Helium recovery compression: how to design it right

Why helium needs specialist compression

Helium is light, inert, expensive and largely non-renewable. It is used in MRI magnet cooling, research cryostats, leak testing, welding and shielding, fibre-optic drawing, semiconductor processes and some laboratory analysers. Many users lose most of the helium they buy to atmosphere. A recovery system captures some of that gas so it can be purified and reused or reliquefied.

Three helium properties drive the compressor design:

• Very small, light atoms: helium escapes through joints, threads, elastomer seals and porous materials that hold air or nitrogen without trouble. Leak tightness is the first requirement.
• High ratio of specific heats: k is about 1.66, against about 1.40 for air. For the same pressure ratio, helium gets much hotter in each stage, so more stages and good intercooling are needed.
• Low density: helium’s molecular weight is about 4, so dynamic machines such as centrifugal compressors are rarely practical at recovery scale. Positive-displacement machines are normal.

What does a complete helium recovery system include?

The compressor is one part of a chain. Each part must be sized and specified together.

1. Collection: return lines from cryostats, magnets, test chambers or process vents, usually at a little above atmospheric pressure. Back-pressure on the user must stay very low, so line sizing and check valves matter.
2. Low-pressure buffer: a gas bag (gasometer) or a low-pressure vessel absorbs surges and lets the compressor run steadily.
3. Pre-treatment: filters, and sometimes a dryer, to protect the compressor from particles and moisture.
4. Compressor: raises the gas from near atmospheric to storage pressure, often in several stages.
5. Purifier: removes air, moisture and oil vapour, often by cryogenic or adsorption methods, to the purity the end use needs.
6. High-pressure storage: cylinder bundles, tube trailers or storage vessels, rated and tested for the pressure.
7. Controls and analysis: purity analysers, pressure and level control, alarms and automatic start and stop.

Define the recovery duty before choosing a compressor

Recovery systems fail most often because the duty was guessed. Before any selection, establish:

• How much helium is returned, in normal m³ per hour or per day, and how the flow varies: steady boil-off, batch fills, quench events or test cycles.
• The return gas purity and its contaminants: air ingress, nitrogen, moisture, oil vapour.
• The buffer volume available, which sets how often the compressor starts.
• Required storage pressure, typically in the high-pressure cylinder range of around 150–200 bar (2,175.6–2,900.8 psig), or whatever the purifier and storage design require.
• Required product purity and what the helium will be used for afterwards.

Which compressor technology suits helium recovery?

The choice follows the flow, the storage pressure and the purity target. Small laboratory systems often use diaphragm or oil-free piston machines; larger industrial systems use multi-stage oil-free reciprocating compressors.

Stopping helium leakage

A recovery system that leaks loses the value it was built to save, and air that leaks in lowers purity. Practical rules:

• Use welded or high-quality compression or face-seal fittings; avoid tapered pipe threads in the gas path wherever possible.
• Choose seal materials with low helium permeability; ordinary elastomers allow slow diffusion.
• Recover packing and seal leakage back to the suction side or buffer where the design allows.
• Leak-test the finished system, ideally with a helium leak detector, and repeat after any maintenance that opens the gas path.
• Keep the low-pressure side slightly above atmospheric so that any leak is outward, not air ingress, unless the design states otherwise.

Keeping the recovered helium pure

Purity is lost in three main ways: air drawn into low-pressure return lines and bags, moisture from users or leaks, and oil vapour from lubricated compressors. A purity analyser on the return gas and on the product tells you where contamination comes from. Low-purity gas can be sent to a separate storage for repurification rather than mixing it into clean stock. Our page on high-purity gas sampling and verification covers how to take samples that represent the gas accurately.

Instrumentation and protection

• Bag or buffer level, or low-pressure vessel pressure, controlling compressor start and stop.
• Low suction pressure trip, so the compressor does not pull a vacuum and draw in air.
• High discharge temperature on every stage.
• Interstage and final pressure, with relief valves on each stage.
• Cooling water or air-cooler fan monitoring.
• Purity analysis with alarm and diversion of off-specification gas.

