Overview of oxygen sources

This article will review the four most common sources of medical oxygen: oxygen cylinders, oxygen concentrators, oxygen plants, and liquid oxygen. While historically, liquid oxygen (LOX) has been relatively more common only in well-resourced settings, in the wake of the SARS-CoV-2 pandemic, significant efforts were set in motion to scale up access to LOX as well as other oxygen supply types in resource-limited settings, too.

Medical grade oxygen must be >82% purity and without contamination. Oxygen used at clinical sites is either: 1) generated offsite and transported via cylinders in gaseous or liquid form; or 2) generated onsite via oxygen concentrators or plants. Oxygen is then delivered to patients via wall medical gas piping—either directly from an onsite oxygen plant or from a manifold supplied by oxygen cylinders—or via oxygen cylinders or concentrators placed at the bedside. 

Here we refer to oxygen sources as low, intermediate and high pressure:

• “Low pressure” – < 2bar/20 PSI
• “Intermediate pressure” – 3.4bar/55PSI
• “High pressure” – >137bar/2000PSI

Choosing the optimal oxygen supply setup can be challenging and limited data exist to guide cost-effectiveness. This article discusses the different sources of medical oxygen, including advantages and disadvantages of each source.

Always consult the manufacturer’s specifications and recommendations prior to using any oxygen sources. Improper use or ‘DIY’ approaches can be dangerous. In most countries, parts must comply with national regulatory body standards.

An overview of oxygen delivery devices and oxygen connector types can be found elsewhere in this O2 Encyclopedia.

The choice of which oxygen source(s) are most effective at a clinical site is dependent on many factors, including the amount of oxygen needed at a clinical site, cost, existing infrastructure, reliability of electricity, and availability of skilled personnel and local supply chains. Another important consideration is timeline for increasing oxygen capacity at a clinical site, as acquiring additional oxygen cylinders or concentrators can usually be accomplished in a much shorter timeframe than installing additional oxygen production plants. 

A comprehensive comparison of the different oxygen sources—including advantages and disadvantages of each—can be found in Table 3.1 of the  WHO-UNICEF technical specifications and guidance for oxygen therapy devices. 

Oxygen cylinders are usually filled at plants which concentrate oxygen via pressure swing adsorption (PSA), vacuum swing adsorptions (VSA), or cryogenic distillation (liquid oxygen – LOX). Cylinders can be either filled at offsite oxygen plants and then distributed to clinical sites or filled at oxygen production plants. Cylinders are most commonly made out of molybdenum steel (for extra corrosion resistance and strength) or aluminum in various sizes. Service pressure for aluminum cylinders is approximately 137-150 bar, while service pressure for steel cylinders may vary more widely. Always check with the manufacturer for specifications.

Oxygen cylinders supply oxygen to patients in one of two ways. They are either placed at the patient’s bedside for direct delivery to patient(s) through a pressure regulator, or they are connected in parallel to a manifold which supplies multiple terminal units via medical gas piping systems at a set pressure. In both cases, a flow meter is also required both to help control the rate of oxygen flow to a patient and to measure it–providing feedback on consumption to the clinician (see this article on oxygen connections for more information).

Advantages & Disadvantages

One distinct advantage of oxygen cylinders (especially in resource limited settings) is that they do not require electricity for use. For this reason, oxygen cylinders are often the primary and/or backup source of oxygen for clinical sites with unreliable electricity. However, oxygen cylinders still require a host of fittings and accessories to control oxygen flow, pressure, and humidity (see this article on oxygen connections for more information). Also, oxygen cylinders are heavy and can be quickly consumed, so transport and refilling can pose logistical challenges.

Cylinder sizes can be confusing, as there is no universal standard in use worldwide. The most commonly used system names cylinders alphabetically, with larger sizes assigned letters appearing later in the alphabet (see chart below). For ISO (and British naming), these sizes are AZ, C, D, E, F, G, H, and J. In many settings, size is listed as the volume in liters of water (e.g. 50L). 

In the US, one naming system begins with the letter “M” for “medical,” followed by a number that is volume of compressed gas in cubic feet (e.g. M2 – M265). An H sized tank may also be referred to as M250 tanks since up to 250 cubic feet of compressed oxygen can be stored in them. In some cases, cylinder sizes are defined by liquid liter capacity.

