In this article we will discuss about:- 1. Simple Impulse Turbine 2. Reaction Turbine 3. Impulse Reaction Turbine.
Simple Impulse Turbine:
The impulse turbine depends completely upon the dynamic action of the steam. The direction of motion of the steam is changed as it passes across the blade. As a result of change of direction of steam across the blade, it will impart a force on the blade. Now if the blade is free, it will move in the direction of force.
In impulse turbine, steam first passes through the nozzle. In nozzle, the enthalpy is reduced and is converted into kinetic energy. The high velocity steam passes over moving blades and force is developed as shown in Fig. 20.1. The numbers of blades are fixed around the circumference of the shaft. So the steam will cause the turbine shaft to rotate. Thus mechanical work is obtained. The velocity of steam is reduced as it moves over the moving blades. But the pressure remains unchanged.
The steam turbine shown in Fig. 20.2 is simple impulse turbine or De-Laval turbine. There is only one set of nozzles. The complete expansion of steam from the steam chest pressure to the exhaust or condenser pressure takes place in the one set of K nozzles. Thus the pressure in the moving blades chamber is approximately equal to condenser pressure.
The steam enters the moving blades chamber with a high velocity. The pressure in the set of moving blades remains constant. The velocity of steam is reduced as it passes over moving blades as some of the kinetic energy of the steam is used in producing work on the turbine shaft.
For simple, impulse turbine, rotational speeds of the magnitudes of 20000 rpm may be obtained. This high speed of rotation will produce high centrifugal force which will restrict the size of the turbine. Thus the De-Laval type turbine is of relatively small size and hence has a small power-output. Also due to the high speed of rotation, a direct drive between the turbine disc and external equipment such as generator is not possible.
For this reason, a reduction gear box is installed between the turbine-shaft and the external equipment. Also the velocity of steam leaving the turbine is quite appreciable resulting in an energy loss. It is as high as 10 to 12% of the initial kinetic energy of steam.
Velocity Diagram for Simple Impulse Turbine:
α = Angle of absolute velocity measured at inlet with the plane of rotation
β = Angle of absolute velocity at outlet
θ = Blade inlet angle or angle of relative velocity at inlet
φ = Blade outlet angle or angle of relative velocity at outlet
Vb = Linear blade speed or blade velocity
Vai = Absolute velocity of steam at inlet
Vae = Absolute velocity of steam at outlet
Vri = Relative velocity of steam at inlet
Vre = Relative velocity of steam at outlet
Force = Rate of change of momentum in tangential direction
= (mass flow rate of steam) × (Change in velocity of whirl)
= m × (Vwi – Vwe)
Diagram Efficiency (or Blade Efficiency):
End Thrust or (Axial Thrust):
= Change of momentum in axial direction
Axial thrust = Mass flow rate x Change of velocity in axial direction
= m × (Vfi – Vfe)
Velocity Diagram for Reaction Turbine:
In case of a reaction turbine, the expansion of steam takes place in fixed blades as well as in moving blades. The figure shows the expansion of steam in a reaction turbine. The steam enters the turbine at pressure Po. In fixed blades it expands to pressure P1.
So (ho – h1) is enthalpy drop in fixed blades. Then steam flows over the moving blades and expands from pressure P1 to pressure P2. The enthalpy drop in moving blades is (h1 – h2). Due to expansion in moving blades, the relative velocity of steam continuously increases (i.e. Vre > Vη).
The term ‘degree of reaction’ is used for reaction turbine. It measures the proportion of the work-done by the effect (due to pressure drop in moving blades) The degree of reaction is defined as,
When D.R. = 0, it is an impulse turbine, since no enthalpy drop is taking place in moving blades.
When D.R. = 1, it is 100% reaction turbine.
For Parson’s reaction turbine, the blades are symmetric and degree of reaction is 50%.
We know that,
Enthalpy drop in moving blades is equal to increase kinetic energy of the steam to corresponding to relative velocity while, the steam flows over the moving blades.
Thus for 50 % reaction turbine i.e. for Parson’s turbine, the fixed and moving blades have the same shape. This gives rise to symmetric velocity diagram.
For 50 % reaction turbine, the velocity diagram is symmetric i.e. inlet velocity triangle is same as outlet velocity triangle.
Impulse Reaction Turbine:
Principal of Operation:
In reaction turbines, the pressure of the steam is reduced as it moves over the moving blades that gives rise to an increase in kinetic energy. This pressure drop also gives rise to a reaction in the direction opposite to that of increased velocity. Thus driving force is obtained. In impulse reaction turbine, impulse principle plus reaction principle is used.
There are number of rows of moving blades fixed to the shaft and equal number of fixed blades attached to the casing. The fixed blades in this arrangement correspond to the nozzles referred to in the impulse turbine. Instead of a set of nozzles, steam is admitted for whole of the circumference and therefore there is all round admission.
In passing through the first row of fixed blades, the steam undergoes a small drop in pressure and, its velocity is increased. It then enters the first row of moving blades and just as in impulse turbine, it suffers change in direction and momentum. This gives an impulse to the blades.
But here the passage of the blades is so designed that there is a small drop in pressure in the moving blades that gives rise to the reaction force. Thus the driving force is a vector sum of impulse and reaction forces. The steam velocities in this type of turbine are comparatively low.