Optimized selection of one- and two-stage ejectors under design and off-design conditions

Abstract

To provide guidance for the optimized selection of one- and two-stage ejectors, the efficiencies of these two ejectors are compared under design and off-design conditions based on a verified mathematic model. Under design condition, the one-stage ejector presents a higher efficiency at low compression ratios but the two-stage ejector is superior at high compression ratios. The superiority of the two-stage ejector is more obvious at low expansion ratios and at higher compression ratio value, due to lesser mixing loss and more boosting loss in the mixing chamber, respectively. The compression ratio of the equivalent entrainment ratio should be defined as the diacritical point between the high and low expansion ratio, whose value is approximately 1.73 for the R141b ejector. The inevitable loss of mechanical energy in the diffuser causes the crossed efficiencies of the one- and two-stage ejectors. Compared with the one-stage ejector, the two-stage ejector possesses more slow change of efficiency under off-design conditions due to a lesser primary vapor passing through the first stage ejector, obtaining the ability to resist the sharpdeterioration in efficiency at single chocking mode. For the typical application of ejector refrigeration system with R141b as the working fluid, it would be best to adopt the two-stage ejector.