Supersaturated Steam

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Abstract
Supersaturation, or the failure of steam to condense when the saturated condition is reached in an expansion, has long been a matter of interest to engineers. This paper presents the account of an investigation of this phenomenon in which the principal objectives were the location of the Wilson line, which indicates the condition at which condensation actually occurs, and the measurement of the size of the drops which are formed when flowing steam condenses. Examination of the flow of low pressure steam through an illuminated nozzle fitted with a glass top revealed that the condensation process could be observed and that the pressure of the steam at the condensation point could be measured with the aid of a search tube. It was found that in a simple convergent-divergent nozzle condensation did not occur until the steam had reached the condition approximately represented by the 3.5 per cent moisture line on the Mollier chart. The droplets which formed at that condition were extremely minute, their radii being of the order of magnitude of 6.2 × 10−8 cm, and there was no evidence of growth during their passage through the nozzle. Additional experiments with nozzles of other designs led to the discovery that under certain conditions condensation could take place when the steam had reached the 2 per cent moisture condition, and that the droplets so formed grew rapidly from about 10.0 × 10−8 cm to about 6.0 × 10−5 cm in radius. It was concluded that supersaturation invariably occurred in the condensation of flowing steam, and that the superheated steam formula should be used to estimate the flow of saturated steam through nozzles.

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