Throughout recorded history mankind has used ice as a method of preserving food by cooling it. Alexander the Great served snow cooled drinks to his soldiers in the 3rd century BC and Khalif Madhi used ice as the refrigerant for his caravans crossing the desert to Mecca in the 8th century AD.

By the 1800s the ice trade had become big business with companies cutting hundreds of thousands of tons of ice from rivers such as the Hudson and shipping it as far as the West Indies in sawdust insulated containers. This practice continued well into the 20th century, even after the development of artificial ice, sold as a $quot;more natural$quot; method of ice production.

In the 18th century it was noticed that evaporating ether cooled the skin and in 1755 evaporating ether was used to produce ice by Professor William Cullen. The principle behind this discovery is that a vapourising liquid requires kinetic energy and draws it from the surrounding area - which loses energy and becomes cooler.

Cooling caused by the rapid expansion of gases is the primary means of refrigeration today. In the modern refrigeration system, the refrigerant gas is compressed using a compressor before entering a condenser where it is cooled and condensed to a liquid. After this it goes through an expansion valve where the pressure rapidly decreases, causing flash evaporation and resulting in a mixture of liquid and vapour at a lower temperature and pressure. The cold liquid-vapour mixture then travels through the evaporator coil or tubes and is completely vaporized by cooling the warm air from the space being refrigerated being blown by a fan across the evaporator coil or tubes. The resulting refrigerant vapour returns to the compressor inlet to complete the thermodynamic cycle.

Having discovered these principles, the race was on to find the ideal refrigerant. A refrigerant needs good thermodynamic properties and to be non-corrosive and safe to use. Since the system operates on the energy required to evaporate the liquid, it is good to have a high enthalpy of evaporation. It should exhibit boiling points at pressures that are appropriate to the materials and seals used to construct the system. It is more economic if the operating pressure in the condenser is well below the critical pressure of the gas as higher operating pressures require more power for compression.

A mixture called chemogene (consisting of petrol ether and naphtha) was patented as a refrigerant for vapor compression systems in 1866 and the first gaseous refrigerant, carbon dioxide, was introduced in the same year. Carbon dioxide systems have the drawback that they must be operated at high pressure - up to five times as high as commonly seen in current technology. This posed certain engineering challenges and required the use of heavy steel tubing.

In the late 1870s sulphur dioxide and ammonia were used as refrigerants due to their availability, high latent heats of vaporisation and low cost and by 1926 methylene chloride was being used. However, due to their toxicity and/or flammability and difficulty in handling, accidents were common and research had begun on a safe, non toxic replacement.

In 1926, Thomas Midgley with his associates Henne and McNary, of General Motors observed that the refrigerants then in use comprised relatively few chemical elements, clustered in an intersecting row and column of the periodic table of elements. The element at the intersection was fluorine, known to be toxic by itself. However, Midgley and his collaborators felt that compounds containing it should be both non-toxic and non-flammable. Within three days of starting, Midgley and his collaborators had identified and synthesized dichlorodifluoromethane (CCl2F2) a non-toxic, non flammable refrigerant with excellent thermodynamic properties.

During the discovery process, Midgley and his colleagues had relied on inaccurate literature and, as a result, they introduced a system to systematically identify the molecular structure of refrigerants made with a single halogenated hydrocarbon. The result of this system was the introduction of the $quot;R$quot; prefix for gases and chemicals used as refrigerants so that dichlorodifluoromethane could be fully described by the acronym R-12 and even traditional gases such as ammonia when used as a refrigerant can be notated as R717.

A number of other compounds followed which had differing properties making them suitable in for different applications including chlorodifluoromethane (R22) used in commercial refrigeration and trichlorofluoromethane (R11) used in low pressure systems. This class of products are known as haloalkanes and where all the hydrogen is replaced by chlorine or fluorine they are known as chlorofluorocarbons (CFCs) and were the hydrogen is only partially replaced they are known as hydrochlorofluorocarbons (HCFCs).

