ntg where ntg is the turbine-generator overall efficiency, stated or specified at "design" or full-load conditions. Some of the factors that define the overall efficiency of a turbinegenerator set are: the inlet volume flow, pressure ratio, speed, geometry of turbine staging, throttling losses, friction losses, generator losses and kinetic losses associated with the turbine exhaust. Most turbine manufacturers provide charts specifying either ASR or ng values. Once the ASR has been established, the net enthalpy of the steam supplied to process (NEP) can be calculated:

where Hi = enthalpy at the turbine inlet conditions (Btu/lb)

3500 = conversion from heat to power (Btu/ kWh), including the effect of 2.6% radiation, mechanical and generator losses Hc = enthalpy of condensate return (Btu/lb)

*See Appendix 1 at the end of the book, "Mollier Diagram for Steam."

Hm = enthalpy of make-up water (Btu/lb) x = condensate flow fraction in boiler feed-water

(1-x) = make-up water flow fraction in boiler feed water

Hence, assuming a straight flow turbine (See Figure 7.5), the net heat to process (Btu/hr) defined in Section 7.2.1 can be obtained by multiplying equation 7.3 by the flow rate in lb/hr. The analysis of the overall cycle would require the replication of complete heat and mass balance calculations at part-load efficiencies. To expedite these computations, there are a number of commercially available software packages, which also produce mass/ heat balance tables. See Example 8 for a cogeneration software application.

Selection of Smaller Single-stage Steam Turbines

There exist many applications for smaller units (condensing and noncondensing), especially in mechanical drives or auxiliaries (fans, pumps, etc.); but a typical application is the replacement (or bypass) of a pressure reducing valve (PRV) by a single-stage back-pressure turbine.

After obtaining the TSR from inlet and outlet steam conditions, Figure 7.6 helps in determining the approximate steam rate (ASR) for smaller (<3000HP) singlestage steam turbines. Figure 7.7 is a sample of size and speed ranges available from a turbine manufacturer. It should be noticed that there is an overlap of capacities (e.g. an ET-30 unit can operate in the ET-25 range and in a portion of the ET-15/ET-20 range). Thus, the graph can help in selecting a unit subject to variable steam flows and/or loads.

Example 1. A stream of 15,500 lb of saturated steam at 250 psig (406°F) is being expanded through a PRV to obtain process steam at 50 psig. Determine the potential for electricity generation if the steam is expanded using a singlestage back-pressure 360.0 RPM turbine-generator.


Enthalpy, hi : 1201.7 Btu/lb (from Mollier chart)

Outlet Steam : 50 psig subcooled (9.67% moisture).

Enthalpy, ho : 1090.8 Btu/lb (from Mollier chart) Turbine Speed : 3600 RPM

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

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