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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 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.

 The following shows a transcritical R744 refrigeration cycle. The main difference lies in the upper horizontal line. The state at the beginning of the cycle is compressed gas. The gas is cooled down in the discharge line and in the gas cooler. Point 4 is the outlet of the gas cooler. There is no liquid. The gas is further cooled down in the line before it expands in the valve. The gas is flashing in a two-phase flow (liquid and flash gas), if it passes the high-pressure valve.

If the operation is above the critical point, the function of the condenser changes to that of a gas cooler and the systems must be designed for an operating pressure well above 100 bars. In this transcritical (supercritical) operation, CO2 can also be used for higher temperatures. It is then also suitable as a refrigerant for heat pumps.

Cayla, let us move on to the basic components of a CO2 system.

At the heart of every CO2 refrigeration system lies a series of carefully designed components, each with a specific function and purpose. From the compressor, where the refrigerant’s journey begins, to the high-pressure valve, the heart of the technology, these components work together seamlessly, creating a cycle that ensures efficient cooling while minimising environmental impact.

The higher gas density of R744 results in a higher volumetric refrigeration effect compared to all other refrigerants. This has an effect on compressor displacement and pipe sizing, evaporators and condensers.

The compressor

The compressor serves as the powerhouse of the refrigeration system. Basically, its primary function is to compress the low- pressure CO2 gas, elevating both its pressure and temperature. This compressed gas, now in a high-energy state, is the driving force that fuels the entire refrigeration cycle.

In a booster system, two suction groups are feeding the compressors:

• Medium-stage compressors process the medium temperature evaporation of the cooling loads, typically running around 28 bar, which corresponds to -10°C
• Low-stage compressors process the low temperature evaporation of the freezing loads, typically running around 13 bar, which corresponds to -32°C

The gas cooler/condenser

In a CO2 booster system, this equipment will work as a condenser when operating in subcritical mode and as a gas cooler, in the transcritical mode.

Inside the condenser, the high-pressure high-temperature CO2 gas is transformed. In this component, heat is released causing the gas to transition into a liquid state. This phase change is crucial, as it signifies the system’s ability to shed the absorbed heat, making the environment cooler.

Above the critical point (31°C; 7 370 kPa) CO2 cannot condense (gas cooler). The heat dissipation is no interrelationship of temperature and pressure. Heat dissipation implies that the temperature of the gas and the specific heat capacity is lowered continuously. In contrast the gas phase of condensing refrigerants has a low heat capacity and the liquid phase has a high heat capacity, while condensing the temperature is constant and the heat capacity rises.

Cayla, the ejector is an expansion device. It recovers energy from the compression work and uses it for a first compression stage. There are many various systems utilising ejectors.

The ejector works like a liquid pump. The motive liquid CO2 expands in a throttle. When the motive liquid expands, it is brought up to maximum velocity and high velocity pressure. The vapour from the evaporator flows into the suction chamber and then it is mixed with the liquid. In the diffusor the mixture retards to slow velocity and slow velocity pressure. Therefore, the pressure increases because the total pressure in a system is constant and the sum of pressure and velocity pressure (Bernoulli equation).

The function of the expansion valve is to regulate the flow of the high-pressure liquid CO2, causing it to undergo a rapid pressure drop. This drop results in a substantial decrease in temperature, turning the liquid into a mixture of liquid and vapour.

• In a booster system for example, there are four expansion valves:
• High-pressure valve: controls the pressure at the gas cooler/ condenser, decreasing the pressure from the high-pressure side (9 000 kPa) to the receiver pressure (3 800 kPa)
• Flash-gas bypass valve: controls the pressure at the receiver, decreasing the pressure down to the 3 800 kPa for medium temperature suction pressure
• Medium temperature expansion valve: responsible for the expansion of the liquid on the receiver from 3 800 to 2 700 kPa
• Low temperature expansion valve: responsible for the expansion of the liquid on the receiver from 3 800 to 1 300 kPa

In the evaporator, the low-pressure and low- temperature CO2 liquid-vapour mixture absorbs heat from its surroundings. This absorption causes the refrigerant to evaporate, transforming it back into a low-pressure gas. The cycle is complete, and the space around the evaporator becomes cooler. In a booster system there are the medium temperature evaporators and low temperature evaporators.

Cayla, hopefully this helps with your understanding of CO2 systems, components and operation.

References:

1. ACRA
2. The Deutsche Gesellschaft für Internationale Zusammenarwbeit GmbH,
3. ASHRAE
4. Carrier

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