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Statement (I): The Supersaturated expansion in nozzle is a process in which the steam expands beyond the saturated vapour line superheated condition. Statement (II): Steam cannot exist in superheated state when the expansion process in nozzle reaches the saturated vapour line

Statement (I): The Supersaturated expansion in nozzle is a process in which the steam expands beyond the saturated vapour line superheated condition. Statement (II): Steam cannot exist in superheated state when the expansion process in nozzle reaches the saturated vapour line Correct Answer Statement (I) is true but Statement (II) false

Explanation:

Supersaturated Expansion in nozzle:

Steam flow through the nozzle may be so fast that sufficient time is not available for heat transfer to take place and so the phase change lags behind the expansion. Generally, time available may be of the order of 10-2 seconds for steam to flow through nozzle along with its condensation. 

The supersaturation phenomenon occurs in the nozzle due to delay in condensation as a result of which mass flow rate coming from the nozzle is more than the designed value.

  • The expansion of the superheated steam from pressure P1 to PB is represented by the line AB.
  • The change of phase will start at pressure P2 where the expansion line meets the saturation line at point C.
  • But the condensation does not occur at point C, as the time available is very small (about 0.001s) due to the high velocity of the steam passing through the nozzle. Thus this phenomenon is delayed and the vapor continues to expand in dry condition even beyond point C. This is presented by CC1 and the condensation is suppressed.
  • The vapor between the pressure P2 and P3 is said to be supersaturated or supercooled and this type of flow in nozzles is known as the supersaturated or metastable flow of steam.

Therefore statement II) is incorrect as the Steam can exist in superheated state when the expansion process in nozzle reaches the saturated vapour line.

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