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By Grant Laidlaw

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 Cayla. Yes, I can help. Let us begin with protection against excessive pressures and preventing explosions. CO2 systems operate at higher pressures than other systems because of the high triple point of the gas. High pressure can be dangerous if incorrect materials are used or if the system is not connected properly. Pipes or the housing of components might burst, and the resulting energy release can be dangerous for people and/or can cause damage equipment in the area.

Examples of pressure levels:

• High pressure = 12 000 kPa
• Intermediate pressure and medium temperature cooling = 4 500 kPa
• High pressure LT (low temperature) = 4 500 kPa

To prevent accidents due to high pressure, it is important to use the right materials for piping and brazing operation and to install appropriate safety relief valves.

Additionally, CO2 has a high coefficient of expansion. This means that trapped liquid or gaseous CO2 expands quickly at increasing temperatures, leading to extremely high-pressure increases. Trapped liquid CO2 expands much more than other refrigerants. As an example, the pressure of trapped liquid CO2 in an evaporator at -30°C will rise to over 10 000 kPa if the temperature rises to -20°C. For every 1°C rise in temperature trapped liquid pressure CO2 increases by approximately 1000kPa.

To prevent this pressure increase CO2 systems need to be equipped with safety relief valves at every pressure level to secure the system against excessive pressures. Some systems are equipped with additional cooling systems that activate during close down/stand-still of the equipment, so that liquid CO2 will not increase its pressure. Trapped liquid refrigerant is dangerous.

Cayla, we begin with risk assessment. A risk assessment includes analysis and evaluation of risks or hazards. There are generally two important types of risk assessment: one relating to the manufacturing of a system and one relating to hazards for employees working on a system.

Risk assessment by manufacturers

Systems and components shall be designed and constructed with the intention to eliminate possible hazards to people, property and the environment. Both excessive as well as too low pressure can lead to considerable damage to a RACHP system, e.g. rupturing the system with a total loss of refrigerant. The pressure in the system during operation and standstill shall not exceed the maximum allowable pressure of the system. Protection against foreseeable excessive pressure must be included as part of the system.

The maximum allowable pressure for each component shall not be less than the maximum allowable pressure of the system or part of the system. The selection of materials for components shall consider the impact strength at all temperatures to which they may be exposed.

Risk assessment by technicians

Closely related to the safety of a system declared by the manufacturer, a risk assessment for health and safety of the workers is extremely important. This can also be called a hazard assessment, and it comes, for example, from the Industrial Safety Regulation. A risk assessment must be conducted by the operator/employer to evaluate the health and safety risks to employees resulting from hazards at the workplace. An occupational health and safety system (OHS system) provides a framework for managing the risks and opportunities.

Let us have a look at the correct refrigerant tubing. The standard material is copper with two main copper alloys being in use. In some cases, stainless steel is used, as higher diameters are possible. There are two standard copper alloys in use for CO2 systems. Cu-DHP is the standard for all refrigeration systems and is widely used with CO2 as well. A newer alloy, CuFe2P is used for high pressure systems. Because of the added iron, the material can withstand much higher pressures.

Cu-DHP is used mainly for the low-pressure side where pressures up to 45 bars are common. If a safety valve that releases at 104 bars is used on the high-pressure side, Cu-DHP can be used up to a diameter of 16 mm. Independent of the stated values, CuFe2P can be used for pressures up to 120 bar. The tubes are marked with the copper alloy, the dimensions and the tensile strength. Consult with your supplier to ensure that the correct tubing class is selected. The stated numbers are standard safety coefficients for brazing operations. In many cases, higher pressures are permitted based on technical regulations and additional testing.

The maximum allowable pressure is the pressure for which the system or component is designed for, as specified by the manufacturer. PS is the limit which should not be exceeded whether the system is working or not.

Cayla, let’s move on to brazing. Generally, there are no differences between the brazing of systems for use with synthetic refrigerants and CO2 systems.

Your supplier should be able to recommend the appropriate product.

