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The table highlights the following key points: • R744 compares reasonably well with the HFCs when subcritical and at low condensing temperatures (e.g., the LT comparison). But at higher condensing temperatures (MT example) and when transcritical (HT example), it does not compare well. • The high suction pressure and high gas density of R744 results in very good evaporator performance. In like-for-like systems the evaporator temperature of an R744 system would, in reality, be higher than for HFC systems. • The index of compression is very high for R744, so the discharge temperature is higher than for the HFCs. This can improve heat reclaim potential in retail systems, although the requirement for heat in the summer when the system is transcritical is limited. • The density of R744 results in very high volumetric capacity. This reduces the required compressor displacement (but not the motor size, which would be similar to that required for HFC refrigerants). • The required suction pipe cross section area is in proportion to the volumetric capacity. For R744 the diameter of the suction line is approximately half that required for R404A. • The compression ratio for R744 is less than for the HFCs. This can result in higher isentropic efficiency. Section 5. R744 hazards R744 is not flammable, but its high-pressures, toxicity at high concentration and potential for dry ice formation must be taken into account when applying and handling. This section explains some of the hazards and provides very general guidance on reducing them. More detailed information relating to the design of systems to minimise the hazards is provided later in this document. Asphyxiation R744 is odourless, heavier than air and is an asphyxiant. The practical limit1 of R744 is lower than HFCs because of its potential for high toxicity (HFCs are non toxic): Practical limit of R744, 0.1 kg/m3 (56.000 ppm); Practical limit of R404A, 0.48 kg/m3 (120.000 ppm) Note – The practical limit is defined in EN378 but may vary in regional regulations. The table below summarises the effect of CO2 at various concentrations in air. Table 4. Effects of CO2 at various concentrations in air ppm of CO2 Effects 370 Concentration in atmosphere 5,000 Long-term exposure limit (8 hours) 15,000 Short-term exposure limit (10 min) 30,000 Can be “tasted” 30,000 Discomfort, breathing, difficulties,headache, dizziness, etc. 100,000 Loss of consciousness, death 300,000 Quick death If a leak of R744 could result in a concentration exceeding the practical limit in an enclosed occupied space such as a cold room, precautions must be taken to prevent asphyxiation. These include the use of permanent leak detection which activates an alarm in the event of a leak. High-pressures System components, pipe work, tools and equipment must be rated for these pressures. It should be noted that the standstill pressure on some systems (e.g., cascade systems) is higher than the maximum rated pressure PS (hence the pressure-relief valve setting). The pressure-relief valve will discharge in the event of a fault such as a power failure. Table 5. R744 standstill and typical system operating pressures Standstill at 10°C ambient 44 bar g Standstill at 30°C ambient 71.1 bar g Low temperature evaporator (frozen food) 10 - 15 bar g High temperature evaporator (chilled food) 25 - 30 bar g Cascade condenser 30 - 35 bar g Cascade high-pressure cut out (high side) 36 bar g Cascade pressure-relief-valve (high side) 40 bar g Transcritical high side 90 bar g Transcritical high-pressure cut out (high side) 108 to 126 bar g Transcritical pressure-relief valve (high side) 120 to 140 bar g 1 EN378 Refrigerating systems and heat pumps – Safety and environmental requirements ISO 5149 mechanical refrigerating systems used for cooling and heating - Safety requirements. 9PDF Image | CO2 Product Guide 2021
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