Classification Of Steam Nozzle

A nozzle is often a pipe or tube of varying cross sectional area, and it can be used to direct or modify the flow of a fluid (liquid or gases). Nozzles are frequently used to control the rate of flow, speed, direction, mass, shape, and/or the pressure of the stream that emerges from them. And the Steam nozzle is a passage of varying cross-section, which converts heat energy of steam into kinetic energy. The main use of a steam nozzle is in steam turbines, which produces a jet of steam with a high velocity.

The phenomenon of super saturation occurs in the steam flow through nozzles. This is because of the time lag in the condensation of the steam during the expansion. The small section of the nozzle is called “throat”. It may be noted that the steam enters the nozzle with a high pressure and negligible velocity. But leaves the nozzle with a high velocity and small pressure. The pressure, at which the steam leaves the nozzle, is known as “Back pressure”. Moreover, no heat is supplied or rejected by the steam during flow through a nozzle. Therefore it is considered as adiabatic flow, and the corresponding expansion is considered as an adiabatic expansion. The expansion of steam in a nozzle follows the “Rankine cycle”.

The follows are the three types of nozzle :

1. Convergent nozzle :

When the cross-section of a nozzle decreases continuously from entrance to exit, it is called “convergent nozzle". It is used in a case where the back pressure is equal to or greater than the critical pressure ratio.

2. Divergent nozzle :

When the cross-section of a nozzle increases continuously from entrance to exit, it is called “divergent nozzle”.It is used in a case where the back pressure is less than the critical pressure ratio. 

3. Convergent-divergent nozzle :

When the cross-section of a nozzle first decreases from its entrance to throat, and then increases from its throat to exit, it is called “convergent-divergent nozzle”. This type of nozzle is widely used these days in various types of steam turbines.

Nozzle Efficiency :

When the steam flows through a nozzle, some loss in its total heat takes place due to friction b/w the nozzle surface and the flowing steam. The effect of friction is to increase the dryness fraction of steam. This due to the fact that the energy lost in friction is transferred into heat, which tends to dry or super heat the steam. The “nozzle efficiency or the coefficient of nozzle” is defined as the ratio of useful heat drop to the isentropic heat drop.
Mathematically, nozzle efficiency, 
k=Useful of heat/Isentropic heat drop


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