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To ensure the pressure does not rise to the relief pressure in the event of such a fault, these systems can be fitted with a small auxiliary cooling system. This typically runs on an auxiliary (uninterruptable) power supply and will switch on when the pressure rises above a set point (this is lower than maximum allowable suction pressure PS, but higher than the normal operating pressure). The auxiliary cooling system is sized to remove sufficient heat to keep the standstill pressure below safe low side limit when there is no load on the system (apart from heat absorbed from the ambient). Care must be taken when charging R744 systems. The maximum operating pressure of some systems (such as cascade systems and parts of transcritical systems) is normally below the R744 cylinder pressure. These systems must be charged slowly and carefully to prevent pressurerelief valves discharging. Further information is given in Chapter 5. Trapped liquid The coefficient of expansion for R744 is significantly higher than for other refrigerants. The practical impact of this on liquid R744 trapped between closed valves is shown in the graph in figure 7: Figure 7. Relationship between temperature and pressure of trapped liquid R744. Source: Danish Technological Institute The example shows the effect of a 20K temperature rise on liquid that is trapped at an initial temperature of -10°C. The pressure will increase from 44 bar g to approximately 240 bar g. This condition could potentially occur in a liquid line of a cascade system, and similar situations can arise in other parts of the system and in other R744 systems. As a rule of thumb, trapped R744 liquid will increase in pressure by 10 bar for every 1K temperature increase. The pressure of trapped liquid refrigerant always increases, but the pressure increase of R744 is much greater than for other refrigerants. This is exacerbated by the potential to trap R744 at low temperatures (LT) and hence for the liquid temperature to rise more than for other refrigerants. Systems should be fitted with pressure-relief protection wherever liquid could be trapped, either during operation or service. Methods of providing this protection are covered in the section on design of R744 systems. Dry ice Dry ice (solid R744) is formed when R744 pressure and temperature is reduced to below the triple point (4.2 bar g, -56°C). This will not occur within a properly working refrigeration system, but can occur when: • A pressure-relief valve discharges if it is venting vapor R744 • Venting R744 during service (component change or replacement, for example) • Charging a system which is below 4.2 bar g (e.g., an evacuated system) Dry ice does not expand when it is formed, but dry ice will become gas as it absorbs heat (e.g., from ambient). If the dry ice is trapped within the system, it will absorb heat from the surroundings and turn into gas. This will result in a significant pressure increase. Dry ice can block vent lines, so care must be taken to ensure that this cannot occur: • Appropriate pressure-relief valves should be used – see the section on system design for more information about these and how safety valves should be applied; • When R744 is vented from a system during service it should be vented as a liquid, and the pressure in the system monitored. R744 should always be vented outside a building. Freeze burns Contact with solid or liquid R744 will cause freeze burns and should be avoided. Suitable gloves and goggles should always be worn when working with R744. Section 6. Comparison of R744 with other refrigerants The table below shows a simple comparison of R744 with other types of refrigerant, including those that are currently commonly used and those that are currently evaluated for future use. It uses a simple “traffic light” system and employs the common HFCs, such as R404A and R134a as a baseline. This provides a very simple introduction to the options – the situation varies globally, especially in the availability of refrigerants, components and expertise. For retail applications a well designed and installed R407A/F system generally has better efficiency than R744 systems. 10PDF Image | CO2 Product Guide 2021
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