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)o)h 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 water/steam heating, with no reheating. The feed pump work term for the relatively low pressure steam cycle is ignored, so that /ij, = /i. For the HRSG two temperature differences are prescribed  

With the gas temperature at turbine exit known (T ), 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 (Pc) is pre-selected as a parametric independent variable, then the temperatures and enthalpies at c and e are found from (b) above the temperature T(, is also determined. If there is no heat loss, the heat balance in the HRSG between gas states 4 and 6 is 

Even for this simplest CCGT plant, iterations on such a calculation are required, with various values of p, in order to meet the requirements set on T, the steam turbine entry temperature, and 7s (the calculated value of 7s has to be such that the dewpoint temperature of the gas (7jp) is below the economiser water entry temperature (7b) and that may not be achievable). But with the ratio /i satisfactorily determined, the work output from the lower cycle Wl can be estimated and the combined plant efficiency obtained from 

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Harvey et al. gave a parametric calculation of the thermal efficiency of this plant, as a function of turbine inlet temperature, the reformer pinch point temperature difference and the pressure level in the reformer (the compressor overall pressure ratio, r). 

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