Regardless of sizing nomenclature employed, the gaseous oxygen volume and the liquid water volume capacity can be estimated using this cylinder size estimator tool. if you know the cylinder dimensions and the pressure of the gas it contains. Cylinders are generally available ranging from 1.2 to 50 Liters water capacity. 

Units used to describe cylinder capacity also vary across settings and include volume (liters) and weight (kilograms). It is important to note that in some settings capacity is expressed as liters of gas whereas in other settings capacity is described in liters of liquid volume.

When comparing different sources of information on cylinder size, it is important to pay attention to the referenced fill pressure used to estimate the capacity.

Note: any cylinder size can be placed at the bedside, though the larger sized cylinders (size J and above) are most commonly connected to manifolds which supply oxygen via wall piping.

Colors for medical gas cylinders are not universal around the world. Since multiple medical gases are stored in cylinders, there are color coding systems in place to indicate which gas(es) they contain. Either the entire cylinder or the entire convex portion at the top of the cylinder will be painted with the color(s) to indicate which type of gas(es) are stored in the cylinder. The international standard for the color coding of gas cylinders is ISO 32: 1977 but it is not universally adopted. Medical gas cylinders are required to be labelled (ISO 7225) to help identify the gas contents, but frequently are not. ISO 13769 requires permanent marking to cylinders (stamping, engraving, casting or other methods). 

Oxygen cylinders may be green, white, black and white, or blue depending on the country, standard and whether medical or industrial gas is being used.

Pure oxygen is indicated by white shoulder and black cylinder body in the ISO coding system and solid green for the US coding system. Additionally, there may be a label or stamp on the cylinder which indicates the gas type(s) and or UN number, pressure of the gas, liquid volume (i.e., water volume), and hazard type(s) among other information. The UN number corresponding to oxygen is UN 1072, which is assigned by the United Nations Committee of Experts on the Transport of Dangerous Goods. 

Important note: it is commonplace that cylinders may be filled with a gas that is different than the indicator color of the cylinder. Thus, it is important to have a reliable supplier and test your supply.

Cylinders require several accessories for safe use in clinical settings. Each cylinder has a valve (e.g. either pin index or bullnose) that must be opened with a specialized key, hand wheel or toggle. For valves that require a key, it is recommended to secure these to the cylinders and to have spares on site. For more information on cylinder regulators and valves, check out our article on Oxygen Connector Types.

Once oxygen leaves the cylinder valve at high pressure (~137bar/2000PSI) it must be lowered to a pressure safe for administration to a patient or an oxygen delivery devices. Regulator are specialized devices used for the purpose of lowering pressure (usually to ~55psi/3.4bar) to a level where a device (e.g. ventilator) or flowmeter can be used to further control flow to the patient.

Important note: it is unsafe to connect a patient directly to an oxygen cylinder without a pressure regulator.

When installing an oxygen cylinder:

• Eye and hand protection should be used when handling oxygen cylinders
• Protect and cover the top of the cylinder with the cap when it is not in use or when being transported for delivery
• Ensure the quality of the oxygen is assured (e.g. supplier quality certificate, PSA plant logbook or onsite analyzer testing)
• Cylinders should be setup in a secure position that cannot be easily knocked over (e.g. chained to the wall; moved in secure trolly)
• Clear regulator and cylinder connections to ensure no particles of dirt in the cylinder outlet.
  • Can be done with compressed air or nitrogen, or by briefly opening the cylinder valve (aka “snifting”)
• Tighten all the connections (e.g. cylinder-regulator, regulator-flowmeter, flowmeter-delivery device
• Never fill one oxygen cylinder from another
• A specialized spanner wrench or key may be needed to tighten or open the regulator

When using an oxygen cylinder:

• Always use a regulator
• Regularly check gauge to ensure adequate supply
• Use aluminum alloy cylinders with compatible accessories for magnetic resonance imaging (MRI)
• Replace a cylinder when residual pressure is ~ 29psi (2 bar, 200kPa) to prevent ingress of contaminants including moisture.

Storage & Transport

• Separate and label full and exhausted cylinders to avoid confusion
• Store in well-ventilated, clean, dry conditions, not exposed to extremes of heat or cold and protected from contamination by oil and grease.
• Only trained personnel should transport cylinders
• Not all vehicles are appropriate for cylinder transport

Fire safety

• Ensure fire extinguishers are nearby and are regularly inspected
• Keep oxygen cylinders at least several meters from possible sources of ignition or heat (e.g. flames, electric devices)
• Ensure “no smoking” sign nearby
• Check all nearby electrical circuit breakers and devices are in safe working condition

Anyone handling oxygen must undertake comprehensive safety training.

