Page 123 - Handbook of Energy Engineering Calculations

efficiency and heat rate of the combined cycle.

Calculation Procedure:

1. Sketch the cycle layout
Figure 1 shows the cycle. Since the gas-turbine exhaust-gas temperature is
usually higher than the bleed-steam temperature, the economizer is placed
after the regenerative feedwater heater. The feedwater will be progressively

heated to a higher temperature during passage through the regenerative heater
and the gas-turbine economizer. The cycle shown here is only one of many
possible combinations of a steam plant and a gas turbine.

2. Sketch the T-S diagram

Figure 2 shows the T-S diagram for the combined gas turbine–steam turbine
cycle. There is irreversible heat transfer Q from the gas-turbine exhaust to
T
the feedwater in the economizer, which helps reduce the required energy

input Q .
a2

3. Sketch the energy-flow chart

Choose a suitable scale for the energy input, and proportion the energy flow
to each of the other portions of the cycle. Use a single line when the flow is
too small to plot to scale. Figure 3 shows the energy-flow chart.

4. Determine the thermal efficiency of the cycle
Since e = W/Q , e = Q − Q /Q = 1 − [Q + Q /(Q + Q )], given the
r1
r2
a2
a1
a
a
a
r
notation in Figs. 1 through 3.