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GRANT LAIDLAW Grant Laidlaw is currently the owner of the Air Conditioning and Refrigeration Academy (ACRA) in Edenvale. He holds a Bachelor of Business Administration and an associate degree in educational administration. He has a National Technical Diploma and completed an apprenticeship with Transnet. He has dual-trades status: refrigeration and electrical. He has been involved with SAIRAC for over two decades and served on the Johannesburg committee as chairman and was also president between 2015 and 2018. Currently he is the SAIRAC national treasurer.

Many people ask for assistance in the understanding of theoretical and practical aspects of the industry. We are going back to basics as I have questions coming in that indicate that the basic understanding necessary to work in the industry is not in place.

Hi Gordon, transcritical carbon dioxide (CO2 ) R744 refrigeration systems have gained acceptance in the supermarket refrigeration industry and there is some movement in the light commercial and other sectors of the industry. National and international refrigerant phase down policies are being expanded to include HFCs.

In South Africa, HFCs for example, are about to enter the start of the phase down process. This will have a massive impact on our industry and in particular the air-conditioning sector. Both R32 and R410a are HFCs and are to be phased down in accordance with the Kigali amendment of the Montreal Protocol. Although R32 has a GWP 32% lower than R410a, the fact remains that R32’s GWP of 675 is above the cut off GWP of 500.

Given that CFCs are long gone, HCFCs are being phased out and HFCs are about to begin being phased down, so where do we go? Naturals are the future, and the selection revolves around ammonia, hydrocarbons, some hydrofluoroolefin (HFO) refrigerants and carbon dioxide. I am aware of some research around using water (H2 O) as a refrigerant – but we have yet to see commercially viable results.

Technology advancements in CO2 systems are making these systems more economically viable, in terms of both equipment and installation cost but also energy and operating costs.

Gordon, looking at some background information, we find that CO2 is a naturally occurring compound in Earth’s atmosphere and it is the fourth most common atmospheric compound, behind nitrogen, oxygen and argon. As we all know carbon dioxide is an integral part of the life cycle of plant and animals, as the primary product of respiration in animals and humans. In turn the carbon dioxide is absorbed by trees and converted back to oxygen.

Through the process of photosynthesis, leaves draw in carbon dioxide and water and use sunlight to convert this into chemical compounds such as sugars that feed the tree.As a byproduct of that chemical reaction oxygen is produced and released by the tree. It has been calculated that one large tree can provide oxygen for up to four people.

Trees also store carbon dioxide in their fibres helping to clean the air and reduce the negative effects that this CO2 could have had on our environment. According to the Arbor Day Foundation, in one year a mature tree will absorb more than 48 pounds of carbon dioxide from the atmosphere and release oxygen in exchange.

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’.

CO2 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. CO2 is also non-flammable and non-toxic with the refrigerant number R744. CO2 is commercially available at several different purity levels. The common names and percent purity recommended for refrigeration systems using CO2 with a purity equal to or greater than Bone Dry Purity. Examples of various grades of CO2 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:

• CO2 used in commercial refrigeration systems must be of a purity level high enough to prevent the introduction of non[1]condensable 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 CO2 is acceptable. Lower grades of CO2 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 CO2 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 CO2 attractive as a refrigerant from an environmental perspective.

Unfortunately, the primary disadvantage of CO2 as a refrigerant is the relatively high operating pressures and fairly complex refrigeration systems. Oil management needs particular attention.

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 CO2 systems this temperature is often exceeded when ambient air is used for condensing.

Critical point on a Mollier diagram. Supplied by Grant Laidlaw

Safety when using CO2

The physical properties of CO2 present a unique set of considerations to ensure safety. CO2 is classified as an A1 refrigerant by ASHRAE 34 meaning it is non-toxic and non[1]flammable. 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, CO2 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, CO2 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 CO2 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 CO2 . 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.
• 0.5%–1%: Characteristic sharp odour noticeable. Breathing increases to approximately four times normal rate and can be very laboured. Symptoms of intoxication become evident, and slight choking may occur. Visual impairment, headache and ringing in the ears. Judgment may be impaired, followed within minutes by loss of consciousness.
• Unconsciousness occurs more rapidly above 1% level. Prolonged exposure to high concentrations may eventually result in death from asphyxiation.

