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Carbon sinks : A carbon sink is anything that absorbs more carbon from the atmosphere than it releases – for example , plants , the ocean and soil . In contrast , a carbon source is anything that releases more carbon into the atmosphere than it absorbs – for example , the burning of fossil fuels or volcanic eruptions .
In recent decades , carbon dioxide has been identified as the most significant greenhouse gas in Earth ’ s atmosphere . It is used as the comparative unit of measure when discussing the global warming impacts of various activities , leading to the term ‘ carbon footprint ’.
CO 2 as a refrigerant has emerged as one of the possible refrigerants of the future . It is environmentally friendly , has good heat transfer properties and a high latent heat of vapourisation .
CO 2 is also non-flammable and non-toxic with the refrigerant number R744 .
CO 2 is commercially available at several different purity levels . The common names and percent purity recommended for refrigeration systems using CO 2 with a purity equal to or greater than Bone Dry Purity . Examples of various grades of CO 2 are shown below : Industrial Grade – 99.5 % Bone-Dry Grade – 99.8 % Anaerobic Grade – 99.9 % Instrument Grade – 99.99 % Research Grade – 99.999 % Ultra-Pure Grade – 99.9999 %
Key factors to note :
• CO 2 used in commercial refrigeration systems must be of a purity level high enough to prevent the introduction of noncondensable gases into the system .
• A build-up of these gases can block the heat transfer surface and cause inefficient operation or malfunction of the system .
Mixing of higher purity grades of CO 2 is acceptable . Lower grades of CO 2 will be less expensive but are not recommended . In addition to non-condensable gases these lower grades contain higher levels of contaminants and water . Higher levels of moisture may react with the CO 2 and form carboxylic acid that can degrade system component integrity .
Carbon dioxide as a refrigerant has an extremely low carbon footprint , compared to common synthetic refrigerants . The absence of ODP and extremely low GWP make CO 2 attractive as a refrigerant from an environmental perspective . Unfortunately , the primary disadvantage of CO 2 as a refrigerant is the relatively high operating pressures and fairly complex refrigeration systems . Oil management needs particular attention .
Supplied by Grant Laidlaw
Critical point on a Mollier diagram .
Let us have a look at a transcritical system . A transcritical system is defined as a system that operates above the critical point . Above this point , the refrigerant is not considered liquid or gas , but an undefined fluid . This can be seen on a Mollier diagram . Fluorinated refrigerant systems operate below the critical point . In the case of CO 2 systems this temperature is often exceeded when ambient air is used for condensing .
SAFETY WHEN USING CO 2 The physical properties of CO 2 present a unique set of considerations to ensure safety . CO 2 is classified as an A1 refrigerant by ASHRAE 34 meaning it is non-toxic and nonflammable . However , like many refrigerants currently in use , a large enough leak in a confined space can displace available oxygen for breathing .
At typical refrigeration temperatures , CO 2 operates at considerably higher pressures than synthetic refrigerants – up to around 10 500kPa but 14 000kPa is possible in some instances . When released at these pressures to the atmosphere , CO 2 can change phase to solid form , causing restrictions in the flow that can lead to a buildup in pressure . The concern remains that a large leak of CO 2 can displace existing air in a space , reducing the oxygen levels . If the oxygen levels are reduced considerably , this can lead to health hazards up to and including asphyxiation / death . Average outdoor air consists of around 400 parts per million ( 0.04 %) of CO 2
. The details below list some additional concentration levels and the effects on the human body .
EFFECTS
• 0.1 – 0.2 %: Breathing rate increases slightly .
• 0.3 %: Breathing rate increases to 50 % above normal level . Exposure can cause headaches , tiredness , weak narcotic effect , impaired hearing , and increased blood pressure and pulse rate .
www . refrigerationandaircon . co . za RACA Journal I June 2024 39