Why process gas is not the same as air
An air compressor works with a gas whose properties barely change from site to site. A process gas compressor may handle natural gas, biogas, nitrogen, carbon dioxide, hydrogen-rich streams, refrigerants or mixed hydrocarbon vapours, and the composition can shift with the season, the well, the reactor or the upstream plant. Each change moves the machine’s performance.
- Molecular weight affects the head a dynamic (centrifugal) compressor can develop and the density a positive-displacement machine handles. A light gas such as hydrogen needs far more stages in a centrifugal machine than air at the same pressure ratio.
- Specific heat ratio (k or γ) sets the discharge temperature for a given pressure ratio. Gases with a higher k heat up more, which can limit the ratio per stage.
- Compressibility (Z) departs from 1 at higher pressures and for heavier gases, so ideal-gas calculations underestimate or overestimate power and volume.
- Dew point and phase behaviour: heavy hydrocarbons, water vapour and CO2 can condense in coolers or at suction, and liquid entering a compressor causes serious damage.
- Corrosive and reactive components such as H2S, chlorides, oxygen traces, ammonia or siloxanes govern materials, lubricant choice and valve life.
- Flammability and toxicity drive seal design, venting, hazardous-area classification and protection systems.
Information needed before any selection
The single most useful document is a gas compression enquiry datasheet that the process owner completes and keeps under revision control. A supplier quoting without it is guessing.
| Item | What to state | Why it matters |
|---|---|---|
| Gas composition | Component-by-component mole percent for minimum, normal and maximum cases, wet or dry basis, sample point and date | Sets molecular weight, k, Z, dew point and materials |
| Trace components | H2S, CO2, O2, water, chlorides, siloxanes, particulates, liquids | Corrosion, fouling, lubricant compatibility, seal and valve life |
| Suction conditions | Pressure range (absolute, in bar a and psia) and temperature range | Mass flow, pressure ratio and discharge temperature |
| Discharge requirement | Required pressure and maximum allowable temperature | Number of stages and intercooling |
| Flow basis | Mass flow, or volume flow with its reference conditions stated (actual, normal or standard) | Prevents the most common sizing error |
| Operating cases | Normal, start-up, turndown, upset, maximum and future cases | Control range, recycle and driver sizing |
| Site and utilities | Ambient range, altitude, power supply, cooling water quality, instrument air, nitrogen | Cooler design, driver rating, purge and seal systems |
| Area classification and codes | Hazardous zone, applicable standards and project specifications | Motor and instrument certification, package layout |
A flow quoted in “Nm³/h” without stating whether normal means 0 °C or 15 °C, 1.013 bar a, dry or wet, can change the selected machine by several percent. Always give the reference conditions alongside every volume flow.
Choosing the compressor technology
Technology choice follows flow, pressure ratio, gas behaviour and the need for oil-free compression. The table below gives the broad pattern used in industry; final choice rests with the compressor specialist using validated gas properties.
| Technology | Typical role | Strengths | Watch points |
|---|---|---|---|
| Reciprocating (lubricated or non-lubricated) | Low to moderate flow, high pressure ratio, CNG, hydrogen, nitrogen boosting, refinery and process gas | Handles changing gas weight well; high pressures possible; efficient at part load with proper unloading | Valve and packing wear, pulsation, foundations, distance-piece venting |
| Diaphragm | Small flows of very pure, toxic or high-value gas | Hermetic separation of gas from oil | Limited flow; diaphragm life and leak detection |
| Oil-flooded screw | Moderate flow, moderate ratio, biogas, landfill gas, some hydrocarbon and refrigeration duties | Tolerant of some liquid and dirt, simple, compact | Oil dilution and compatibility with the gas, oil separation and downstream contamination |
| Dry (oil-free) screw | Moderate flow where gas must stay oil-free | No oil contact with gas | Discharge temperature, coatings, seal gas |
| Liquid ring | Wet, dirty or condensable gas at low pressure ratios | Isothermal, tolerant of liquid carry-over | Seal-liquid system, lower efficiency |
| Centrifugal | Large flows, steady duty, lower ratio per stage | High capacity, long running periods, oil-free gas path | Surge and choke limits, sensitivity to molecular weight change |
Normal, start-up and upset cases
Many process gas compressor problems come from a case nobody listed. A machine sized only for the normal case may be unable to start, may surge or overload during a composition swing, or may see liquid at suction during a plant upset.
- Start-up gas: the plant may start on nitrogen, air (only where safe and permitted) or a different gas from the normal stream. Molecular weight and density change, so driver load and discharge temperature change.
- Settle-out pressure: when the machine trips, suction and discharge sides equalise. The machine and driver must be able to restart against that pressure, or a blow-down sequence is needed.
