What are refrigerants?
According to standards EN378-1 or ISO817 a refrigerant is a "fluid, that is used for heat transfer in refrigeration systems and that absorbs heat at low temperature and low pressure and releases heat at higher temperature and pressure, while usually a change of state of appearance happens". The change of state happens, amongst others, in vapour compression refrigeration systems, which are dominantly dealt with in the named standards and this document.
Early refrigeration machines used working fluids that were known to cool while evaporating. Amongst those were ethers, sulfur dioxide (SO2), ammonia (NH3), carbon dioxide (CO2), methyl chloride (CH3Cl) and hydrocarbons. The hydrocarbons showed favourable thermodynamic properties, easing the design of refrigeration systems, compared to other fluids. Additionally, they are non toxic.
The denomination of e.g. propane as refrigerant 290 or R290 or ammonia as R717 according to ISO817 also implies the applicability in a refrigeration system. This means, the denomination R290 implies a defined purity that allows to determine or calculate the thermodynamic properties with sufficient accuracy and enables reliable operation.
Synthetic refrigerants
During the development of the first synthetic refrigerants the target was to reach thermodynamic properties similar to those of hydrocarbons, while avoiding flammability. An exchange of hydrogen atoms in hydrocarbons by halogenes, mainly chlorine and fluorine, partly bromine, led to the first "safety refrigerants" R11 and R12, later R22, R13B1 and so on. These fully halogenated chloro-fluoro-carbons (CFC) came to the market from 1930 on. The simple and safe handling led to the replacement of all other fluids in commercial and household refrigeration within approximately 20 years.
As the chlorine and bromine parts contribute heavily to the depletion of the ozone layer, today's synthetic refrigerants are mainly fluorinated hydrocarbons. Most of those are partly fluorinated and some are based on unsaturated hydrocarbons, like propene, some on ethers. By this, the atmospheric lifetime is shortened and the global warming potential reduced.
Water as refrigerant
R718 Water is basically suitable as refrigerant in vapour compression systems. Because of the high triple point of 0,01°C and the very low vapour pressure in the temperature range of usual refrigeration and heat pump applications, it is not very interesting for such standard applications. For high temperature heat pumps, process cooling and server cooling it is in use. Because of the very low volumetric capacity, usually very high volume flows are necessary. In this document, R718 is not dealt with in detail.
Refrigerants for other refrigeration processes – gas cycles
In gas cycle refrigeration systems, a gas far above its critical temperature is used, staying gaseous during the complete cycle. The temperature increase during compression and the decrease during expansion with extraction of mechanical power is utilised. Thus gas cycle machines usually add an expansion machine, called expander, to the compressor.
Gas cycle machines need a working fluid that has a large temperature change during these process steps corresponding to a high isentropic coefficient. Well suited as refrigerant are R704 helium and R729 air.
Known gas cycle machines are Stirling machines and air cycle machines:
- Stirling machines typically work with helium.
- Air cycle machines can be designed as closed circuits, but also as open systems. The open system takes in the air from the room to be refrigerated, compresses, cools it in heat exchange with ambient air, and supplies it back to the room after the expander.
With the cold side temperature close to ambient temperature, gas cycles are usually considered less efficient than vapour compression systems. At temperatures below ca. -50°C, they can have efficiency advantages, depending on the application. For air, this mostly requires an open circuit system. With gas cycles, large temperature lifts can be reached with low to moderate pressure ratio.
At higher cold side temperatures, like AC, the use can be beneficial in terms of efficiency, if not only the refrigeration system is looked at. When evaluating e.g. aircraft air conditioning systems, also the energy use for transport of the system has to be taken into consideration.
Refrigerants for gas cycles are not dealt with in more detail in this document.