STEAM TURBINES

Turbines

• We shall consider steam as the working fluid
• Single stage or Multistage
• Axial or Radial turbines
• Atmospheric discharge or discharge below atmosphere in condenser
• Impulse/and Reaction turbine

Impulse Turbines

Impulse turbines (single-rotor or multirotor) are simple stages of the turbines. Here the impulse blades are attached to the shaft. Impulse blades can be recognized by their shape. They are usually symmetrical and have entrance and exit angles respectively, around 20 ° . Because they are usually used in the entrance high-pressure stages of a steam turbine, when the specific volume of steam is low and requires much smaller flow than at lower pressures, the impulse blades are short and have constant cross sections.

The Single-Stage Impulse Turbine

The single-stage impulse turbine is also called the de Laval turbine after its inventor. The turbine consists of a single rotor to which impulse blades are attached. The steam is fed through one or several convergent-divergent nozzles which do not extend completely around the circumference of the rotor, so that only part of the blades is impinged upon by the steam at any one time. The nozzles also allow governing of the turbine by shutting off one or more them.

The velocity diagram for a single-stage impulse has been shown in Fig. 22.1. Figure 22.2 shows the velocity diagram indicating the flow through the turbine blades.

 


Figure 22.1   Schematic diagram of an Impulse Trubine

and = Inlet and outlet absolute velocity

and = Inlet and outlet relative velocity (Velocity relative to the rotor blades.)

U = mean blade speed

= nozzle angle, = absolute fluid angle at outlet

It is to be mentioned that all angles are with respect to the tangential velocity ( in the direction of U )

and = Tangential or whirl component of absolute velocity at inlet and outlet

and = Axial component of velocity at inlet and outlet

Tangential force on a blade, 

	(22.1)

(mass flow rate X change in velocity in tangential direction)

or,

	(22.2)

Power developed =	(22.3)

Blade efficiency or Diagram efficiency or Utilization factor is given by

or,

	(22.4)