- Turndown: state minimum flow. Capacity control by recycle, speed, clearance pockets, valve unloaders or slide valves each has its own limit.
- Composition swings: list the heaviest and lightest cases and any wet-gas case.
- Upset and emergency: liquid slugs, high suction temperature, loss of cooling, blocked discharge. Each needs a protective response and a named owner.
Materials, seals and lubrication
Materials selection is driven by the most aggressive credible composition, not the average. Sour gas containing H2S needs materials suitable for sulphide stress cracking; wet CO2 attacks carbon steel; chlorides threaten stainless steels; hydrogen can embrittle some high-strength steels. These choices belong to a qualified materials engineer working to the project’s standards.
Sealing determines how much process gas reaches the atmosphere or the crankcase. Reciprocating machines use rod packing with vents and, for hazardous gases, a distance piece with purge. Rotary and centrifugal machines use mechanical seals, labyrinths or dry gas seals with seal-gas supply and leak monitoring. Every vent needs a safe destination: recovery, flare, treatment or a safe atmospheric location as the process safety study decides.
Lubricants must be compatible with the gas. Hydrocarbon gases dilute mineral oils and lower viscosity; some gases react with or dissolve in particular oil types. Oil-free designs remove this question but bring their own temperature and wear limits.
Coolers, separators and protection
Intercoolers and aftercoolers lower gas temperature between stages and at discharge, saving power and protecting materials. They also condense water and heavy components, so each cooler needs an effective separator and a reliable drain. Liquid that is not removed reaches the next stage. Suction scrubbers or knock-out drums protect the first stage.
Protective devices usually include high discharge temperature, low suction pressure, high discharge pressure, low lube-oil pressure, high vibration, high liquid level in separators and, where relevant, gas detection. Relief valves on every stage protect against overpressure. Trip settings come from the manufacturer and the process safety review.
How Reddy Compressors helps
Reddy Compressors sells new and used/refurbished gas compressors, rents them, services them and supplies their spares, covering CNG, nitrogen, process gas and booster duties. We work across many makes as an independent multi-brand specialist, not an authorised dealer. We have tie-ups with all major and reputed brands. Our office is at Tarnaka, Secunderabad; our team of more than 20 service engineers and technicians serves the whole of Telangana and Andhra Pradesh, and we supply equipment and spares across India.
- Helping you build the enquiry datasheet so suppliers quote against the same, complete duty.
- Supply of new or refurbished reciprocating and screw gas compressors where the duty suits them.
- Service, overhaul and spares for valves, packing, rings, seals, coolers and lubrication systems, identified from the nameplate and the old part.
- Rental compressors for nitrogen boosting, purging support or temporary duties where the gas and site allow.
Oxygen, hydrogen and toxic-gas duties follow the gas-specific codes and the manufacturer’s design rules; final process design, hazardous-area compliance and safety approval stay with the qualified project parties.
What to send us
- Gas analysis for normal, minimum and maximum cases, with trace components.
- Suction and discharge pressure (bar a or psia) and temperatures.
- Required flow with reference conditions, and minimum turndown.
- Operating hours, start-up gas and any upset cases you know of.
- Site location, hazardous-area zone, power supply and cooling arrangement.
- For an existing machine: make, model, serial number and nameplate photo.
Frequently asked questions
Can I use an air compressor to compress natural gas or biogas?
Not without careful review, and usually not at all. An air compressor’s seals, materials, lubricant, motor and controls are not designed to contain a flammable gas or to resist its contaminants. Leakage, oil dilution, corrosion and ignition risk all rise. Gas compressors are designed for containment, venting, hazardous-area electrics and the gas’s properties. Use a machine built and certified for the actual gas.
What information does a gas compressor supplier need to quote?
At minimum: full gas composition with its range, suction pressure and temperature, discharge pressure, required flow with reference conditions, operating cases including start-up and turndown, site ambient and utilities, and the hazardous-area classification. Without these the supplier must assume values, and two quotes may then describe very different machines that cannot be compared fairly.
Why does molecular weight matter in compressor selection?
Molecular weight changes gas density and the head needed for a given pressure ratio. Centrifugal compressors are especially sensitive: a lighter gas needs more stages or more speed to reach the same pressure. Reciprocating machines cope better with changes, but driver load and discharge temperature still shift. That is why the heaviest and lightest cases must both appear on the datasheet.
Which is better for process gas, reciprocating or screw compressors?
It depends on duty. Reciprocating compressors suit high pressure ratios, smaller flows and changing gas weights, such as CNG and nitrogen boosting. Oil-flooded screws suit moderate pressures, steadier flows and dirty or wet gas such as biogas, provided the oil is compatible. The right answer comes from the datasheet, maintenance capability and lifecycle cost, not from habit.
