How gas properties change compressor behaviour

Why gas properties matter more than machine size

Air compressor buyers are used to a simple question: how many cfm at what pressure? Gas compression is less forgiving. A machine sized for natural gas can overheat on a heavier gas, stall in surge on a lighter one, or condense liquid in its cylinders when the composition shifts. Gas properties are therefore part of the design basis, and they must be stated for every operating case, not only the normal one.

The key properties are composition, molecular weight, the ratio of specific heats k (cp/cv), the compressibility factor Z, the dew point of water and hydrocarbons, and any trace components such as hydrogen sulphide, carbon dioxide, oxygen, mercury or solids. Change any of these and the compressor’s performance changes.

Composition is an operating variable, not a fixed number

Real gas streams vary. Biogas methane content changes with the digester’s feed. Natural gas at a well site changes over the life of the field. Process off-gas changes with plant load. A compressor specified on a single “typical” composition often struggles with the lean and rich cases that actually occur.

Ask for and specify at least these cases:

• Normal composition at rated flow.
• Lightest credible gas (lowest molecular weight), which affects head and surge on dynamic machines.
• Heaviest credible gas, which affects power, discharge temperature and condensation.
• Start-up gas, which may be nitrogen or air for purging and testing.
• Upset cases: water carry-over, a step in carbon dioxide or hydrogen sulphide content, or air ingress.

Typical properties of common gases

The values below are rounded textbook figures near ambient conditions, given only to show how much gases differ. Design must use properties calculated for the actual composition, pressure and temperature.

How the k value sets discharge temperature

For an ideal adiabatic (isentropic) compression stage, the discharge temperature is estimated from:

T2 = T1 × (P2 / P1)^((k − 1) / k), with temperatures in kelvin and pressures absolute.

The exponent (k − 1)/k is about 0.29 for air and nitrogen, about 0.40 for helium and only about 0.12 for propane. A higher k means a hotter gas for the same pressure ratio. Real machines run hotter than the ideal figure because of their efficiency, so the actual discharge temperature is found by dividing the ideal temperature rise by the isentropic efficiency.

Worked example: a stage takes gas at 35 °C (308 K) and 2 bar(a) and discharges at 6 bar(a), a pressure ratio of 3.

• Air or nitrogen, k = 1.40: 3^0.286 ≈ 1.37, so the ideal T2 ≈ 422 K, about 149 °C.
• Helium, k = 1.66: 3^0.398 ≈ 1.55, so the ideal T2 ≈ 477 K, about 204 °C.
• Methane, k ≈ 1.31: 3^0.237 ≈ 1.30, so the ideal T2 ≈ 400 K, about 126 °C.

The same pressure ratio produces discharge temperatures nearly 80 °C apart. This is why helium and hydrogen machines usually need more stages and intercoolers, and why limits on discharge temperature, set by valve materials, packing, lubricant or the gas itself, often decide the number of stages.

Compressibility factor and real-gas density

The ideal-gas law, PV = nRT, works well for air at plant pressures. At higher pressures or near a gas’s condensation region, real gases deviate. The compressibility factor Z corrects for this: PV = ZnRT. Natural gas at high pressure typically has Z below 1, so it is denser than the ideal-gas law predicts. Hydrogen generally has Z slightly above 1.

Z matters in three places: converting between standard and actual flow, calculating the mass that each cylinder or impeller actually moves, and calculating absorbed power. Using Z = 1 when the real value is 0.85 can misstate density by more than 15%, which flows straight into wrong flow and power figures. Z should come from a recognised equation of state for the actual composition, such as Peng–Robinson, Soave–Redlich–Kwong, or GERG-2008 and AGA 8 for natural gas.

Standard, normal and actual flow: get the basis right

Many gas-compressor disputes start with a flow figure on the wrong basis. Agree which one is meant and write it on every data sheet.

To convert between them you need pressure, temperature, Z and molecular weight. Mass flow is the safest basis when compositions vary, because a volume figure without its reference conditions can mean almost anything.

Molecular weight and dynamic compressors

Centrifugal compressors generate head, not pressure. The pressure rise for a given head is proportional to gas density, so a lighter gas produces a much smaller pressure rise through the same impeller. Hydrogen-rich gases therefore need many more stages or higher tip speeds. A centrifugal machine designed for a heavy gas, run on a light one, may fail to reach discharge pressure; run on a heavier gas than designed, it may overload the driver. Positive-displacement machines, such as reciprocating, screw and diaphragm compressors, are less sensitive to molecular weight for pressure, but power still changes with density.

Water, condensables and the phase envelope

Compression raises pressure, and higher pressure raises the dew point. Gas that is dry at suction can condense water or hydrocarbons after an intercooler, where the temperature has dropped but the pressure has risen. Liquids in a reciprocating cylinder can break valves, rods and pistons; in a screw or centrifugal machine they cause erosion and imbalance.

