Page 353 - Steam Turbines Design, Applications, and Rerating
P. 353
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