Why hydrogen is different from air and other gases
Hydrogen is the lightest gas. Its low molecular mass and high diffusivity mean it escapes through joints, packing and seals that would hold air or nitrogen comfortably. It has a wide flammability range in air and needs very little energy to ignite, so a small leak in a poorly ventilated enclosure can become a serious hazard.
Those properties change how a compressor behaves:
- Performance: for a given impeller tip speed, dynamic machines achieve a far lower pressure ratio per stage on hydrogen than on heavier gases, which is why most hydrogen duties use positive-displacement machines.
- Containment: sealing, packing vents and instrument connections must be designed for a gas that finds every leak path.
- Materials: some steels and high-strength alloys lose ductility in hydrogen service, depending on alloy, strength level, stress, temperature, pressure and cycling.
- Safety systems: ventilation, gas detection, isolation and venting become part of the equipment boundary, not optional extras.
Neither “stainless steel” nor “oil-free” is an unconditional approval for hydrogen. Each needs evidence for the exact service.
What duty are you actually compressing for?
The first step is to name the application. Industrial transfer between storage vessels, compression downstream of an electrolyser, recovery and recirculation of process hydrogen, cylinder or tube-trailer filling and laboratory supply all impose different demands on flow, pressure, purity and starts.
For each application, describe every operating case, not just the normal one:
- Normal, minimum and maximum flow, and how quickly flow changes.
- Start-up, shutdown, purge and blocked-outlet conditions.
- Gas composition, including moisture, oxygen, nitrogen and any hydrocarbons.
- Suction and discharge pressure in bar(a), and suction temperature.
- Running hours per day, number of starts and pressure cycles per year.
- Required product purity and where it will be measured.
A supplier who receives only “200 bar, 20 kg/h” cannot confirm suitability. The missing cases are often the ones that decide staging, cooling and controls.
Pressure ratio and stage screening
Pressure ratio is always calculated from absolute pressures: r = P2(abs) ÷ P1(abs). Using gauge values gives the wrong answer, and the error is largest at low suction pressure.
Consider a hypothetical transfer from 10 bar(a) suction to 200 bar(a) discharge. The overall ratio is 200 ÷ 10 = 20. If that ratio were split equally across three stages, each stage ratio would be 20^(1/3), about 2.71. The intermediate pressures would then be roughly 27.1 bar(a) and 73.7 bar(a).
These figures are screening values only. Real stage pressures depend on gas properties, interstage cooling, pressure losses, rod-load and temperature limits of the chosen machine, and the supplier’s own selection. Treat them as a way to check whether a quotation is sensible, not as settings.
| Screening item | How it is worked out | What it tells you |
|---|---|---|
| Overall ratio | Discharge bar(a) ÷ suction bar(a) | Whether one, two, three or more stages are likely |
| Equal stage ratio | Overall ratio to the power 1/number of stages | A first estimate of each stage’s duty |
| Interstage pressure | Suction pressure multiplied by the stage ratio, stage by stage | Estimated normal interstage operating pressure only; equipment design pressure requires a separate rated design |
| Transferred mass | Mass flow multiplied by running hours | A sense check against storage and filling plans, not storage sizing |
Flow, gas properties and temperature
Hydrogen flow should be stated as mass flow (kg/h) wherever possible. If volume flow is used, state the reference pressure, temperature and whether it is a dry or wet basis. Mass flow equals reference density multiplied by reference volume flow, and the density must match the gas and those reference conditions.
A short worked example: an industrial transfer at 10 bar(a) suction, 200 bar(a) discharge and 30 °C inlet, running at a nominal 20 kg/h for eight hours, moves about 160 kg of hydrogen. That figure checks the logistics. It does not size storage, define power or confirm discharge temperature.
Discharge temperature, absorbed power and cooler duty need gas-specific calculations from the compressor supplier. Ideal-gas and equal-stage assumptions are useful for a first look, but they cannot underwrite an equipment guarantee.
Which compressor technology suits hydrogen?
No single technology is best. The choice depends on flow, pressure, purity, containment and duty cycle.
| Technology | Typical strengths | Points that need verification |
|---|---|---|
| Diaphragm compressor | Gas is separated from the hydraulic side by metal diaphragms, which suits high purity and high pressure at modest flow | Diaphragm integrity monitoring, failure response, flow limits, materials and maintenance intervals |
| Reciprocating piston (lubricated or non-lubricated) | Wide range of flows and pressures, multi-stage designs well established | Cylinders, rings, rod packing, distance pieces, pulsation, packing vent routing and the lubrication boundary |
| Ionic liquid or other specialist designs | Used for some refuelling and storage duties | Maker’s evidence for the exact duty, liquid carry-over and service support |
| Dynamic (centrifugal) machines | Very large flows in refinery and process plants | Many stages for modest ratio, seal systems, operating range and specialist aerodynamic design |
Ask every supplier to explain why their technology fits your duty and what its limits are. A good answer names the trade-offs.
