INTRODUCTION

The steam nozzle is a passage of varying cross-section by means of which a part of the enthalpy of steam is converted into kinetic energy as the steam expands from a higher pressure to a lower pressure. Therefore, a nozzle is a device designed to increase the velocity of steam. Steam nozzles are of three types, namely convergent nozzle, divergent nozzle, and convergent–divergent nozzle. If the cross-section of the nozzle decreases continuously from the entrance to exit, then it is called a convergent nozzle, as shown in Fig. 6.1(a). If the cross-section increases, then it is called a divergent nozzle, as shown in Fig. 6.1(b). If the cross-section of the nozzle first decreases and then increases, it is called a convergent–divergent nozzle, as shown in Fig. 6.1(c). The least area of cross-section of the nozzle is called the throat. The divergent section has to be long as the divergent angle is limited to about 7° in order to prevent separation at the wall.

The main purpose of steam nozzles is to produce a high velocity jet of steam which is used in steam turbine injectors for pumping feed water into boilers and to maintain high vacuum in power plant condensers or steam jet refrigeration condensers.

Figure 6.1 Types of nozzles: (a) Convergent nozzle (b) Divergent nozzle (c) Convergent-Divergent nozzle