Velocity Compounding of Impulse Turbine: The velocity compounded impulse turbine has moving and fixed blades. The moving blades are keyed to turbine shaft and fixed blades are fitted to casing. The high pressure steam from boiler is expander in nozzle where pressure energy is converted into kinetic energy.
The high velocity steam is directed on first set of moving blades and as steam flows over the blade it imparts some of its momentum to blades and loses some velocity. Some part of high K.E is absorbed by blades and there is no change in velocity of steam as it passes through fixed blades. The steam then goes to next set of moving blades and this process is repeated until all the energy of steam is absorbed. The figure below shows the velocity compounding of impulse turbine.
Pressure Compounding of Impulse Turbine(Rateau turbine):
This is used to solve the problem of high blade velocity in the single-stage impulse turbine.
It consists of alternate rings of nozzles and turbine blades. The nozzles are fitted to the casing and the blades are keyed to the turbine shaft. In this type of compounding, steam is expanded more than once(as in velocity compounding).