Page 147 - Advanced Gas Turbine Cycles

118 Advanced gas turbine cycles

Within the steam plant % depends on several factors:
the boiler and condenser pressures;
0 the turbine and boiler feed pump efficiencies;
0 whether or not there is steam reheat;
0 whether or not there is feed heating and whether the steam is raised in one, two or three
stages.
On the other hand 778 depends on some of the following features of the gas turbine
plant:
the gas turbine final exit temperature;
0 the specific heat capacity of the exhaust gases; and
0 the allowable final stack temperature.
The interaction between the gas turbine plant and the steam cycle is complex, and has
been the subject of much detailed work by many authors [5-81. A detailed account of
some of these parametric studies can be found in Ref. [l], and hence they are not discussed
here. Instead, we first illustrate how the efficiency of the simplest CCGT plant may be
calculated. Subsequently, we summarise the important features of the more complex
combined cycles.

7.5.1. A Parametric calculation
We describe a parametric ‘point’ calculation of the efficiency of a simple CCGT plant,
firstly with no feed heating. It is supposed that the main parameters of the gas turbine upper
plant (pressure ratio, maximum temperature, and component efficiencies) have been
specified and its performance (T& determined (Fig. 7.3 shows the T, s diagram for the
two plants and the various state points).
For the steam plant, the condenser pressure, the turbine and pump efficiencies are also
specified; there is also a single phase of watedsteam heating, with no reheating. The feed
pump work term for the relatively low pressure steam cycle is ignored, so that hb = ha. For
the HRSG two temperature differences are prescribed:
(a) the upper temperature difference, AT& = T4 - T,; and
(b) the ‘pinch point’ temperature difference, ATk = T6 - T,.
With the gas temperature at turbine exit known (T4), the top temperature in the steam
cycle (T,) is then obtained from (a). It is assumed that this is less than the prescribed
maximum steam temperature.
If an evaporation temperature ( p,) is pre-selected as a parametric independent variable,
then the temperatures and enthalpies at c and e are found; from (b) above the temperature
T6 is also determined. If there is no heat loss, the heat balance in the HRSG between gas
states 4 and 6 is

(7.21)

where Mg and M, are the gas and steam flow rates, respectively. Thus, by knowing all the
enthalpies the mass flow ratio p = MJMg can be obtained. As the entry water temperature
Tb has been specified (as the condenser temperature approximately), a further application