Materials, sealing and leakage
Build a gas-contact inventory covering casings, cylinders, valves, piston rods, seals, gaskets, fasteners, piping, filters and instruments. For each item, record the material specification and the pressure, temperature and cycling it will see. Hydrogen embrittlement is not a single yes-or-no property; it depends on all of these together.
Then build a leakage inventory. List every credible release point: rod packing, flanged joints, instrument fittings, vents, relief outlets and maintenance connections. For each, decide where the gas goes, how it is detected, how the area is ventilated and who owns the protective system. Detector set points and relief settings belong to the project’s safety design.
- Route packing vents and relief outlets to a safe location, not into an enclosure.
- Keep the number of threaded joints in hydrogen service to a minimum.
- Confirm the hazardous-area classification before choosing motors, instruments and panels.
- Plan purging with inert gas before maintenance and before first introduction of hydrogen.
Purity, pretreatment and contamination
State the feed purity and the maximum allowable oxygen, moisture, oil and particulate content for your process. Define where samples will be taken and who is responsible for each treatment step, such as drying, deoxo units or filtration.
An oil-free compressor does not by itself guarantee zero hydrocarbons in the product. Maintenance materials, cleaning agents, seal wear and poorly purged pipework can all introduce contamination. Include cleanliness checks after maintenance in the operating procedure.
Commissioning, acceptance and diagnostics
Before hydrogen is introduced, confirm that approved drawings, serial numbers, gas-contact materials, pressure-equipment documents, isolation and vent connections, cooling, protective functions and leak-tightness evidence are all in place. Hydrogen introduction and pressure testing follow qualified, site-specific procedures.
During acceptance, record pressure, temperature, flow, vibration, leakage indicators and gas-quality results against the supplier’s and the project’s criteria. Those readings become the baseline for later fault-finding.
| Symptom | Where to look first |
|---|---|
| Low delivered flow | Suction pressure and temperature, valve condition, control state, internal leakage |
| High discharge temperature | Stage ratio, inlet temperature, cooler fouling, cooling-water supply |
| Leak indication | Packing, joints, instrument fittings, vent routing |
| Purity deviation | Feed quality, lubrication boundary, recent maintenance, purge practice |
These are directions for investigation. Protective functions must never be bypassed, and the maker’s limits always apply.
How Reddy Compressors helps
Reddy Compressors sells new and used or refurbished gas compressors, rents them, services them and supplies their spares, including CNG, nitrogen, process-gas and booster machines, across a wide range of brands as an independent multi-brand specialist. Hydrogen duties still follow the gas-specific codes and the maker’s design rules, so our role is practical and clearly bounded.
- We help you turn an outline requirement into a complete enquiry: operating cases, absolute pressures, mass flow, purity and duty cycle.
- We screen pressure ratio and staging so you can compare supplier proposals on a like-for-like basis.
- We supply and service the supporting compressed-air and nitrogen systems that hydrogen plants rely on for instrument air, purging and inerting.
- Our team of more than 20 service engineers and technicians, based at Tarnaka in Secunderabad, covers Telangana and Andhra Pradesh, and we supply equipment and spares across India.
Final selection, materials approval and safety design stay with the equipment maker and your project’s qualified engineers.
What to send us
- Application and site location.
- Gas composition with impurities and the analysis method.
- Suction and discharge pressure (minimum, normal, maximum) in bar(a).
- Suction temperature range and mass flow range in kg/h.
- Running hours, starts and pressure cycles.
- Purity requirement and sampling point.
- Existing equipment nameplate photos, if any.
Separate operating cases from the rated hydrogen design
A stage-screening worksheet estimates a running condition. It does not establish the allowable pressure of a cylinder, cooler, separator or connected instrument. The supplier must separately evaluate blocked flow, settle-out, isolation, temperature and protective-system cases before issuing equipment ratings.
Keep three records distinct: the process operating-case table, the manufacturer selection, and the pressure-equipment protection schedule. A value derived from equal stage ratios belongs in the first record until the selected design is confirmed. This separation prevents a useful preliminary calculation from being copied into a purchase order as an equipment limit.
Frequently asked questions
Can an ordinary air compressor compress hydrogen?
No, not without documented confirmation from the maker that the machine, its materials, seals and electrical equipment suit the exact hydrogen service. Air compressors are not designed for hydrogen’s leakage behaviour, flammability or material effects. Reaching the required pressure on air proves nothing about hydrogen. Use a machine selected and rated for hydrogen, installed to a hazardous-area design.
Is an oil-free compressor automatically suitable for hydrogen?
No. Oil-free construction removes one contamination route, but suitability also depends on gas-contact materials, sealing, leakage routing, pressure rating, cooling and hazardous-area design. Oil-free machines can still introduce contamination through seal wear or maintenance materials. Ask the supplier for evidence covering the full hydrogen duty, not just the lubrication arrangement.
What type of compressor is best for hydrogen?
