Compressors for cryogenic plants: interfaces and selection

Where compressors sit in a cryogenic plant

Air separation units, nitrogen and oxygen liquefiers, LNG and biogas upgrading plants, helium and carbon dioxide recovery systems and cryogenic freezers all rely on compression somewhere. Very few of these compressors actually see cryogenic fluid. The low temperatures are produced inside the cold box, heat exchangers and expansion turbines; the compressors usually work at or near ambient temperature on either side of that cold section.

The first engineering task is therefore to draw the process flow diagram and mark, for each compressor, where the gas comes from, what state it is in at the suction flange and where it goes next. Two machines on the same site can be completely different services.

What the enquiry must define

A request for “a compressor for our cryogenic plant” cannot be priced or engineered safely. The process designer or licensor needs to supply, for each duty:

• The actual gas or mixture, with composition range, molecular weight, moisture, and any oxygen, hydrocarbon, hydrogen or corrosive content.
• Suction and discharge pressures in absolute terms at the compressor flanges, with the lowest credible suction pressure.
• Suction temperature range, including cold-start and upset cases.
• Mass flow (kg/h) for minimum, normal and peak cases, plus any recycle, warm-start, defrost or standby changeover cases.
• Required discharge temperature limit and the cooler or downstream equipment that receives the gas.
• Purity and contamination limits, including whether any lubricant contact is acceptable.
• Hazardous-area classification, utilities available and the plant’s safe-state philosophy.

Where data are missing, they should be recorded as open items with a named owner, not filled with assumptions.

Absolute pressure and pressure ratio

Compressor performance depends on absolute pressures. Many process sheets give gauge pressures, and mixing the two causes serious errors. P(abs) = P(gauge) + local atmospheric pressure.

Example: a duty lists suction at 0.30 bar(g) (about 4.4 psig) and discharge at 5.30 bar(g) (about 77 psig), with 1.00 bar atmospheric pressure assumed. Absolute suction is 1.30 bar(a) and absolute discharge 6.30 bar(a), so the pressure ratio is 6.30 / 1.30 = 4.85. Dividing the gauge figures (5.30 / 0.30) would give 17.7, a completely different machine. At inland sites in Telangana the local barometric pressure is noticeably below sea level, which should be taken from the site, not assumed.

Mass flow versus actual suction volume

Process plants specify mass flow; compressor makers size on actual inlet volume. For screening, Q(actual) = ṁ × R × T / P, with R the gas’s specific constant, T in kelvin and P absolute.

Example: nitrogen (R ≈ 296.8 J/(kg·K)) at 500 kg/h, 300 K and 1.50 bar(a) gives (500 / 3,600) × 296.8 × 300 / 150,000 ≈ 0.082 m³/s, about 297 m³/h actual. If suction falls to 1.20 bar(a) at the same temperature, the actual volume rises to about 371 m³/h, an increase of 25%. The low-suction-pressure case often governs machine size.

Normal or standard volumes (Nm³/h) are useful for process balances but must state the reference temperature, pressure and moisture basis. The contract should say clearly whether acceptance is on mass throughput, normal volume or actual suction capacity.

These ideal-gas equations are unit checks. Near condensation, at high pressure, or with mixtures, the real-gas compressibility factor Z and a validated property model are needed, and the compressor maker’s own calculation governs.

Choosing the compressor technology

An industrial air compressor data sheet is not evidence that a machine is suitable for oxygen, hydrogen, hydrocarbon or cold boil-off gas. Those services need the maker’s specific gas-service confirmation, materials and seal design. Oxygen service in particular needs specialist cleaning and compatible materials, following the gas-specific codes and the maker’s design rules.

Cold piping, liquids and protection

Even a warm compressor can be connected to cold piping. Thermal contraction, flange loads, support positions and insulation terminations must be checked over the full cycle, including cool-down and warm-up. Steel that is tough at ambient temperature can become brittle at low temperature, so minimum design metal temperature matters wherever cold gas can reach.

Liquid is the enemy of most compressors. Cryogenic droplets, condensed hydrocarbons, water, ice or solid carbon dioxide can damage valves and rotors. Suction knock-out vessels, liquid detection, heating where specified and a clear shutdown response protect the machine. “Dry gas duty” on a data sheet must be backed by evidence that the plant actually delivers dry gas in start-up and upset cases.

• Seals: define the seal type, buffer or barrier gas, vent destination and acceptable leakage.
• Pressure protection: deadhead, blocked discharge, reverse flow and trapped cold liquid warming up are all credible overpressure sources, and relief design belongs to the process safety authority.
• Check valves: confirm their location so downstream storage cannot drive reverse flow or reverse rotation after a trip.
• Safe state: define what the compressor does on loss of cooling, seal gas, power or instrument air.