• Know the water and hydrocarbon dew points at every stage pressure, not just at suction.
• Keep intercooler outlet temperatures above the dew point with a margin, or provide separators and drains that remove the liquid formed.
• Check start-up and settle-out, when cold gas sits in pressurised piping.
• For carbon dioxide, check whether the gas will pass near or above its critical point (about 31 °C and 74 bar), where density changes sharply with small temperature changes.

Trace components cause their own problems: hydrogen sulphide affects material choice and seal life, siloxanes in biogas form abrasive deposits, oxygen in hydrocarbon streams raises flammability concerns, and solids and polymers foul valves and coolers.

Turning gas properties into a reliable specification

1. List every operating case with full composition, pressure (bar(a) and psig) and temperature.
2. State flow as mass flow, plus volume flow with its reference conditions.
3. Name the property method or equation of state to be used, so vendor and purchaser calculate the same numbers.
4. Give the maximum allowable discharge temperature per stage and the reason for it.
5. State dew points and liquids handling requirements at each stage.
6. Ask each vendor to show calculated power, discharge temperature and flow for every case, not only the rated one.

How Reddy Compressors helps

We sell (new and used/refurbished), rent and service gas compressors, including CNG, nitrogen, process gas and booster machines, and we supply their spares. As an independent multi-brand specialist based at Tarnaka, Secunderabad, we serve sites across Telangana and Andhra Pradesh with more than 20 service engineers and technicians, and supply equipment and spares across India.

For gas-property questions we help in practical ways: checking whether an existing machine suits a changed gas or pressure, reviewing discharge temperature and intercooling on a running unit, comparing vendor offers on the same basis, and identifying why a machine is overheating, short of flow or condensing liquid. For a new duty we use your process data and the maker’s selection tools, not guesswork. Oxygen, hydrogen and toxic-gas duties still follow the gas-specific codes and the compressor maker’s design rules, and final design calculations remain with the licensor, consultant or manufacturer responsible for the package.

What to send us

• Gas composition (mole %) for normal, lean and rich cases, plus water content and trace components.
• Suction and discharge pressure in bar(a) or psig, and suction temperature.
• Required flow as mass flow or with the reference conditions stated.
• Existing machine make, model, serial number and nameplate photo, if any.
• Cooling water or ambient air temperature, including Indian summer peaks.
• Any limits on discharge temperature, lubricant or materials from the process licensor.

Frequently asked questions

Why does my gas compressor run hotter on a different gas?

Discharge temperature depends strongly on the gas’s ratio of specific heats, k. Gases with a high k, such as helium, heat up much more for the same pressure ratio than gases with a low k, such as propane or heavier natural gas. A change in composition, suction temperature or pressure ratio can therefore push a machine past its temperature limits even when nothing mechanical has changed.

What is the compressibility factor Z in gas compression?

Z is a correction that shows how far a real gas departs from ideal-gas behaviour at a given pressure and temperature. When Z is below 1, the gas is denser than the ideal-gas law predicts; above 1, it is less dense. Z affects flow conversion, the mass moved per stroke or revolution and the power absorbed, so it should be calculated with a recognised equation of state.

What is the difference between Nm³/h and actual m³/h?

Nm³/h is volume flow corrected to 0 °C and 1.013 bar absolute, so it represents a fixed quantity of gas. Actual m³/h is the volume the gas occupies at real suction pressure and temperature, which is what the cylinders or impellers must handle. Converting between the two needs pressure, temperature and compressibility, so always state which basis a flow figure uses.

Can an air compressor be used for nitrogen or other gases?

Nitrogen behaves much like air, so some air compressors can handle it, but the casing must be gas-tight, seals and vents must be suitable, and the maker must accept the duty. Flammable, toxic or high-value gases need a compressor designed for gas service, with the right seals, materials, hazardous-area electrics and leakage control. Never convert an air machine to such gases without the maker’s approval.

Why does liquid form between compressor stages?

Compression raises the gas’s dew point. When gas is cooled in an intercooler at a higher pressure, water or hydrocarbons can condense even if the gas was dry at suction. Separators with working drains, or intercooler outlet temperatures held above the dew point with a margin, prevent liquid reaching the next stage, where it can damage valves, pistons or rotors.

Which flow basis should I use in a gas compressor enquiry?

Give mass flow in kg/h where possible, because it does not depend on pressure or temperature. Add volume flow with its reference conditions clearly stated, for example Nm³/h at 0 °C and 1.013 bar absolute. Include the full gas composition for each case, so that every vendor converts and sizes the machine on the same basis and offers can be compared fairly.

Related pages

• Process gas compressor selection requirements
• Gas compressor interstage cooling and separation
• Gas compressor capacity control
• Carbon dioxide compression interfaces
• Hydrogen compression equipment boundaries
• CNG and process-gas reciprocating compressors