The table below provides guidance for routine maintenance of oxygen cylinders. Preventive maintenance for oxygen cylinders should be carried out by the gas supplier at regular intervals, including every 5 years for hydrostatic pressure testing, weight loss testing (>5%), and corrosion testing as well as daily for visual inspection prior to use. With proper care, an oxygen cylinder can be expected to last 20 or more years. Flowmeters and valves can be expected to last 5-10 year.

Portable oxygen concentrators are electrically-powered units which produce oxygen using pressure swing adsorption (PSA) or sometimes vacuum pressure swing adsorption (VPSA) technology. 

Oxygen concentrators commonly produce 3, 5, 8, or 10 LPM continuous flow output, though some may produce >10 LPM. Most high quality devices should produce oxygen at >90% purity. Most concentrators have an adjustable flowmeter.  

Advantages & Disadvantages

Some distinct advantages of oxygen concentrators over other oxygen supply types, is that they are portable and can be moved between patient care areas according to need, and as long as power is uninterrupted, they can provide continuous supply for long periods of time. Like other sources of oxygen, their output flow can be split to provide oxygen to multiple patients. Moreover, they—like oxygen cylinders—can be acquired to quickly increase the oxygen delivery capacity at a clinical site. 

However, they require a continuous, reliable supply of electricity as well as preventive maintenance—particularly if operating in humid climates—to operate effectively. Make sure that oxygen concentrators are efficient (<70W/LPM) and compatible with your local power supply and outlet adapters. Many oxygen concentrators do not have backup batteries that will power the device in the event of power failure. Consider using with a voltage stabilizer, surge protector and/or UPS. 

There are multiple things to consider when procuring an oxygen concentrator, including:

• Supply shortages – Everyone in the world has been trying to buy these since the beginning of the pandemic. Be wary of large stockpiles of ‘available’ devices from unknown manufacturers.
• Flow rates – Concentrators commonly produce 3, 5, 8 or 10 LPM continuous flow output, though some may produce 10 or more LPM. All devices should have an adjustable flowmeter (and if planning to use for pediatric patients, a minimum flow rate of 0.5 LPM may be needed. Make sure the FiO2 is adequate (>~82-90%) at the max flow rate, as many concentrators make claims of high flows but deliver limited FiO2 (and thus limited benefit) at those flow rates.
  • For stable patients on stable amounts of oxygen who are using the concentrators in climate controlled rooms (e.g. COPD at home, one of the original use scenarios for portable concentrators),  a device with relatively low output < 5LPM may be sufficient
  • For hospitalized patients who may have fluctuating needs (e.g. acute respiratory illness, COVID19), concentrators with higher outputs (e.g. 10-15 LPM, or higher) may be more desirable
• Oxygen concentration – WHO Technical Specifications stipulate output of >82% concentration of oxygen at max rated flow rate at 40 degrees C and 95% relative humidity (RH). Most high quality devices should have >90% output. NOTE: many concentrators, especially smaller (<10kg) units only produce 30% FiO2 at max flow rates, which is inadequate to care for most patients with acute respiratory illnesses.
• Power – ensure compatibility with local power supply and outlet adapters; and ensure efficiency <70W/LPM. Many oxygen concentrators do not have backup batteries that will power the device in the event of power failure. Consider using with a voltage stabilizer and surge protector.
• Noise – <50 dBA; many noisy units on a single ward can make it difficult to hear patient alarms
• Durability – Consider IP21 or greater to offer special protection against moisture
• Outlet pressure – must generate at least 55kPa at all flows and ensure stable (narrow range) of output pressure (+/- 1 psi)
• Operating environment – ensure safe operation in 0 to 40 degree C, and RH 15 to 95%; ensure certified to altitude suitable for destination (many are not certified above 3000m due to partial pressure changes)
• Alarms – device must have alarms for low oxygen concentration alarm (<82%), no flow, high/low pressure, low battery, power supply failure, high temperature. NOTE: be cautious of devices with prolonged warm up periods, maximum operating durations (good units should work continuously), or auto-shutoff functions.
• Spare parts – ensure spare parts for one year – including spare battery for power failure alarms (more recent models may use a capacitor and do not have a battery), intake filter, and internal filter
• Delivery devices – don’t forget to ensure access to delivery devices (e.g. nasal cannula, face masks and humidification bottles)
• Access to maintenance tools – ideally facilities should have pressure gauge, FiO2 meter oxygen analyzer, and spare parts for routine maintenance; consider a local service contract or identifying a provider
• Training – recipients of concentrators should be provided with training on operation, troubleshooting and maintenance
• Be wary of devices with auto-shutoff timers as these can increase likelihood for errors

For more technical specifications for portable oxygen concentrators check out our Database for oxygen concentrator specifications.