Standby, redundancy and restart

State the redundancy honestly. If two machines are both needed at peak demand, the system is not N+1 at peak. If a 700 kg/h machine serves a 650 kg/h peak, the nominal margin is under 8% before allowing for warm, low-pressure suction conditions. A standby machine must be tested along its whole path, including cooling, seal gas, controls and power, and shared utilities such as one cooling-water valve are a common-cause weakness.

Restart after a cold-box trip deserves its own procedure. Residual cold inventory, uncertain suction phase and seal-gas pressure must be confirmed before a compressor is restarted. That procedure is written by the process authority and the compressor maker together.

How Reddy Compressors helps

Reddy Compressors is an independent multi-brand compressor specialist at Tarnaka, Secunderabad, serving industry since 2002. We sell new and used/refurbished gas compressors (CNG, nitrogen, process gas and booster duties), centrifugal compressors and the air compressors that support cryogenic plants, rent them, service them and supply their spares. For axial compressors we provide service and spares. We are not an authorised dealer of any brand, and we do not design cold boxes, liquefiers or plant safety systems. We have tie-ups with all major and reputed brands.

• We help you prepare a complete compressor enquiry: gas data, absolute pressures, mass flow cases and interface boundaries.
• We supply and service the warm-end machines: main air compressors, nitrogen boosters, product compressors and instrument-air packages with dryers.
• Our 20+ service engineers and technicians cover Telangana and Andhra Pradesh, and we supply machines and spares across India.
• Rental compressors from our own fleet, including oil-free air, can support shutdowns, commissioning and breakdowns where the duty allows.

Oxygen, hydrogen and toxic-gas duties still follow the gas-specific codes and the maker’s design rules, and we work within them.

What to send us

• Process flow diagram or sketch showing where the compressor sits.
• Gas name and composition range, with moisture and contaminant limits.
• Absolute suction and discharge pressures and temperatures for each case.
• Mass flow at minimum, normal and peak, and any recycle or start-up cases.
• Existing machine nameplate photo and service history, if replacing or repairing.
• Site location, utilities available and hazardous-area classification.

Frequently asked questions

What type of compressor is used in an air separation plant?

The main air compressor in a medium or large air separation unit is usually a multi-stage or integrally geared centrifugal machine, because it handles high, steady flows with an oil-free gas path. Smaller plants may use oil-free or oil-injected screw compressors with suitable purification. Product gases such as nitrogen or oxygen may then be raised to delivery pressure by separate reciprocating, centrifugal or diaphragm boosters chosen for that gas.

Does a compressor in a cryogenic plant handle cryogenic gas?

Usually not. Most compressors in cryogenic plants work on warm gas, before cooling or after the gas has been warmed in heat exchangers. Boil-off gas recovery is a notable exception, where vapour may reach the compressor very cold unless it is warmed first. The suction flange temperature, not the plant’s name, decides the materials and machine design needed.

Why must compressor pressures be given in absolute terms?

Pressure ratio, density and machine capacity all depend on absolute pressure. A suction of 0.3 bar(g) and discharge of 5.3 bar(g) is a ratio of about 4.85 in absolute terms, while dividing the gauge figures wrongly suggests 17.7. Stating absolute pressures, with the local barometric pressure used, removes this error and lets the maker size the machine correctly.

Can an oil-injected screw compressor be used for nitrogen boosting?

For many nitrogen booster duties, yes, provided the purity requirement allows for the oil carry-over that remains after separation and filtration, and the machine is designed for nitrogen service. Where very high purity is required, an oil-free reciprocating or diaphragm machine is often preferred. Oil-injected compressors are not used for oxygen service because of fire risk.

Why does the low suction pressure case often govern compressor size?

For a fixed mass flow, lower suction pressure means lower gas density and therefore more actual volume entering the compressor. Dropping from 1.5 to 1.2 bar(a) increases the actual suction volume by about a quarter. The machine must be sized for the worst credible suction case, and the pressure ratio also rises, which increases power and discharge temperature.

What causes high discharge temperature on a gas booster compressor?

Common causes are a higher pressure ratio than designed, often from low suction pressure, fouled or under-performing intercoolers, high cooling-water temperature, leaking valves on reciprocating machines that recompress hot gas, or a gas with a higher heat capacity ratio than assumed. Check absolute pressures, cooler approach temperatures and valve condition before changing settings, and keep within the maker’s limits.

Related pages

• Process gas compressor selection requirements
• Gas properties and compression behaviour
• Nitrogen booster compression
• Gas compressor interstage cooling and separation
• Integrally geared centrifugal compressors
• Specialist compression and vacuum engineering centre