Page 346 - Steam Turbines Design, Applications, and Rerating

320 Chapter Fifteen

Using the basic definition of TSR:
3600 3600
ΔH j = h 1 − h 2 = = = 281 kJ/kg
TSR 12.8
From Example 15.6, the optimum isentropic heat drop per Rateau stage =
135.2 kJ/kg.
280
Approximate stages required = = 2.1 (thermodynamically)
135.2
The turbine will require two Rateau stages.
Assuming 80 percent stage efficiency:
280 × 0.80 = 224 kJ/kg
3034.9 − 224 = 2810.9 kJ/kg
Example 15.8: Curtis and Rateau staging Conditions: p g = 103.4 bar (p a = 104.4
bar), 510°C, exhaust p g = 10.4 bar (p a = 11.4 bar). Find the number of 890-mm-
diameter Rateau stages required, at 5000 r/min, when using one 635-mm diame-
ter Curtis stage, assuming optimum stage efficiencies. (See Fig. 15.8.)
From Example 15.5 we found that a Curtis stage removes 282.8 kJ/kg
3394.1 − 282.8 = 3111.3 kJ/kg
Exhaust pressure p a = 40.6 bar

Assuming 70 percent stage efficiency:
282.8 × .70 = 197.9 kJ/kg
3394.1 − 197.9 = 3196.2 kJ/kg
From this end point to 11.4 bar, the isentropic heat drop for the Rateau stages is:
3196.2 − 2876.0 = 320.2 kJ/kg
From Example 15.6, the optimum isentropic heat drop per Rateau stage =
135.2 kJ/kg.
320.2
Approximate stages required = = 2.4 (thermodynamically)
135.2
The turbine will therefore require one 635-mm Curtis stage and two 890-mm
Rateau stages.
Assuming an overall Rateau efficiency of 81 percent, the end point will be:
3196.2 − .81 (320.2) = 2936.8 kJ/kg

15.3 Extraction Turbine Performance
Today’s fuel costs demand that the maximum amount of energy be
squeezed from each pound of steam generated. To help in this effort,
when both process steam and shaft power are required, many plant
designers are turning to extraction turbines.