Page 352 - Steam Turbines Design, Applications, and Rerating
P. 352
326 Chapter Fifteen
3600
TSR = = 16.6 kg/(kWh)
3211.8 − 2995.0
TSR to exhaust (p a = 150 mbar): From steam table or TSR tabulation (Table
14.4), TSR = 3.52 kg/(kWh).
Now assume that the efficiency of the entire turbine (inlet-to-exhaust) is 75 per-
cent and the efficiency of the inlet-to-extraction section is 70 percent. Therefore:
Approximate SR (inlet-to-extraction) will be:
16.6
= 23.7 kg/(kW⋅h)
0.70
Approximate SR (inlet-to-exhaust) will be:
3.52
= 4.69 kg/(kW⋅h)
0.75
Point A is the first point to be located on the diagram by multiplying:
Total power × approximate SR (inlet-to-exhaust) = 18,500 kW × 4.69 kg/(kW⋅h)
= 86,750 kg/h
Locate Point A at 18,500 kW and 86,750 kg/h throttle flow.
Point B is located at zero kW and a throttle flow of 5 percent of A. This 5 percent
flow is cooling steam going to the extraction section. Thus,
B = 0.05 × 86,750 kg/h = 4338 kg/h
The zero extraction line results from connecting Points A and B.
Point C is located by dividing:
Extraction flow requirement/approximate steam rate (inlet-to-extraction):
70,000 kg/h ÷ 23.7 kg/(kW⋅h) = 2955 kW
Locate Point C at 2955 kW and 70,000 kg/h throttle flow. Now draw line C-D par-
allel to A-B and another line C to B. Label A-B Zero Extraction and C-D 70,000
kg/h Extraction.
Notice that the general shape of the diagram and the slopes of the
lines are determined mainly by the steam conditions used. The turbine
investigated here could be built, but one must remember that we have
dealt only with the thermodynamic aspects of this application. The
mechanical aspects, such as blade stresses, nozzle flow limits, cooling
steam and other factors must also be checked.
This example can also be carried further to determine the number of
stages in each section using the procedures outlined earlier. To do so
requires finding the energy available in each portion of the turbine.
The energy available (ΔH i ) to the inlet-to-extraction section is: