Page 294 - Steam Turbines Design, Applications, and Rerating

Shortcut Graphical Methods of Turbine Selection 271

Using an actual Mollier diagram (Fig. 14.1 or 14.2), find (h 1 ) at the
intersection of inlet pressure and temperature lines. Extend a line ver-
tically down until it crosses the pressure line associated with the
exhaust pressure. This is indicated by line 1. Line h 1 − h 2 corresponds to
the theoretical enthalpy drop. The actual exhaust enthalpy will be at a
higher value because of the turbine efficiency. Therefore, the actual pro-
cess will follow line 2. From the equations on this chart, a theoretical
steam rate (TSR) can be determined. The actual steam rate (ASR) can
be determined by knowing the turbine efficiency. Efficiencies for single-
stage turbines will range from 30 to 60 percent, and multistage turbines
will range from 50 to 80 percent depending on operating conditions.

14.2 Estimating Steam Rates
Estimating steam rates is best illustrated by using the example of a
plant that has a surplus of 50,000 lb/h of 300 psig steam and requires
15 psig steam elsewhere in its process operations. The pressure reduc-
tion could be accomplished across a turbine and it is assumed that this
turbine would have an efficiency of 65 percent. It would drive an elec-
tric generator through a speed-reducing gear; their respective efficien-
cies are assumed to be 94 and 98 percent. Here would be the result:

h 1 = 1203.5 Btu/lb

h 2 = 1026.4 Btu/lb
2547*
TSR = = 14.38 lb/hp⋅h
1203.3 − 1026.4
14.38
ASR = = 22.12 lb/hp⋅h
0.65
kW produced =

50,000 lb/h × 0.98 gear eff. × 0.94 gen. eff.

22.12 lb/hp⋅h × 1.341 hp/kW

= 1550 kW
Steam turbine selection is also possible by making extensive use of
vendor-supplied data.
For simple single-stage machines, Tables 14.1 and 14.2 give perfor-
mance and physical data (dimensions, etc.) that closely approximate

*1 hp⋅h = 2547 Btu.