Commissioning, acceptance and troubleshooting

Commission in a planned sequence: pressure test and leak test each section; purge with clean helium to remove air; run the compressor on recirculation to check temperatures and pressures stage by stage; then connect users and measure recovered volume and purity over a representative period.

Interface points along the recovery chain

Most helium is lost not inside a machine but at the joins between parts supplied by different makers. Each interface below needs an agreed pressure, an agreed purity and one named party who owns it. When nobody owns an interface, losses there go unmeasured.

Fitting a flow totaliser at the purifier outlet and another at the point of supply is the most useful step for optimisation. Without them, recovery rate is a guess.

Recovery rate and purity: a worked example

To optimise helium recovery, split the losses into stages and find the largest. One percentage cannot tell you where the gas went. The example below is (illustrative), and all its inputs are assumptions.

Basis. A site buys 1,000 litres of liquid helium a month. Liquid helium has a density of about 0.125 kg/litre at its normal boiling point. Helium gas at 15 °C and 1.01325 bar(a) has a density of about 0.1693 kg/m³. So 1 litre of liquid gives 0.125 ÷ 0.1693 = about 0.738 m³ of gas, and 1,000 litres gives about 738 m³ of gas a month. All volumes in this example are at 15 °C and 1.01325 bar(a). Conversion figures quoted at 20–21 °C come out about 2% higher, so check the basis before comparing invoices with meter readings.

Loss per regeneration cycle. Assume a purifier vessel with 20 litres of free volume at 150 bar(g), which is 151.0 bar(a), and 30 °C. Helium’s compressibility factor Z is about 1.07 at that pressure. The gas vented each cycle is 151.0 × 20 ÷ 1.07 × (288.15 ÷ 303.15) ÷ 1.01325, which is about 2,650 litres, or 2.65 m³ at 15 °C. If that gas goes back to the gas bag instead of to atmosphere, most of this loss is avoided.

Why purity drives the regeneration count. The purifier fills up with impurities, so how often it must regenerate depends on the impurity load. In Case A the gas returned is 97% helium, so the impurity carried with 664.2 m³ of helium is 664.2 × 3 ÷ 97 = about 20.5 m³ a month. In Case B it is 701.1 × 1 ÷ 99 = about 7.1 m³. The ratio is 7.1 ÷ 20.5 = 0.35, so 10 cycles fall to about 3.5. Improving return purity from 97% to 99% cuts the impurity load to about one third. That reduces regeneration losses, purifier energy and adsorbent wear.

Value and payback (illustrative). Case B recovers 677.8 − 617.8 = about 60 m³ more gas a month. At an assumed value of ₹1,000 per m³ (illustrative; use your own contract price), that is about ₹60,000 a month (illustrative). If the improvements cost an assumed ₹6,00,000 (illustrative), for user-side return lines, bag repair and an inlet purity analyser, simple payback is 6,00,000 ÷ 60,000 = 10 months.

Compression energy is minor. Ideal-gas isothermal compression from 1.05 to 201.3 bar(a) needs 101,325 × ln(201.3 ÷ 1.05) ÷ 3,600,000 = about 0.148 kWh per m³. That is the theoretical minimum. Assume a real multi-stage machine uses about 0.4 kWh/m³. Then 677.8 m³ needs about 271 kWh a month, or about ₹2,440 at an assumed ₹9/kWh (illustrative). Capture and purity have far more effect on the result than compressor efficiency.

Leak-tightness: quantifying it and testing it

A leak specification needs a number. Leak rates are usually stated in mbar·litre/s, which is a pressure multiplied by a volume flow (a pV throughput). One mbar·L/s is 0.001 bar·L/s. Over a year of 31,557,600 s that is 31,558 bar·litres, or about 31.1 m³ of gas at 1.01325 bar(a).

