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7th European Thermal-Sciences Conference (Eurotherm2016) IOP Publishing Journal of Physics: Conference Series 745 (2016) 032157 doi:10.1088/1742-6596/745/3/032157 service these units. For commercial refrigeration applications in warm climates, ejector systems enabling flooded evaporator operation all year long are commercially available. The first pilot systems applying additional compressor unloading by pre-compressing of medium pressure vapour from chilled food display cabinets, as shown in figure 3, have been successfully operated for several years in Switzerland. For traditional ejector cycles, where all the vapour from the evaporators has to be sucked by the compressor, proper lubricant return can be a challenge. However, this problem has been solved. Applying ejectors does not mean to neglect the traditional high side pressure control to achieve maximum energy efficiencies. The ejector(s) must be able to control the high side pressure actively. Similar strategies as controlling the cooling capacity by adjusting the compressor displacement by either applying a frequency converter and/or apply several compressors in parallel can be applied also with ejectors. The usage of an adjustable ejector with variable position needle to control effective nozzle size and high side pressure is one way. However, as with frequency-controlled compressors, the efficiency and operating envelope of the adjustable ejectors are limited and reduced in off-design conditions. The multi-ejector concept, applying several fixed motive nozzles in parallel with binary- scaled ejector capacities, represents an alternative solution. Since 2014 this concept has been implemented in several pilot supermarkets. The multi-ejector is an add-on enhancement technology especially for warm climates, denoted the third-generation CO2 supermarket refrigeration systems, with a simplified circuit shown on the right hand side of figure 2. It enables increasing the energy efficiency at high ambient temperatures compared to a state of the art CO2 booster system with parallel compression, denoted the second- generation CO2 supermarket refrigeration systems, central circuit in figure 2. The parallel compression booster system represents a development step towards higher energy efficiency compared to the traditional booster systems with flash gas by-pass, denoted the first-generation CO2 supermarket refrigeration systems, as shown on the left hand side in figure 2. The second and third generation are also opening for an efficient integration of the AC function often required for the buildings, even in the Northern part of Europe. However, in regions with very low ambient temperature during the winter season, it is advisable to split the gascooler, avoiding too low inlet temperatures to the high-pressure control devices. 4PDF Image | R744 ejector technology
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