REFRIGERANTS
A pragmatic way forward
John Poole of Refrigerant Solutions argues for stability in terms of GWP limits.
E
arly commercially used refrigerants were either flammable, toxic, high pressure or a combination of each of these properties. Products included
hydrocarbons, ammonia, carbon dioxide, methyl chloride and others. Subsequently new products were invented which were non- flammable, low toxicity, lower pressure and more energy efficient , namely chlorofluorocarbons (CFCs), hydrofluorocarbons (HCFCs, primarily R22) and hydrofluorocarbons (HFCs). Discovery that CFCs destroyed the ozone layer led to their extinction & replacement largely by HFCs but, because of their potent ability to increase global warming (compared to CO2
in the UK and several other countries, eg USA, Canada, Belgium, France, India, Malaysia & others. Hydrocarbons are good refrigerants but their use is severely limited by their high degree of flammability (A3 ASHRAE safety classification), notwithstanding their positive properties as refrigerants with low GWPs. Some other technologies have found only limited applications in the market place despite years of research because of low efficiencies, eg magnetic, absorption, evaporative cooling, Stirling cycle, thermoelectric & thermoacoustic. Conventional HFC systems are based on the successive
) when released
to the atmosphere, HFCs are now subject to phasedown globally and possible ultimately to phase-out. Safety is a key driver, indeed it could be argued THE key driver to future technology development. The safety issue will severely limit the extent to which hydrocarbons, ammonia and high pressure carbon dioxide can be used to replace HFCs. Fear of continuing future legislation restricting the use of products with higher GWPs, which particularly applies in the case of large equipment with a long plant life, is a strong factor in decision making of Original Equipment Manufacturers. There is also the issue of corporate image & the wish to be seen as the early mover to lower GWP solutions. Ammonia is one of those low GWP solutions & one of the early refrigerants used, primarily in industrial applications. But ammonia is both toxic & flammable with a safety classification from ASHRAE of B2. In Canada in 2017, a British Columbia MP called on the Canadian government to phase out ammonia refrigeration in the wake of fatalities during routine maintenance of the ammonia refrigeration systems at an ice rink. There have been several fatalities involving ammonia
22 August 2020
condensation of refrigerant vapour at high pressure which allows heat to be rejected at high temperature, followed by evaporation at low pressure which requires the absorption of heat at low temperature. The system is powered by a compressor, typically driven by an electric motor, which sucks refrigerant vapour from the evaporator and compresses it to high pressure to enter the condenser. This sequence is called the Rankine or ‘vapour compression’ cycle. Although carbon dioxide (CO2
) has most of the
properties desirable in an HFC replacement, its critical temperature is only 310C. This means that it does not exist in the liquid phase at higher temperatures so cannot be used in the vapour compression cycle. Transcritical operation with heat rejection in the high temperature heat exchanger is possible but needs maximum operating pressures in the range 120 to 150 bar, i.e. ~5 greater than in existing HFC equipment, requiring substantial developments in design, construction and maintenance. Equipment using CO2
is often more complicated in design
and higher capital cost, and of course certainly not suitable for retrofitting existing installations. So, what does the future hold? At least, one certainty
www.acr-news.com
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