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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. In the last RACA Journal issue we looked into protection against excessive pressures and preventing explosions. We will start with looking at the comparison between CO2 to other natural as well as synthetic refrigerants which are listed below.

The properties of CO2 are often quite different from other refrigerants. This results in several differences in the use and applicability of CO2 as refrigerant:

• The molecular weight is low compared to that of synthetic refrigerants and similar to propane (R290)
• The latent heat is high. This results in a lower mass flow than is expected for HFCs and HFOs
• The volumetric cooling capacity is higher than for any other refrigerants. This results in low tube diameters and small compressor sizes. Piping is typically one or two sizes smaller than other direct expansion piping systems
• Because of the low critical temperature, transcritical operation is expected for single-stage systems in warmer climate zones. Two-stage systems may be designed for subcritical operation
• The high triple point is the reason dry ice sublimates at
• -78°C. This characteristic makes dry ice a good direct refrigerant, e.g. for the transport of refrigerated food in solid state
• The heat transfer properties of CO2 are better than for other refrigerants (HCFCs, HFCs or HFOs). This is due to a variety of characteristics and depends on the application. It is possible to reduce the charge by about 50% compared with HFCs
• There is an excellent material compatibility for CO2
• Generally, a good energy balance for subcritical operation can be expected

Let us move on to the thermodynamics relating to CO2.

The thermodynamic properties of refrigerants are determined by their properties – such as their aggregation state (phase) at certain temperatures and pressures, their volumetric cooling capacity, pressure ratio and others. A phase is a range in which the chemical composition and the determining physical dimensions are constant. Phase diagrams show the state of phase for a given refrigerant. The transition from one phase to another – such as from solid to liquid – is marked with a curve called ‘line of equilibrium’ or ‘phase boundary’.

There are three boundaries between phases:

1. Solid-liquid: melting
2. Liquid-vapour: evaporating
3. Solid-vapour: sublimation

The transition from solid to liquid is called melting point. The phase boundary shows us the melting points at different pressures. The phase boundary between liquid and vapour shows all boiling points at various pressures.

Two important points in the chart below are the triple point (A) and the critical point (B).

Triple point: Three phase boundaries coincide at this point as three states of matter coexist: solid, liquid, vapour. Small changes in temperature or pressure will move the phases in one direction. In the vapour compression cycle, it is not possible to use a refrigerant below the triple point because of the potential presence of solid frozen refrigerant.

Vapour refrigeration systems operate either in subcritical mode, i.e. below the critical point (classic refrigeration cycle) or in transcritical mode, i.e. above the critical point. The transcritical mode is relevant mainly for CO2.

The temperature pressure diagram (general)

The phase of a substance is influenced by both its temperature and pressure. A temperature pressure diagram therefore shows the pressure curve as a boundary of the different aggregation states.

Some diagrams show only the vapour pressure curve between the triple point and critical point.

This curve is important for the comparison of refrigerants. If a replacement refrigerant is needed, it shows which refrigerants work under similar or equal conditions.

The evaporation and condensation conditions are shown for each refrigerant and can be used for choosing the appropriate refrigerant. This generally depends on the chosen application and the chosen condensing temperature. Not every refrigerant is suited for every application.

The enthalpy pressure diagram (full version)

The diagram shows the boundaries between the different phases and allows one to calculate the systems, cooling load, mass flow, pipe dimensions and more. All information regarding the design of the components can be found in the diagram. Both kinds of energy (heat and mechanical work) are shown as differences of enthalpy. The diagram allows us to visualise the cycle process as changes of the thermodynamic state in a closed loop.

The triple point is high at -56.6°C/520 kPa compared to other refrigerants. It is important to note that the pressure at the triple point is higher than the pressure of normal atmosphere, which is at about 101.325 kPa. This means that at atmospheric pressure, CO2 is present as dry ice or gas as it sublimates at -78°C.

The critical point on the other hand is low at 31°C at 7,383 kPa.

The low critical temperature requires very low condensing temperatures during subcritical operation. Ambient temperatures above 20°C require transcritical operation and a gas cooler, because the condensing temperature has to be at least 10K above the ambient temperature.

Most graphs therefore show the regions above the triple point, as shown here for CO2.

Subcritical and transcritical

Differentiation between “subcritical” and “transcritical” operation of CO2 (R744) refrigerating systems:

• Subcritical operation means that the refrigerant circuit operation takes place below the critical point
• Transcritical means that operation is above the critical point

The critical point is the thermodynamic property and varies depending on the type of refrigerant. For the refrigerant CO2 (R744), the critical point is at a temperature of approx. 31°C. Below this temperature (i.e. subcritical operation) the usual vapour compression process with evaporation and liquefaction takes place.

Above the critical temperature of 31°C, liquefaction of the CO2 is no longer possible. Instead of a condenser in the standard process, the supercritical (transcritical) process has a gas cooler.

Cayla, let us look at the expected pressures found in the various systems:

High pressure

• Subcritical system: up to 45 bar (4,500 kPa)
• Transcritical system: up to 120 bar (12,000 kPa)
• Medium pressure (of a)
• Transcritical system: up to 90 bar (9,000 kPa)

Low pressure

• Subcritical system:4 to 35 bar
• Transcritical system
• With medium pressure receiver: up to 35 bar (3,500 kPa)
• Without medium pressure receiver: up to 90 bar (9,000 kPa)

Cayla, as you can see there are differences to be found when dealing with CO2 systems. In the next issue I will explain more about the workings of CO2 systems.

Reference:

1. ACRA
2. The DeutscheGesellschaft für Internationale ZusammenarbeitGmbH,
3. ASHRAE
4. Carrier
5. A-Gas