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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 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 this subject by looking at types of systems.
There are many configurations, but I will try to look at the more common systems.
SUBCRITICAL SYSTEMS – CASCADE A subcritical cascade is a two-stage system with two refrigerants. The low-pressure or first stage is filled with R744 and the highpressure second stage is filled with another second refrigerant, for example, propane, ammonia, HFC or HFO.
Both stages are coupled with a heat exchanger. On the
respective systems we find on the first stage subcritical CO2 condenser and on the second stage an evaporator.
The subcritical CO2 cycle is similar to other refrigerant cycles, except a situation which occurs when the system cycles. The CO2 must be continuously subcooled, and to achieve this, a small plant with a secondary refrigerant is used. If there is no off cycle, cooling the low-pressure stage requires a pressure resistance up to 90 bar( 9 000kPa).
In a cascade the first stage with the lowest evaporation pressure the vapour of the refrigerant condenses in a heat exchanger, which is the evaporator of the next stage. With the refrigerant R744, normally only two stages are built. In the second stage, the refrigerant evaporates 3K or 4K below the condensing temperature of the first stage. For example, if the condensing temperature of R744 equals 20 ° C, then the evaporating temperature of R290 equals 16 ° C.
This system is used in supermarkets and cold rooms. Today, the second stage is often charged with a HFC, R717( NH3) or hydrocarbons( e. g. R290).
FLOODED SYSTEMS In flooded systems, the refrigerant does not evaporate completely. Generally, the vapour quantity is between 25 %– 50 %. This mixture can flow in the piping. There are two kinds of flow generation: liquid pump or gravity. Both systems need a receiver, which at the same time, is a separator of the vapour. There is no superheating in the evaporator.
TRANSCRITICAL SYSTEMS The transcritical system operates at higher pressure and some designs are as follows:
Simple single stage: This is a simple system. There is only one regulation valve-Q2, the high-pressure valve. This regulation valve ensures the optimal high pressure in the gas cooler-E2. Before this valve, only supercritical gas exists in the high-pressure side of the system.
The CO2 is expanded in this valve, therefore, behind this valve subcooled liquid and flash gas exists. The liquid absorbs the heat input of the evaporator-E1 and evaporates. The gas is suctioned and compressed from the transcritical compressor-T. The low-pressure receiver is placed in the suction line. This system is suitable for small plants and for training units.
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RACA Journal I October 2025 www. refrigerationandaircon. co. za