installation of topping turbines in retrofit applications.

The high net thermal efficiency of topping-cycle applications makes them highly competitive with any form of electric generation. The key factor is the availability of a continuous low-pressure heat sink. A 1,000 kW topping turbine would typically serve a low-pressure steam load of 30,000 to 40,000 lbm/h (14,000 to 18,000 kg/h). As the heat sink becomes smaller, so does the return on investment.

An extraction turbine is a multi-stage unit having one or more outlets to allow intermediate-pressure steam to be withdrawn. Extraction turbines are generally designed for applications in which there is a need for steam at different pressures or where there are varying low-pressure process steam requirements. Commonly, though not always, extraction turbines are condensing turbines. Fixed extraction turbines may sometimes be used as an alternative for two smaller turbines (one back-pressure and one condensing). Varying extraction turbines can function as either condensing or non-condensing units in different periods or as both simultaneously, depending on load.

A bottoming cycle can be used for industry applications that discharge high-pressure steam or waste heat. Admission turbines operating on a bottoming cycle can utilize steam as available at multiple pressures.

While the steam generation source is commonly an independent boiler system, the turbine generator sets are often delivered to the site in compact skid-mounted packages, factory-designed and -assembled for the application. Larger systems may be assembled on site, though various components may be prepackaged.

Steam Turbine System Performance

The thermal efficiency of a steam turbine system varies far more than with reciprocating and gas turbine engines and is mostly a function of turbine design and steam supply and exhaust conditions. Turbine mechanical efficiency typically ranges from 30 to 60% for single-stage turbines and 50 to 85% for multi-stage turbines. System simple-cycle electric generation (thermal) efficiency typically ranges from 20 to 30% for condensing systems and 5 to 15% for non-condensing systems.

Tables 26-3 and 26-4 show representative steam rates for condensing steam turbines, in lbm/kWh (kg/kWh), under various inlet conditions, each at an exhaust condition of 3 in. HgA (10.2 kPa), in English and SI units, respectively. Included are theoretical steam rates and actual steam rates (excluding generator losses) with various steam turbine mechanical efficiencies. Tables 25-5 and 25-6 show representative heat rates (HHV basis) in English and SI units, respectively. Heat rates are calculated under the assumption of 83 Btu/lbm (193 kJ/kg) hotwell enthalpy and 83% boiler efficiency.

The distinguishing performance feature of non-condensing (topping-cycle cogeneration) applications is that while simple-cycle thermal efficiency is low, net thermal efficiency approaches 100% (excluding boiler losses) because virtually all unused heat energy (in the form of lower pressure steam) is passed directly to process. The topping-cycle turbine is, therefore, an extremely efficient power generator. While the energy input to the topping-cycle steam turbine, per unit of power output, is far greater than with a reciprocating or combustion turbine engine, net thermal efficiency, assuming effective use of all low-pressure output steam, is generally equal to or greater than either of the engine types.

Table 26-7 shows representative steam rates, in lbm/kWh, for non-condensing turbine operation under various inlet and exhaust conditions. Included are theoretical steam rates and steam rates for turbines with mechanical efficiencies of 35%, 50%, and 60%. Table 26-8 shows the steam rates from Table 26-7 in SI units.

Figure 26-31 represents performance of a large industrial condensing turbine with a variable medium-pressure extraction. This turbine's power output (in thousand kW, or MW) varies depending on the total steam entering the turbine (throttle steam) and the amount of steam that leaves via the extraction port. The straight parallel lines show how power output varies with these two steam flow rates. Note that the turbine has several physical limits: the maximum throttle (or inlet) flow, the maximum exhaust

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