1Unit Fuel Consumption Output Characteristics

These depend on the unit design and manufacturer. The actual specific fuel consumption or efficiency and output also depend on (a) ambient temperature, (b) pressure ratio and (c) part-load operation. Figure 7.11 shows performance data of a gas turbine. Vendors usually provide this kind of information.

2. Exhaust Flow Temperature. This data item allows the development of the exhaust heat recovery system. The most common recovery system are HRSGs which are classified as unfired, supplementary fired and fired units. The amount of steam that can be generated in an unfired or supplementary fired HRSG can be estimated by the following relationship:


steam flow rate exhaust flow rate to HRSG

specific heat of products of combustion gas temperature-after burner, if applicable

saturation temperature in steam drum a factor to account radiation and other losses, 0.985

enthalpy of steam leaving superheater saturated liquid enthalpy in the steam drum HRSG effectiveness = (T1-T2)/(T1-T3), defined by Fig. 7.10.

fuel factor, 1.0 for fuel oil, 1.015 for gas.

total gas flow to the HRSGs, if the system must produce a maximum of 160,000 lb/hr of 615 psia/750°F steam. How many HRSG-gas turbine sets are needed?

DATA (See notation of Equation 7.5 and Figure 7.10)


3. Parametric Studies for Off-design Conditions. Varying the amount of primary or supplementary firing will change the gas flow rate or temperature and the HRSG steam output. Thus, according to the varying temperatures, several iterations of equation [7.5] are required to evaluate off-design or part load conditions. When this evaluation is carried over a range of loads, firing rates, and temperatures, it is called a parametric study. Models can be constructed or off-design conditions using gas turbine performance data provided by manufacturers (See Figure 7.11).

4. Exhaust Pressure Effects on Output and Exhaust Temperature. Heat recovery systems increase the exhaust backpressure, reducing the turbine output in relation to simple operation (without HRSG). Turbine manufacturers provide test data about inlet and backpressure effects, as well as elevation effects, on turbine output and efficiency (Fig 7.11).

Example 4. In a combined cycle (Fig 7.4), the steam for the turbine of Example 2 must be generated by several HRSGs (See Fig 7.10). To follow variable CHP loads and to optimize overall system reliability, each HRSG will be connected to a dedicated 10-MWe gas turbine-generator (GTG) set. The GTG sets burn fuel-oil and each unit exhausts 140,000 kg/hr of gas at 900°F. Estimate the

Cp f

140,000 kg/hr per gas-turbine unit 0.26 Btu/lb/°F, average specific heat of gases between T1 and T4 900°F, hot gas temperature fuel factor, 1 for fuel oil.

The inlet steam conditions required in the steam turbine of Example 3 are: (615 psia/750°F) and hi = 1378.9 Btu/ lb (from Mollier chart). Thus,

HRSGs Ws = 160,000 lb/hr from all HRSGs

e = HRSG effectiveness, 0.9

Therefore, hSh = hi = 1378.9 Btu/lb and, hsat = 474.7 Btu/lb (Sat. Water @ 615 psia)

From equation 7.5, the total combustion gas flow required is

Water |

Water |

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