With just two protons in its nucleus and two electrons in its principal energy level, helium is the second lightest element. It is inert, electrically very stable, and has a high ionisation potential as well as a high capacity to absorb heat energy. With a boiling point of -269Â°C, just 4Â°C above absolute zero, liquid helium is the coldest substance on Earth. These properties combine to make helium the most versatile of the inert gases.
The small size of the helium molecule means helium is an ideal gas for leak detection in vacuum and other gas-tight systems. By evacuating a pipe with a vacuum pump, and directing a stream of helium onto the areas where leaks are suspected, any helium entering the system can be detected by an appropriately tuned small mass spectrometer.
In order to help the refrigeration industry to comply with strict European rules governing the containment of fluorinated gases, gas companies are introducing oxygen-free mixes of nitrogen and helium to replace the oxygen-free nitrogen normally used for pressure testing refrigerant systems. This makes it much easier to detect very small leaks.
Helium's super inertness (and the fact that it releases more of its available energy in the arc than argon or carbon dioxide) makes it the shielding gas of choice for high speed welding of stainless steels, aluminium, copper and titanium. It is also used as a shielding gas in laser welding. In both these applications, the use of helium produces a superior weld penetration and a lower profile weld bead than can be achieved with other shielding gases. The higher ionisation potential of helium also serves to suppress the formation of a plasma, or charged cloud, of metallic ions that can spoil the weld.
Helium also plays a role in the lasing gas for CO2 lasers - for example, the powerful, workhorse lasers used for very accurate cutting in industries such as the automotive sector. The lasing gas for CO2 lasers is made up of nitrogen, helium and CO2. In these lasers the nitrogen is first excited by an electrical discharge, and then transfers its energy to the CO2. As the CO2 molecules decay, they give off laser light. Helium is then used in this process to remove excess energy and bring the CO2 molecules down to their ground state, and back into the cycle.
But some of the major applications of helium involve cooling and heat transfer. Helium increasingly forms a component in the gas atmospheres used in heat treatments such as vacuum carburising, a processes used to manufacture components such as automotive gears. For this application the optimum atmosphere mixture contains 30-40 percent nitrogen or argon in helium. Also, thanks to recent developments in furnace design and new gas recycling systems (see text box: Supply and Demand), the use of pure helium gas is becoming a very attractive environmentally friendly alternative to oil for the quenching, or rapid cooling, used to achieve the desired metallurgical characteristics in metal components.
Helium also plays an important cooling role in the production of optical fibres. These are made by placing a large glass preform at the top of a high tower, heating it until the glass begins to flow, and then drawing a fine continuous fibre. Early in the process, helium is used in the manufacture of the preform as a carrier gas for removing the chlorine used to dry the glass. However, its greatest use is for quick cooling of the drawn fibres. The most effective way to do this is to pass them through a chamber containing a helium/nitrogen mixture. $quot;Although optical fibre production has dramatically decreased in Europe and the US, there is a growing manufacturing base - and hence the demand for helium - in India and the Far East,$quot; notes Pete Ward, senior business consultant for helium at The Linde Group.
But, he stresses, the greatest demand for helium continues to come from manufacturers of the superconducting magnets that lie at the heart of MRI scanners. Liquid helium plays a key role in both the manufacture and maintenance of these machines.
The use of superconductors in the magnets allows very large amounts of current to travel through very small wires (low resistance), making it possible to generate very strong magnetic fields. However, superconductivity can only be achieved below a critical temperature. The niobium-titanium superconductor used in the magnets has a critical temperature of around 10K (-263Â°C). This is achieved and maintained by first pre-cooling the magnets to 77K (-196Â°C) using liquid nitrogen, then further cooling them down to around 4K (-269Â°C) using liquid helium before shipping them as sealed units.
Although improvements in the designs of cryostats - vacuum vessels with a multi-layered super insulation that work like a vacuum flask to maintain the low temperature and minimise heat leaks into the magnet from outside - mean that heat transfer, and thus vaporisation of the helium, is reduced, most magnets still require topping up with liquid helium every 6-12 months. This service is generally provided by industrial gases companies, who enter into contracts with hospitals, MRI suppliers and other MRI users to provide a topping up service on a regular basis.
The manufacture and maintenance of the superconducting magnets and the maintenance of the MRI scanners currently account for around 40 percent of helium use. However, technological developments suggest that the MRI market for liquid helium may eventually decline. Work on higher temperature superconductors is an active area of research, and the magnet designs are constantly being upgraded to help to reduce operating costs for MRI scanners. For example, developments in cryostat design and improvements to minimise the heat load on the magnet mean that some scanners include superconducting magnets that are sold as 'sealed for life'. If used correctly these should never need topping up.
However, any predicted decline in demand for helium for MRI applications is likely to be offset by the growing use of helium in the manufacture of flat panel liquid crystal displays (LCDs). LCD manufacturing technology is, explains Steve Pilgrim, global marketing manager - electronics at The Linde Group, 'like a blown-up version of semiconductor manufacturing, but using bigger substrates and fewer processes.' As in semiconductor manufacture the thermal properties of helium are put to use for back cooling, or cooling the underside of the substrate. An additional - and potentially greater use for helium - is in the helium/nitrogen gas mixtures used to cool the substrates when they are in the load locks, the devices used on process tools to load and unload the substrates as they are passed by robots from chamber to chamber during the manufacturing process. 'LCD manufacture,' notes Pilgrim, 'is a fast evolving industry that will be with us for some time.'
While LCD display manufacture is centred mainly around the Pacific Rim in countries such as Taiwan, Korea and China, the hot bed for helium use in the UK remains in the so-called 'cryogenic basin' within a 16-mile radius of Oxford. This area is home to a number of magnet manufacturers and superconductor users, including companies such as Siemens and Magnex as well as to the scientific labs at Culham and Harwell.
Major helium suppliers, currently Linde Gas UK and Air Products, have helium transfill sites nearby and BOC expects to have a new UK Â£8.5 million helium production plant in Thame, Oxfordshire, in full production by the middle of 2007. The new plant, which replaces BOC's old helium facility in Leeds, will be the largest helium plant in the UK.
Helium in tight supply
2006 was notable for major shortages in helium supply. The causes ranged from problems that delayed production at new helium plants in Skikda, Algeria and Ras Laffan Industrial City in Qatar; a series of events that led to a reduction of pressure in the Bureau of Land Management crude helium pipeline in the US which, in turn, led to a reduction in helium production; and a series of planned maintenance shutdowns in plants around the world.
But the good news, reports Phil Kornbluth, executive vice president at Matheson Tri-Gas Global Helium, writing in Speciality Gas Report, is that 'by early December (2006), all of the world's existing helium plants were operating normally.' By summer of 2007, he predicts, the Skikda and Ras Laffan plants should be producing at near current capacity, helping to alleviate the current tight helium supply situation.
gasworld believes that as we go to press, most major suppliers are very much short of helium and while there may be good news on the operation of the new plants over the next month or two, supply will remain tight through to the last quarter in 2007.
As helium is a relatively expensive gas and, in general, is widely used in applications where costs are major considerations, there has been an increasing trend towards recovery, purification and recycling of helium.
Helium recycling systems are readily available. The basic recycling method involves compression of the used gas, followed by purification. Depending on the purities required this can be carried out by means of pressure swing adsorption (PSA), membrane or cryogenic separation. By developing separate collection methods and purification systems that work together as two batch processes, gases companies have been able to develop efficient systems to separate out the maximum amount of helium from the helium/air mixture and recycle the purified helium for re-use.