What is settle-out pressure in a gas compressor?
Settle-out pressure is the equalised pressure that forms across the compressor and its connected volumes after a trip or shutdown, when suction and discharge sides balance through the machine or bypass. If the compressor and driver cannot start against it, the system must be depressurised first. It should be calculated early because it affects driver sizing, start-up sequence and relief design.
Do you service gas compressors outside Hyderabad?
Yes. Our service teams cover the whole of Telangana and Andhra Pradesh from our Secunderabad office, and we supply gas compressors, rentals and spare parts across India. Project-based site support elsewhere in India is arranged where feasible. Send the machine details and site location and we will confirm what we can do.
Related pages
Field guide
Choosing a process gas compressor: Start with the gas, not the catalogue
A gas compressor is a process asset. It moves gas that may be flammable, toxic, corrosive, reactive or very hard to seal, and it sits inside a larger train of separators, coolers, seals and controls. Selection starts with a process study. Atmospheric air is fairly constant; process gases are not.
The train around the machine
What the gas does to the design
- Molecular weight. Sets density, leakage and the pressure rise a stage can give. Hydrogen is very light, escapes through small clearances and needs specialist machines or more stages.
- Compressibility. Real gases depart from ideal behaviour at high pressure and near phase boundaries, which changes power, discharge temperature, capacity and density.
- Heat-capacity ratio. Decides how fast temperature rises on compression, so the same pressure ratio gives different discharge temperatures for different gases.
- Condensation. Intercooling can condense water or heavy hydrocarbons. Liquid reaching the next stage causes erosion, valve damage or mechanical failure.
- Hazard. Flammable, toxic, sour and oxygen service each demand their own containment, materials and cleanliness. CO₂ with moisture is corrosive.
Technology by duty
| Technology | Strength | Watch for |
|---|---|---|
| Reciprocating | Very high pressure, high ratio per stage, flexible capacity | Pulsation, piping vibration, valve and rod-packing wear |
| Centrifugal | Large, steady, pulse-free flow | Surge and choke; recycle energy away from design point |
| Rotary screw | Wet, dirty or variable gas; compact; good turndown | Oil separation (flooded) or rotor clearances (dry) |
| Diaphragm | Near leak-tight, oil-free, high purity and pressure | Lower flow; diaphragm fatigue; specialist maintenance |
| Axial | Very large flow at modest pressure rise | Precise aerodynamics and control |
As a first match: low flow at very high pressure points to reciprocating or diaphragm; large, stable, continuous flow to centrifugal; wet or variable gas at moderate pressure to screw; extremely large flow at modest rise to axial.
Inputs to collect before any quotation
- Gas composition, molecular weight and compressibility.
- Suction and discharge pressure, suction temperature and pressure ratio.
- Minimum, normal and maximum flow, and the turndown needed.
- Liquid content, solids, corrosiveness, flammability, toxicity and any oil-free requirement.
- Driver available (motor, gas engine, turbine), redundancy, site climate and altitude, maintenance capability, future expansion and energy cost.
Protect the compressor before compression
Many failures begin upstream. Use an inlet separator or scrubber for bulk liquid and slugging, filtration for solids, heating where hydrates or condensation threaten, and gas treatment to remove moisture, H₂S, CO₂ or heavy ends where required.
Sealing, cooling and drivers
- Seals depend on the machine: rod packing, mechanical seals, labyrinths, dry-gas seals, buffer and separation gas. A seal-system fault can stop a healthy compressor.
- Multistage compression with intercooling lowers power and discharge temperature but needs knockout drums and level control.
- Electric motors suit steady power supply; gas engines suit remote gas sites; turbines suit high-power pipeline and LNG duty; steam turbines suit plants with steam networks.
Hydrogen and CO₂
Hydrogen needs material compatibility, embrittlement checks, seal design, leak detection, ventilation and hazardous-area classification. It is not a natural-gas machine with a new label. CO₂ needs water removal, corrosion control, interstage cooling and control of the phase envelope, and compression is a large part of capture energy.
Reliability in service
Watch suction and discharge temperature and pressure, vibration, axial position, bearing temperature, driver power, oil and seal-gas pressure, filter differential pressure and separator level. A single reading is a snapshot; a trend shows deterioration. Common failure modes are liquid carryover, valve failure, rod-packing leakage, surge, rotor fouling, seal failure, cooler fouling, bearing damage and piping vibration.
Our scope for this equipment is stated on the enquiry page. Send the gas analysis and duty points so we can confirm whether the job is within what we supply and service.
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer, Reddy Compressors. Last updated 17 October 2026. How our engineering content is reviewed.