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Elliott Shortcut Selection Method for Steam Turbines 313

For example, find the approximate steam rate when the 18,500 kW,
5000 r/min turbine we’ve discussed is operated at 14,800 kW and 4500
r/min.

% Power = 14,800 kW/18,500 kW = 80%

% Speed = 4500/5000 = 90%
From Fig. 15.3, the power correction is 1.04 and the speed correction
1.05. Total correction is 1.04 × 1.05 = 1.09. The part load steam rate is
therefore 4.57 × 1.09 = 4.98 kg/(kWh).

15.2 Stage Performance Determination
The following examples illustrate the performance of various combina-
tions of impulse staging. It should be understood, of course, that stage
selection and overall turbine efficiency are affected by many important
considerations other than stage efficiency. Speed limitations, mechani-
cal stresses, leakage and throttling losses, windage, bearing friction,
and reheat must be factored into the ultimate turbine design. That’s
the job of the vendor’s turbine specialist.
The approximate efficiencies of a Curtis stage and a Rateau stage
are shown in Fig. 15.4.

Figure 15.4 Approximate effi-
ciencies of Curtis and Rateau
stages. (Elliott Company, Jean-
nette, Pa.)