Safety relief valves

To secure the system against excessive pressure, CO2 systems are generally equipped with safety relief valves at all pressure levels. Only smaller systems with restricted charge sizes do not require safety relief valves. We differentiate between main and secondary safety valves. Safety relief valves release CO2 in case a certain design pressure is reached. If CO2 is released into a room, the room itself must be sufficiently ventilated to prevent dangerous concentrations. CO2 safety relief valves generally release enough CO2 to reach safe pressures in the system and close again without releasing the complete charge of the system.

The piping and installation diagram for each system contains an overview of both main and secondary safety valves and can be used to familiarise oneself with their location. A word of caution: in case of dry ice formation, no pipes may be installed behind the valves. The outlet of the valves should be directed towards the floor or horizontally.

Main valves

The main safety relief valves protect the system during normal operation. It is especially important that each receiver is equipped with a safety relief valve. In comparison to other systems, CO2 systems often have more receivers to deal with expanding gas in case of increasing temperatures. Each pressure level needs its own safety relief valve. A booster system for example has a minimum of three safety relief valves: high pressure level of the gas cooler, medium pressure of the receiver, low pressure of the low temperature evaporators.

Secondary valves

The secondary relief valves protect the system during maintenance. These valves are only activated in those parts of the system where liquid CO2 is trapped during work on the system. These valves allow isolation of certain parts, for example evaporators or liquid lines for servicing and repair without evacuating the whole system. In case of over pressurisation these secondary safety valve vent to the ambient. There are special ball valves with internal overflow devices. This valve does not vent the refrigerant, but trapped liquid could expand to the suction line or liquid line.

Looking at options to handle increase in pressure and explosion prevention, the increase of pressure in CO2 systems can especially be a problem if the system is in standstill, for example during a break-down of the system, general switch-off due to maintenance or even when the system pauses during normal operation when enough cooling capacity has been provided. There are two options of handling this: either by supplying an additional cooling system specifically for standstill operation, or by limiting the charge size to ensure that the maximum allowable pressure will not be exceeded.

Increase in pressure during standstill can be prevented by installing a small additional (auxiliary) refrigeration system. This usually activates automatically, usually based on pressure readings in the system that indicate standstill during normal operation or breakdowns (emergency cooling) in the system due to different reasons, such as power outages. The system should be filled with a different refrigerant, such as propane, have a cooling load of about 1 kW and have an evaporation temperature below the triple point of CO2. It should always run on a different power supply.

Sub-critical cascade-systems are often equipped with additional cooling units or safety operation functions. The upper stage is activated in case the pressure in the CO2 system exceeds the limit. The charge of the receiver is limited to 80% of the receiver volume.

A further option is to limit the charge. Smaller, simple one-stage systems can be designed without additional cooling if the design limits the charge size and pressure in the system below a dangerous level. These systems also do not have safety relief valves.

During the design stage of the total volume, or piping and components, refrigerant charge and maximum pressure for expected ambient temperatures is determined. The receiver of such a system must be in the middle or low-pressure stage.

The filling factor and the ambient temperature determine the pressure in the system. This method may be applied only for the receiver or for a simple one-stage system. The filling factor is the quotient of the charge mass and complete volume of the system. A system with a volume of 2 dm³ and a charge of 560g carbon dioxide has a filling factor of 280 kg m-3. If the ambient temperature is 40°C, the expected maximum pressure is 8 000 kPa (80 bar). If the filling factor is only 200 kg/m-3, the expected maximum pressure is 7 000 kPa.

As CO2 is not damaging to the ozone layer and does not contain fluorine, regulations on ozone depleting or fluorinated substances do not affect the use of CO2 as refrigerant. SANS 10147 is the primary standard set in South Africa. Additional standards include the OHS Act and the pressure vessel regulation. South Africa follows the ASHRAE classifications for CO2 as a refrigerant (A1). All persons working with refrigerants should have knowledge of SANS 10147.

Cayla, I hope that this assists you with your understanding of R744. We will continue with the thermodynamics relating to CO2 in the next issue.

References:

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

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