Evaluation of sodium incipient superheat measurements with regard to the importance of various experimental and physical parameters

The published results of the various laboratories on the parameters having an influence on superheat are not definite. There has even been inconsistent and conflicting experimental evidence on the effect of some parameters.The most consistent agreement in the trend is seen in the influence of oxide impurity and gas entrainment. In both cases superheat decreases with increasing gas entrainment and oxide level.The most inconsistent and conflicting experimental evidence concerns the effect of heat flux and of temperature ramp. The data of bulk superheat as far as the trend is concerned are in good agreement. The bulk superheat definitely decreases with increasing velocity. An influence of the heating wall material on the superheat is uncertain. Experiments on materials compatible with sodium did not exhibit significant effects. Most of the experiments in which the influence of the operation time has been observed were short time tests. It is supposed that the effect of operating conditions was masked by the degassing of the heating surface during heating up or boiling. There is experimental evidence that for given test conditions a characteristic threshold superheat range is not exceeded. The threshold superheat level is presumably determined by the heating surface finishing, the oxide level and the gas loading of the sodium.A general evaluation of the results of loop experiments shows an average value of superheat for technically clean sodium of 80 ± 30°C (O₂-impurity of about 5–10 ppm) and for sodium with impurity of about 40 ppm of about 25 ± 20°C.