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5.3.3 Hydrocarbons Hydrocarbons are not widely used in industrial refrigeration except where the additional safety measures required to ensure that leaking refrigerant cannot be ignited are required anyway, for example in a petrochemical plant. They offer excellent efficiency, and compatibility with most materials and lubricants. However the precautions required to prevent ignition are significantly more expensive than those required for R-717 systems, although the whole system cost may be comparable. HC- 290 is generally similar in performance to HCFC-22 and R-717 in terms of operating temperatures and pressures, and requires similarly sized compressors. 5.3.4 R-744 (carbon dioxide) R-744 cannot be used in exactly the same way as other industrial refrigerants. It needs to be coupled with a higher temperature refrigerant in a cascade system due to the low critical temperature of 31oC or else used in a transcritical system. Transcritical systems have been used in commercial and small systems, but there are no compressors on the market to provide the necessary high operating pressures to run an industrial R-744 system in this way. A medium- sized distribution centre with an installed capacity of 1500 kW has been operating in Denmark since 2008, using multiple commercial-sized compressors (Madsen, 2009). R-744 is particularly suitable for use in freezer systems because it is liquid at positive pressure down to -56oC and the gas is extremely dense, giving very high rates of heat transfer. The pressure drop characteristics are also very favourable at low temperatures, so R-744 freezer systems have been found to be significantly more efficient than any other alternative, even R-717 (Pearson, 2009). In slightly higher temperature applications, such as cold storage, R-744 cascade systems are likely to be slightly less efficient than two-stage R-717, but still on a par with a single stage economised system, and more efficient than any system using a secondary fluid due to the much lower pumping cost for R-744 compared to glycol, brine or other heat transfer fluids (ASHRAE, 2010). In Article 5 countries, Saudi Arabia and Jordan (HAT countries) have cold store and processing applications where cascade systems utilise R-744 and R-717 as a working pair. In higher temperature applications, for example IT cooling, R-744 is attractive as an alternative to chilled water because it is electrically non-conductive, does not cause fabric damage in the event of a small leak and enables use of smaller heat exchangers. The major challenge in these systems is that the operating pressure is approximately 50 bar. 5.3.5 R-718 (water) Water and air are the two most used secondary refrigerants. In general R-718 is not suitable as a primary refrigerant for most industrial applications because the triple point is very slightly above 0oC. The advantage of R-718 as a primary refrigerant is the very high latent heat, and the disadvantage is the swept volume required for a typical cooling duty is extremely high. There are a few notable exceptions: R-718 has been used for a few deep mine cooling projects where a vacuum system is used to create a mix of solid and liquid water (ice slurry) at the triple point. Similar systems have been used for large plastics moulding coolers. Ice slurry is used routinely for fish cooling onboard larger fishing vessels. Heat pumps based on R-718 are also emerging the market (Larminat, 2014). The advantage of water is the high critical point, which is much higher than any other refrigerant. Sakamoto (2017) shows a water vapour chiller that can match the capacity of a HFC based chiller and reduce the primary CO2 emissions by 17%, claimed to better efficiency. Also Madsboell (2016) shows good results with water as refrigerant for positive temperatures. These water based chillers can be used 110 2018 TOC Refrigeration, A/C and Heat Pumps Assessment ReportPDF Image | Heat Pumps Technical Options
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