Pressure decay test with temperature correction (illustrative). A 500-litre storage section is filled to 150.0 bar(g) at 30 °C. Twenty-four hours later it reads 148.8 bar(g) at 28 °C. The raw drop is 1.2 bar. Temperature alone changes absolute pressure in proportion to absolute temperature. So the expected reading is 151.01 × (301.15 ÷ 303.15) = 150.01 bar(a), or 149.0 bar(g). Temperature therefore explains about 1.0 bar of the drop, and the real leak is about 0.2 bar. At this pressure every 1 °C change moves the reading by about 0.5 bar. If you ignore temperature, a test can hide a real leak or show one that does not exist. Read the gas or vessel-wall temperature, not shade air, and use a gauge or transmitter that resolves 0.1 bar or better, because a 0.2 bar result is within the error of an ordinary process gauge.

The 0.2 bar loss is about 0.2 × 500 ÷ 1.07 × (288.15 ÷ 301.15) ÷ 1.01325 = about 88 litres of gas at 15 °C a day, or about 32 m³ a year. That is about 88 × 1,013 ÷ 86,400 = 1.0 mbar·L/s. A helium sniffer can then find which joint is leaking.

On the low-pressure side the risk is the opposite: air getting in. Any point that falls below atmospheric pressure can draw air in through a joint that holds pressure perfectly. Test the bag, header and suction pipework at their working pressure of a few millibar, using a pressure-hold test or soap solution. A test at high pressure will not prove these parts. Also record the bag’s permeation as its maker states it, because helium diffuses through many elastomers and fabrics.

How compressor type changes contamination risk

The page’s technology table covers pressure and flow. The table below covers only where contaminants enter, which is often what limits purity in practice. No type is free of risk. The aim is to know which risks to monitor.

Condensate drains are often missed. Every time an interstage drain opens, it releases some helium with the water. A timer drain that opens on a fixed cycle loses gas even when there is no water. A level-sensing drain, or a drain piped back to the gas bag, avoids that loss. For purity sampling at each stage, see high-purity gas sampling and verification. For diaphragm machines in more detail, see diaphragm process gas compressors.

Helium recovery optimisation checklist

1. Measure before changing anything. Record supplied volume, recovered volume (from a totaliser) and return purity for at least one full operating month.
2. Split the losses by stage. Use the stages in the worked example: capture, regeneration, leaks and drains, and off-specification rejection. Work on the largest first.
3. Raise the capture fraction. Connect every user vent, close bypass vents, fit non-return valves, and make sure staff do not vent gas during fills or maintenance.
4. Protect return purity. Keep the low-pressure side above atmospheric, set the low-suction trip, and inspect the bag and header joints on a schedule.
5. Return regeneration and drain gas to the bag wherever the purifier and drain design allow it.
6. Leak-test with numbers. Run temperature-corrected pressure decay on high-pressure sections and a sniffer survey on all joints. Record leak rates in mbar·L/s so you can compare results over time.
7. Match the buffer to the compressor. Count compressor starts per hour. Frequent cycling wears valves and packing and increases leakage.
8. Keep analysers calibrated. A purity analyser that drifts will either reject good gas or pass bad gas.
9. Review every month. Track recovery percentage, regeneration count and starts per hour. A change in any one of them is an early warning.

How Reddy Compressors helps

We sell (new and used/refurbished), rent and service gas compressors, including process gas and booster compressors, and supply their spares. As an independent multi-brand specialist working from Tarnaka, Secunderabad, with more than 20 service engineers and technicians covering Telangana and Andhra Pradesh and supply across India, we can help a hospital, research institution or industrial user in several ways: reviewing the recovery duty and staging, comparing compressor offers on the same basis, servicing oil-free reciprocating and diaphragm machines, checking leak tightness and supplying identified spares such as valves, rings and packing after confirming fit from the machine data.

Purifiers, reliquefiers and pressure-vessel storage come from their specialist makers, and we work alongside them rather than claiming their scope. Helium systems must follow the maker’s pressure ratings and the applicable pressure-vessel and cylinder rules.

What to send us

• Helium users and how they return gas: steady boil-off, batch or test cycles.
• Estimated recovered flow (Nm³/h or per day) and its peaks.
• Return pressure, buffer type and volume, if any.
• Required storage pressure (bar and psig) and storage type.
• Purity required after purification.
• Existing compressor make, model, serial number and nameplate photo, if any.