The new CFCs were extremely easy to handle and could be easily combined in azeotropic mixtures giving tailor-made refrigerants for specific applications such as R502 - a 48.8/51.2 blend of R22 and R115 which is used in low-temperature compression refrigerating facilities.

In addition to their original application as refrigerants, chlorofluoroalkanes have been used as propellants in aerosol cans, cleaning solvents for circuit boards, and blowing agents for making expanded plastics (such as the expanded polystyrene used in packaging materials and disposable coffee cups).

As a result of their near perfect performance, safety and handling characteristics, almost all other refrigerants disappeared - with the exception of ammonia, which was still used in industrial applications - and it seemed that these products would from the basis of modern refrigeration systems for many years to come.

However, in 1974's Molina and Rowland published a paper on the journal Nature claiming that a build up of released CFCs was leading to a depletion in the Earth's ozone layer. The very property of CFCs that made them so attractive as a refrigerant - namely their lack of reactivity was causing them to build up in the stratosphere where the sun's ultraviolet rays caused the chlorine atom to react with the ozone leading it to convert to oxygen and resulting in ozone depletion.

Initial indifference from the academic community prompted the pair to hold a press conference at a meeting of the American Chemical Society in Atlantic City in September 1974, in which they called for a complete ban on further releases of CFCs into the atmosphere.
Scepticism from scientists and commercial manufacturers persisted, however, and a consensus on the need for action only began to emerge in 1976 with the publication of a review of the science by the National Academy of Sciences. This led to moves towards the worldwide elimination of CFCs from aerosol cans and refrigerators, and it is for this work that Molina later shared the Nobel Prize in Chemistry.

By 1987, scientific evidence that CFCs were causing ozone depletion had become overwhelming. By 1989 a resolution was passed called the Montreal Protocol. This banned the production of some CFCs, including the most used R11 and R12, by 1996.

The result of this was that refrigerant manufacturers turned towards less damaging compounds. They started to replace CFCs with HCFCs and HFCs (hydrofluorocarbons, which have no chlorine atoms) such as R134a, which could replace R12 in car air conditioning systems. HCFCs have approximately 10% of the ozone depletion potential (ODP) of CFCs and they will be phased out by 2030 under the Montreal Protocol.

So, where does the future for refrigerants lie? OEMs need products which give them the correct properties for their refrigeration equipment but which do not have any environmentally damaging effects. This has lead suppliers down two paths: reversion to some older refrigerants used before CFCs were invented, and a new class of zero ODP products known as fluoroiodocarbons (FICs).

FICs have attractive physical properties (similar to CFCs) and, because they undergo photolysis within two days when released into the atmosphere, they never reach the stratosphere and thus have negligible ozone-depletion potential. This short lifetime also gives them very low global warming potential. It is likely that FICs will be blended with hydrocarbons and HFC's to give refrigerant blends which appear to be nonflammable, non-ozone-depleting, near-azeotropic, compatible with mineral oil and other lubricants, and with very low acute toxicity.

For older refrigerants there has been an upsurge in ammonia systems due to the relatively low toxicity of ammonia and also movement back towards CO2 now that engineering practices have improved and systems can be designed to safely handle the increased pressure.

As well as this, $quot;forgotten$quot; gases such hydrocarbons have made a substantial return to use in this application. It has been known since 1867 that hydrocarbons such as propane and isobutane can be used as refrigerants but concerns about their flammability and the availability of the new range of $quot;safe$quot; CFCs caused their disappearance in the 1920s.

However, these zero ODP products have found new uses in applications as wide as domestic refrigerators, car air conditioning and refinery refrigeration. In Europe virtually 100% of domestic refrigerators have isobutane (R600a) as the refrigerant and the extremely small charge of refrigerant means that they can operate safely. Similarly in Australia some states allow the use of hydrocarbons for vehicle air conditioning.

However, politics abound in this area. It is thought that powerful HCFC refrigerants manufacturers are influencing bodies such as the EPA in the US, preventing the use of hydrocarbon refrigerants as they are currently illegal.