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Support
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.
WELCOME TO THE SOLUTIONS PAGE
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.
This is a continuation of a question asked by Cayla in RACA Journal August issue: Grant, could you please look into the use of R744 and how these systems work. It is quite difficult to understand the operation of these systems. Also, a particular concern is the high pressures. Thank you.
Hi Cayla, yes I can help. In the last RACA issue we looked into CO2 as a refrigerant as well as the critical and triple point. We began looking at subcritical and transcritical systems and what is meant by these terms. Let us continue on this subject.
The‘ critical’ point is a thermodynamic property that varies depending on the type of refrigerant. For the refrigerant CO2( R744), the critical point is at a temperature of approximately 31 ° C. Below this temperature( i. e. subcritical operation) the usual vapour compression process where 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. In the chart below the bottom horizontal line represents the evaporation( point 6 to 7) in an air cooler or chiller and the superheating in the suction line( point 7 to 1). The line on the right represents the compression( point 1 to 2). The upper horizontal line represents the de-superheating of the discharge gas in the discharge line and the condensing and subcooling in the condenser( point 2 to 5). The left vertical line represents the process in the expansion valve or high-pressure valve.
All images by ACRA
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.
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RACA Journal I September 2025 www. refrigerationandaircon. co. za