Principle of reaction turbine can well be explained from the following figure.
Figure shows a water sprinkler in gardens and is an arrangement wherein steam is passed into the drum supported on bearings. It comes out of the nozzle shaped spouts fitted on its periphery. Steam issues out in the form of jet. In both the cases illustrated above we find that the direction of rotation of shaft is opposite to the direction of jet. This is due to reactive force produced by the jet of fluid. At this instant phenomena of recoil of gun, jet propulsion etc., can be recalled. A reaction turbine is one in which the steam pressure decreases gradually while it flows over moving blades as well as through the fixed blades.
Fixed blades are also called guide blades. They direct or guide the path of steam flow on to the moving blades. The passage between a pair of guide blades serves the purpose of nozzle. The expansion of steam is adiabatic. Figure shows the variation of pressure and velocity of steam as it flows over the blades.
IMPULSE TURBINE Vs REACTION TURBINE :
1. In impulse turbine steam expands completely in the nozzles and flows over the blades at constant pressure. In reaction turbine steam expands partially in the nozzle and on the blades.
2. Relative velocity of steam to the blade remains almost constant or slightly reduces due to friction in impulse turbine. Due to continuous expansion of steam, the relative velocity at exit, in reaction turbine, slightly increases.
3. Impulse blades are symmetrical and are of plate or profile type. The area of blade channels is constant. Reaction turbine blades are asymmetrical and are of aerofoil section. The area of blade channels is varying (converging) type.
4. Blades of impulse turbine are in action only when they face the jet, but the blades of reaction turbine are in action all the time.
5. Impulse turbines have the same pressure on the sides of the rotor blades; different pressures exist on the two sides of moving blades of reaction turbine.
6. For a given power output, impulse turbine has less number of stages than the reaction turbine because of larger pressure drops in the latter.
7. Absolute velocity of steam and blade velocity, in an impulse turbine, are high as compared to those in reaction turbine.
8. De-Laval, Curtis, Rateau and Zoelly are examples of Impulse Turbine.
Hero and Parson Turbines are reaction turbines.
9. Impulse Turbines are suitable for small power requirements. Reaction turbines are suitable for medium and higher power requirements.
PARSON’S REACTION TURBINE:
Parson, an Irish engineer, invented the modern reaction turbine combining the principle of impulse as well as reaction. This differs from Hero's pure reaction turbine. It is, therefore, called impulse - reaction turbine. Parson's reaction turbine consists of a rotor to which rings of blades are mounted. These are moving blades. Fixed or guide blades are provided on the rings fitted internally to the stationary casing (Figure).
Superheated steam from the boiler enters the turbine and passes alternatively through the fixed and moving blades (Fig below).
There is a continuous fall of pressure as the steam expands through the blades. Volume of steam gradually increases and hence size of low pressure side (stage) should be bigger. It may be noted that the fixed blades serve as guide blades as well as nozzles. The passage between fixed blades is narrowed for this purpose towards moving blades. As steam passes through fixed blades, its velocity increases, thus K.E. increases. As it passes over moving blades velocity falls implying absorption of K.E. by the rotor. Thus mechanical energy is developed.
The steam, after passing through the turbine, exhausts into the condenser through a draft tube attached to the tail end (exit) of the turbine.
In a Parson's turbine fixed and moving blades are made identical; that is
Due to continuous expansion of steam over the blades, relative velocity does not remain constant; it slightly increases.