2. Steam turbines may be of two kinds, namely
(i) impulse turbine
In Impulse turbine, the enthalpy drop (pressure drop) completely
occurs in the nozzle itself and when the fluid pass over the
moving blades it will not suffer pressure drop again.
Hence pressure remain constant when the fluid pass over the rotor
blades. Fig. shows the schematic diagram of Impulse turbine.
3. (ii) Reaction turbine.
`
• In Reaction turbines, addition to the pressure drop occurs in the
nozzle there will also be pressure drop occur when the fluid passes
over the rotor blades. Fig. shows the Reaction turbine.
• Most of the steam turbine are of axial flow type devices except
Ljungstrom turbine which is a radial type.
8. Analysis on single stage turbine:
1. General velocity diagram of Impulse turbine:
If there is no frictional loss, then the axial thrust is zero as
Vf1 = Vf2 and Vr1 = Vr2
10. 5. Condition for Maximum Utilization Factor or Blade
efficiency with equiangular Blades for Impulse Turbine:
Consider the velocity diagram shown in fig (a).
Due to the effect of blade friction loss, the relative velocity at
outlet is reduced than the relative velocity at inlet. Therefore
Vr2 = Cb Vr1. Corresponding velocity triangle shown if fig (b)
11. Analysis on two stages:
1. Condition for maximum efficiency for velocity compounded
Impulse turbine (Curtis Turbine)
12. Then the maximum blade efficiency is,
Maximum total work done per kg of steam,
13. Reaction turbines:
These are of axial type. But pure reaction turbine are not in
general use, only impulse-reaction turbines are used.
1. The velocity triangle for general case:
In reaction turbine steam continuously expands as it flows over
the blades thereby increases the relative velocity of steam,
i.e., Vr2 > Vr1
14. 2. Degree of reaction (R):
The degree of reaction for reaction turbine stage is defined as
the ratio of enthalpy drop in the moving blades to the total
enthalpy drop in fixed and moving blades (i.e., static
enthalpy drop to total enthalpy drop), as shown in fig.
15. The variation ηb with blade speed ratio Φ for the reaction stage
is shown in fig.
16. Effect of Reheat Factor and Stage Efficiency:
Consider a turbine with n number of stages, say n = 4 as shown
in fig.
17. Isentropic Turbine Efficiency
►For a turbine, the energy rate
balance reduces to 1 2
)
(
2
)
V
(V
)
(
0 2
1
2
2
2
1
2
1
cv
cv z
z
g
h
h
m
W
Q
► If the change in kinetic energy of flowing matter is negligible,
½(V1
2 – V2
2) drops out.
► If the change in potential energy of flowing matter is negligible,
g(z1 – z2) drops out.
► If the heat transfer with surroundings is negligible, drops out.
2
1
cv
h
h
m
W
cv
Q
the left side is work developed per unit of mass flowing.
where
18. ►The isentropic turbine efficiency
is the ratio of the actual turbine
work to the maximum theoretical
work, each per unit of mass
flowing:
(Eq. 6.46)
19. Isentropic Compressor and Pump Efficiencies
►For a compressor the energy rate
balance reduces to
1
2
)
(
2
)
V
(V
)
(
0 2
1
2
2
2
1
2
1
cv
cv z
z
g
h
h
m
W
Q
► If the change in kinetic energy of flowing matter is negligible,
½(V1
2 – V2
2) drops out.
► If the change in potential energy of flowing matter is negligible,
g(z1 – z2) drops out.
► If the heat transfer with surroundings is negligible, drops out.
1
2
cv
h
h
m
W
cv
Q
the left side is work input per unit of mass flowing.
where
20. ►The isentropic compressor
efficiency is the ratio of the
minimum theoretical work input
to the actual work input, each per
unit of mass flowing:
(Eq. 6.48)
►An isentropic pump efficiency is defined similarly.