Frequently asked questions

Is helium recovery worth it for a small user?

It depends on how much helium you lose, how steady the losses are and how pure the returned gas is. Users with continuous boil-off, such as research cryostats, often recover a useful share. Users with small, irregular losses may find leakage and contamination reduce the benefit. Measure your actual consumption and venting pattern before deciding, rather than relying on general estimates.

Why do helium compressors need so many stages?

Helium’s high ratio of specific heats makes it heat up much more than air for the same pressure ratio. Raising helium from near atmospheric to cylinder pressure in too few stages would produce discharge temperatures beyond what valves, seals and lubricants can tolerate. Four or five stages with intercooling keep each stage’s temperature within safe limits.

Can a normal air compressor be used to recover helium?

No. A standard air compressor is not leak-tight enough for helium, may add oil to the gas and is not designed for helium’s discharge temperatures or storage pressures. Helium recovery needs a compressor designed or adapted by its maker for helium service, normally an oil-free reciprocating or diaphragm machine with suitable seals and fittings.

How do you stop air getting into recovered helium?

Keep the low-pressure collection side slightly above atmospheric pressure, use a low suction pressure trip so the compressor never pulls a vacuum, check gas bags and return lines for damage, and leak-test joints. A purity analyser on the return gas shows quickly when air is entering, so the source can be found and fixed before clean stock is contaminated.

What pressure is recovered helium stored at?

Recovered helium is commonly compressed into high-pressure cylinder bundles or tubes, typically around 150–200 bar (2,175.6–2,900.8 psig), though the right figure depends on the storage vessels, the purifier design and how the helium will be reused. All storage must be rated and tested for its working pressure under the applicable pressure-vessel and gas-cylinder rules.

Which spares are most important for a helium recovery compressor?

For oil-free reciprocating machines, keep stage valves, piston rings, rider rings and packing sets, plus filter elements. Diaphragm compressors need spare diaphragms and check valves. Identify each part from the make, model and serial number, and confirm fit before ordering. Holding critical spares avoids long stoppages during which helium would otherwise be lost to atmosphere.

How do you optimise a helium recovery system?

Start by measuring. Fit flow totalisers so you know how much gas was supplied and how much was recovered. Then split the losses into capture at users, purifier regeneration, leaks and drains, and off-specification rejection, and work on the largest first. Capture and return purity usually matter most. Improving return purity from 97% to 99% cuts the purifier’s impurity load to about one third. That reduces regeneration losses. Compressor efficiency has much less effect.

What is a good helium recovery rate?

It depends on how many users are connected and how steady their return flow is. A well-run system with good capture and high return purity can recover a large share of the gas supplied. One poorly connected user or a leaking bag can pull the figure down sharply. Compare against your own measured baseline rather than a published figure. Look at each loss stage separately, because one overall percentage hides where the gas is going.

How much helium gas is in one litre of liquid helium?

Liquid helium at its normal boiling point has a density of about 0.125 kg per litre. Helium gas at 15 °C and 1.01325 bar(a) has a density of about 0.1693 kg/m³. So one litre of liquid gives about 0.74 m³ of gas at those conditions. At 0 °C it is about 0.70 m³, and at 21 °C about 0.75 m³. Always state the reference temperature when comparing supply and recovery volumes. A mismatch in reference conditions can look like a loss of several percent.

How do you leak test a helium recovery compressor and storage?

For high-pressure sections, use a pressure decay test and correct the reading for temperature. At 150 bar, each 1 °C change moves the reading by about 0.5 bar. Then use a helium sniffer leak detector on every joint to find individual leaks. Test the low-pressure bag, header and suction pipework separately at their working pressure. At that pressure the risk is air getting in, not gas getting out. Record results in mbar·L/s so you can compare surveys over time.

Related pages

• Gas properties and compression behaviour
• Diaphragm process gas compressors
• Oil-free process gas reciprocating compression
• Cryogenic process compression interfaces
• High-purity gas sampling and verification