1. Position unit 0.5m away
   1. Avoid positioning near objects that may obstruct air inlet (e.g. curtains or the wall). 
   2. Keep far away from fire ignition sources (e.g. smoking, cooking, etc)
2.   Consider humidification bottle*
   1. *Often prescribed for flow rates >4 Liters per minute, though evidence of benefit is limited
   2. Must use distilled or sterile water 
   3. Replace water and inspect bottle daily
3. Attach oxygen device tubing 
   1. Smooth bore oxygen tubing should connect to concentrator outlet (or the humidifier bottle if used)
4. Inspect air filter
   1. Ensure the filter is in place, clean and dry
   2. Usually on the back of the device
   3. Inspect the filter daily 
   4. Remove & clean filter  weekly (or more frequently if dusty environment) by washing with warm soap-water, rinse and dry with towel 
   5. Consider having a spare filter to avoid interruptions in concentrator use while cleaning the filter
   6. *Some devices have internal filters that require periodic changing (see user manual)
5. Test alarms and power on
   1. Always check back of the device (and user’s manual) to ENSURE YOU HAVE THE CORRECT POWER SUPPLY (voltage and frequency) otherwise you may destroy the unit
   2. Consider a voltage stabilizer and backup power supply if frequent outages or flickering/dimming lights are common 
   3. Avoid extension cords to minimize fire risk and tripping hazard
   4. For each new patient, with device unplugged, flip the power switch on and listed for continuous power failure alarm 
   5. For each new patient, with the device plugged in, flip power switch on and listen for alarm test (which should self terminate) & sound of the compressor
   6. Some devices may require 2-20 min warm up to reach desired O2 concentration 
6. Adjust flow
   1. Follow your clinician’s advice (e.g. adjust flow to oxygen saturation by pulse oximeter of SpO2 >90% or context appropriate goal)
   2. Turn the flowmeter nob counter-clockwise to increase flow
   3. Middle of the ball indicates the set flow rate
   4. DO NOT EXCEED the max rated flow for the device even if the flowmeter allows, this can damage the device
   5. Ensure you feel flow coming from flowmeter
7. Secure oxygen delivery device
   1. Follow your clinician’s advice 
   2. Usually a nasal cannula or simple face mask are used to deliver oxygen from a concentrator with output <10LPM)
   3. Inspect the tubing of the device to ensure no kinks, and listed for leaks (tighten or fix as needed)

A 10 L/min oxygen concentrator requires ~350-600W of power which is not variable with flows. A voltage stabilizer is important to prevent device failure.

Download the full, modifiable job aids for:

• Oxygen concentrator setup, operation and maintenance
• Guide to splitting or combining oxygen concentrators

Oxygen concentrators require regular inspection and maintenance though if done properly on a high quality machine, these devices can be reliable with minimal resources for many years. Because these devices are kept at the bedside, users must adhere to proper infection prevention control measures.

1. 1. Inspect & clean air inlet filter 1-2x weekly (A second air filter is needed to ensure continuous use while cleaning and drying one air filter)
   2. Inspect humidifier bottle outlet for obstruction daily 
   3. Replace humidifier water daily 
   4. Check tubing for kinks & leaks daily 
   5. Ensure air inlet filter is not obstructed daily
   6. Check power and O2 concentration alarms with each new patient or power up
   7. Consider cable ties to secure connectors
   8. Avoid running at or above max rated flow as this can cause premature failure due to excess moisture building up in the sieve bed
   9. Avoid >15m of total tubing to the patient
   10. Oxygen can cause fires! Keep away from open flames, heat sources & smoking.
   11. Zeolite does not last forever – needs to be replaced around 25,000 hours, though may be considerably less depending on use, maintenance and humidity

Oxygen generating plants are comprised of multiple pieces of equipment and generally are categorized by their mechanism of operation as either a pressure swing adsorption (PSA), vacuum swing adsorption (VSA), or vacuum-pressure swing adsorption plant (VPSA). The entire oxygen plant setup should be certified for medical use.

