Velocity Compounded (Curtis)
For a two stage system U/Ci = 1/4, for a three stage system U/Ci = 1/6There is no pressure drop except in the nozzle ( although in practice some drop occurs due to losses as the steam passes over the blade). Dividing the velocity drop across the stages leads to a loss of efficiency but gives a more acceptable blade speed reducing centrifugal stress and simplifying gearing arrangement.
For a three row system, the steam speed at inlet to the first row is 6 times the blade speed, reducing the velocity makes the conditions at the final stages close to ideal.
To maintain the same mass flow for the reducing velocity, blade height is increased to the limit of centrifugal forces. Taper-twisting and flattening of the blade angle is then given to the final stage blades.
Some reheating occurs due to friction of the fixed blades associated with a loss of velocity of about 12%
Theoretically efficiency is independent of the row number. However in practice efficiency and work done in final stages reduces and therefore overall efficiency drops with increase rows.
- Typical values for efficiency are
- two wheel curtis 68%
- three wheel curtis 50%
- Single wheel rateau 85%
Pressure-Velocity Compound
This system gives the advantage of producing a shortened rotor compared to pure velocity compounding. In addition it also removes the problem of very high inlet steam velocities and the reduction in efficiency and work done in the final stages.In this design steam velocity at exit to the nozzles is kept reasonable and thus the blade speed (hence rotor rpm) reduced.
Typical applications are large astern turbines