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Critical point: The phase boundary between liquid and vapour / gas terminates in the critical point. The liquid and gaseous phases become indistinguishable. Both phases have the same density and cannot be further compressed. Condensation is not possible.
TYPICAL PHASE DIAGRAM
Traditionally only parts of this graph are used to describe the vapour compression cycle. The relevant regions are those that show liquid( yellow), vapour( white) and liquid / vapour( blue), as refrigerants cannot be used in solid state in the vapour compression cycle. A full version is shown below. While the temperature is not marked on the axis, the triple point is represented by the red dashed line. Below this line is a region where the solid phase and the gaseous phase exist simultaneously in equilibrium as shown in the green region.
If the state of carbon dioxide changes resulting in the crossing of the dashed line, dry ice forms.
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.
Schematic representation of a full pressure enthalpy diagram
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.
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RACA Journal I August 2025 www. refrigerationandaircon. co. za