A PSA oxygen plant is functionally a larger (immobile) version of a portable oxygen concentrator. Though sometimes built on steel frames or pallets for easier en bloc transport, they are generally stationary units. These centralized sources of oxygen are found onsite at hospitals and oxygen is delivered either via medical gas pipeline systems to bedside terminals or used to fill cylinders via a cylinder filling terminal (which includes a filling ramp and booster compressor).

Many of the same advantages and disadvantages as for portable oxygen concentrators apply, including the need for continuous power. Variable maintenance and quality of oxygen generating plants commonly results in variable quality of oxygen concentration output (i.e. in many resource-limited settings, PSA plants if not maintained, may deliver oxygen concentrations far below 90%.

Important Note: oxygen plants require reliable power and regular maintenance. It is highly recommended to have ample oxygen cylinder backup systems in place if an oxygen generating plant is the primary source of oxygen for a facility.

The primary components of an oxygen generating plant include an air compressor or blower, dryer, filters, compressed air tank, dual separation chambers (sieve beds), product tank/reservoir, and control and alarm systems. These components may very by system design and whether a PSA, VSA or VPSA.

Pressure swing adsorption (PSA) plants convert ambient air (nitrogen 78.09%, oxygen 20.95%, argon 0.93%) to high concentration oxygen (>93% +3%) by:

1. Filtration – air is entrained into the system through filters (>5 micron pre filter; 0.01 micron coalescing filter; and coal or activated carbon filter)that remove gross contaminants (e.g. dust and hydrocarbons) as well as water vapor (by optional drier for the air compressor)
2. Compression – air is compressed into an adsorbent sieve container of zeolite which selectively adsorbs nitrogen more than oxygen. Site altitude must be considered when choosing the appropriate size compressor to ensure a plant can reach rated output capacity. The higher the altitude, the lower the atmospheric pressure (i.e. less air density) and thus the compressor must pull in more air to each the same output Read more about the effect of altitude on compressor output here.
3. High pressure venting – gas is purged from the sieve container under high pressure thereby keeping the nitrogen trapped in the zeolite, and the emerging gas is richer in oxygen and enters a reservoir tank (with pressure regulator)
4. Low pressure venting – when the zeolite adsorbent bed is saturated, the incoming ambient air is diverted to a secondary sieve bed for compression, while the primary sieve bed is depressurized (i.e. pressure is ‘swung‘ from high to low) and purged to release the trapped nitrogen via a muffler, and regenerate the zeolite so it is ready for another cycle of gas to enter.
5. Oxygen delivery – O2 from the oxygen plant can enter a compressed oxygen received tank and then distributed directly to the facility via medical gas piping, or it can be further compressed to fill cylinders (for portable low pressure PSA systems, the oxygen is delivered to a flowmeter for delivery to the patient). Oxygen leaving the plant should be 93% +3%, and output varies depending on plant design from 2 to >200 Nm3/hr (at 0 degrees C, 101.3 kPa). Of note output may also be reported as Sm3/hr, which is standard reference conditions (20 degrees C and 101.3 kPa).

Using two adsorbent beds allows for nearly continuous generation by alternating compression and purging cycles. There are multiple adsorbents used for PSA though zeolite is among the most common. Most are porous materials with different affinities for various gases.

A plant may be setup in multiple ways (e.g. single, duplex, multiplex) and may or may not have an optional booster compressor to fill high pressure gas cylinders.

Vacuum swing adsorption (VSA) is similar to PSA in that it separates gases from a gas mixture. Unlike PSA, VSA operates near ambient pressures and uses a vacuum (not a compressor) to separate the gases and regenerate the adsorbent. A VSA uses an oil-free blower which can decrease filtration requirements, is less prone to reduced efficiency at altitude, less risk of oil carry over downstream of the oxygen system, and is more durable. A VSA uses only one adsoprtion tower which may reduce the footprint of the plant. A VSA facility may be more energy efficient than PSA, but have higher upfront capital costs than PSA and produce oxygen at lower pressures (i.e. may not be enough to feed a distribution piping system without an additional booster). A VSA plant may have higher efficiency in humid environments because it does not need the air dryer component. A VSA, in some cases, may also reduce potential for water condensation by cooling the entrained air for the sieve beds to within 10 degrees C of ambient temperature. Generally, a VSA may also require less maintenance than PSA and have longer lifespans. A VSA has lower output pressure (130-420 kPa, 19-60 psi) than PSA, thus the VSA needs an oil-free booster compressor to reach 689 kPa (100psi) for medical gas pipeline networks. Capital expenditure for a VSA plant is ~15% higher than a PSA plant because of this booster. When generating >100Nm3/hr, VSA cost-effectiveness is more in line with that of a PSA plant. Relatively few manufacturers make VSA technology.

