Page 254 - Chemical process engineering design and economics

Compressors, Pumps, and Turbines 235

Thus,

W T = - Ah (5.39)
The turbine efficiency,

Ah
T!T= —— (5-40)
Ah s
where Ah s is the change in enthalpy for an isentropic expansion.
Therefore,

W T = - T| T Ah s = - TVT (has - hi) = i\r(hi - h 2S) (5.41)
After multiplying Equation 5.41 by the steam flow rate, m, we obtain

= Ti T m(h 1 -h 2S ) (5.42)
Because power is the rate of doing work, P T = m WT, Equation 5.42 be-
comes

P T = Tl T m(h 1 -h 2S ) (5.43)

When sizing steam turbines, Molich [34] recommends a safety factor of
10%.
Efficiencies for single-stage turbines are given in Figure 5.19 for noncon-
densing, dry, saturated steam. As it can be seen, the turbine efficiency, which in-
cludes mechanical as well as hydraulic losses, depends on brake or shaft power,
steam pressure, and turbine speed. To take into account the reduction in efficiency
caused by condensation, an arbitrary method, quoted in Reference 14, is to multi-
ply the turbine efficiency by the average of the vapor mass fraction entering and
leaving the turbine. Also, the effect of superheated steam on the turbine efficiency
is taken into account by dividing by a correction factor, c s, given in Figure 5.20.
Thus, the turbine efficiency of a single-stage turbine, given by Neerkin [31], is

( X "l T) B
TI T = I - — — (5.44)
|
I 2) c s
where x is the mass fraction of water, and T|B is the single-stage isentropic effi-
ciency from Figure 5.19.

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