There is no universal best type. Diaphragm compressors suit high purity at modest flow, reciprocating machines cover a broad range of flows and pressures, and dynamic machines serve very large process flows. The right choice follows from the stated flow, pressure ratio, purity, containment, duty cycle and the supplier’s evidence for that duty.
Why should hydrogen pressures be given in absolute units?
Pressure ratios and gas properties are calculated from absolute pressure. If gauge pressures are used, the ratio is wrong, which distorts the number of stages, temperatures and power estimates. The error is greatest at low suction pressure. Give suction and discharge in bar(a) and note where each pressure is measured.
How many stages does a hydrogen compressor need?
That depends on the overall pressure ratio and the machine’s temperature and load limits. Divide discharge bar(a) by suction bar(a) to find the ratio, then the supplier splits it across stages with cooling between them. For illustration, dividing a ratio of 20 equally across three assumed stages gives roughly 2.7 per stage. This calculation does not establish that three stages suit the selected machine.
Who approves the final hydrogen compressor selection?
The equipment maker approves the machine for the stated duty, and your project’s qualified engineers approve materials, pressure protection and the hazardous-area design, each within their own scope. We help prepare a clear, complete enquiry and compare proposals, but we do not replace those approvals or certify the installation.
Related pages
Calculators and worked examples
These tools give indicative figures for screening, comparison and for preparing an enquiry. They run in your browser and send nothing. No prices or tariffs are assumed: enter your own to see costs. Efficiencies and other figures shown are example values: replace them with your own. Results are not a performance guarantee or a design certificate; final selection is confirmed by a service engineer against the machine data sheet and the site.
Gas compression stage screening
Convert suction and discharge to absolute pressure, find the overall and per-stage pressure ratio, the interstage pressures for an equal split, the ideal discharge temperature by the gas’s k value, and, from the mass flow, the actual suction volume, normal volume and mass transferred.
The worked example below shows the method with fixed numbers. Turn on JavaScript to change the inputs.
Worked example
Inputs
- Suction pressure (in the unit below): 10
- Suction pressure unit: bar a
- Discharge pressure (in the unit below): 200
- Discharge pressure unit: bar a
- Site atmospheric pressure (bar a; for gauge entries): 1.013
- Gas: Hydrogen (k 1.41, MW 2)
- Other gas: k = cp/cv (used only for Other): 1.3
- Other gas: molecular weight (kg/kmol; used only for Other): 17
- Suction temperature (°C): 30
- Number of stages (screening, equal ratio): 3
- Stage ratio for the temperature check (0 = equal split): 0
- Isentropic efficiency (%; 100 = ideal): 100
- Maximum allowable discharge temperature (°C; 0 = not checked): 0
- Mass flow (kg/h; 0 = not used): 20
- Compressibility factor Z at suction (1 = ideal gas): 1
- Running hours for the transferred mass (h; 0 = not used): 8
- Lowest credible suction pressure (same unit as suction; equal to suction = no comparison): 10
Result
- Suction, absolute: 10 bar a (8.987 bar g, 145 psia)
- Discharge, absolute: 200 bar a (198.99 bar g, 2,886 psig)
- Overall pressure ratio: 20 absolute discharge ÷ absolute suction
- Equal stage ratio: 2.714 over 3 stages
- Interstage pressures (equal split): 27.1, 73.7 bar a estimated operating pressures only, not design pressures
- Exponent (k − 1)/k: 0.291 k = 1.41 for hydrogen; R = 4,124.2 J/(kg·K)
- Ideal discharge temperature per stage: 132.1 °C (405.3 K) at a stage ratio of 2.714, suction 30 °C
- Actual suction volume: 25 m³/h (0.417 m³/min, 14.7 acfm) at 10 bar a, Z = 1
- Normal volume: 222.4 Nm³/h (0 °C, 1.013 bar a; density 0.0899 kg/Nm³)
- Mass transferred: 160 kg in 8 h (a logistics check, not storage sizing)
Method. P(abs) = P(gauge) + site atmospheric pressure. Overall ratio r = P₂(abs) ÷ P₁(abs); dividing gauge figures gives a wrong ratio, worst at low suction pressure. Equal split: stage ratio = r^(1/n); interstage pressure after stage i = P₁ × (stage ratio)^i. Ideal (isentropic) discharge temperature T₂ = T₁ × (stage ratio)^((k−1)/k), kelvin, assuming intercooling back to the suction temperature before every stage; with an isentropic efficiency η, T₂ = T₁ + (T₂ideal − T₁) ÷ η. Gas constant R = 8,314.46 ÷ molecular weight. Actual suction volume = ṁ × Z × R × T₁ ÷ P₁; normal volume uses 0 °C and 1.01325 bar a with Z = 1. Transferred mass = mass flow × running hours. Rounded textbook k values near ambient; real-gas Z and the maker’s calculation for the actual composition govern. These are screening values, not equipment ratings or settings.
What to take from it. Always work in absolute pressure, and check the lowest credible suction pressure: it gives the highest ratio and discharge temperature and the largest actual suction volume for the same mass flow.
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.