A hybrid vacuum-pressure swing adsorption (VPSA) also exists. A VPSA plant uses pressure to separate air and a vacuum to purge the gas. This setup may be preferred when O2 demand is high, though cost and complexity are higher.

PSA and VSA plants require significant, continuous power supply that is voltage stabilized to avoid interruptions. Power consumption varies by size of the plant, hours of operation and hourly output. A PSA plant may require approximately 1.2 kWh per Nm³  oxygen produced (VSA 0.39 per kWh per Nm³).

Oxygen generating plants generate considerable heat and noise and placement in an enclosed structure with good ventilation and protection from weather (Read more on Infrastructure Requirements below).

Preventative maintenance for PSA/VSA plants can be considerable and often provided as part of the contract with the supplier. Such PM requires highly trained technicians. Full discussion of maintenance of PSA/VSA plants is beyond the scope of this article. To learn more on PSA Maintenance, select the ‘advanced’ version of this article using the slider at the top right of the screen.

Bulk liquid oxygen generated at cryogenic air separation units (ASUs) off-site from the hospital, then transported by truck to the hospital and stored in large tanks. Liquid oxygen is released by the vacuum insulated evaporator (VIE) to gaseous oxygen to supply a facility via a central piping mechanism.

Liquid oxygen requires proximity to an ASU for supply, onsite VIE equipment, maintenance and significant safety infrastructure. With heavy use, VIE equipment can become iced over and require additional maintenance to avoid output failure.

Other limitations of LOW include max flow restrictions due to plumbing resistance and maximum zone and total facility storage capacity challenges.

Air separation units (ASUs) are essentially distilleries – that is they use the different properties (i.e. different boiling points) of different liquids (i.e. oxygen – bp 90.2 K, nitrogen – bp 77.4 K, argon – bp 87.3 K) to separate each from the mix. In the case of oxygen distillation, the process is cryogenic (cold) fractional distillation and it has been around since the late 19th century. The process starts with air, which has multiple components including: nitrogen (78.09%), oxygen (20.95%), argon (0.93%), carbon dioxide, carbon monoxide, neon, helium, methane, acetylene, krypton, nitrous oxide, hydrogen, ozone, and xenon.

Modern ASUs can produce more than 4000 tons of oxygen per day at up to 99.5% purity (and 10,000 tonnes per day of liquid nitrogen for various scientific and commercial applications).

How do ASUs convert gaseous air to pure liquid oxygen (LOX):

1. Filtration 1 – contaminants must be removed from the air (e.g. dust)
2. Compression – filtered, atmospheric air is compressed to 5-10 bar (72-144 psig) and water vapor is removed in interstage coolers
3. Filtration 2 – compressed air is passed through a molecular sieve adsorber (which must be regenerated), remaining contaminants (e.g. water vapor, hydrocarbons, CO2) are removed
4. Cooling– gaseous, decontaminated air is first cooled by heat exchange then refrigeration and further compression to produce a mixture of liquid oxygen and nitrogen. Cooling and compression systems are often coupled to improve energy efficiency.
5. Distillation – the compressed and cooled mixture enters a two column (high pressure -hp; and low pressure-hp) system that is used to separate the primary components of the mix – oxygen, nitrogen and argon. (for more detailed explanation see Castle, 2022.). The ASU can produce gas, liquid or a combination of purified O2 and N2.
6. Storage – depending on the intended use and transport method, the purified oxygen may be stored as either liquid oxygen in insulated storage tanks (most commonly), or as gas in tanks/cylinders.

There are many factors to consider in the construction of an ASU that we beyond the scope of this discussion. Of note, these facilities consume significant power (e.g. 2400 metric tons per day requires 32 megawatts of power), though there have been marked improvements in efficiency in recent decades.  power consumption is likely the majority of an ASU’s lifetime costs. These facilities also require significant staffing 24/7 and initial capital investment ($25-$125m USD).

