The first refrigeration cycles that were made around 1875 used fluids such as ammonia, sulfur dioxide or carbon dioxide. Subsequently, in the first half of the twentieth century emerged new fluids derived from methane and ethane (chlorofluorocarbons CFCs) and hydrochlorofluorocarbons (HCFCs). For many reasons, both technical (thermodynamic performance, compatibility with oils, seals, metals, acceptable pressures etc.) and social acceptability (low flammability and toxicity etc.), these fluids have gradually taken the place of the old, in the exception of ammonia, still used in industrial facilities, including food.

In particular, a CFC and an HCFC, R12 (CCl2F2) and R22 (CHClCF2), have come to be used in 75% of the French fleet of refrigeration (1998), while all of these fluids represented over 90% of the total.

Two environmental concerns came abruptly to question the widespread use of CFCs: the breakdown of the ozone layer and the increasing greenhouse effect. As explained below, it was very quickly decided to stop production of CFCs and halons, and to challenge that of HCFCs, as they still contain chlorine. All these measures led to the industrial refrigeration major technological revolution that began in 1994 and is not complete.

Although the contribution of fluorinated compounds to the greenhouse effect increase is much lower than that of other gases such as CO2 or CH4, the strong growth of concentration in the last century brought the international community to question their use, because of their role in the destruction of the ozone layer.

The way in which the ozone layer depletion issue was tackled at the international level is particularly exemplary: after some hesitations, far from adopting a conservative attitude, manufacturers generally supported mandatory regulations, in the frame of the Montreal Protocol which became effective in 1989.

This is not common and the reasons for this behavior are numerous. However, it is clear that this threw manufacturers into a technological race where those who had invested in research substitutes for CFCs sought to capitalize on their advance by advocating a strict regulatory framework, thus limiting the market share of their competitors. Behind the apparent convergence of views there was a battle between a few large industrial companies in industrialized countries and their counterparts in the "South", particularly in India and China.

CFC replacement fluids can be grouped into three broad categories: transition fluids, zero ODP fluids, and zero ODP fluids with low GWP.

The first solution that came to mind to replace CFCs were hydrochlorofluorocarbons or HCFCs, relatively close on the chemical level and less harmful to the ozone layer. However, these non-zero ODP fluids are either already banned or will soon be by the signatory countries of the Montreal Protocol and the agreements that followed, so that they are only a short-term solution, hence their name: “transition fluids”.

In the longer term, the only halogenated fluids that are acceptable vis-à-vis the ozone layer are hydrofluorocarbons (HFCs), which do not contain chlorine atoms and have a zero ODP value.

The main drawback presented by HFCs is that while their ODP is zero, their GWP is very high (1300 for R134a, 3200 for R125, 580 for R32, 4400 for R143a), and their molecules, very stable, have atmospheric lifetimes of several decades or even hundreds of years.

Their contribution to the greenhouse effect is thus potentially significant, and their production is therefore likely to be questioned sooner or later.

HFOs are fluorinated fluids containing at least one carbon double bond. The main advantage of HFOs is their low impact on the greenhouse effect. Indeed, stability is less than that of HFCs due to the presence of the double bond. These synthetic fluids have been rediscovered since the first decade of the 21st century and are experiencing new developments to enable them to fully or partially replace HFCs (chemists now offer mixtures of HFO and HFCs).

Some of these new fluids are already available on the market as the R1234yf or R1234ze.

The only alternative fluids that do not present drawbacks regarding either the ozone layer or the greenhouse effect are non-halogenated fluids such as ammonia (R717), propane (R290), isobutane (R600a), carbon dioxide (R744) and water (R818).

The first three have flammability constraints (as well as toxicity and compatibility issues with some metals such as copper does for ammonia) that limit their use. The latter cannot be used for negative temperatures, whereas R744 cycles have efficiencies significantly lower than the others.

It is also possible to use blends as replacement fluids. In fact, manufacturers of refrigerants consider that with the exception of R134a, it is now unlikely that pure fluids will be discovered whose thermodynamic properties would allow them to perfectly replace the old ones. However, by mixing pure fluids in well-chosen proportions, it is possible to obtain more suitable features, which explains the interest in the blends.

To characterize a refrigerant, a nomenclature has been established. The following notation is used: R-WXYZ.The meanings of the letters are:

• R: refrigerant, but sometimes HFC, HFO etc. is used;

• W: represents the number of double bonds;

• X: represents the number of carbon atoms - 1;

• Y: represents the number of hydrogen atoms + 1;

• Z: represents the number of fluorine atoms.

