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

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 TechnicalDiploma 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.

Hi Riaan. I have looked at R744 previously but let us investigate R744 in more detail. As you are aware refrigeration has many applications in the industrial, commercial and domestic sectors. With a rising increase in global temperatures, one must consider the impact that refrigerants have on the environment. According to the IIR, refrigeration systems represent a major challenge to sustainable development. The approximately three billion refrigeration systems worldwide consume 17% of global electricity.

There is concern as both the increased energy consumption and the current use of refrigerants have led to environmental problems.

An efficient use of more environmentally friendly natural refrigerants, such as CO2 (R744) as a refrigerant provides an approach that meets the challenge of a growing demand for air conditioning and refrigeration and the urgent need to limit its impact on the environment.

There has been considerable change of late introducing new refrigerants as a replacement for HCFCs: a third class of fluorinated chemicals was introduced, hydrofluorocarbons (HFCs). These have no ozone depletion potential (ODP), but GWP (global warming potential) still up to several thousand times higher than CO2. The latest amendment to the Montreal Protocol, the Kigali Amendment (2016), requires the phase-down of this group of chemicals.

It is important to note that South Africa as a signatory to the Montreal Protocol and subsequent amendment has now entered the first phase of the phase down process with regard to HFC refrigerants. There is no doubt that the phasing down of CFC refrigerant usage will have considerable impact on the industry. Consider the extensive use of HFC refrigerants in unitary air conditioning systems (R410a / R32):

Ultra-low GWP natural refrigerants

Focus has now switched from ozone depleting refrigerants to those causing global warming and there is movement towards natural refrigerants.

Natural refrigerants are substances that occur naturally in the environment. Natural refrigerants are composed of the elements hydrogen, oxygen, carbon and nitrogen and include hydrocarbons, carbon dioxide, ammonia, water and air – ‘the natural five’.

The strong advantages of natural refrigerants are that they have zero ODP, and a zero or negligible GWP. As part of the natural biogeochemical cycles, natural refrigerants do not form persistent substances in the atmosphere, water or biosphere.

Carbon dioxide or CO2, can be used as the refrigerant in the vapour compression refrigeration cycle as well as in direct refrigeration in its ‘dry ice’ form. CO2, when used as refrigerant is also called R744.

The GWP of CO2 is defined as the reference value 1 for a period of 100 years. The GWP is calculated for a specific time horizon, arbitrarily over 100 years. In some contexts, other periods are chosen, i.e. 5, 20 or 100 years.

The diagram below illustrates the GWP values for natural refrigerants:

Focusing on R744; the use of carbon dioxide as refrigerant was first suggested by Alexander Twinning in a British patent in 1850. It was then first applied in the manufacturing of ice in the US when Thaddeus Lowe modified hydrogen pumps to compress CO2 in 1866. In Europe, Carl von Linde built the first CO2 compressor in 1882.

CO2 had its peak of use in the middle of the 1920s after it was successfully used to freeze meat on long shipping routes, for the first time by J&E Hall on a trip from the Falkland Islands to the UK in 1886. J&E Hall further developed a two-stage CO2 compressor and later supplied refrigeration equipment to the National Skating Palace.

In the first half of the 20th century, CO2 was increasingly replaced with more efficient ammonia systems. Air cooled condensers required higher temperatures to a CO2 system and complicated subcritical operation. With the development of CFCs and their use in all refrigeration and air conditioning sub-sectors, the use of CO2 decreased more and more until it ended in the early 1960s.

CO2 as refrigerant experienced a revival in the 1990s because of its low environmental impact when used as refrigerant and its favourable properties regarding a variety of applications. CO2 has particularly good performances for low-temperature applications, e.g. in freezer plants. It has high heat transfer rates and can therefore provide rapid cooling.

The first modern CO2 system was developed in Scotland, where a small ammonia plant was used to condense CO2, which then circulated as a secondary refrigerant in a thermo-siphon system without a compressor. It is now being used in low-temperature applications, heat pumps for heating water and has gained some traction in the motor industry.

Hazards of carbon dioxide and safety measures

CO2 is part of many biological processes and naturally occurs in the atmosphere. It is non-flammable and non-toxic in small concentrations. However, when used as refrigerant, there are several hazards associated with its use.

