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REAL Alternatives Module 2 Design Differences for Alternative Refrigerants | 15 2.4 Pressures Typical pressures in R744 systems are shown in table 2 below. Table 2, Typical R744 pressures Typical pressure Bar g (MPa) Setting of PRV in high side of transcritical system (i.e. PS) 120 (12) High side of transcritical system, operating above the critical point 90 (9) Intermediate pressure in a transcritical system 35 to 65 (3.5 to 6.5) Setting of PRV in high side of the low stage of a cascade system (i.e. PS) 40 (4) High side pressure in the low stage of a cascade system 30 (3) Low temperature (LT) evaporator 15 (1.5) High temperature (HT) evaporator 30 (3) Cylinder standing outside in an ambient of 5OC 40 (4) Plant at stand still in an ambient of 20OC 55 (5.5) The high pressure of R744 can lead to an increase in leakage with a consequent increase in energy consumption and indirect environmental impact. To minimise leak potential the pipe work and components must be suitable for the PS of that part of the system. In many cases this entails using different components compared to those used for HFC systems, and using pipe with a thicker wall, or using steel pipe work. Joints should be brazed or welded and mechanical joints should be avoided wherever possible. Components such as cascade heat exchangers could potentially operate with a high temperature difference between the inlet and outlet. This will cause thermal shock which leads to leakage so this should be taken into account when selecting the component. The temperature difference can be reduced by de‐superheating the gas before it reaches the condenser. Loss of refrigerant also occurs because of issues associated with pressure relief valves (PRVs). There should be sufficient difference between PS (and therefore the PRV vent pressure) and the typical operating pressure for that part of the system so that venting of R744 through PRVs is minimised. In many systems this is not the case, and even a small increase in the operating pressure causes the PRV to vent. This is made worse because R744 pressure can increase very rapidly, reaching the PRV vent pressure before the high pressure switch has operated and shut off the system (as with other systems the setting of the high pressure switch should not be more than 90% of PS). If multiple vents occur the PRV spring weakens and the vent pressure reduces, increasing the incidence of venting. In addition to this problem, leakage occurs if the PRV does not then re seat correctly, even after a single vent.PDF Image | Alternative Refrigerants
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