Examples:

• Nomenclature / chemical composition of refrigerant:

• R22 / CHClF2

• R134a / CF3CH2

• FR1234yf / CF3CF = CH2

Among these examples, we see the addition of suffix "a" or "yf". These letters represent organic fluid isomers, which have different physical and chemical properties.

Similarly, in order to specify a cyclic hydrocarbon, regulations require to insert the letter "C" to the identification number of the refrigerant.

Particular cases exist, to represent mixtures and inorganic compounds.

Among the mixtures are usually distinguished zeotropic mixtures and azeotropic mixtures.

A zeotropic blend is a mixture whose compositions in vapor and liquid phase are different when the two phases coexist, the most volatile substance evaporating more easily than the less volatile one. Therefore, the boiling temperature in the evaporator is not constant. This temperature difference is called the temperature glide.

Series 400 is attributed to zeotropic mixtures. The identification numbers are recording serial numbers of the mixtures. The mixtures having the same components have the same number, only a capital letter suffix characterizes the mass distribution of the blend components.

Thus, the compound R410A is, by mass, 50% of R125 and 50% of R32, and R410B is composed of 55% of R125 and 45% of R32.

An azeotropic mixture is a fluid mixture which behaves like a pure substance. Series 500 is attributed to these mixtures.

The 600 series is attributed to various hydrocarbons. For example, the R600 is butane.

Series 700 is attributed to inorganic fluids. The rule of nomenclature consists of adding the molecular weight of the fluid of the number series 700. For example, R717 denotes ammonia.

The two main international regulations are the Montreal Protocol and the Kyoto Protocol. The following paragraphs provide some essential information about the contents of these agreements and their historical contexts. Then, we present the safety standard ASHRAE 34 for classifying refrigerants according to their dangerousness.

Montreal Protocol

The "Montreal Protocol on Substances that Deplete the Ozone Layer" aims to reduce and eliminate substances that deplete the ozone layer after highlighting the responsibility of synthetic refrigerants (CFCs, HCFCs). Based on the Vienna Convention on the Protection of the Ozone Layer adopted in 1985, the Protocol has been signed by 24 countries and by the European Community in 1987.

The protocol requires the elimination of the use of substances that deplete the ozone layer: are concerned chlorinated fluids such as CFCs and HCFCs. Since 2001, the production and use of CFCs are permanently banned. The new HCFC equipments are banned since 1 January 2010. Today, HCFCs may be used in maintenance to complete the load of existing installations. Their production will be banned from 2015.

Europe, anticipating the prohibition of 2010 by a decade has contributed to the development of HFCs from the 90s.

To measure the impact of a fluid on the ozone layer, the ODP (Ozone Depleting Potential) is defined as the ratio of the capacity to destroy the ozone layer of a given fluid to that of R11 .

Kyoto Protocol

The signing of the Montreal Protocol led to the gradual disappearance of fluids with high impact on the ozone layer. However due to the confirmation of climate greenhouse warming by gas emissions, the impact of refrigerants on the greenhouse is more and more deemed to be important by regulations. The Kyoto Protocol is an international treaty whose objective is to reduce greenhouse gas emissions.

Signed in 1997 in Kyoto, this protocol aims to reduce by 5.2% the emissions of six greenhouse gases. Gases such as carbon dioxide, methane, nitrous oxide and CFC substitutes are concerned. This reduction in greenhouse gas emissions should take place between 2008 and 2012, having as a reference the level in 1990.

To assess the impact of global warming gases, the GWP (Global Warming Potential) is defined as the ratio of the contribution of a unit of gas (x) to the contribution of a unit of the reference gas (r), namely CO2.

GWP expresses the global warming potential of a greenhouse gas emissions compared to that of carbon dioxide. This is an indicator of the mass of CO2 released giving a warming effect equivalent to that of the mass of refrigerant. Because life in the atmosphere of these gases varies, so that their impact varies over time, this indicator is defined on a 100 years time period.

The draft amendment to the European regulation 842-2006 (F-gas) published in 2012 tends to impose more stringent short-term constraints:

• charging of existing facilities will be closed for fluids whose GWP is greater than 2500;

• it will be prohibited to use fluids with GWP higher than 150 for new installations.

These restrictions lead to the phasing out of HFCs and their partial or total replacement by 4th generation HFOs.

ASHRAE classification of refrigerants

ANSI/Ashrae 34 classes safety features of refrigerants on the basis of their toxicity and flammability.

Notation security group is composed of a capital letter and a number. The letter and number represent the level of toxicity and flammability of the fluid.