Lesson 8 - Refrigeration Technology

Primary refrigerants are used for the generation of refrigeration (primary refrigeration), normally through mechanical means. They are primarily chosen for their ability to be condensed and evaporated effectively, under the conditions required by the refrigeration equipment.

Further considerations for choosing a primary refrigerant include:

• safety (flammability and toxicity potential);

• its compatibility with materials;

• its monetary cost; and

• its environmental impact.

Many primary refrigerants are no longer used due to their harmful impact on the environment. The following are the main criteria against which the environmental impact is measured:

• Ozone depletion potential (ODP) – this gives an indication of a refrigerant’s impact on the ozone layer. The values range from 0 (where the substance has no impact on the ozone layer) to 1 (where the substance has the most harmful impact on the ozone layer).

• Global warming potential (GWP) – this indicates how harmful the refrigerant is in terms of its potential contribution to global warming. It is measured in relation to CO_2, which is deemed to have a GWP of 1. The values range from 0 to several thousand. It is essentially an indication of how ‘bad’, or how potentially damaging, a greenhouse gas is.

These terms may seem similar to you, but ODP means that the refrigerant causes damage to the ozone layer at high levels in the earth’s atmosphere, which allows the sun to do more damage. GWP means that the refrigerant acts as a greenhouse gas that blankets the earth at lower levels in the atmosphere, and traps heat in.

Primary refrigerants – naming convention

Before we classify refrigerants, let’s quickly look at the naming convention used for them. The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) adopted an R numbering system for refrigerants. Each refrigerant is assigned an R number.

Refrigerant numbering

The naming convention is based on four numbers, preceded by the letter ‘R’, e.g. R1270. However, as the convention drops any zeros at the start of numbers, they are most often shortened to two- or three-number codes, for example R0022 becomes R22, which was a very common refrigerant until it was banned from use by the Montreal Protocol.

Each digit has a meaning that is related to the chemical structure of the refrigerant. So, looking at these in turn:

1^st number – this digit is the number of carbon-carbon double bonds in the structure (C=C).

2^nd number – this is the number of carbon atoms in the molecule minus 1.

3^rd number – this is the number of hydrogen atoms plus 1. For methane (CH₄) this digit would be five (4 + 1 = 5), and for ethane (C₂H₆) it would be 7 (6 + 1 = 7).

4^th number – this is the number of fluorine atoms in the molecule.

It is assumed that any spare atoms not accounted for in the above code numbers will, by default, be chlorine atoms.

If bromine atoms are present, they are denoted by ‘B’, followed by the number of atoms of bromine.

If you would like to see the full list of refrigerants and their numbers, do follow the link below.

https://www.ashrae.org/technical-resources/standards-and-guidelines/ashrae-refrigerant-designations

Natural refrigerants are refrigerants that are found in nature. However, this doesn’t mean that they are not manufactured. They fall into two categories:

• Inorganic (meaning they do not contain carbon-hydrogen bonds) refrigerants such as CO₂ (R744), ammonia (R717), and water (R716). 

• Organic refrigerants such as hydrocarbons. Examples of those commonly used are propane (R290), isobutene (R600a), and propylene (R1270). These hydrocarbons have no ODP and low, or no, GWP. However, these substances are highly flammable, so they are very rarely used on an industrial scale.

Synthetic refrigerants are man-made and do not exist in nature. The most important categories of these refrigerants are:

• Chlorofluorocarbon refrigerants (CFCs). CFCs contain carbon, fluorine, and chlorine. Examples are R11 and R12. Due to their highly detrimental impact on the ozone layer, CFCs have been banned from use or production under the Montreal Protocol and they all have an ODP of 1.

• Hydrochlorfluorocarbon refrigerants (HCFCs). HCFCs contain hydrogen, carbon, fluorine, and chlorine. An example of this is R22. Whilst their ODP values are lower (R22 has an ODP of 0.055), they are still greenhouse gases and are thus being phased out.

• Hydrofluorocarbon refrigerants (HFCs). HFCs contain hydrogen, carbon, and fluorine. Examples of these are R32, R134a, and R143a. They have an ODP of 0, but they are still greenhouse gases. These gases are commonly referred to as F-gases. Their use is controlled under legislation such as EU Regulation No 517/2014.

The link below leads to the regulations on F-gases, along with other useful information.

https://www.fluorocarbons.org/f-gas-regulation/

• Synthetic refrigerants are man-made and include CFCs, HCFCs and HFCs.

The table below summarises the important parameters for several refrigerants.

Table: Properties of various refrigerants

You can see from this table why R600a would be excluded from consideration, based on its high flammability. If it wasn’t for this, it would be an ideal choice based on the other parameters.

R717 (ammonia) is widely used despite its toxicity classification; this is because it has a low environmental impact as a gas. Special precautions are taken with its use, and when a secondary refrigerant circuit is used, the ammonia can be contained within one area.

Before choosing a refrigerant, several other general criteria such as corrosiveness, ease of leak detection, and compatibility with compressor oils (we will learn about these later on in the lesson) should also be considered.

Now that we have reviewed some of the general refrigerant properties, let’s have a quick look at what properties a primary refrigerant should have, in relation to its performance.

Based upon what you learned in the lesson on Refrigeration Theory and Cycle, what properties would make for a  good primary refrigerant?

Give yourself 5-10 min to think about this, and then click ‘Next’ to continue with this lesson.