Energy Management Civil Engineering

Figure 7.7 Power/speed ranges for single stage turbines. Courtesy Skinner Engine Co.

Figure 7.6 Estimation of steam rates for smaller (<3,000 HP) single-stage turbines. Courtesy Skinner Engine Co.

a) Calculate TSR using equation 7.1

3412 Btu/kWh

= 30.77 lb/kWh b) Obtain steam rate from Fig. 7.6, ASR = 38.5 lbs/HP-hr.

c) Calculate potential generation capacity (PGC): Ws(0.746 kW/hp)

Figure 7.7 shows that units ET-15 or ET-20 better match the required PGC. Next, the generator would

Figure 7.7 Power/speed ranges for single stage turbines. Courtesy Skinner Engine Co.

have to be sized according to a commercially available unit size, e.g. 300 kW.

Selection of Multi-Stage Steam Turbines

Multistage steam turbines provide more flexibility to match various pressure levels and variable flow rates in larger cogeneration applications. Figure 7.5 describes a variety of back-pressure and condensing multi-stage steam turbines.

For turbine selection, a Mollier diagram should be used to explore various multi-stage turbine alternatives. In general, a preliminary analysis should include the following:

— Approximation of actual steam rates.

— Defining number of stages.

— Estimation of stage pressure and temperatures.

— Calculation of full-load and part load steam rates.

— Estimation of induction and/or extraction pressures, temperatures and flow rates for various power outputs.

For larger units (>3000 kW), Figure 7.8 shows a chart to determine the approximate turbine efficiency when the power range, speed and steam conditions are known. Figure 7.9 gives steam rate correction factors for off-design loads and speeds. However, manufacturers of multistage turbines advise that stage selection and other thermodynamic parameters must be evaluated taking into account other important factors such as speed limitations, mechanical stresses, leakage and throttling Enthalpy, hi losses, windage, bearing friction and reheat. Thus, after a preliminary evaluation, it is important to compare notes with the engineers of a turbine manufacturer. Outlet Steam

1378.9 Btu/lb (from Mollier chart or steam tables)

Example 2. Steam flow rate must be estimated to design a heat recovery steam generator (a bottoming cycle). The steam is to be used for power generation and is to be expanded in a 5,000 RPM multistage condensing turbine to produce a maximum of 18,500 kW. Steam inlet conditions is 600 psig/750°F and exhaust pressure is 4" HGA (absolute). Additional data are given below.

Data (for a constant entropy steam expansion @ S = 1.61 Btu/lb/°R)

Enthalpy, ho : 935.0 Btu/lb (from Mollier chart or ASME steam tables)

Turbine Speed : 5000 RPM

Calculate TSR using equation 7.1 (TSR can also be obtained from ASME Tables or the Mollier chart):

Figure 7.8 Approximate steam turbine efficiency chart for multistage steam turbines (>3,000 kW. Courtesy Elliot Co.

3412 Btu/kWh hi - ho Btu/lb

7.68 lb/kWh

Using the data above, Figure 7.8 gives ntg = 77%.

Combining equations 7.2 and 7.4 and solving for Ws, the total steam flow required is

PCG x TSR/ntg 18,500 kW x 7.68 lb/kWh

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