www. refrigerationandaircon. co. za RACA Journal I August 2025 39
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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.
Cayla asks: 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 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.
www. refrigerationandaircon. co. za RACA Journal I August 2025 39