204
6 Steam Turbines
The interpretation is:
v 1
2
2
: kinetic energy supplied to the rotor,
w
w
1
2
2
2
2
2
−
: rotor loss,
v 2
2
2
: outlet kinetic energy.
It seems appropriate to define rotor efficiency by
(6.6)
Such a definition is only relevant with an impulse turbine. As there is no pressure
drop, the kinetic energy supplied to the rotor is the only source of work. We define
a rotor velocity coefficient by
6.4.3 Stage Efficiency Definitions
In order to define stage efficiency, we must formulate a statement about the usefulness of the kinetic energy at the stage outlet. Rotor work is first written as
(6.7)
When outlet kinetic energy is completely lost, it becomes obvious from Eq. (6.7) together with Fig. 6.7 that the isentropic enthalpy drop supplied to the stage for work
production is ∆h h
h
v
s
s
s
=
−
=
00
1
1 2 1
2
. Internal efficiency is then
(6.8)
It becomes relatively simple to reason how this efficiency may be optimised (see
Sect. 6.4.6). If outlet kinetic energy is useful, e.g. because it is supplied to a next
stage, with Eq. (6.7) and Fig. 6.7, the enthalpy drop available to the stage for work
production actually is
2
2
1
1
1s
2
2
2
v
v
−
. This enthalpy difference may be visualised on
the h-s diagram by h
h s
00
02
−
, where the total state h s
02 is obtained by adding the
outlet kinetic energy to the state 1 2
s
s
= . Efficiency is then defined as
r
2
1
W .
v / 2
=
D
h
.
2
r 2s
r 1
w
w
w
f
f
=
=
∆W
v
v
v
w
w
v
s
s
=
−
−
−
−
−
1
2
1
2
1
2
1
2
2
2
2
2
2
2
2
2
2
2
(
) (
)
.
i
ss r
s
00
1s
00
2s
W
W
W
.
h
h
h
h
h
=
=
=
=
−
−
D
D
D
h
h h
D