Page 351 - Steam Turbines Design, Applications, and Rerating
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Elliott Shortcut Selection Method for Steam Turbines 325
The extraction-to-exhaust portion of this turbine therefore operates
on steam conditions of 250 psig/600°F/4-in Hga. The TSR (extraction-
to-exhaust) is 9.35 lb/kWh. The energy available to the extraction-to-
exhaust section is therefore:
ΔH i = (3413 Btu/kWh) ÷ (9.35 lb/kWh) = 365 Btu/lb
The blade velocity for 35-in nominal diameter staging with a 1-in blade
height will be:
V b =π(35 + 1)(4500)/720 = 706 ft/s
If all staging is of the Rateau type in this portion of the turbine:
V j = 706/.46 = 1535 ft/s
ΔH i per stage is therefore:
2 2
Vj 1535 2
ΔH i = = = (6.85) = 47.0 Btu/lb
224
224
The number of Rateau stages in this section would therefore be:
Total energy available 365
= = 7.7 (say 8)
Energy removed per stage 47.0
Our turbine will, therefore, contain one 25-in diameter Curtis stage
followed by eight 35-in diameter Rateau stages with the extraction
opening after the Curtis stage.
Using metric data, we would observe Fig. 15.10 and proceed as fol-
lows:
Assume: Shaft power and speed: 18,500 kW at 4500 r/min
Steam conditions: 40 bar (gage)/400°C/150 mbar (abs.)
Extraction requirements:
70,000 kg/h at 17.25 bar (gauge)
First tabulate the TSR for an absolute inlet pressure of 41.0 bar at 400°C:
TSR to extraction (p g = 17.25 bar, p a = 18.26 bar):
h 1 = 3211.8 kJ/kg at s = 6.756 kJ/(kg K)
From steam table, at the same entropy,
at p a = 18.0 bar, h = 2991.5 kJ/kg
at p a = 18.5 bar, h = 2998.2 kJ/kg
Therefore, at 18.26 bar, h 2 = 2995.0 kJ/kg