Meghe Group of Institutions Department for Technology Enhanced Learning 1UNIT IV

Department of Mechanical & Engineering VI SEM ENERGY CONVERSION-1 UNIT- IV DTEL 2UNIT II Steam Nozzles & Steam Turbine

Syllabus 3UNIT IV Steam Nozzles : Adiabatic Expansion in Nozzles, Maximum Discharge Critical Pressure Ratio and effects of Friction, Calculation of Throat and Exit Areas, Supersa Flow, Wilson Line. Steam Turbines Principals of Working of Steam Turbines, Classification of Steam Turbines, Comparison of Impulse and Reaction Turbines, Compounding of Steam Turbines.

Learning Objectives To study the Adiabatic Expansion in Nozzles, What should be Maximum Discharge Critical Pressure Ratio and effects of Friction, To Calculation of Throat and Exit Areas, Introduction to Supersa Flow, Wilson Line. 4UNIT IV

Learning Objectives Steam Turbines Principals of Working of Steam Turbines, Classification of Steam Turbines, Comparison of Impulse and Reaction Turbines, Compounding of Steam Turbines. 5UNIT IV

Steam Nozzles A steam nozzle may be defined as a passage of varying cross-section, through which heat energy of steam is converted to kinetic energy. UNIT IV6

TYPES OF NOZZLE There are three types of nozzles: Convergent nozzle Convergent-Divergent nozzle Divergent nozzle UNIT IV7

Cross-section of a nozzle tapers to a smaller section i.e. throat. UNIT IV8 Convergent nozzle Convergent Nozzle

Divergent nozzle The nozzle in which the cross-sectional area of the nozzle increases continuously from the inlet to exit, then this type of nozzle is called as divergent nozzle. UNIT IV9 Divergent Nozzle

Cross-section of a nozzle at first tapers to throat and then it diverges to a large diameter. UNIT IV10 Convergent-divergent nozzle Convergent-Divergent Nozzle

Adiabatic Expansion in Nozzles UNIT IV11 CONVERGENT THROAT DIVERGENT s2 s1=s2 No heat No work done

UNIT IV12 Expression for the velocity of steam at exit of the nozzle: The expansion of steam through the nozzle is considered as isentropic, hence Q = 0, and W = 0, gz 1 and gz 2 = 0 Consider a nozzle as shown in figure. Applying steady flow energy equation to the nozzle,

UNIT IV13 The velocity at the inlet of nozzle is very small as compared to outlet, hence it can be neglected. Where, and are the enthalpies of the steam at the inlet and outlet in J/kg respectively.

Condition for Maximum Discharge UNIT IV14 Where, m, Discharge A, C/S Area V1,Inlet Velocity

Critical Pressure ratio UNIT IV15 Its value is 0.58 when steam is saturated and when steam is super heated as n for as for steam is saturated n= super heated n= 1.3

WILSON LINE And METASTABLE EXPANSION UNIT IV16

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Steam Turbine A steam turbine is a thermo-mechanical device that extracts thermal energy from pressurized steam, and converts it into rotary motion. UNIT IV18

Steam Turbines A steam turbine is mainly used as an ideal prime mover in which heat energy is transformed into mechanical energy in the form of rotary motion. A steam turbine is used in 1.Electric power generation in thermal power plants. 2.Steam power plants. 3.To propel the ships, submarines. In steam turbines, the heat energy of the steam is first converted into kinetic (velocity) energy which in turn is transformed into mechanical energy of rotation and then drives the generator for the power generation. UNIT IV19

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Steam Steam is vapourized water. It is a transparent gas. At standard temperature and pressure, pure steam (unmixed with air, but in equilibrium with liquid water) occupies about 1,600 times the volume of an equal mass of liquid water. Saturated steam is steam at equilibrium with liquid water at the same pressure and temperature. Superheated steam is steam at a temperature higher than its boiling point at a given pressure UNIT IV21

Rankine Cycle There are four processes in the Rankine cycle, these states are identified by number in the diagram to the right. Process 1-2: The working fluid is pumped from low to high pressure, as the fluid is a liquid at this stage the pump requires little input energy. Process 2-3: The high pressure liquid enters a boiler where it is heated at constant pressure by an external heat source to become dry saturated vapour. UNIT IV22

Process 3-4: The dry saturated vapour expands through a turbine, generating power. This decreases the temperature and pressure of the vapour, and some condensation may occur. Process 4-1: The wet vapour then enters a condenser where it is condensed at a constant pressure and temperature to become a saturated liquid. The pressure and temperature of the condenser is fixed by the temperature of the cooling coils as the fluid is undergoing a phase-change. UNIT IV23 Rankine Cycle

Expansion Phase Steam travels down main steam piping Turbines convert thermal energy -> mechanical energy (nozzles) and then work (blading) -> turn rotor/shaft Pressure drops as steam goes through Work performed on turbine blading – Main Engines (ME) -> propulsion – Ship’s Service Turbine Generators (SSTG) -> electricity UNIT IV24

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T- s Diagram UNIT IV26

How does the steam turbine work? Impulse stage – whole pressure drop in nozzle (whole enthalpy drop is changed into kinetic energy in the nozzle) Reaction stage – pressure drop both in stationary blades and in rotary blades (enthalpy drop changed into kinetic energy both in stationary blades and in the moving blades in rotor) UNIT IV27

