Page 353 - Steam Turbines Design, Applications, and Rerating
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Elliott Shortcut Selection Method for Steam Turbines  327

                            ΔH i = 3211.8 − 2995.0 = 216.8 kJ/kg


                         V b =π(635 + 25) (4500)/60,000 = 155.5 m/s

            V j for an ideal Curtis stage would therefore be:
                                           155.5
                                    V b
                               V j =    =        = 676 m/s
                                   0.23    0.23
            V j for an ideal Rateau stage would be:
                                           155.5
                                    V b
                               V j =    =        = 338 m/s
                                   0.46    0.46
            With a ΔH i of 216.8 kJ/kg, V j for the inlet-to-extraction section with
            one-stage would be:









                          V j =  2000 Δ H i = 44.72  216.8  = 658 m/s
            This is seen to be very close to the 676 m/s for an ideal Curtis stage. We
            will therefore assume that the inlet-to-extraction section will contain
            one 635-mm Curtis stage.
              Now for the extraction-to-exhaust section. To find the energy avail-
            able to this section we need the temperature of the steam entering this
            portion of the turbine (extraction steam temperature). The enthalpy of
            this steam will be:
                 inlet steam enthalpy −ΔH i (inlet-to-extraction) ×η (inlet-to-
              extraction) = 3211.8 − 216.8 × 0.70 = 3211.8 − 151.8 = 3060.0 kJ/kg

              From the Mollier diagram, Fig. 14.2, at 18.26 bar and 3060 kJ/kg the
            extraction steam temperature is found to be close to 315°C.
              The extraction-to-exhaust portion of this turbine therefore operates
            on steam conditions of 17.25 bar (gauge)/315°C/150 mbar (abs.). The
            energy available to the extraction-to-exhaust section is therefore (from
            steam table):

                            ΔH i = 3060.0 − 2228.0 = 832.0 kJ/kg

              The blade velocity for 890-mm nominal diameter staging with a
            25-mm blade height will be:


                          V b =π (890 + 25) (4500)/60,000 = 215 m/s
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