Introduction to Turbines & Steam Turbines - Types of Steam Turbines PDF Print E-mail
Written by Norrie   
Wednesday, 13 January 2010 10:06
Article Index
Introduction to Turbines & Steam Turbines
Steam Turbines
Steam Control to a Turbine
Types of Steam Turbines
Removal of Non-Condensibles
Turbine Seal-Steam System
All Pages

 

TYPES OF STEAM TURBINE

(Figure. 10)
For any steam turbine to operate, a pressure difference must exist between the steam supply and the exhaust. Where the exhaust steam is above atmospheric pressure, the turbine is classed as a ‘Back Pressure Turbine' or 'Non-Condensing Turbine'.
I. BACK-PRESSURE STEAM TURBINES
As an example, taking a 600 Psi steam supply to a turbine, the turbine speed is controlled by the steam input. If we have an exhaust pressure of say 125 Psi (a D.P. of 475 Psi), this exhaust steam will still contain a lot of heat and pressure energy and may be used to drive other smaller turbines and for heating purposes in re-boilers, heaters, vaporisers...etc. In this type of turbine, the exhaust must be maintained at a constant pressure by a PCV control system downstream of the turbine exhaust to prevent changes in the exhaust pressure that would affect the turbine speed by changing the pressure drop across it. The governor would be fighting against these pressure fluctuations and speed control would be erratic.
II. CONDENSING STEAM TURBINES
In a condensing steam turbine, the maximum amount of energy is extracted from the steam. This is achieved by passing the exhaust steam into a condenser (called a Surface Condenser). The steam is condensed by surface contact with bundles of tubes through which cooling water is passing. As the steam condenses, its volume, on changing to water, decreases by about 1800 times. This great decrease in volume causes a vacuum to form in the condenser. Due to this, the pressure drop across the turbine and therefore the turbine power is maximised. The steam condensate (water) is level controlled in the condenser and pumped back to the steam generation plant. However, although the water for the steam generation is purified and treated, the steam will still contain some Non-condensibles. These will build up in the surface condenser and gradually destroy the vacuum, thereby decreasing the P.D. across the turbine and thus decreasing its efficiency and power. In order to maintain the vacuum, the non-condensibles must be removed from the surface condenser. This is carried out by a system of ‘STEAM EJECTORS' and ‘Ejector Condensers' which pull the gases from the surface condenser and eject them to the atmosphere.

TYPES OF STEAM TURBINES

Figure. 10



Last Updated on Wednesday, 24 February 2010 20:09