, fouling from liquids, polymers or dust, leaking seats | Inspection, reconditioning, valve plate and spring kits; leaking valves show as high stage temperature and wrong interstage pressure |
| Rod packing and wiper rings | Leakage to the vent, scored rods | Packing sets, vent flow checks, rod run-out and surface checks |
| Piston and rider rings, liners | Capacity loss, blow-by, wear from poor lubrication or dirty gas | Ring sets, liner measurement, honing or replacement |
| Intercoolers, aftercoolers and separators | Fouling, tube leaks, liquid carry-over to the next stage | Cleaning, pressure testing, drain and level-control repair |
| Lubrication | Wrong oil for the gas, cylinder lubricator faults, oil dilution by hydrocarbons | Gas-compatible oil, lubricator overhaul, oil analysis |
| Capacity control and protection | Unloaders, bypass and recycle valves, trips that fail to act | Unloader kits, safety valve testing, trip function tests |
Further reading: interstage cooling and separation, capacity control, pulsation and vibration and reciprocating valve engineering.
Sales, rental and project work: what to expect
Gas compression is quoted per project. There is no standard gas compressor that can be picked from a list and delivered, because the gas, pressures, flow and site rules differ every time. A reliable quote follows a defined sequence: the enquiry data above, a check of staging, temperatures and materials, a defined package boundary (who supplies the motor, panel, coolers, detection and vents), and then the commercial offer. Rental of gas compressors depends on a suitable machine for your gas and classification being available, so it is also arranged per project. For many gas sites, such as nitrogen generation, instrument air and purging, the compressed-air side is a large part of the job and is something we supply and hire routinely; see compressed air and nitrogen for oil and gas.
How Reddy Compressors helps
- Independent multi-brand suppliers since 2002, based in Tarnaka, Secunderabad, with tie-ups with all major makes.
- Sales, rental, service and spares for CNG, nitrogen, process-gas and booster compressors, quoted per project.
- More than 20 service engineers and technicians for valves, packing, rings, coolers and overhauls.
- Coverage across Telangana, Andhra Pradesh and districts within about 300 km of Hyderabad; supply across India and service elsewhere on a project basis.
- The air side of gas plants: PSA and membrane nitrogen generators and their feed air, instrument air, dryers and temporary air.
What to send us
- Gas analysis in mol%, with water, H2S and other contaminants.
- Suction and discharge pressures (minimum, normal, maximum), stating gauge or absolute.
- Flow in Nm³/h or kg/h with reference conditions, normal and peak.
- Gas and ambient temperatures, cooling available and power supply.
- Area classification and any approvals your project requires.
- For service or spares: nameplate photos, model and serial number, and the fault or running log.
Frequently asked questions
What is a gas compressor?
A gas compressor raises the pressure of a gas other than atmospheric air, such as natural gas, nitrogen, biogas, carbon dioxide or a process gas. Compared with an air compressor it is designed around the gas’s properties, which change temperature, staging and power, and it contains leaks and vents through rod packing, distance pieces and relief lines. It must also suit the area classification where it is installed.
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How many stages does a CNG compressor need?
For about 2 bar(g) suction to 250 bar(g), the overall ratio at Hyderabad’s atmosphere is about 85. Three stages give discharge temperatures above 200 °C in a hot summer, which is too high for reliable continuous duty. Four stages bring each stage to roughly 140–165 °C, and five give extra margin. Higher suction pressure, as at an online station on a medium-pressure line, reduces the stages needed.
What is a nitrogen booster compressor used for?
A nitrogen booster raises nitrogen from a PSA or membrane generator, usually at 5–8 bar(g), or from a liquid vaporiser, to the higher pressure needed for laser cutting, pressure testing, purging or filling cylinders. It needs a buffer receiver to protect generator purity, a tight suction to keep air out, and a high-pressure receiver for peaks. Purity-sensitive uses need oil-free or well-separated machines.
What is a booster compressor?
A booster compressor takes gas or air that is already under pressure and raises it further. Because its inlet is already pressurised, it needs fewer stages for a given final pressure than a machine starting from atmosphere, but its output depends on the inlet pressure and flow being held. Boosters are used on natural gas lines, CNG daughter stations, nitrogen systems and high-pressure air for PET blowing.
What compressor is used for biogas?
Low-pressure biogas transfer to engines or flares usually uses blowers or screw machines. Upgrading to biomethane and filling bio-CNG cylinders use reciprocating compressors up to about 250 bar(g). In every case the gas must be pretreated: H2S removal, drying and, for engines, siloxane removal. Materials and oil must suit wet, slightly acidic gas, and condensate must be drained from every cooler.
Do you provide gas compressor service and spares in Hyderabad?
Yes. We service CNG, nitrogen, process-gas and booster compressors across Telangana and Andhra Pradesh, and elsewhere in India on a project basis. Work covers valves, rod packing, piston and rider rings, liners, coolers, lubricators, unloaders and safety valves. Send nameplate photos and model and serial numbers so we can identify the correct spares and confirm fit before quoting.
Can an ordinary air compressor compress gas?
Not for flammable, toxic or valuable gases. Air compressors are not built to contain gas leakage, are not designed for hazardous areas and are staged for k = 1.40 air. Nitrogen is sometimes possible with a gas-tight machine and the maker’s approval, but the purity and asphyxiation risk still have to be managed. Natural gas, biogas and process gases need machines designed for gas service.
What is the price of a CNG or gas compressor?
There is no list price because each gas compressor is engineered for the gas, suction and discharge pressures, flow, drive and area classification. The main cost drivers are the number of stages, flow and power, materials for the gas, the hazardous-area electrics and instruments, the cooling system and the package scope. Send the enquiry data on this page and we will quote per project.
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Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 7 November 2026. How our engineering content is reviewed.