Monday, January 5, 2009
The Main Criteria of Curtis Wheel design is it manage to bring down steam velocity in lesser number of stages.
The Curtis stage (see Figure 2) is designed to be a power rotor, extracting a large amount of energy out of the steam.
As main steam enters the HP turbine, it first passes through the nozzle block.
The nozzle block contains the nozzles.
The velocity of steam is increased and the pressure is decreased as the steam passes through these nozzles.
On an impulse turbine, the only time a pressure drop occurs is when steam passes through a nozzle. After steam passes through the nozzles, it passes through the first set of moving blades. In the first set of moving blades, work is extracted from the steam causing the velocity to drop. After passing through the moving blades, the steam then passes through the non-moving blades. The only purpose the non-moving blades serve is to redirect steam from the first set of moving blades to the second set of moving blades. On an impulse turbine, non-moving blades do not have any effect on the pressure or the velocity of the steam passing through them. After leaving the non-moving blades the steam passes through another set of moving blades.
This setup of a nozzle followed by a set of moving blades, non-moving blades, and moving blades makes up a single Curtis stage.
After steam exits the nozzle there are no further pressure drops. However, across both sets of moving blades there is a velocity drop. This causes the Curtis stage to be classified as velocity compounded blading.