Liquid oxygen (LOX) for clinical purposes is produced at ASUs and then transported by specialized tanker truck to clinical facilities. Strict international and national guidelines exist for safe transport of cryogenic pressure vessels. At the clinical facility, the truck fills an on-site, insulated LOX storage tank.

Liquid oxygen storage tanks are most commonly two layers (1. outer carbon steel layer; and 2. inner stainless steel layer) separated by a vacuum to provide insulation. These bulk tanks are capable of 16 bar (and in some cases up to 35 bar) operating pressures and usually come in 2, 3, 5, 10 and 20 m^3 sizes (size must be carefully chosen – if oversized this can result in excess pressure build up and waste via off gassing). Oxygen hubs may require ~50 ton tank (1 ton = ~875 liquid liters of oxygen). See Table: Oxygen Tank Sizing below from the EpIC Planning Guide for Setting up liquid oxygen (LOX) systems in hospitals in low- and middle-income countries. Approximately 10% of the volume in a cryogenic storage tanks is unusable because it is required to maintain operating pressure. This volume as well as a buffer to avoid shortage in the setting of delayed refill or surge in demand, should be considered when choosing tank size. If estimating LOX volumetrically for an unknown tank size, the volume of the smaller inner layer must be accounted for. The storage tank often sits on a weighing scale. A LOX tank is not cooled, but relies on insulation, evaporation and pressure to keep oxygen in its liquid form. Tanks must be certified for medical oxygen use and comply with national/international regulator standards (see “Regulatory” section below).

LOX tanks are heavy and require specialized infrastructure and placement on reinforced concrete slabs. There are numerous considerations for location placement of LOX tanks including: accounting for environmental factors such as seismic effects and extreme winds, available power supply, safe distance from power lines, diesel generators (and other ignition sources), fenced/secured, accessible to tanker trucks, central pipeline distribution system access, and space for backup vaporizers, among other considerations.

At sea level, an unpressurized (1 atmosphere), Oxygen boils or liquifies (i.e. changes from gas to liquid or from liquid to gas) at -183 degrees C (-297 degrees F). Liquid oxygen must be stored below its critical temperature (-118 degrees C), thus it is usually stored at -160 to -180 degrees C and high pressure to prevent keep it in liquid form.

The LOX insulated storage tank is usually connected via valves to a vaporizer and pressure control system/manifold. The vaporizer allows evaporation of the liquid oxygen into gaseous oxygen at >99% purity which can then be distributed through the hospital’s central medical gas pipeline distribution system (Configuration 1 in figure below). The manifold monitors and manages pressure, fill level and vacuum. Pressure is reported as a gauge pressure (i.e. relative to ambient atmospheric pressure not always 1 standard atmosphere). Before entering the MGPS, the pressure is reduced from ~10 bar to 4 bar (150psi to 60 psi). Collectively, the LOX tank, valves, evaporator and pressure control systems are called a vacuum-insulated evaporator (VIE) system. The VIE system must be spaced at least 8m from the facility for safety. Although a VIE technically does not require power to evaporate LOX, the overall system including alarms, fill pumps and monitoring systems do require power. Oxygen leaving a VIE is extremely cold and at high pressure, and thus must be warmed to ambient temperature (by the superheater) and pass through a pressure regulator prior to delivery to patients.

• When demand suddenly drops and VIE pressure rises suddenly, a safety pressure control valve opens to prevent explosions and vent excess gaseous O2
• When demand increases, a rapid evaporation of large amounts of oxygen cause a drop in temperature which then causes a drop in vapor pressure and therefore reduced supply. To overcome this, an electronically controlled valve opens so that liquid oxygen can enter the superheater coils, where the liquid is exposed to ambient temperature, then rapidly becomes gas to supply the facility. In settings with lower a ambient temperatures, bulk tank vaporizers may freeze more quickly
• When there is no demand, a VIE is always producing some amount of liquid oxygen evaporation due to heat entering the liquid; the evaporation helps keep the liquid cold but also increases pressure in the system which must subsequently be relieved to prevent explosion.

A VIE system must be appropriately sized to account for demand and desired refill frequency. It is rare and inefficient, but possible to use liquid oxygen cylinders with built in vaporizers to connect directly to manifold systems (See configuration 2 in the figure below). Such cylinders are sensitive to vibration (e.g. during transport) leading to off gassing as well as static losses, thus only ~2/3 of the cylinder volume is usable.