Perhaps we should at this point note that as CO2 at ambient pressure is heavier than air, leak detection systems should be placed low, approximately 200mm from the floor.

Gordon, let us have a look at the pressures expected when using CO2 as a refrigerant. We know that CO2 operates at higher pressures than typical HCFCs or HFCs, due to the inherent thermodynamic properties of the substances. HFC direct expansion (DX) refrigeration systems mechanical safeties and control set points shut the system down around 2 400kPa (depending on the refrigerant) discharge pressure. The entire piping system is rated for safe working conditions above this maximum pressure, so no secondary relief devices are necessary.

In addition, if the fluorinated refrigeration system shuts down due to power outage or servicing, the internal pressures do not climb any higher to exceed the maximum system design pressure. In fact, the pressure tends to fall as temperatures drop and equalisation occurs.

In the case of CO2 , the high saturated pressure of CO2 at summertime ambient conditions exceeds the pressure rating of type K copper piping, along with most standard DX refrigeration valves. This requires the “high side” of the CO2 system to be constructed using higher pressure rated materials and installation practices, at a higher cost.

To reduce overall system installation cost, the ‘low side’ portions of a CO2 system are designed for the lower operating pressures, allowing copper to be used for the low side piping. When the system is operating normally, pressures are maintained below the rated pressure of the system. The CO2 system pressure becomes a safety concern when liquid becomes trapped in a portion of the system that is not rated for the full pressure at higher temperature.

It is therefore critical not to allow liquid CO2 to become trapped in the system without means of pressure relief. As the temperature of a saturated mixture rises, pressure will rise until it reaches the saturation pressure in the table above. If the refrigerant pressure exceeds the rating of the piping, valves, or other components of the system, this can lead to leaks and possibly bursting of system components.

Measures must be taken in system design to ensure that pressure cannot build up in any portion of the system. All components, valves, piping, fittings and joining methods are to be verified to ensure pressure ratings above the maximum anticipated system pressures.

In CO2 systems, pressure relief devices must be appropriately located to allow the system to vent safely in the event of a system shutdown or other event that causes pressures above system ratings. All points within the system must be allowed to vent back to the pressure relief valves without restriction. Check valves are typically utilised to allow portions of the system to vent back to receivers, where pressure relief valves are located. Any portion of the system that cannot vent back to the receiver must have its own pressure relief valve.

Gordon, a particular issue with CO2 is the formation of dry ice, which is simply CO2 in solid form. In a CO2 refrigeration system, there are two common conditions where this may occur.

The first and potentially dangerous location is at a pressure relief valve. When a pressure relief valve is open, the refrigerant is undergoing a rapid drop in pressure from system pressure to atmospheric pressure. If liquid CO2 is being released, the CO2 release in a solid and vapour mixture causes the formation of dry ice. Therefore, pressure relief valves should not have any piping installed downstream of the valve. If the pressure drop happens inside the pipe, dry ice will form, blocking flow and preventing pressure from being released.

The second condition where dry ice may form is when charging the system. If the system vacuum is broken with liquid, dry ice can form inside the system, again restricting flow. This condition is less dangerous because it does not cause pressure buildup beyond system ratings but should still be avoided. Gordon thanks for the question, I will go deeper into R744 systems in the following issues.

Thanks to everybody for the overwhelming response. I receive many questions each month and cannot publish all of them. But keep them coming, as I may answer you directly. Looking forward to hearing from you.

References

1. SETA training
2. ASHRAE
3. ACRA
4. A-Gas
5. Hussman training
6. Arbor Day Foundation