Impulse Turbines An impulse turbine has fixed nozzles that orient the steam flow into high speed jets. These jets contain significant kinetic energy, which the rotor blades, shaped like buckets, convert into shaft rotation as the steam jet changes direction. A pressure drop occurs across only the stationary blades, with a net increase in steam velocity across the stage. UNIT IV28

Impulse & Reaction Turbines UNIT IV29

Reaction Turbine In the reaction turbine, the rotor blades themselves are arranged to form convergent nozzles. Turbine also makes use of the reaction force produced as the steam accelerates through the nozzles formed by the rotor. A pressure drop occurs across both the stator and the rotor, with steam accelerating and decelerating. no net change in steam velocity across the stage but with a decrease in both pressure and temperature, UNIT IV30

Working of Steam Turbine Based Power Plant UNIT IV31

Working Steam is generated in steam generator and supplied to steam turbine at high pressure. (Usually sub critical pressure) This steam enters the high pressure rotor and expands, to produce work. Low pressure turbine stage is provided to extract more amount of work from the steam leaving into the condenser. UNIT IV 32

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Pressure-velocity changes over Impulse steam turbine NOZZLE EXHAUST STEAM TURBINE SHAFT MOVING BLADES HIGH PRESSURE STEAM Schematic of Impulse Turbine VLVL PHPH Q PLPL VHVH R C B Nozzle Rotor Blades Velocity Variation Pressure Variation Pressure-Velocity diagram in Impulse Turbine A P UNIT IV38

Reaction steam Turbine Principle of working - In this type of turbine, the high pressure steam does not initially expand in the nozzle as in the case of impulse turbine, but instead directly passes onto the moving blades. UNIT IV39

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Forces acting on a reaction blade Reaction force: is due to the change in momentum relative velocity of the steam while passing over the blade passage. Centrifugal force: is the force acting on the blade due to change in radius of steam entering and leaving the turbine. Resultant force: is the resultant of Reaction force and Centrifugal force. UNIT IV41

Pressure-Velocity change in reaction turbine Fixed Blade Moving Blade UNIT IV42

Impulse Turbine Reaction Turbine The steam expands (pressure drops) completely in nozzles or in the fixed blades The steam expands both in the fixed and moving blades continuously as it flows over them The blades have symmetrical profile of uniform section The blades have converging (aerofoil) profile The steam while passing over the blades remains constant The steam pressure while passing over the blades remains constant The steam pressure while passing over the blades gradually drops Because of large initial pressure drop, the steam and turbine speeds are very high Because of gradual pressure drop, the steam and turbine speeds are low The nozzles are fitted to the diaphragm (the partition disc between the stages of the turbine) The fixed blades attached to the casing serve as nozzles Difference between Impulse & Reaction Turbines UNIT IV43

Impulse TurbineReaction Turbine Power is obtained only due to the impulsive force of the incoming steam Power is obtained due to impulsive force of incoming steam as well as reaction of exit steam Suitable for small capacity of power generation & occupies less space per unit power Suitable for medium & high capacity power generation and occupies more space per unit power Efficiency is lesser Efficiency is higher Compounding is necessary to reduce speed Compounding is not necessary UNIT IV44

Compounding of Impulse Turbines As the complete expansion of steam takes in one stage (i.e., the entire pressure drop from high pressure to low pressure takes place in only one set of nozzles), the turbine rotor rotates at very high speed of about 30,000 rpm (K.E. is fully absorbed). High speed poses number of technical difficulties like destruction of machine by the large centrifugal forces developed, increase in vibrations, quick overheating of blades, impossibility of direct coupling to other machines, etc. To overcome the above difficulties, the expansion of steam is performed in several stages. UNIT IV45

Utilization of the high pressure energy of the steam by expanding it in successive stages is called Compounding. Methods of Compounding: Velocity compounding (Curtis Impulse Turbine) Pressure compounding Pressure-velocity compounding UNIT IV46

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Velocity Compounding (Curtis Impulse Turbine) N – Nozzle M – Moving Blade F – Fixed Blade UNIT IV48

Pressure compounding Consists of two stage of nozzles followed by two rows of moving blades. Consists of two stage of nozzles followed by two rows of moving blades. UNIT IV49

Pressure Compounding UNIT IV50

Pressure-Velocity Compounding (Combined Impulse Turbine) Total pressure drop is divided into two stages & the total velocity obtained in each stage is also compounded. A – Axial clearance, N – Nozzle, M – Moving Blade, F – Fixed Blade P i and P e – Pressure at inlet & exit, V i and V e - Velocity at inlet & exit UNIT IV51

Advantages of turbines Large power achieved by relatively small size High efficiency Simple design High revolution UNIT IV52

Summary Nozzle is a critical device which increase the velocity of steam to rotate turbine by striking on turbine blade It also govering the turbine rotation. 53UNIT IV

Summary UNIT IV54

 Thermal Engineering by P.L. Ballaney  Thermal Engineering by Mathur & Mehtra  Thermal Engineering by Vasaudani & Kumar  Power Plant Engineering by V.M. Domkundwar  Thermal Engineering by R.K. Rajput UNIT II55 Citations/References

Thank You 56UNIT IV