Power Plant Engineering

STEAM TURBINE 205

In the first set of nozzles, there is some decrease in pressure which gives some kinetic energy to
the steam and there is no drop in pressure in the two rows of moving blades of the first wheel and in the
first row of fixed blades. Only, there is a velocity drop in moving blades though there is also a slight drop
in velocity due to friction in the fixed blades. In second set of nozzles, the remaining pressure drop takes
place but the velocity here increases and the drop in velocity takes place in the moving blades of the
second wheel or rotor. Compared to the pressure-com-pounded impulse turbine this arrangement was
more popular due to its simple construction. It is, however, very rarely used now due to its low efficiency.

6.8 Impulse-Reaction Turbine

As the name implies this type of turbine utilizes the principle of im-pulse and reaction both. Such
a type of turbine is diagrammatically shown. There are a number of rows of moving blades attached to
the rotor and an equal number of fixed blades attached to the casing.

Boiler

steam

Nozzle

Exhaust

Shaft

MGMNMGMN

Steam chest pressure

Initial steam velocity

Lost velocityCondenser pressure

Fig. 6.10. Impulse Reaction Turbine.
In this type of turbine, the fixed blades which are set in a reversed manner compared to the
moving blades, corresponds to nozzles mentioned in connection with the impulse turbine. Due to the
row of fixed blades at the entrance, instead of the nozzles, steam is admitted for the whole circumfer-
ence and hence there is all-round or complete admission. In passing through the first row of fixed
blades, the steam undergoes a small drop in pressure and hence its velocity somewhat increases. After
this it then enters the first row of moving blades and just as in the impulse turbine, it suffers a change in
direction and therefore in momentum. This momentum gives rise to an impulse on the blades.

But in this type of turbine, the passage of the moving blades is so designed (converging) that
there is a small drop in pressure of steam in the moving blades which results in a increase in kinetic
energy of steam. This kinetic energy gives rise to reaction in the direction opposite to that of added
velocity. Thus, the gross propelling force or driving force is the vector sum of impulse and reaction
forces. Commonly, this type of turbine is called Reaction Turbine. It is obvious from the Fig. 6.10 that
there is a gradual drop in pressure in both moving blades and fixed blades.