One liter of liquid oxygen produces approximately 861 liters of gaseous oxygen. Thus, transport and storage and of LOX may be much more efficient. Tanks commonly range from 500 to 25,000 liters of LOX, though larger sizes may be encountered. A 25,000 liter LOX tank can generate 21,500,000 gaseous liters of oxygen. Of note, output flow capacity differs by VIE design and can range from 150 to 20,000 liters per minute. Output efficiency and capacity may be impacted by surging demand, adequacy of maintenance and weather (see images below).

Some VIEs can be used to fill cylinders, with the need of an external compressor (gas filling) or cryogenic pump (liquid filling) depending on the use case.

Cost for a VIE can vary widely ($10,000 to $100,000 USD) with substantial annual maintenance costs, of up to 40% the initial capital investment cost. The cost of central O2 piping for the facility is an additional, substantial cost.

Advantages of LOX systems:

• Provide pure oxygen
• Provide high volumes (relative to space requirement)
• Some systems may be utilized during temporary power outages
• Compact storage for volume of oxygen

Disadvantages of LOX systems:

• Supply cannot be regenerated onsite (in contrast to a PSA plant)
• Require transport/supply chain/roads
• Require proximity to an ASU
• Require careful maintenance of central medical gas piping system
• Require backup system
• Require technical knowledge
• Require large, well ventilated spaces for storage – free of potential ignition sources (e.g overhead power lines, gas powered generators)
• Require special considerations in settings with extreme temperature and humidity
• Safety systems for VIE operation require power
• Inefficient if demand is low (i.e. vents/wastes oxygen due to constant oxygen evaporation even if not being used)

Liquid oxygen VIE systems require regular maintenance by highly trained personnel. This may come as part of a service contract with the LOX provider who also may lease the VIE equipment to the facility. Outsourcing maintenance may be the best option for facilities without LOX technicians.

Preventative maintenance includes cleaning any grease/oil from the system and regularly inspecting all system components including valves, fittings, level and pressure gauges, relief valves, and VIE bursting disc replacement.

De-icing and the prevention of ice accumulation are important considerations for VIE design and operation. High flow volumes especially in hot and humid environments can lead to ice build up on the vaporizer (See LOX Freezing image below). Increasing vaporizer size, twinning the vaporizer or de-icing are all strategies that can help mitigate this risk. The vaporizer may use ambient airflow to absorb heat and prevent icing, though in cold climates, an actively heated vaporizer may be required to prevent icing. A two vaporizer setup is also commonly used so that the system can alternate between vaporizers to prevent significant ice accumulation. Pressure reducing safety valves, ancillary shutoff valves, and pressure flow regulators are also required to complete a LOX system with central piping of gaseous oxygen.

Refills should be done be certified suppliers with protocols that meet international standards. Of note, LOX suppliers may only fill and maintain tanks owned externally if the tanks meet certain safety and operational requirements. A power source is required for passive VIE systems to power the pump when the truck delivers LOX (this may be supplied by the truck if a diesel powered electrical generator is on the truck).

A more in depth discussion can be found in the PATH CHAI Oxygen Generation and Storage publication and should be obtained from the equipment manufacturer.

• WHO-UNICEF technical specifications and guidance for oxygen therapy devices. Geneva: World Health Organization and the United Nations Children’s Fund (UNICEF), 2019 (WHO medical device technical series). License: CC BY-NC-SA 3.0 IGO
• Technical specifications for Pressure Swing Adsorption (PSA) Oxygen Plants: Interim guidance. Geneva: World Health Organization, 2020. License: CC BY-NC-SA 3.0 IGO
• Castle, W. F. “Air separation and liquefaction: Recent developments and prospects for the beginning of the new millennium”. International Journal of Refrigeration. 25: 158–172. January 2002 doi:10.1016/S0140-7007(01)00003-2
• PATH-CHAI Oxygen Generation and Storage, July 2021
• WHO technical consultation on oxygen access scale-up for COVID-19. WHO CC-BY-NC-SA, July 2021
• Oxygen sources and distribution for COVID19 treatment centres. WHO, April 2020
• Technical specifications for pressure swing adsorption (PSA) oxygen plants. WHO, January 2020
• Technical specifications for oxygen concentrators. WHO, September 2015
• Foundations of Medical oxygen Systems, WHO, February 2023
• Meeting Targets and Maintaining Epidemic Control (EpiC). Planning guide: Setting up liquid oxygen (LOX) systems in hospitals in low- and middle-income countries. Durham (NC): FHI 360; 2023.