The main hazards of carbon dioxide are:

• Asphyxiation
• High pressures
• Rapid expansion of trapped liquid or gas
• Solidification – dry ice formation during (instantaneous) venting or discharge from safety valve

As mentioned, in small quantities CO2 is non-toxic, non-flammable and does not irritate skin or eyes and has no odour. However, at moderate to high concentrations it is dangerous being denser than air and can stratify displacing oxygen which will lead to asphyxiation.

The danger of R744 is to be estimated differently than that of halogenated refrigerants (HCFCs and HFCs) of the same safety group A1. Carbon dioxide is dissolved in the blood, influences the respiratory centre in the brain and the oxygen absorption capacity of the red blood cells. Even at a carbon dioxide content of just a few percent in the air, respiration is accelerated and concentrations of 8% or more are already life-threatening, even if there is still enough oxygen present.

HFC refrigerants generally have only a weak effect on the body. Concentrations from approximately 6% have a narcotic effect. It usually only becomes life-threatening at approximately 20 % by volume due to oxygen displacement.

According to the standards ISO 817, EN 378 and AHRI 700, R744 is classified as A1 refrigerant. This means that it is non-flammable and of lower (chronic) toxicity.

The material data safety sheet (MSDS) includes all relevant information for the user of CO2 as refrigerant.

CO2 refrigerant bottles should generally be marked with the following information:

• Classification: Gas under pressure, liquefied gas
• Hazard statement: Contains gas under pressure, may explode if heated
• Storage: Protect from sunlight, store in a well-ventilated place
• CAS Number: 124-38-9
• Hazard class: 2.2 (non-flammable, non-toxic gas)

For the provision with refrigerant CO2, cylinder sizes from 13.4 to about 50kg charging amount are in use. For liquid and gas charging the cylinder is equipped with a dual port cylinder valve. Generally, cylinders are fitted with a residual pressure device (RPD) to ensure used refrigerant or any other substance cannot be reintroduced into a cylinder.

Riaan, with regards to safety equipment and measures, employers are obliged to protect their employees against any possible risk associated with the employee’s job performance. These include the provision of adequate and regularly serviced technical protection measures, such as installed gas sensors of the equipment, appropriate personal protective equipment (PPE) and organisational measures, such as supervision, coordination, rules of conduct, operating instructions, safety instruction, design of workplaces.

Emergency procedures must be established for the work with R744. Each technician working on R744 systems must be trained so that they are capable of immediately following the relevant emergency procedures in case of an alarm.

The main safety equipment for the safe use of CO2 refrigerant are gas sensors, which have to be installed wherever large amounts of CO2 can escape in closed or sealed areas. The sensor gives an alarm if critical concentration is reached and warns people from entering the space or indicates that it has to be left immediately.

• Position gas sensors close to racks/storage shelves or evaporators as this is the most likely position of a leak and the resulting accumulation of refrigerant.
• Position gas sensor about 20 to 30 cm above floor level as CO2 is heavier than air and will accumulate close to the floor.
• Choose robust gas sensors with a self-test function. It is important that unauthorised people cannot change the settings.

Every machinery room where the leakage of CO2 can lead to a concentration exceeding the refrigerant concentration limit according to EN 378-1:2016 or local standards has to be equipped with an emergency mechanical ventilation system.

The system requires two independent controls and a gas sensor system. The ventilation can thereby be started from outside the room using an emergency switch or automatically based on the gas sensor readings. An additional emergency switch on the inside of the room is required.

Mechanical ventilation has to have a power supply that is independent of the refrigeration system. If a failure of the power system of the refrigeration system causes a leak, the ventilation will then still work. As CO2 is heavier than air, the system should be situated close to the floor.

The necessary volume flow of the ventilation is based on the charge size of the largest sized equipment in the machinery room. It is also possible to calculate an air exchange at the rate of 15 air changes per hour.

• In case of a gas alarm:
• Leave the affected area immediately
• Inform people that are close by
• Ensure proper ventilation

Enter the affected area only if the gas alarm is off and gas alarm system shows normal operation

In the case of someone needing to be rescued, only enter the area while using a self-contained breathing apparatus. As with other refrigeration systems, with the installation in a closed area it should be ensured that appropriate ventilation is provided to release the CO2 to the outside and to provide the required fresh air.

Riaan, I hope that this helps with your understanding around CO2 refrigerant and natural refrigerants in general.

References:

1. ACRA
2. The Deutsche Gesellschaft für Internationale Zusammenarbeit GmbH,
3. ASHRAE
4. World Health Organisation
5. A-Gas

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