Mechanical Engineering Science

CHAPTER 3 Steam Turbines

Prime Movers Prime Mover is a device, which converts the available natural source of energy into mechanical energy, to drive other machines. Prime mover can also be defined as a device in which the potential energy of steam is transformed into kinetic energy & later the kinetic energy is converted into mechanical energy in the form of rotary motion of the turbine.

Parts of the Turbine

1. Nozzle2. Rotor3. Blades 4. Shaft 5 25. Casing 4

1 3

Nozzles are used to convert the pressure energy of the steam to kinetic energy & they direct the steam to the blades.Blades are mounted on the circumference of the turbine rotor. The kinetic energy is converted into the rotary motion i.e. mechanical energy. Rotors are the circular discs on which the blades are mounted. Shaft is a metal rod on which the rotor is fixed. Casing is the one which encloses the entire set up.

NOZZLE Converges Diverges

Steam at high Steam at high pressure velocity Entry Exit

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Impulse Turbine

In an impulse turbine, the steam is initially expanded in a nozzle, so that high pressure steam is converted into low pressure steam. The high velocity jet of steam, coming out of the nozzle is made to impend on the blades or the curved vanes of the turbine. This in turn results in the motion of the rotor. The change in energy takes place as :Pressure energy Kinetic energy Mechanical energy ( rotary motion )

If the pressure is lowered in a single step & complete energy transformation ( shown in the previous page ) takes place from only one set or one row of blades, then the rotor of the turbine rotates at a very high speed. Such a high speed posses a no. of technical problems such as increase in vibrations, quick overheating of bearings, difficulty in lubrication, etc Hence, the expansion of steam is performed in several stages. The utilization of the high pressure energy of the steam by expanding it in successive stages is called as Compounding.

High velocity Low velocity steam steam

Resultant of all the centrifugal forces

curved vanes ( blades )

In an Impulse turbine, the Impulse force which is the propelling or the driving force is defined as the resultant of all the centrifugal forces as a result of the change in momentum due to the change in the direction of the steam flow. Example : Delaval turbine In the Pressure - Velocity graph, (next page) if the turbine is 100% efficient, Vin = Vout i.e. P & R will be in the same line. It is a straight line from B to C coz the pressure remains a constant, hypothetically, as the area b/w the blades is a constant. The velocity decreases from Q to R due to the conversion of kinetic energy to mechanical energy

Pressure Velocity diagram

Nozzle Moving blades Boiler pressure

Velocity P BRQAC

Reaction Turbine

In a Reaction turbine, energy transfer takes place across the rotating element due to both Static pressure energy change & Dynamic pressure energy change. In this type of turbine, the high pressure steam isnt passed through the nozzle, initially, as in the case of the Impulse turbine. But it directly passes onto the blades.

It contains 2 types of blades, namely Fixed blades & Moving blades. The blades are designed in such a way that when the steam passes over it, the steam experiences a same effect as in the nozzle. Hence, the pressure drops continuously as the steam passes over the set of blades causing simultaneous increase in the velocity of steam. The fixed blades act as nozzle & they direct the steam to the moving blades. The moving blades convert the kinetic energy to the mechanical energy & cause the motion of the turbines.

The fixed blades are mounted on the casing while the moving blades are mounted on the rotor. The blades are of Aerofoil type in shape. The moving blades also act as nozzles as the pressure is never constant when the steam passes over them coz the area b/w the blades isnt a constant. They are either of Converging or Diverging type.

Moving blade Resultant force (1) (2)

(1) Reaction force (2) Centrifugal force

The force due to the backward reaction to the force causing the motion of the jet is called the Reaction force. In addition to the reaction force, there is also a centrifugal force exerted by the steam due to the change in its momentum as a result of the change in its direction while passing over the blades. The propelling force in case of the Reaction turbines, is the resultant of the reaction force as well as the centrifugal force. Hence, it is more effective than the Impulse turbine.

Pressure Velocity diagram

Fixed blades Moving blades

Boiler pressure

Velocity P BRQAC

Pressure Velocity diagram( Multiple Fixed & Moving Blades ) F M F M Exit Velocity Exit PressureBoiler pressureInput Velocity

Differences