Power Plant Engineering

204 POWER PLANT ENGINEERING

Nozzle

Moving

blades

Re-entry

passage

Steam re-entry

Fig. 6.9. Flow of Steam on Blades.
The whole expansion of steam from the steam chest pressure to the exhaust pressure takes place
in the nozzles only. There is no drop in either in the moving blades or the fixed i.e. the pressure remains
constant in the blades as in the simple impulse turbine. The steam velocity from the exit of the nozzle is
very high as in the simple impulse turbine. Steam with this high velocity enters the first row of moving
blades and on passing through these blades, the Velocity slightly reduces i.e. the steam gives up a part of
its kinetic energy and reissues from this row of blades with a fairly high velocity. It then enters the first
row of guide blades which directs the steam to the second row of moving blades. Actually, there is a
slight drop in velocity in the fixed or guide blades due to friction. On passing through the second row of
moving blades some drop in velocity again occurs i.e. steam gives up another portion of its kinetic
energy to the rotor. After this, it is redirected again by the second row of guide lades to the third row of
moving blades where again some drop in velocity occurs and finally the steam leaves the wheel with a
certain velocity in a more or less axial direction. compared to the simple impulse turbine, the leaving
velocity is small and it is about 2 percent of initial total available energy of steam.

So we can say that this arrangement is nothing but splitting up the velocity gained from the exit
of the nozzles into many drops through several rows of moving blades and hence the name velocity-
compounded

This type of turbine is also termed as Curtis turbine. Due to its low efficiency the three row wheel
is used for driving small machines The two row wheel is more efficient than the three-row wheel.

velocity compounding is also possible with only one row of moving blades. The whole pressure
drop takes place in the nozzles and the high velocity steam passes through the moving blades into a
reversing chamber where the direction of the steam is changed and the same steam is arranged to pass
through the moving blade of the same rotor. So instead of using two or three rows of moving blades,
only one row is required to pass the steam again and again; thus in each pass velocity decreases.

6.7 Pressure and Velocity Compounded Impulse Turbine

This type of turbine is a combination of pressure and velocity compounding and is diagrammati-
cally. There are two wheels or rotors and on each, only two rows of moving blades are attached cause
two-row wheel are more efficient than three-row wheel. In each wheel or rotor, velocity drops i.e. drop
in velocity is achieved by many rows of moving blades hence it is velocity compounded. There are two
sets of nozzles in which whole pressure drop takes place i.e. whole pressure drop has been divided in
small drops, hence it is pressure-compounded.